EPA/600/R-95/173
Assessment of Nitrogen Loads
        to Aquatic Systems

                     by

        A.S. Patwardhan and A.S. Donigian, Jr.
            AQUA TERRA Consultants
             Mountain View, CA 94043
            Contract Number 68-CO-0019
                 Project Officer

              Thomas O. Barnwell, Jr.
        National Exposure Research Laboratory
                Athens, GA 30605
        National Exposure Research Laboratory
          Office of Research and Development         yt-nv---
         U.S. Environmental Protection Agency      DRINKING ^^^r
                Athens, GA 30605

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                                      NOTICE
The U.S. Environmental Protection Agency through its Office of Research and Development funded
and managed the research described here under Contract No. 68-CO-0019 to AQUA TERRA
Consultants.  It has been subjected to the Agency's peer and administrative review and has been
approved for publication as an EPA document.

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                                     FOREWORD

The Ecosystems Research Division of EPA's National Exposure Research Laboratory conducts
research on organic and inorganic chemicals, greenhouse gas biogeochemical cycles, and land use
perturbations that create direct and indirect, chemical and nonchemical, stressor exposures and
potential risks to humans and ecosystems. Comprehensive models based on fundamental studies of
stressor behavior are developed to predict exposures in mutimedia environments, to simulate the
interactions of the climate system and the terrestrial biosphere, and to evaluate the aggregate causes
of ecological stress, including land use change/management, within a watershed/regional context.
Field and laboratory experiments are conducted to quantify and model greenhouse gas fluxes between
the atmosphere and the terrestrial biosphere and to understand abiotic and biotic pollutant fate
processes in soils, sediments and water.

In the study reported here, work by the Environmental Defense Fund to estimate the various sources
of nitrogen to  Chesapeake Bay is reviewed and replicated. An improved methodology was then
developed for estimating atmospheric nitrogen contributions to aquatic systems by refining some of
the EDF procedures and assumptions  used in estimating the contributions  from various nitrogen
sources within the  watershed.  The  refined  procedures  were applied to the Chesapeake Bay
watershed,  to  Galveston  Bay (Texas) and to Tampa Bay (Florida) using commonly available
databases.

                                               Rosemarie C. Russo, Ph.D.
                                               Director
                                               Ecosystems Research Division
                                               Athens, Georgia
                                        111

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                                     ABSTRACT
Nitrogen is ubiquitous in the environment, released by many mobile and stationary sources, and
is a major plant nutrient applied as fertilizers and  animal wastes.   EPA has recognized the
importance of nitrogen in the environment for both beneficial (e.g. soil fertility, plant nutrition)
and deleterious effects, ranging from the control of ozone distribution, greenhouse gas impacts,
air  pollution, acidification  and eutrophication  of surface  waters,  and contamination  of
groundwater drinking supplies.

In 1988 the Environmental Defense Fund  (EDF) estimated the various sources of nitrogen to the
Chesapeake Bay;  these included 23% from point sources, 39% from atmospheric deposition
(including both nitrate and ammonia), 34%  from fertilizers, and 4% from animal wastes. They
further projected that with atmospheric nitrogen emissions to increase by 44% by the year 2030,
atmospheric nitrogen contributions to aquatic systems are likely to become even more significant.

The objective of the study is to estimate the contributions of atmospheric nitrogen deposition to
Chesapeake Bay, Galveston Bay, and Tampa Bay with respect to the nitrogen contributions from
nonpoint and point sources.  The EDF methodology was reviewed and refined to develop the
current  project  methodology.   A Lotus  123  spreadsheet was developed based on the new
methodology, and the spreadsheet was used in estimating  nitrogen loadings from the various
nitrogen sources found in each of the three study  watersheds.

The results obtained from the application of the methodology to the Chesapeake Bay watershed
were  in agreement  with the  EDF results.  Sensitivity  analysis was performed  on  three
spreadsheet assessment parameters  — ratio  of wet to dry  atmospheric deposition,  first-order
riverine nitrogen decay rate, and amount of fertilizer application. During the sensitivity analysis
we  analyzed  the  sensitivity  of the  ratio of wet  to  dry deposition  because dry atmospheric
deposition data  is not readily available,  and changes to this ratio effectively change the total
deposition. It was concluded from the sensitivity  analysis that, as atmospheric deposition is an
important  source  of nitrogen input  on all  the land  use categories in the watershed, it has a
significant effect on the nitrogen loadings that are delivered to the aquatic systems.

The results obtained from the application of the methodology to the three study sites indicate that
over 40%  of the total nitrogen load delivered to aquatic systems may originate from atmospheric
deposition.   Recommendations to  improve the  current methodology estimates and  further
research issues are provided.

This report was submitted in fulfillment  of Contract No. 68-CO-0019, Work Assignment No.
27(11), by AQUA TERRA Consultants of Mountain View, CA under sponsorship of the U.S.
Environmental Protection Agency.   This report covers the period from October 1992 through
September 1993, and work was completed as of 30 September 1993.  Final report revisions were
performed under Purchase Order No. 5D2086NASA, and were completed September 30, 1995.

                                          iv

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                                  CONTENTS

                                                                          Page
Disclaimer	      ii
Foreword	     iii
Abstract	     iv
Figures	     vii
Tables	    viii
Acknowledgments	      x

1.0   INTRODUCTION  	      1
      1.1    Study Goals and Objectives  	      1
      1.2    Summary of Study Results	      2
      1.3    Study Conclusions and Recommendations	      4
      1.4    Format of Report	      5

2.0   REVIEW OF EOF ANALYSIS  	      6
      2.1    EDF Procedures Used for the Chesapeake Bay Watershed	      6
            2.1.1  Equal Retention Procedure	      8
            2.1.2  Differential Retention Procedure	     10
      2.2    EDF Study Conclusions	     14
      2.3    Assumptions Used in the EDF Study	     14

3.0   METHODOLOGY USED IN ESTIMATING NITROGEN LOADS TO AQUATIC
      SYSTEMS	     17
      3.1    Calculational Steps in Current Methodology	     17
      3.2   Define NPS Loads by Land Use	     18
      3.3    Define Point Source Loads	     20
      3.4   Subbasin Loads	     20

4.0   ASSESSMENT OF NITROGEN LOADS FROM STUDY SITES	     25
      4.1    Application of the Methodology to Chesapeake Bay
            Watershed	     25
            4.1.1  Comparison of New Methodology Results with
                  EDF Results	     28
            4.1.2  Comparisons of Percent Loadings from Individual
                  Nitrogen Sources  	     35
            4.1.3  Conclusions from Chesapeake Bay Comparison	     36

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      4.2   Application of Methodology to Galveston Bay	     37
            4.2.1  Available Data for Analysis	     37
            4.2.2  Results Obtained from Galveston Bay	     43

      4.3   Application of Methodology to Tampa Bay	     47
            4.3.1  Available Data for Analysis	     47
            4.3.2  Results Obtained from Tampa Bay	     50

      4.4   Sensitivity Analysis  	     53
            4.4.1  Sensitivity of Wet and Dry Atmospheric Deposition
                  Ratios	     53
            4.4.2  Sensitivity to Riverine Decay Parameter 	     56
            4.4.3  Sensitivity to Fertilizer Application Amounts	     58
            4.4.4  Conclusions from Sensitivity Analysis	     61

5.0   CONCLUSIONS AND RECOMMENDATIONS	     63

6.0   REFERENCES  	     66

7.0   APPENDIX A - Description of the Lotus 123* Spreadsheet   	    A-l
                                       VI

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                                    FIGURES

2.1   Chesapeake Bay Watershed	      7
2.2   EDF's Differential Retention Procedure Flowchart	     13

3.1   Methodology Flow Chart for Estimating Nonpoint and Point
      Source Loads  	     22
3.2   Example of NPS Load Computations from each Subbasin	     23

4.1   Chesapeake Bay Watershed with Subbasin Boundaries	     26
4.2   Galveston Bay Watershed with Subbasin Boundaries	     38
4.3   Tampa Bay Watershed with Subbasin Boundaries	     48
                                        vn

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                                     TABLES

1.1   Comparison of Percent Nitrogen Contributions from Various
      Sources to Chesapeake Bay, Galveston Bay
      and Tampa Bay	      3

2.1   Calculated Nitrogen Loadings to the Land and Water Surfaces
      of the Chesapeake Bay Watershed in 1984	     10
2.2   Nitrogen Loadings to the Chesapeake Bay (US EPA, 1983)   	     11
2.3   Equal Retention Estimates of Nitrogen Sources to the Bay  	     11
2.4   Retention Factors and N Loadings Using Differential
      Retention Procedure   	     15

4.1   Comparison of Current Nitrogen Inputs to EDF Nitrogen Inputs	     27
4.2   Estimated Mean Annual Total Nitrogen Loads from the Subbasins
      in the Chesapeake Bay Watershed (from Donigian et al., 1994)	     30
4.3   Land Use Areas in  the Chesapeake Bay Watershed (from
      Donigian et al., 1991)	     31
4.4   Above Fall Line Nitrogen Point Loads (from Donigian et al., 1994)	     31
4.5   An Example of the Input Requirements for the Lotus 123 Spreadsheet  ....     33
4.6   Comparison of Nitrogen Loads  to the Chesapeake Bay	     34
4.7   Comparison of Percent Nitrogen Contributions from Various
      Nitrogen Sources to the Chesapeake Bay	     35
4.8   Land Use Areas in  the Various  Subbasins of the Galveston Bay
      Watershed (modified from Newell et al., 1992)	     39
4.9   Total Edge-of-Field Nitrogen Loads from the Subbasins of the
      Galveston Bay Watershed (modified from Newell et al.,  1992)	     42

4.10  Unit Area Nitrogen Loads from the Various Land Use Categories
      in the Galveston  Bay Watershed	     44
4.11  Point Sources, Wet Atmospheric Deposition, and Travel Time for
      the Galveston Bay Watershed Subbasins 	     45
4.12  Estimated Total Nitrogen Loads to the Galveston Bay from
      Various Nitrogen Sources	     46
4.13  Percent Loadings from Various Nitrogen Sources to the
      Galveston Bay	     46
4.14  Total Edge-of-Field Nitrogen Loads from the Subbasins  of the
      Tampa Bay  Watershed	     49
4.15  Point Sources, Wet Atmospheric Deposition, and Travel Time for
      the Tampa Bay Watershed Subbasins  	     51
                                        vin

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4.16  Estimated Total Nitrogen Loads to the Tampa Bay from
      Various Nitrogen Sources	     52
4.17  Percent Loadings from Various Nitrogen Sources to the
      Tampa Bay	     52
4.18  Retention Factors for the Base Case for the Chesapeake Bay
      Watershed	     55
4.19  Sensitivity of Ratio of Dry to Wet Atmospheric Deposition
      Rates to Nitrogen Loadings to Chesapeake Bay	     57
4.20  Percent Contribution to the Chesapeake Bay with Varying
      Ratios of Wet to Dry Atmospheric Deposition Rates	     57
4.21  Sensitivity of Riverine Decay Rates to Nitrogen Loadings
      to Chesapeake Bay	     59
4.22  Percent Contribution to the Chesapeake Bay with Varying
      Riverine Decay Rates	     59
4.23  Sensitivity of Fertilizer Application Rates to Nitrogen
      Loadings to  Chesapeake  Bay  	     60
4.24  Percent Contribution to the Chesapeake Bay with Varying Base
      Scenario Fertilizer Rates	     60
4.25  Effect of Changing Atmospheric Deposition and Fertilizer
      Application of Total Nitrogen Load from the
      Chesapeake Bay Watershed	     62
                                         IX

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                              ACKNOWLEDGMENTS
This document was prepared under Work Assignment No. 27(11) of Contract No. 68-CO-0019
by AQUA TERRA Consultants for the U.S. Environmental Protection Agency, Environmental
Research Laboratory in Athens, GA.  Mr. Thomas Barnwell of the EPA-Athens Laboratory was
the EPA Project Manager providing technical and administrative guidance, and project direction.
The EPA Office of Air Quality and Standards, through Mr.  John  Bachmann and his staff, is
acknowledged for  its interest, support, and funding of this work.

A number of individuals provided data and information vital to the completion of this work. Dr.
Charles Newell and Elaine Higgins of Groundwater Services, Inc., provided all of the nonpoint
loading data and information about the Galveston  Bay watershed. Dr Neil Armstrong provided
point source loading data for the Galveston Bay watershed. Herbert Hudson from the Galveston
Bay National Estuary Program (NEP) program provided initial information about the Galveston
Bay. Ms. Holly Greening of the Tampa Bay NEP provided general background on the Tampa
Bay watershed, and Mr. Hans Zarbock of Coastal Engineering, Inc. provided nonpoint and point
source  data, and travel time estimates for the subbasins in the  Tampa Bay watershed.  Ms. Gail
Huff of Soil Conservation Service provided estimates of fertilizer application rates in  the Tampa
Bay watershed.

For AQUA TERRA Consultants, Mr. Avinash Patwardhan acted as the Project Manager, and
applied the developed methodology  to the three study sites, analyzed  results, and wrote the
report.    Mr.  Anthony Donigian provided  input  in  the conceptualization  of the  project
methodology, and  provided technical review and  guidance.

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                                     SECTION 1.0

                                   INTRODUCTION
 1.1  STUDY GOALS AND OBJECTIVES

 Nitrogen is ubiquitous in the environment, released by many mobile and stationary sources, and
 is a major plant nutrient applied as fertilizers and  animal wastes.  EPA has recognized the
 importance of nitrogen in the environment for both beneficial (e.g. soil fertility, plant nutrition)
 and deleterious effects, ranging from the control of ozone distribution, greenhouse gas impacts,
 air pollution, acidification  and eutrophication  of surface  waters,  and contamination  of
 groundwater  drinking supplies.  EPA  has mandates to evaluate and regulate these effects of
 nitrogen under the Clean Air Act, the Clean Water Act, the Safe Drinking Water Act, and under
 numerous international agreements.

 In  1988 the Environmental Defense Fund (EDF) (Fisher et al., 1988) estimated the various
 sources of nitrogen to the Chesapeake Bay; these included 23% from point sources, 39% from
 atmospheric deposition (including both nitrate and ammonia), 34% from fertilizers, and 4% from
 animal wastes.  They further projected that the atmospheric component would increase to 55%
 by  the year 2030.  In EPA Region III, the Chesapeake Bay Program is currently evaluating
 strategies for attaining a 40% reduction in nutrient loads (including nitrogen and phosphorous)
 reaching the Chesapeake Bay.  The modeling systems developed as part of that study include
 performing mass balance modeling of nitrogen on various land categories to explore and quantify
 both the sources of nitrogen and potential  reduction  alternatives.

 The objective of this work was to assess the applicability of the EDF approach and associated
 findings to other parts of the country, in particular to the Gulf Coast region. The approach was
 to review and evaluate the EDF methodology, to improve and refine the EDF methodology and
 assumptions,  and to implement these refined procedures to reproduce their results (in a Lotus
 123  spreadsheet  format) using commonly available data sources  (e.g. EPA/State waste
 discharges, emission  inventories, soils  and meteorologic data bases).  The refined procedures
 were then  to be applied to the Chesapeake Bay watershed; Galveston  Bay,  Texas; Tampa Bay,
 Florida; and the Ohio River Drainage Basin.  However, due to lack of nonpoint and point source
 data for the Ohio River Drainage Basin, this site was not included  in the study.

The first component of this study deals with  reviewing the EDF analysis and assumptions, and
then replicating the EDF results for the Chesapeake Bay watershed.  The second component of
the  study deals with  developing an improved methodology to estimate atmospheric nitrogen
contributions to aquatic systems by refining some of  the EDF procedures and assumptions used
in estimating the contributions from various nitrogen sources within a watershed.

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To  formalize and facilitate the refined project  methodology, we developed a  Lotus 123*
spreadsheet to estimate the contributions of atmospheric nitrogen deposition to the study sites
listed above with respect to the nitrogen contributions from nonpoint and point sources. The
land use categories from which nonpoint  source loadings were calculated consist of  forest,
cropland,  pastures, and urban area.   The point source  loadings consist of contributions of
nitrogen from industrial and sewage treatment plant discharges.  The annual wet atmospheric
nitrogen deposition data was obtained from the National Atmospheric Deposition Program
(NADP).

The Lotus 123 spreadsheet, which was developed by refining some of the assumptions that were
used in the EDF study, was applied to the Chesapeake Bay watershed. The total nitrogen loads
from each land use obtained by the current methodology were compared to the results obtained
from  the  EDF analysis.  The  spreadsheet was  then used to calculate the contributions of
atmospheric nitrogen deposition as it relates to other sources of nitrogen for the Galveston Bay
and Tampa Bay study sites.

1.2  SUMMARY OF STUDY RESULTS

The percent contributions from various nitrogen sources in the watershed for all three study sites
are presented in Table 1.1.  For the Chesapeake Bay, the results for the current  analysis are
based on dividing the watershed into fourteen subbasins, whereas the EDF results  (also shown
in Table 1.1) were obtained by considering the  watershed as a single basin.  As  can be seen
from Table 1.1, the  current methodology indicates that 43%  of the total nitrogen load to the
Chesapeake Bay results from atmospheric deposition, which is in close agreement with the 39%
estimate obtained by the EDF study; the total nitrogen loads of 140 million kg per year from the
EDF study was similar, but slightly higher than the 115  million kg per year from the current
study.  It can also be concluded from Table 1.1 that the percentage contributions calculated from
point sources and manure loadings to the Chesapeake Bay using the current methodology and
the EDF estimates are also in close agreement.  The current methodology estimated  slightly
higher contributions  of nitrogen from  point sources as compared to the EDF analysis. This
could be due to the data used to calculate point source loads.  In the current study we used an
average (1984-1987)  point source loadings as estimated  by  the U.S.  EPA Chesapeake Bay
Program Office (Donigian et al., 1994), while the EDF analysis used point source data  for the
year 1985  only.  The percent contribution from manure was 3.2% higher than the EDF estimate.
In the EDF analysis manure was only applied to the pastures, and the authors used a retention
factor 97.5% which  resulted in very low nitrogen  loadings to the Bay as a result of manure
application.  However, in our analysis, manure was applied to both cropland and pastures, and
as cropland has lower retention  of nitrogen than pastures, a slightly higher percentage nitrogen
load to the Bay was estimated.

The percentage nitrogen loadings to the Chesapeake Bay from fertilizer applications as estimated
by the two procedures do not agree. In the EDF analysis the  authors used the county fertilizer
sales data to estimate the total amount of fertilizer applied to the entire watershed and also used
a single constant retention factor for all the agricultural cropland in the Chesapeake Bay

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       Table 1.1    Comparison of Percent Nitrogen Contributions from Various Sources to
                   Chesapeake Bay, Galveston Bay, and Tampa Bay.
       Nitrogen                   EDF         Percent Using Current Methodology
       Source                     Analysis
                                 Chesapeake   Chesapeake   Galveston    Tampa
                                 Bay          Bay          Bay         Bay

       Point Source              23%         30%        48%          19%
       Atmospheric Deposition    39%         43%        49%          67%
       Fertilizer                  34%         20%          3%          14%
       Manure                    4%          7%
watershed.  However, in our analysis we used fertilizer application data, estimated by state and
county extension officials (Donigian et al., 1994).   In the present analysis the total fertilizer
application  was 13  million kilograms greater than the amount applied in the  EDF study.
However, using variable retention parameters (computed from unit area loads as reported by
Donigian et al., 1994) for all 14 subbasins resulted in lower fertilizer loads to the Chesapeake
Bay as compared to the EDF estimated load.

The results obtained from the application of the project methodology to the Galveston Bay study
site support the findings that are obtained from the Chesapeake Bay  study.  The nonpoint
nitrogen loadings that were estimated by Newell et  al. (1992) and used in this project indicate
that over 50% of the total nonpoint load is due to the land use area occupied by urban dwellings.
In the Galveston Bay watershed we estimated the total nitrogen load to the Bay to equal 36
million kilograms.  From the total nitrogen load that was delivered to the Galveston Bay we
estimated the  following distribution:  48%  to  originate from  point sources,  49% due  to
atmospheric deposition,  and only 3% from  fertilizer application (Table 1.1).   The direct
atmospheric deposition onto the Galveston  Bay water surface accounted for 39% of the total 49%
atmospheric nitrogen deposition  load; and  the  watershed  contributed  the  remaining  10%.
Fertilizer loadings to the Bay were quite low as only 22%  of the watershed was under  crop
production.

The Tampa Bay watershed results support the findings that were obtained from the Chesapeake
Bay and  Galveston  Bay study.   The  nonpoint  and point  source nitrogen  loadings for the
watershed were provided by Greening (1993, personal communications).   In the Tampa Bay
watershed we estimated the total nitrogen load to the Bay to equal 4.5 million kilograms. From
the total  nitrogen load that  was  delivered to the Tampa  Bay we estimated  the following
distribution: 19% to originate from point sources,  67% due to atmospheric deposition, and  14%
from fertilizer application (Table 1.1).  The direct atmospheric deposition onto the Tampa Bay
water surface accounted for 40% of the total 67% atmospheric nitrogen deposition load, while
the watershed  contributed  the remaining 27%.    The relatively  large contribution due  to

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atmospheric deposition may be due to the total nitrogen load being considerably lower than the
loads in the other two studies. Also, the relatively small watershed area and large water surface
area of Tampa Bay will also magnify  the relative contributions from atmospheric sources.
1.3 STUDY CONCLUSIONS AND RECOMMENDATIONS

In summary,  the study results indicate that atmosphere deposition of nitrogen over portions of
the United States  may  be a significant contribution of nitrogen  to  aquatic  systems.   The
application of the project  methodology to  Chesapeake Bay, Galveston Bay, and Tampa Bay
supports this conclusion; we estimated that over 40% of the total nitrogen load that is delivered
to these water bodies is due to atmospheric nitrogen deposition. Although not confirmed in this
study, increased emissions of nitrous oxides to the atmosphere may be a source of the increased
nitrogen inputs to aquatic systems through the deposition pathway.

One salient point that the user should be aware of while using the estimates derived in this report
is that the results obtained  from the project  methodology are based on certain assumptions, and
the computations are performed for mean annual flow conditions. The assumptions used in the
application of the current methodology are:

       1.     Dry atmospheric deposition is equal to wet atmospheric deposition;
             however,  users of the spreadsheet developed  in this effort can
             apply  a different relationship if desired and  supported by dry
             deposition data.

       2.     The nitrogen inputs to the land use categories of pastures, urban,
             and  wetlands is only from  atmospheric deposition.  Normally
             pastures  receive manure application, urban  areas receive some
             fertilizer application, and the nitrogen inputs to wetlands can also
             be due to runoff from neighboring land uses.  These are assumed
             to be small,  negligible, sources in this  study.

       3.     The riverine nitrogen  loss  parameter  is constant for all the
             streams/rivers in  all the three study sites.  Again, spreadsheet
             users can employ different loss rates if supported by the available
             data.

       4.     Biological fixation was ignored for crops and forests.  It has been
             reported that symbiotic fixation in forests  can range from 30 to
              100kg/ha/yr(Tarrant, 1983). However, there is no indication that
             fixation should have a significant effect on nitrogen retention.

       5.     The retention parameter for fertilizer application and atmospheric
             deposition on the cropland is assumed to be the same.  In reality,

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             fertilizers are applied two to three times in a year; and atmospheric
             deposition occurs on a daily basis.  It is unclear what impact, if
             any, this has on the retention parameter.

There is a need to further investigate the assumptions that were used in this report.  Following
is a list of some of the areas which need further investigation:

       1.     Evaluate nitrogen loadings under transient conditions.

       2.     Investigate and better characterize total nitrogen deposition, and
             the relationship between wet and dry atmospheric deposition.

       3.     Evaluate possible changes in the first-order nitrogen loss rate with
             the travel time associated  with various river/strearns found in the
             watershed.

       4.     Investigate how changing climate, crops, soil type, and topography
             affects the retention parameters for the various land use practices
             in the watershed.

       5.     Evaluate how retention parameters for atmospheric nitrogen that
             is deposited  on the  watershed may  differ from those for other
             human nitrogen  inputs, such as fertilizers and  animal waste.

       6.     Study the effects of nitrogen  deposition and its delivery from the
             watershed to the watershed outlet.   We can  account  for  the
             contribution  of atmospheric nitrogen deposition  on  the  water
             surface; however, there is a paucity in the information regarding
             the amount  of  atmospheric  nitrogen that is deposited on  the
             watershed and is finally delivered to the aquatic system of interest.

       7.     Investigate the possible  impacts,  if any,  of biological nitrogen
             fixation on retention parameters, especially for crops and forests.

1.4  FORMAT OF  REPORT

Section 2 describes the EDF methodology in detail.  The results obtained from the EDF analysis
along with the major assumptions used in their study are presented. The EDF procedures were
reviewed and refined to develop the project methodology.  The project methodology is discussed
in Section 3. In Section 4 of this report we describe and analyze the results obtained from the
application of the project methodology to the Chesapeake Bay,  Galveston Bay, and Tampa Bay
watersheds.  Sensitivity analysis was performed on key nitrogen load assessment parameters; the
results obtained from the sensitivity analysis are also presented in Section 4.  In  Section 5, the
study conclusions and recommendations for future work are presented.

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                                    SECTION 2.0

                            REVIEW OF EDF ANALYSIS
In this section we provide a detailed review and description of the procedures, findings, and the
assumptions of the Environmental Defense Fund's (Fisher et al.,  1988) study of atmospheric
nitrogen deposition as it relates to the total nitrogen load to the Chesapeake Bay Watershed.  In
Section  2.1, the EDF methodology and  the results  obtained from  the  application  of the
methodology to the Chesapeake Bay watershed are presented.  The conclusions derived from the
EDF study are presented in Section 2.2, and assumptions used in the EDF study are listed in
Section 2.3.

It has been reported that in the past century anthropogenic emissions of nitrogen oxides to the
atmosphere  of North America  have increased  (Fisher et al., 1988).  The changes in the
composition of the atmosphere have resulted in an increased nitrate content of precipitation and
of surface waters.  The coastal  waters of the United States are reported to be receiving large
inputs of nitrogen. Fisher et al., (1988) analyzed the contributions of nitrogen to the Chesapeake
Bay above the Fall Line by considering the various possible sources of nitrogen, and how these
sources relate to  one another in relation to the total  nitrogen delivery to the Bay. The Fall Line
represents the topographic transitional area between the coastal plain and piedmont physiographic
divisions.  The above Fall Line watershed segments of the Chesapeake Bay drain through the
river/stream network into the Chesapeake Bay. The below Fall Line segments, which were not
analyzed in the EDF study, drain directly into the  Chesapeake  Bay.

The EDF report lists the possible nitrogen sources resulting from human activity along with their
contributions.  The sources reported consist of atmospheric deposition,  runoff from agricultural
land,  the outflows from sewage treatment plants, and industrial discharges.  The contributions
of nitrogen  to the Chesapeake  Bay are estimated for  the year 1984.  The Chesapeake Bay
watershed is depicted in Figure 2.1.

2.1 EDF PROCEDURES USED FOR THE CHESAPEAKE BAY WATERSHED

Fisher et al. (1988) reported that atmospheric nitrogen deposition, a component of acid rain, is
a major source of nitrogen to the Chesapeake Bay and other Atlantic  coastal waters,  and is a
major factor in the decline of coastal water quality.  To derive the above conclusion, the authors
performed nitrogen load computations to the Chesapeake Bay using two alternative procedures,
referred to as the equal retention and differential retention methods.

In the equal retention procedure,  the  authors first calculated  the total nitrogen inputs to  the
Chesapeake Bay  watershed from point and nonpoint sources.  The nonpoint source inputs to  the

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         WEST BRANCH
         SUSOUEHANNA
           .JUNIATA
                                                           LOWER
                                                           6USQUEHANNA
UPPER POTOMAC
SHENANDOAH
 EAST BRANCH
 SUSOUEHANNA
PATUXENT
     LOWER
     POTOMAC

      RAPPAHANNOCK
                                                                  MATTAPONI
                                                                PAMUNKEY
        Figure 2.1  Chesapeake Bay Watershed

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watershed were comprised of atmospheric deposition, fertilizers and animal waste. The authors
then calculated the percent contribution from individual nonpoint sources using the ratio of the
input from the  nonpoint  source of interest to total nonpoint inputs.  The U.S. EPA (1983)
estimated  the total nonpoint and point source loads to the Chesapeake Bay.  For the equal
retention procedure the nonpoint load to  the bay was differentiated  based  on the ratios of
individual nonpoint source loads calculated above to estimate the contribution from that source
to the Chesapeake Bay.  Additional contributions to the Chesapeake Bay were calculated by
direct atmospheric deposition to the water surface of the Bay.

The differential retention procedure was  a  refinement of the equal retention procedure,  as the
procedure considered the effects of alternative land use categories in the watershed.  The land
uses evaluated consist of forest, cropland, urban, and pastures.  The authors calculated nitrogen
inputs to the above listed land uses, and then using literature reported retention factors and a
50% loss  of nitrogen due to riverine uptake the nitrogen loads to the Chesapeake Bay were
estimated.  In the sections below, we describe the two procedures in greater detail and present
the results obtained from both methods.

The anthropogenic nitrogen budget for the entire watershed was calculated using data from U.S.
EPA, Chesapeake Bay Program, the National Atmospheric Deposition Program (NADP), State
Agricultural Agencies, and U.S. Bureau of the Census.  The authors used the NADP observed
data for calculating atmospheric nitrogen loadings to the watershed land surface as well as on
the water surface of the Bay. The atmospheric deposition to the land  surface of the Chesapeake
Bay watershed  was computed by dividing the watershed into nine segments (North Branch
Susquehanna, West Branch  Susquehanna,  Susquehanna below Sunbury,  West Chesapeake,
Patuxent, Eastern Shore, Potomac, York-Rappahannock, and James River; see Figure 2.1). All
other nitrogen  load calculations were done for the entire watershed.  The  total  fertilizer
application in the watershed was calculated using fertilizer sales data from the State Agricultural
Agencies, and the Agricultural  Census data was used  to calculate  animal population  in the
watershed. Data on point source nitrogen loadings were obtained from U.S. EPA, Chesapeake
Bay Program.
2.1.1 Equal Retention Procedure

The steps used in calculating nitrogen loads to the watershed using the equal retention procedure
are listed below:

1.    The atmospheric nitrogen deposition to the Chesapeake Bay watershed was calculated
      using the National Atmospheric Deposition Program's (NADP) wet atmospheric nitrogen
      deposition data. Based on the atmospheric deposition trends in the watershed the authors
      divided  the watershed into nine basins — North Branch Susquehanna, West Branch
      Susquehanna, Susquehanna below Sunbury, West Chesapeake, Patuxent, Eastern Shore,
      Potomac, York-Rappahannock, and James River.  The Chesapeake Bay water surface
      area was used to calculate direct atmospheric deposition to  the Bay water body. Total


                                           8

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       atmospheric deposition was assumed to be two times wet deposition to account for
       unavailable dry deposition data.

2.     The data on total nitrogen fertilizer applied  in  a county were obtained  from  state
       agricultural reports.  These data were used to calculate commercial fertilizer usage for
       the states of Pennsylvania  and Virginia.  For counties which were partially  in the
       watershed the authors used  a planimeter to calculate the area and the percent of the
       county in the watershed. The total amount of fertilizer consumed in the county was then
       multiplied by the percentage  of the county  in  the  watershed to estimate nitrogen
       consumed in the part of the county which was in the watershed.  For those states where
       the 1984 fertilizer consumption was unavailable, the authors used the 1982 Census of
       Agriculture data.

3.     Using the Agricultural Census data, the authors calculated the type and animal population
       in each county of the watershed. The total population of a given farm animal was then
       multiplied by the average amount of nitrogen that the animal excretes  each year to
       calculate the total nitrogen produced by that species of animal in the county.

4.     The authors  obtained  the  point  source total nitrogen input  from  the U.S. EPA,
       Chesapeake Bay Program for the year 1985. The point sources included discharges from
       sewage treatment plants as well as industrial sources. All STP's with flow rates over 0.3
       million gallons per day (1136 m3 per day) were included in calculating nitrogen loads;
       this accounted for 96% percent of the total flow from all STP's.  For industrial sources,
       observed nitrogen  loads were  used whenever  data were available, or were estimated
       based on a typical value for  that industry type.

The total nitrogen loads calculated  by  EDF to the land and water surfaces of the Chesapeake
Bay Watershed using the steps listed above are presented in Table 2.1.

As  mentioned above, in this  procedure the authors  assumed that the contribution of each
nonpoint source to the Bay is proportional to  the loading of each nonpoint  source  to the
watershed.  The nitrogen loadings to the Chesapeake Bay as estimated by the US EPA (1983)
are listed in Table 2.2.  Table 2.3 below lists the results obtained from the equal  retention
procedure.   As shown  in Table 2.3, the  point source  loadings  and the sum of  nonpoint
contributions is the same as estimated  by EPA (1983) and as listed in Table 2.2.   The  total
nonpoint contribution to the watershed as listed in Table 2.2 was multiplied by the last column
in Table 2.1 to calculate loadings from individual sources of nonpoint pollution.  The values of
direct atmospheric deposition to the Bay were kept the  same as in Table 2.1, as the EPA
estimates of N loads to the Bay in Table 2.2 do not account for direct atmospheric deposition
to the Bay.

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      Table 2.1  Calculated Nitrogen Loadings to the land and water surfaces of the Chesapeake
                 Bay Watershed in 1984

             Source               106 kg N yr1  % of Total   % of NFS

             Precipitation-Land
                 Nitrate              143           23            25
                Ammonium           79           13            14

             Animal Waste           195           31            34

             Fertilizer               158           25            27

             NFS subtotal            575            --           100

             Point Sources            41            7

             Precipitation-Bay
                 Nitrate              8.3           1.3
                Ammonium          4.6           0.7

             Total                   629           100
2.1.2 Differential Retention Procedure

In the Differential Retention procedure, the authors calculated the nitrogen loadings from each
N source to the various land types found in the watershed as shown in Figure 2.2. The authors
then estimated the amount of nitrogen that will be lost in runoff based on existing nitrogen
retention studies.  In the following paragraphs we describe the literature sources used by  the
authors to estimate retention parameters, i.e. the fraction of the nitrogen loading that remains,
from the various land uses in the watershed.

2.1.2.1  Cropland

Total fertilizer application to the  cropland acreages of the Chesapeake  Bay Watershed  was
calculated as  158 x 106 kg N yr1.  The product of average atmospheric deposition (the authors
did not use the watershed delineation as used  for the equal retention procedure) rate with  the
cropland area was used  to  calculate atmospheric  N deposition on croplands.   The  authors
reported that  25.6 x 106 kg N yr1 was deposited as nitrate deposition and 14.1 x 106 kg N  yr1
as ammonium.  The total N load on cropland as reported by the authors is the sum of fertilizer
and atmospheric deposition,  and is equal to 197.7 x  106 kg N yr1, with 13% from atmospheric
nitrate deposition, and 6% from atmospheric ammonium.


                                           10

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Table 2.2   Nitrogen Loadings to the Chesapeake Bay (US EPA, 1983)

      At Fall Line                     Average Year*
             Point Sources                    6.3
             Non-Point Sources               50.6
             Sub Total                       56.9
      Below Fall Line
             Point Sources                   26.6
             Non-Point Sources               16.2
             Sub Total                       42.8
      Watershed Total
             Point Sources                   32.9
             Non-Point Sources               66.8
      Total                                 99.7

      The EPA estimated the loadings for the period May-October,  EDF multiplied the
      EPA (1983) values by 1.5 to obtain yearly nitrogen loads to the Bay.

Table 2.3  Equal Retention Estimates of Nitrogen Sources to the Bay

      Source                                      106 kg N yr1

      Point Source                                       32.9

      Non Point Sources:
             Atmospheric Nitrate (N)             16.6
             Atmospheric Ammonium (N)         9.2
             Fertilizer (N)                      18.4

      Animal Waste                            22.6
      NPS Contribution                                   66.8

      Subtotal, Excluding Direct Deposition                 99.7

      Deposition to Surface of Bay:
             Atmospheric Nitrate (N)             8.3
             Atmospheric Ammonium (N)         4.6
      Subtotal, Direct Deposition                           12.9

      TOTAL                                            112.6

Note: Point Source and NPS Contributions are from EPA Analysis (1983).
      The EPA estimates are modified to account for direct deposition
      to the Bay.
                                  11

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The authors reported that the EPA estimates of total nitrogen loadings to the Bay originating
from cropland was 59.8 x 106 kg N yr1.  It was assumed that  13% of runoff from cropland can
be  attributed to atmospheric nitrate and  6% to atmospheric ammonium.   Based  on this
assumption, the authors divided the EPA cropland loading to differentiate the source of nitrogen
in cropland runoff.

Runoff N loadings from cropland, based on a series of field measurements were reported  to
range from 0.8  to 20 kg N ha'1 yr1 (Smullen et al., 1982).  The authors used 20 kg N  ha'1 yr1
as the runoff loading rate; this rate when multiplied by the cropland area (2952855 ha)  resulted
in 59.1 x 10* kg N yr1 load from cropland.  The authors calculated a 0.70 cropland retention
factor which also included river uptake. This retention factor can be obtained by the following
equation:

                      Bay Loading (59.1 x 106 kg N yr1)
Retention Factor =  1	= 0.70  (2.1)
                      Watershed Loading (197.7 x 106 kg N yr1)
2.1.2.2 Forests

Nitrogen retention factors as reported in the literature for forested watersheds range from 0.52
to 0.99 (Kelly and Meagher, 1986; Likens et al., 1977; Weller et al., 1986; Schreiber et al.,
1976; and Linthurst, 1988).  In the EDF study the authors used a retention parameter of 0.8 for
forested watersheds.   The forest runoff was further reduced by 50% to account for riverine
uptake thus resulting in a overall retention  factor of 0.90. i.e. only 10% of the N input to forests
reached the Bay.

2.1.2.3 Pastures

The two sources of nitrogen inputs  to pasture land consist of atmospheric deposition and the
application of animal waste.  The atmospheric nitrogen deposition on the pastures was calculated
using the area occupied by pastures in the  watershed.  The loads were reported to be 23.6 x 106
kg N yr"1 from atmospheric nitrate and 13.0 x 106 kg N yr"1 from atmospheric ammonium. Data
from Robbins et al.  (1972) cited in the EDF report indicates that only 1.5% to 5% of nitrogen
in animal waste applied to land reaches the stream via runoff. The authors assumed that  5% of
nitrogen in animal waste runs off the land, and this runoff load was further reduced by 50% to
account for riverine uptake, thus resulting in a overall retention factor of 0.975. The retention
factor  for atmospheric nitrogen loads were assumed to be 0.70, which was reported to  be the
same as the cropland retention factor.
                                           12

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Figure 2.2  EDF's Differential Retention Procedure Flowchart




                                        13

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Urban

The cities located in the Chesapeake Bay Watershed were assumed to drain directly into the Bay,
hence the river uptake losses were assumed to be zero. It was expected that the urban lands will
retain between 0% (complete washoff) to 70% (same as cropland) of nitrogen inputs.  The study
assumed  a 0.35 retention factor for urban areas which is an average of 0% retention and 70%
retention.

The retention factors used for the various N sources are listed in Table 2.4 along with calculated
nitrogen loadings to the Bay.
2.2 EDF STUDY CONCLUSIONS

Atmospheric nitrate deposition is reported to be the major factor in the decline of coastal waters.
From the estimates of various sources of nitrogen to the Chesapeake Bay,  the  individual
contributions  under  the differential retention procedure were quantified as 23% from point
sources,  39% from atmospheric deposition  (includes  nitrate  and ammonium), 34% from
fertilizer, and 4% from animal wastes.  It  is reported by the authors that, in the absence of
further nitrogen oxide emission controls, emission in the Chesapeake Bay region will increase
by 44% by the year 2030.

The Chesapeake Bay Program is currently evaluating strategies for attaining a 40% reduction
in nutrient loads (nitrogen and phosphorus) reaching the Bay.  The EDF study indicates that if
nitrogen  loads from other sources are reduced  by  40%,  but  atmospheric loadings  are  not
changed, then the overall nitrogen reduction to the Bay will be only 16% instead of 40%.
2.3 ASSUMPTIONS USED IN THE EDF STUDY

The following is a list of the assumptions that were used in the EDF study, as documented in
their report (Fisher et al., 1988):

       1.     Dry nitrogen deposition is equal to wet deposition.

       2.     N source from forest runoff is due to atmospheric N.

       3.     There is a 50% reduction in the nitrogen load reaching the bay due to riverine
             uptake.

       4.     For the equal retention procedure  it was assumed that  the loading to the bay are
             proportional  to inputs of fertilizer, animal waste and atmospheric deposition on
             the land surface of the watershed.
                                         14

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Table 2.4     Retention Factors and N Loadings Using Differential Retention Procedure.
      Source                                Fraction           Nitrogen
                                            Retained           Loadings
                                                               (106 kg N yr1)

      Point Source
            Below Fall Line                 0.00                 26.6
            Above Fall Line                 0.50                  6.3
         Subtotal                                                32.9

      Direct Atmospheric Deposition to Bay
            Atmospheric Nitrate Nitrogen      0.00                  8.3
            Atmospheric Ammonium Nitrogen 0.00                  4.6
         Subtotal                                                 12.9

      Cropland
            Fertilizer                       0.70                 47.7
            Atmospheric Nitrate Nitrogen      0.70                  7.8
            Atmospheric Ammonium Nitrogen 0.70                  4.3
         Subtotal                                                 59.8

      Forest
            Atmospheric Nitrate Nitrogen      0.90                  9.0
            Atmospheric Ammonium Nitrogen 0.90                  5.0
         Subtotal                                                 14.0

      Pastures
            Animal Waste                   0.975                 4.9
            Atmospheric Nitrate Nitrogen      0.70                  7.1
            Atmospheric Ammonium Nitrogen 0.70                  3.9
         Subtotal                                                 15.9

      Urban
            Atmospheric Nitrate Nitrogen      0.35                  2.8
            Atmospheric Ammonium Nitrogen 0.35                  1.6
      Subtotal                                                    4.4

      TOTAL                                                  140.0
      Note:  Retention factors from various land uses include a 50% river uptake.
                                  15

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      5.     All fertilizer sold is applied to the crops  i.e. applications are derived from
             fertilizer sales.

      6.     Atmospheric deposition from individual monitoring sites represent larger areas.

      7.     Storage, handling, and volatilization losses of animal waste are ignored.

      8.     Other fluxes of N from the watershed are not altered by increased atmospheric
             N deposition.

      9.     Assumed 70% nitrogen retention from all croplands.

      10.    Assumed 97.5%  retention percentage  for animal  waste applied to cropland,
             however, the retention of atmospheric deposition was assumed to be 70%.

      11.    Assumed 35 % nitrogen retention from urban areas.

In Section 3, we describe the current methodology which was developed by refining some of the
EDF assumptions that are listed above.
                                           16

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                                    SECTION 3.0

     METHODOLOGY USED IN ESTIMATING NITROGEN LOADS TO AQUATIC
                                     SYSTEMS
In Section 2.0 the EDF procedures were reviewed and discussed in detail. Based on the EDF
assumptions that needed to be reevaluated, the following tasks were included in this effort:

       1.     Investigate retention parameters for cropland, urban, and pasture
             areas.
       2.     Investigate the assumption of 50% loss of nitrogen due to riverine
             uptake.
       3.     Evaluate application amounts  of fertilizer and animal waste in
             relation to agronomic application rates by crops (i.e. based on crop
             needs).
       4.     Divide watershed into  subbasins  to account for varying NFS
             loadings from alternative land uses and riverine travel time to the
             Bay (or watershed outlet).
       5.     Investigate  relationship  between wet  and   dry  atmospheric
             deposition.

In this section we describe the steps  and procedures of the current  methodology used in
estimating nitrogen loads to Chesapeake Bay, Galveston Bay and Tampa Bay.  The following
is a list of the EDF assumptions that are modified using the current methodology:

       1.     The nitrogen loadings from the watershed are calculated by dividing the watershed
             into smaller subbasins to better represent the variation  in land use in  the
             watershed, rather than lumping all the land uses together, as was done  in the EDF
             analysis.
       2.     Fertilizer applications are calculated using crop nitrogen demand, rather than
             fertilizer sales data.
       3.     Variable relationship between wet and dry atmospheric deposition is  used.
       4.     Riverine losses are calculated using travel time and nitrogen losses during
             travel to the watershed outlet.

3.1 CALCULATIONAL STEPS IN CURRENT METHODOLOGY

The steps used in estimating nitrogen loadings to aquatic systems using the current  methodology
are presented  below:


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STEP 1.     Define Study  Region.  Drainage Areas. Subbasins and Drainage
             networks, and Land Use Distribution -  Define and calculate the
             drainage areas of the three study sites, i.e. the area that directly
             drains into Chesapeake Bay, Galveston Bay, and Tampa Bay. Once
             the  study  regions  are defined  and  their  total drainage  area
             calculated, the next step was to map and  list  the stream/river
             network  that carries the loadings from the watershed segments to
             the watershed outlet.  The stream/river network aids in dividing
             the watershed into subbasins.  For each subbasin we then calculate
             the land  use distribution.

STEP 2.     Identify  Data  Needs - The following data items and categories
             were needed for estimating nonpoint and point source contributions
             of nitrogen:

             a.  Fertilizer
             b.  Meteorological
             c.  Point source
             d.  Animal waste
             e.  Land use
             f.  Sewage plants discharge
             g.  Industrial discharges
             h.  N deposition (wet and dry)
             i.  Channel network and morphology

STEP 3.     Calculate Loadings from Subbasins and Local Tributary Areas

             The two sources of nitrogen loads at the subbasin outlet consist of
             nonpoint sources and point sources. The nitrogen loads to the bay
             or  watershed  outlet will be estimated by calculating  individual
             subbasin loads and  then routing this load to  the bay or watershed
             outlet using travel  time and riverine uptake parameters.  In the
             following subsections we describe the procedures that will be used
             to calculate the loadings from nonpoint and point sources.

3.2  DEFINE NPS LOADS BY LAND USE

Nonpoint loads to the Bay or Watershed Outlet are a function of retention by the land use and
loss due to riverine uptake.  Calculation of nitrogen loads from all the land uses was a two step
procedure: the first step was to estimate the total nitrogen inputs for all the land uses found in
each subbasin,  and the second step then involved the calculation of the edge-of-field nitrogen
loads from each  land use based on  the retention factor of the land  use.   For each land use
practice, we calculated the percent contribution of each nitrogen source with respect to the total
nitrogen input to the land use.  This percent  contribution  was used to calculate the source of


                                           18

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nitrogen from the total nonpoint nitrogen load generated from the land use.

             Nitrogen Inputs:  Following is a list of typical land use categories for which
             nitrogen inputs were calculated:
             Forest
             Croplands
             Wetlands
             Pasture/Idle
             Urban
Nitrogen inputs primarily consist of atmospheric deposition.
Using the unit area observed wet and dry deposition data
in conjunction with the area occupied by the forest in the
subbasin,  we calculated  the total nitrogen input to forests.

The nitrogen inputs to croplands consist of fertilizer
application, manure application, biological fixation
(for legume crops), and atmospheric deposition.  For the
major crop grown in each subbasin we calculated the typical
agronomic fertilizer application rate. It was assumed that
manure is only applied to croplands and pasture/idle lands.
Based on the ratio of cropland area to pasture/idle area, an
area-weighted manure application rate was calculated.
The atmospheric deposition of nitrogen was calculated using
observed unit area wet and dry atmospheric N deposition
data.  The area occupied by the cropland was then used to
calculate the total nitrogen input to croplands.

Nitrogen inputs to wetlands consist of atmospheric
deposition.

The two primary sources of nitrogen inputs consist of
atmospheric deposition and manure application.  Manure
application amounts were  calculated using the area
weighted procedure described above.

Nitrogen inputs consist of atmospheric deposition only.
             For each of the land uses listed above we calculated the ratio of each individual
             nitrogen source to the total nitrogen input to the land use; these ratios were called
             Input Source Ratios. An example snowing the calculation of the Input
             Source Ratio is given below:
             Total Nitrogen Input to Cropland (TN)     =
                           Fertilizer (F)  -I- Manure (M)
                           + Biological Fixation (BF)
                           + Atmospheric Deposition
                           (AD)
                                           19

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             Atmospheric Deposition Input      =     [Atmospheric Deposition (AD)]/[Total
             Source Ratio for Cropland               Nitrogen Input to Cropland (TN)]
             These calculated Input Source Ratios were then used to differentiate the sources
             of nitrogen from each land use, adding identical nitrogen sources resulted in the
             nitrogen load from each source at the subbasin outlet.

             Nitrogen Outputs:  Literature reported nonpoint loading retention parameters (e.g.,
             Kelly and Meagher, 1986; Liken et al., 1977; Weller et ah, 1986; Schreiber et
             al.,  1976 for forest land use; and Robbins et al.,  1972 for pastures) for each of
             the land use categories found in the subbasin were used  to estimate the amount
             of nitrogen loading from the land use of interest.  If retention parameters for all
             the land uses were unavailable, then literature studies (e.g., Donigian et al., 1994)
             which report NFS edge-of-field loadings for the land use of interest were used.
             These loading rates were then used  to back-calculate the retention factor from
             equation 2.1  in Section 2.1.2.

3.3 DEFINE POINT SOURCE LOADS

The point sources of interest consist of industrial and sewage treatment plant discharges.  The
point sources discharge directly into the stream/river and the retention or loss of nitrogen from
point sources is due to riverine uptake only.  The point sources that discharge directly into the
bay or watershed outlet were not subject to riverine losses.

3.4 SUBBASIN LOADS

The total nitrogen load at the subbasin outlet is the summation of nonpoint and point source
loads.  However,  we routed the nonpoint and point source loadings individually from  each
subbasin in the watershed.  We computed the total NFS load from the  subbasin by multiplying
the edge-of-field export from each land use by its corresponding area.

The source of nitrogen  resulting from  nonpoint  source  loadings from each  land use was
calculated by multiplying the  input source ratios by the total land use nitrogen load.   This
procedure was used for all the land uses in the subbasin. The total subbasin load was computed
by summing the loads from all the land uses. Summing up identical nitrogen sources from all
the land uses in the subbasin resulted in the total loads from individual  nitrogen sources in each
of the subbasins.

After the total subbasin NFS nitrogen load was calculated, the next step was to calculate the ratio
of N from each source (fertilizer, biological fixation, manure, and atmospheric deposition) to
the total NFS nitrogen load at the subbasin outlet.  This  ratio was called the Output Source
Ratio. This ratio is used to differentiate the various sources of nitrogen at the bay or watershed
outlet.


                                           20

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STEP 4.      Calculate Stream Losses and Contributions to Bay or Outlet

             The flow of nitrogen from the watershed subbasins to the bay or
             watershed outlet is shown in Figure 3.1.  The load from each
             subbasin was routed to the bay or watershed outlet  using Travel
             Time from each subbasin.  The nitrogen loadings to the bay or
             watershed outlet were assumed to be dependent on riverine uptake
             and the travel time of the stream/river to the outlet.  The nitrogen
             load to the bay or watershed outlet from all  the subbasins in the
             watershed were calculated using the following first-order decay, or
             exponential   relationship   (Barnwell,   1993,   personal
             communications):
                         Subbasins
                   TN =    V     { Subbasin^ *exp [ - k *
                            a-l
             where:

             TN           =     Total Nitrogen load to the bay or watershed outlet
             Subbasin;      =     Total Nitrogen load from subbasin i (includes NFS and
                                 point sources)
             Tj            =     Travel time from subbasin i
             k             =     First-order riverine N loss constant (literature values of
                                 riverine loss will be used)

             Riverine nitrogen  losses include  settling  (sedimentation)  of
             particulate nitrogen, denitrification,  ammonia volatilization,  and
             some algal and plant uptake (where subsequent settling removes
             the nitrogen).

             The nonpoint and point sources were routed  separately using the
             above equation. The above equation was  not used for subbasins or
             local tributary areas that  drain  directly  to the bay or watershed
             outlet,  as the loadings  from  these  local  subbasins  or local
             tributaries are  not subject to riverine losses.

             The individual nitrogen source in the nonpoint source loadings to
             the bay or watershed outlet from each subbasin was calculated by
             multiplying the total load (as  calculated  by Equation 3.1) by the
             Output Source Ratios calculated for each subbasin.  The procedure
             showing the calculation of nonpoint nitrogen loads from a subbasin
             is depicted in Figure 3.2.
                                          21

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Figure 3.1  Methodology Flow Chart for Estimating Nonpoint and Point Source Loads
                                        22

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                 Example of NFS Load Computations  from  each Subbasin
       Landuae  A
                                       Landuae B
                                                                          Landuae C
        N-lnputa

     Atm.  Depo.  « 10
     Manure * 40

     Total Input  « 50
        N-lnputa

    Atm. Depo. - 10
    Manure  - 40
    Fertilizer  « 100
    Total Input • 150
    N-lnputa

Atm. Depo  • 10


Total Input •  10
                           I Calculation of Input Sourca Ratio* [
   Atm Input Ratio • 10/50 •  0.2     Atm.  Input  Ratio • 10/150 •  0.07    Atm. input Ratio  • 10/10  • 1
   Manure Input Ratio •  40/50 • 0.8  Manure Input Ratio • 40/150 • 0.27
                                  Fert. Input Retio - 100/150  • 0.66

    Assume 80%  Retention           Aaaume  70% Retention           Assume 60% Retention

                    Unit Area  Load • N-lnput - (N-lnput x Fraction Retained)

    Unit  Area Load •  5                   Unit  Area  Load •  45              unit Area Load «  4

   Distribution of Loads             Distribution of Loads             Distribution of Loads

                   Load from Each N Source  • N  Source Ratio x Unit Area Load

                                                                      Atm  Load • i ' 4  • 4
   Atm. Load • 0.2 '5-1
   Manure Load • 0 8  ' 5 •  4
Atm.  Load • 0 07 * 45 •  3.15
Manure Load • 0 27  • 45 • 12.15
Pert  Load •  0 66 •  45 • 29 70
                   Total Subbasin Load « Unit Area Load 1rnm Landuse  A->B->C

                               Total Subbesm Load • 5  •* 45 » 4 • 54

     Nitrogen Sources of  Subbasin Load

          Atm  Load • Atm Load from  Landuse  (A-B-C) • 1-3 15*4-8.15
          Manure Load • Manure Load (torn Lenduse (A-B)  * 4*12 15-16 15
          Fert  Load • Pert. Load trom Landuse A  • 29 70
                         Calculation of Output Source Ratios
   Output Source Ratios in  Subbasin Load • Source Load / Total Subbasin  Load
          Aim  Subbasin Ratio  • 8 15/54  • 0 15
          Manure SuDbasm Ratio •  16 15/54  -  0 30
          Fert  Subbasin Ratio • 29 7/54 • 0 55

    Assume 50% Subbasin Load Lost Due to Riverine Uptake

           Loading to Bay  or Watershed Outlet •  54/2 • 27

     Source Contributions in  Bay  or Watershed Load from Subbasin

            Atm  Contribution • 0 15*27 • 4 05
            Manure Contribution • 0 30'27 • 6 10
            Fertilizer Contribution • 055*27 • 14.85
Figure 3.2   Example  of NFS  Load  Computations  from each Subbasin

                                             23

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             The total loadings to the bay or watershed outlet is the sum of
             loads  from  all the  subbasins  in  the  watershed  plus  direct
             atmospheric  deposition  on the water  surface.  The sum  of all
             identical nitrogen source loadings to the bay or watershed outlet
             from all the  subbasins in the watershed divided by the total Load
             from all the  subbasins was used in calculating the contribution of
             individual nitrogen sources at the bay or watershed outlet.

STEP 5.     Perform sensitivity analysis

             After initial computations of nitrogen loadings from the three sites
             of interest,  additional   analyses  were performed   to test  the
             sensitivity of the nitrogen output to the following parameters and
             assumptions  used in the methodology:

             a.     Vary retention parameters for all land uses
             b.     Vary river/stream loss (k in equation 3.1)
             c.     Evaluate range of literature reported loading rates for all land uses
             d.     Investigate relationship between wet and dry deposition.
             e.     Evaluate sensitivity to fertilizer application amounts, i.e. varying fertilizer
                    applications to 1.25 and 1.50 times the agronomic application rates.

             The results from the sensitivity analysis are presented in Section 4.4.
                                            24

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                                    SECTION 4.0

              ASSESSMENT OF NITROGEN LOAD FROM STUDY SITES
The methodology developed for this project was described in detail in Section 3.  In this section
the results from the application of the methodology to the three study sites are presented. In
Section 4.1 we first describe the results obtained from the application of the project methodology
to the Chesapeake Bay watershed.  These results are then compared to the EDF results obtained
using the differential retention procedure only.  The results obtained from the application of the
methodology to the remaining two study sites — Galveston Bay, and Tampa Bay — are presented
in Sections 4.2, and 4.3 respectively.  Sensitivity analysis was performed on the following
parameters:

       1.     Ratio of Wet to Dry Atmospheric Deposition;
       2.     First-order nitrogen loss rate in  rivers; and
       3.     Fertilizer application amounts (varying agronomic recommended rates).

The results obtained from the sensitivity analysis are presented and discussed in Section 4.4.

4.1 APPLICATION OF THE METHODOLOGY  TO CHESAPEAKE BAY WATERSHED

For the application of the  methodology to the Chesapeake Bay watershed, we divided the
watershed into  fourteen subbasins ~ East Branch Susquehanna, West  Branch Susquehanna,
Juniata, Lower Susquehanna, Conowingo,  Upper Potomac,  Shenandoah,  Lower Potomac,
Rappahannock,  Mattaponi, Pamunkey, James, Appomattox, and Patuxent (see Figure4.1).  The
input data (nonpoint and point sources) for the above listed subbasins were obtained  from the
watershed modeling  study  conducted by Donigian  et  al. (1994)  for the Chesapeake  Bay
watershed.  The nonpoint, point, and atmospheric deposition data of total nitrogen inputs  used
in the Chesapeake Bay application along with  the EDF nitrogen inputs are presented in Table
4.1.

The land uses from which nonpoint source loads were computed consisted of cropland, forest,
pasture, and urban land use.  The nonpoint source inputs to the Lotus 123* spreadsheet consist
of atmospheric deposition from all land uses, manure application to cropland and pastures, and
fertilizer application to cropland.  We used the average annual atmospheric nitrogen deposition
data as reported by Fisher et al. (1988). The manure and fertilizer application rates used for all
the subbasins in the watershed were calculated by Donigian et al. (1994), based on fertilizer and
manure application data provided by local and  state agencies.
                                         25

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         WEST BRANCH
         SUSQUEHANNA
            JUNIATA
                                                            LOWER
                                                            6USOUEHANNA
UPPER POTOMAC
SHEKANDOAH
 EAST BRANCH
 SUSOUEHANNA
PATUXENT
     LOWER
     POTOMAC

      RAPPAHANNOCK
                                                                   MATTAPONI
                                                                 PAMUNKEY
 Figure A.I  Chesapeake  Bay Watershed with Subbasin Boundaries
                                      26

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      Table 4.1     Comparison of Current Nitrogen Inputs to EDF Nitrogen Inputs

                                            Current Inputs      EDF Inputs
                                            106 kg/yr            106 kg/yr
Direct Bay Inputs
      Point Sources Above Fall Line              12.7               12.6
      Point Sources Below Fall Line              27.0               27.0
      Atmospheric Deposition                    13.0               13.0

Forest
      Atmospheric Deposition                   112.0              139.0

Cropland
      Atmospheric Deposition                    38.1               39.7
      Fertilizer                                171.0              158.0
      Animal Waste                            49.5                0.0

Pasture
      Atmospheric Deposition                    19.0               36.6
      Animal Waste                            32.8              195.0

Urban
      Atmospheric Deposition                    14.2                6.7

GRAND TOTAL                               489.0              627.0
                                        27

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Other inputs needed for the spreadsheet consist of edge-of-field loads to the streams and travel
time from each subbasin to the watershed outlet.  Donigian et al.  (1994) in their Hydrological
Simulation Program-Fortran (HSPF) (Johanson  et al.,  1984) model application study of the
Chesapeake Bay watershed computed the total nonpoint loads from all  the subbasins in the
watershed; these loads  from each land use for all the subbasins are listed below in Table 4.2.
The land use areas for all the subbasins are listed in Table 4.3 and the above Fall Line point
source loads are listed in Table 4.4. Travel time from each subbasin to the watershed outlet was
calculated by Bicknell (1993, personal communications). In Table 4.5  a sample input data set
to the Lotus 123  spreadsheet for the East Branch Susquehanna Subbasin  is presented.

Nonpoint  and point  source data were input to the spreadsheet  for all  the subbasins in the
Chesapeake Bay  watershed.   The  dry atmospheric deposition  was  assumed  to equal wet
deposition; this was done to replicate the EDF assumption.  As discussed in the methodology,
the nonpoint and point source loads  were routed separately to the Chesapeake Bay.  Input and
Output Source Ratios (as  discussed in Section 3) were calculated for all the subbasins. Output
Source Ratios were used  to compute the source of nitrogen from the  subbasin load that was
delivered to the Chesapeake Bay.  The first-order rate constant for nitrogen loss in streams, k,
was determined to be 0.08/day to replicate the 50% riverine loss rate used by EDF; this rate is
typical of nitrogen  loss/decay rates  reported  in  the  literature  (Barnwell,   1993,  personal
communications).  Identical sources of nitrogen resulting  from all the subbasin loads to the
Chesapeake Bay were added together to obtain the total contributions resulting from each source
of nitrogen. The nitrogen loads from various land uses and nitrogen sources are listed in Table
4.6. The percent contributions to the Chesapeake Bay from the various nitrogen sources in the
watershed are presented in Table 4.7.

4.1.1 Comparison of New Methodology  Results with EDF Results

The total nitrogen load computed using the current methodology is  115 million kilograms, which
compares favorably with the EDF estimate of 140 million kilograms, a difference of 25 million
kg of nitrogen  or 18% of the EDF value.  One of the reasons for this is the  larger watershed
area used in the EDF analysis.  The watershed  area above the Fall Line as estimated by The
Chesapeake Bay Program U.S. EPA, as reported by Donigian et al.  (1994) was 13 million
hectares as compared to 16 million hectares estimated by EDF, a difference of 20% of the EDF
value.  The EDF study reported larger acreages under agricultural, pasture, and forest land uses
by 0.5, 1.3, and  2.2 million  hectares respectively.  However, EDF reported  0.5  million less
hectares  under the urban land  use.   In the following  sections we  compare  and discuss the
nitrogen  loadings  from individual  land  use practices  in  the Chesapeake Bay watershed as
estimated by the current methodology and the EDF analysis.

4.1.1.1  Cropland

As noted in Section 3.0, nitrogen loads resulting from the cropland area of the Chesapeake Bay
watershed  were due to three sources of nitrogen, -- fertilizer,  atmospheric deposition, and
manure.   In the EDF analysis the authors assumed that nitrogen  loadings from cropland were


                                          28

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only due to fertilizer and atmospheric deposition.

The total fertilizer load to the Chesapeake Bay as estimated by the current methodology was 25
million kg less than that estimated by the EDF study.  The reasons for this discrepancy in the
loading estimates are as follows:

       1.    The nitrogen loads were calculated for the current methodology by
             dividing the watershed in  fourteen subbasins;  however,  EDF
             results are based on considering the entire watershed as one basin
             with four different land uses.

      2.    The  cropland  area used by the EDF study was  greater  than the
             current  methodology area by 0.48 million hectares.  The current
             methodology area is 16%  less than the EDF estimated area.

      3.    In the EDF analysis the authors calculated the nitrogen loads to the
             Chesapeake Bay using a constant nitrogen retention parameter for
             the  cropland   in   the  watershed;  whereas,   in  the   current
             methodology we calculated fourteen different nitrogen retention
             parameters which  were based  on  the estimated nitrogen loads
             calculated for  the  fourteen subbasins by Donigian et al.  (1994).
             The range of retention parameters used in the current methodology
             range from  0.80 to 0.90;  however, in the EDF  study a constant
             retention parameter of 0.70 was used.

      4.    In the EDF analysis a 50% loss of nitrogen load from the cropland
             segment was  assumed due to riverine  uptake;  however, in the
             current  analysis we  routed the nitrogen loads from  each of the
             fourteen subbasins to the Bay  using travel  time and  a decay
             parameter calculated  for  average  annual flow conditions.   The
             decay parameter of 0.08/day used in the current methodology
             replicated the  EDF assumption  of 50% nitrogen  loss  due to
             riverine uptake (Barnwell,  1993,  personal  communications);
             however, in our analysis the total nitrogen loss in the rivers was
             calculated as a product of the first-order loss parameter  and the
             estimated travel time in each subbasin.

The EDF study assumed no manure application to cropland,  hence we cannot compare manure
loadings  from cropland.   The fertilizer and manure applications to cropland as reported by
Donigian et al.,  1994, were derived  from application rates  supplied  by state and county
personnel.  The total nitrogen load to the Chesapeake  Bay as a result of fertilizer and manure
application was estimated  as 27.5 million kg, which is less than the 48 million kg fertilizer
nitrogen  load estimated  by the EDF study. Atmospheric loads to the Chesapeake Bay due to
deposition on cropland are 8.85 million  kg less than the EDF estimates, and this can be

                                          29

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       Table 4.2     Estimated Mean Annual Total Nitrogen Loads from the Subbasins in the
                    Chesapeake Bay Watershed (from Donigian et al., 1994)

                                 Cropland     Forest        Pasture       Urban
                                 106kg        lO'kg        106kg       106kg

East Branch Susquehanna          9.71          1.87         9.83         3.93
West Branch Susquehanna         5.37          0.70         7.55         0.88
Juniata                           2.73          0.49         5.07         0.51
Lower Susquehanna               8.93          1.11         5.36         2.03
Conowingo                       2.71          0.76         0.65         0.96
Upper Potomac                   1.16          1.50         3.25         0.52
Shenandoah                      2.32          1.19         1.63         0.34
Lower Potomac                   8.77          4.07         6.78         2.51
Rappahannock                    1.15          0.77         0.94         0.15
Mattaponi                        0.30          0.01         0.11         0.03
Pamunkey                        0.74          0.07         0.23         0.09
James                            2.96          1.53         1.89         0.53
Appomattox                      0.98          0.15         0.27         0.13
Patuxent                         0.21          0.03         0.05         0.33
explained by the reasons listed above, i.e. different retention parameters and different loss rates
by subbasins.

4.1.1.2 Forest

The nitrogen inputs to the forest land use were assumed to  be from atmospheric nitrogen
deposition only. The current methodology nitrogen loads from forested land use are higher than
7.7 million kg as compared to the EDF estimate.  This suggests that the EDF assumed a high
nitrogen  retention parameter (0.80) for the forest land use.  The retention parameters  for the
fourteen subbasins calculated using load estimates from Donigian et al.  (1994) ranged from 0.40
to 0.92, with an average retention value of 0.68. Fisher et al. (1988) reported that the literature
cited nitrogen retention values for forested land areas  range  from 0.52  to  0.99 (Kelly and
Meagher, 1986; Likens et al., 1977; Weller et al., 1986; Schreiber et al., 1976; and Linthurust,
1988).

4.1.1.3  Pasture

The two  sources of nitrogen inputs to pastures consisted of manure and atmospheric deposition.
The nitrogen loads to the Bay from these two sources using  the  current methodology  were
estimated to be less than the EDF estimates.  The atmospheric deposition  was estimated to be
less than 6.5 million kg, while the manure component was estimated to be lower by 1.11  million
kg, compared to the EDF values.  The reason for lower loading to the Bay using the


                                          30

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Table 4.3    Land Use Areas in the Chesapeake Bay Watershed (from Donigian et al., 1994)
East Branch Susquehanna
West Branch Susquehanna
Juniata
Lower Susquehanna
Conowingo
Upper Potomac
Shenandoah
Lower Potomac
Rappahannock
Mattaponi
Pamunkey
James
Appomattox
Patuxent

Total  Area
Cropland
Itfha
602
209
173
386
140
81
134
350
74
19
42
190
54
15
Forest
lO'ha
258
99
56
83
34
147
167
164
91
4
19
224
34
8
Pasture
Itfha
1730
1430
594
492
54
778
443
453
233
121
195
1270
241
29
Urban
ICPha
332
68
37
114
40
48
37
173
16
8
20
63
18
34
2470
8070
1389
1008
      Table 4.4     Above Fall Line Nitrogen Point Loads (from Donigian et al., 1994)

                                              Point Loads
                                                   106kg
                   East Branch Susquehanna
                   West Branch Susquehanna
                   Juniata
                   Lower Susquehanna
                   Conowingo
                   Upper Potomac
                   Shenandoah
                   Lower Potomac
                   Pamunkey
                   James
                   Patuxent

                   Total
                   3.50
                   1.20
                   0.51
                   1.88
                   0.74
                   0.34
                   0.86
                   2.19
                   0.03
                   0.92
                   0.53

                  12.70
                                        31

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current methodology can be attributed to the following:

       1.     Nitrogen inputs to the Chesapeake Bay watershed as estimated by
             the EDF study are higher than the estimates derived from Donigian
             et al., 1994 (Table 4.1). The reason for higher inputs is because
             the land use area under pastures as calculated by EDF was greater
             than the area estimated by the current methodology by 1.33 million
             hectares, the current methodology area is 51% of the EDF area.
             Fisher et al. (1988) estimated that the pasture area occupied 16.5%
             of the total watershed area, while in the current methodology we
             estimate that pastures occupy 10.6% of the watershed area.

       2.     The EDF study  assumed that there were no losses during  the
             handling and storage of manure,  and  hence all the animal waste
             produced  in the county  was applied to the watershed.  However,
             in the current methodology the manure application accounted for
             manure storage, handling, and volatilization losses.  The animal
             waste application accounts for 16.8% of the  total nitrogen  input
             under the current methodology; however,  for the EDF study it
             accounts for 31.1% of total  nitrogen application.

       3.     The  EDF  study  calculated  an  average  rate  of atmospheric
             deposition (wet+dry) using wet deposition data from the four sites
             located in the watershed; the atmospheric deposition data ranged
             from 11.40 to 17.00 kg/ha.   The average rate (15.06 kg/ha) of
             atmospheric  deposition  was used to calculate the total nitrogen
             input to the land use from atmospheric deposition.  In the current
             methodology we used data from each  of the four sites, applied to
             the adjacent watershed areas, to calculate the atmospheric nitrogen
             input to each land use.

4.1.1.4 Urban

The nitrogen inputs to  the urban  land  use were assumed  to be from atmospheric nitrogen
deposition only. The current methodology estimated nitrogen loads from the urban land use are
higher by 2.76 million kg, or  63%  greater than the EDF estimate.   The higher loadings
calculated by the current methodology can be explained by:

       1.     Nitrogen inputs to the Chesapeake Bay watershed as estimated by
             the current methodology are higher than the EDF estimates by 7.5
             million kg; which is 210% of the  EDF nitrogen input (Table 4.1).
             The reason  for higher inputs  is because the urban land use area
             calculated by the current methodology is greater than the area
             estimated by the  EDF study by 0.50 million hectares.


                                           32

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          Table 4 5 An Example of the Input Requirements for the Lotus 123 Spreadsheet
          EAST BRANCH SUSQUEHANNA

          NONPOINT SOURCES

                     FORESTS
                              Aim. Wet Deposition kg/ha
                              Aim. Dry Deposition kg/ha

                    CROPLANDS
                              Atm. Wet Deposition kg/ha
                              Aim. Dry Deposition kg/ha
                              Fertilizer kg/ha
                              Manure kg/ha
                              Biological Fixation kg/ha
                    WETLANDS
CO
OJ
Atm. Wet Deposition kg/ha
Atm. Dry Deposition kg/ha
                    PASTURES
                      URBAN
          POINT SOURCES
                              Atm. Wet Deposition kg/ha
                              Atm. Dry Deposition kg/ha
                              Manure kg/ha
                              Atm. Wet Deposition kg/ha
                              Atm. Dry Deposition kg/ha
                              Sewage Treatment Plants kg
                              Industrial Discharge kg




Pert. Rates
Factors kg/ha

1.00

1.00
1.00 99



1.00

1.00


1.00


INPUTS




kg/ha
7.74
7.74
7.74
7.74
99.00
14.00
0.00
0.00
0.00
7.74
7.74
11.00
7.74
7.74
1748864
1748864
INPUTS INPUTS

Total
Inputs Input
Source
kg Ratios
15.48 0.50
0.50
128.48 0.06
0.06
0.77
0.11
0.00
0.00 0.00
0.00
26.48 0.29
0.29
0.42
15.48 0.50
0.50
3497727

INPUTS INPUTS ATM
NPS Total Load
Study Using
Loads Landuse NPS Study
Site Areas
kg/ha ha kg
5.67 1.73E+06 9.81 E+06

16.06 6.03E+05 9.68E+06




0.00 O.OOE+00 O.OOE+00

7.21 2.58E*05 1.86E+06


11.80 3.32E+05 3.92E+06



INPUTS INPUTS
Sources of Travel
NPS Time
Loads days

kg
4.90E+06 8.60
4.90E+06
5.83E+05 8.60
5.83E+05
7.46E+06
1.06E+06
O.OOE+00
O.OOE+00 8.60
O.OOE+00
5.4SE+05 8.60
5.45E+05
7.74E+05
1.96E+06 8.60
1.96E+06


INPUTS
River
Decay
/day


0.08

0.08




0.08

0.08


0.08




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      Table 4.6     Comparison of Nitrogen Loads to the Chesapeake Bay

                                      Total Load 106 kg/yr
                                Current                  EDF
                                Methodology             Procedure
Source

Point Sources (AFL)                7.38                       6.3

Point Sources (BFL)               26.60                      26.6

Direct Atmospheric Deposition
to the Bay                        12.90                      12.9

Cropland
      Fertilizer                   23.00                      48.0
      Atmospheric Dep.            3.25                      12.1
      Manure                     4.48                       0.0

Forest
      Atmospheric Dep.           21.70                      14.0

Pasture                 .  -
      Manure                     3.79                       4.9
      Atmospheric Dep.            4.50                      11.0

Urban
      Atmospheric Dep.            7.16                       4.4
Total Load                      114.76                    140.2
                                  34

-------
       2.     The EDF study assumed a constant nitrogen retention factor of
             0.35 for urban land use; whereas, our estimates for the range of
             nitrogen urban retention factors varied from 0.14 to 0.59 due to
             the different travel times for the fourteen subbasins.

       3.     The same procedures as described above were used to calculate
             atmospheric deposition on urban  land, thus contributing to the
             differences in the estimates.

4.1.2  Comparisons of Percent Loadings from Individual Nitrogen Sources

The percent contributions to the Chesapeake Bay from the various nitrogen sources in the
watershed are presented in Table 4.7.  As can be  seen from Table 4.7, the current methodology
indicates that 43% of the total  nitrogen load to the Chesapeake Bay results from atmospheric
deposition.   This estimate is in close agreement with the 39% estimate  obtained by the  EDF
study.  It can also be concluded from Table 4.7 that the percentage contributions calculated  from
point sources and manure loadings to the Chesapeake Bay using the current methodology and
the EDF estimates are also in close agreement.  The current methodology estimated slightly
higher  contributions of nitrogen from point sources as compared to the EDF analysis; this could
be due to the data that were used to calculate point source loads.  In the current study we used
average (1984-1987) point source loadings  as estimated by the U.S. EPA Chesapeake Bay
Program Office (Donigian et al., 1994), whereas the EDF analysis used point source data for
the year 1985 only.  The percent contributions from manure was 3.2%  higher than the  EDF
estimate. In the EDF analysis manure was only applied to the pastures, and the authors used
a retention factor of 97.5 % which resulted in very low nitrogen  loadings to the Bay as a result
of manure application.  However, in our analysis, manure was applied  to both cropland and
pastures, and as cropland has  lower retention  of nitrogen than  pastures  a slightly  higher
percentage nitrogen load to the Bay was estimated using the current methodology.
       Table 4.7     Comparison  of Percent Nitrogen Contributions from Various Nitrogen
                    Sources to the Chesapeake Bay

                                              Percent Using
       Nitrogen Sources           Current Methodology       EDF Analysis

       Point Sources                       29.6                23.5
       Atmospheric Deposition              43.1                38.9
       Fertilizer Application                20.1                34.1
       Animal Waste                        7.2                 3.5
                                          35

-------
The percentage nitrogen loadings to the Chesapeake Bay from fertilizer applications as estimated
by the two procedures are not in agreement.  In the EDF analysis the authors used the county
fertilizer sales data to estimate the total amount of fertilizer applied to the entire watershed and
also used a single constant retention factor for all the agricultural cropland in the Chesapeake
Bay watershed.  However, in our analysis we used fertilizer application data estimated by state
and county extension officials (Donigian et al., 1994).  In the present analysis the total fertilizer
application was 13 million kilograms greater  than  the amount  applied  in the EDF study:
however, using variable retention parameters (computed from unit area loads as reported by
Donigian et al.,  1994)  for all the fourteen  subbasins resulted in lower fertilizer load to the
Chesapeake Bay as compared to the EDF estimated load.

4.1.3  Conclusions from Chesapeake Bay Comparisons

The application of the current methodology  yielded favorably close results to the EDF study.
There were some minor discrepancies from loads estimated for individual land uses, which could
be attributed to the following reasons:

       1.     Total watershed area used in  the EDF analysis was 3.41  million
              hectares larger than the area used in the current methodology. The
              greater area used by the EDF study represents a 26% differential
              in watershed area as compared to the current methodology area.

       2.     EDF used a constant average atmospheric deposition rate for the
              entire watershed,  however, in our analysis we used actual field
              data collected by  NADP at four sites (Aurora, NY; Penn State,
              PA; Leading Ridge, PA; and  Wye, MD) in the watershed.

       3.     The EDF study used constant retention factors for each of the four
              land uses  in the watershed; whereas,  in  the current analysis the
              retention  parameters varied for the four land uses in the fourteen
              subbasins.

       4.     A constant 50% nitrogen loss was assumed by the EDF study due
              to riverine fate  and transport.  In our analysis the nitrogen loss  in
              streams was calculated as a function of decay rate and travel time
              in individual subbasins.

       5.     EDF assumed  no loss of nitrogen from animal manure due  to
              handling,  storage, and application,  whereas these  losses  were
              accounted for in our analysis.

       6.     Fertilizer  application in the current  methodology  was based on
              fertilizer  application information provided by the local and state
              agricultural agencies, not fertilizer sales as was done in  the EDF
              analysis.

                                           36

-------
The objective of this project was to assess the applicability of the EDF approach and associated
findings to the other parts of the country, in particular the Gulf Coast region.  The approach
consisted of reviewing and refining  the  EDF methodology and then developing  a Lotus  123
spreadsheet using the refined procedures. The first component of the study dealt with applying
the Lotus 123  spreadsheet to the Chesapeake Bay watershed and comparing the results with the
EDF analysis.  From  the results and discussions that are presented in Section 4.1, it can be
concluded that  the first  component  of  the study  was  completed  successfully.   The EDF
methodology was reviewed and refined and the new methodology yielded favorable results as
compared to EDF results.

The current methodology yielded a better estimate of nitrogen loadings to the Chesapeake Bay
watershed for the following reasons:

       1.     The nitrogen load computations from various land use practices in
             the watershed were not lumped together, as in the case of the EDF
             study.

       2.     We used  varying  retention  parameters  for  the  same land  use
             practice in different subbasins of the watershed. This allowed us
             to  better   represent   soil,   climate,  crop   and   topographic
             characteristics of the watershed.

       3.     Riverine decay was calculated based on travel time and first-order
             decay rate, rather than a fixed 50% loss of nitrogen as assumed by
             the EDF analysis.
4.2  APPLICATION OF THE METHODOLOGY TO GALVESTON BAY WATERSHED

The Gulf of Mexico region ranks first among those of the U.S. in total estuarine drainage area
and in total water surface area (Rabalais, 1992).  Some of the water quality problems affecting
the Gulf Coast estuaries result  from man-made pollution  both  within and  upstream of the
drainage area.  As the Gulf of Mexico waters are a habitat for marine and biological life, which
is affected by the nitrogen concentration in waters, it is important to control the nitrogen loads
reaching the coastal waters.  The Galveston Bay watershed is an integral part of the area that
drains into the Gulf of Mexico.

4.2.1 Available Data for Analysis

Newell et al.  (1992) have reported that the watershed area  that drains into the Galveston Bay
occupies 10.84 million hectares.  The water surface area occupied by the Galveston Bay is 1.40
million hectares (NOAA, 1990).  Newell et al. (1992) conducted a study for the Galveston Bay
National Estuary Program to estimate the nonpoint loadings  to the Galveston Bay. The authors
divided  the watershed into 21 subbasins (Figure 4.2), 3 of the 21 subbasins drained into an


                                          37

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                                                                                                                                                         Houston, Texas

                                                                                                                                                         Rice University
                                                                                                                                                         Houston, Texas
                                                                                                                                               CSljobNo:  C-1220
                                                                                                                                               Dale:
                                                                                                                                                        10/25/91
                                                                                                                                                                 FIGURE   E.2

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 Table 4.8 Land Use Areas in the Various Subbasins of the Galveston Bay Watershed (modified from Newell et al., 1992)
U)
              Subbasin
              Name
Armand/Taylor
Bastrop/Austin*
Brays Bayou
Buffalo Bayou
Cedar Bayou
Chocolate Bayou
Clear Creek
Dickinson Bayou
East Bay*
Greens Bayou
North Bay*
San Jacinto River
Sims Bayou
South Bay*
Trinity Bay*
Trinity River
West Bay*
White Oak Bayou

Total Area
% of Watershed Area 20.1 %
                                                   Land Use Areas (ha)
Urban

6734
5180
20979
52836
6734
2849
9324
3626
10101
23310
2849
4144
10101
8029
6475
12432
13727
18648
218078
20.1%
Open/
Pasture
7252
15022
6734
31339
12950
8288
17353
11655
18648
14245
2590
4403
8806
5698
17871
34965
27195
6475
251489
23.2%
Cropland

2590
22792
4144
41699
20720
24605
11655
5439
19166
4662
259
2072
2849
1813
20461
37555
20461
2590
245532
22.6%
Wetlands

2331
10878
1036
10360
8029
6734
7252
4921
23051
3626
518
2072
2072
3108
17353
39109
24346
111
167573
15.5%
Fore

777
777
0
1554
5957
1295
111
259
2072
8029
259
3885
259
0
16317
158767
518
0
201502
18.6%
                - Subbasins that drain directly into the Galveston Bay (Higgins, 1993, personal communications).

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adjacent subbasin (Addicks Reservoir, Barker Reservoir, and Ship Channel drain to Buffalo
Bayou,  Higgins, 1993, personal communications).  Thus in the analysis presented below we
divided the watershed into 18 subbasins.  It was also reported that 6 of the 18 subbasins drain
directly into the Galveston Bay. The six subbasins that drain directly into the Galveston Bay are
Bastrop/Austin, East Bay, North Bay, South Bay, Trinity Bay, and West Bay (Higgins,  1993,
personal communications).

The nonpoint loading data used in our analysis are derived from Newell et al., 1992.  The land
uses in  the watershed are comprised  of high density urban, residential urban, open/pasture,
agriculture, barren, wetlands, and forest.  In our analysis we grouped the high density urban,
residential urban, and barren land use into  the category of urban.   The percent of the total
watershed area occupied by high density urban, residential urban, and barren land use categories
are 10%, 9%, and 1% respectively. The modified land use categories and the areas associated
with each land use are listed in Table 4.8.

The nonpoint nitrogen loading data available consisted of the  total nitrogen load  that was
generated from each subbasin.  The procedure that was used for estimating  the total nitrogen
load from the subbasin was not obvious from the results presented in the report.  It was reported
that EMC (Event Mean Concentration) data for each land  use in conjunction with respective
runoff rates from the land use of interest were used to calculate the total nitrogen load from the
subbasin.

Moreover, the runoff data that the authors used to calculate loadings from each land  use were
not available.  Thus one of the major tasks during the evaluation of the nonpoint load data
consisted of estimating what portion of the total nitrogen load resulted from the various land use
categories found in the subbasin.  The steps used for estimating nitrogen loads for each land use
within a subbasin are described below:

       1.     Newell et al. (1992), provided the following EMCs for the various
              land uses found in the watershed:

              Land Use            EMC (mg/1)                Normalized EMC
                                                             to Forest

              Urban*              2.35 to 3.18  mg/1          2.84 to 3.83
              Open/Pasture         1.51 mg/1                  1.82
              Agriculture          1.56 mg/1                  1.88
              Wetlands            0.83 mg/1                  1.00
              Forest               0.83 mg/1                  1.00

              *  - For the urban land  use category we calculated area weighted
              EMC values for all the eighteen subbasins, as the area occupied by
              the three land uses of high density urban, residential urban, and
              barren changed from  one subbasin to another.   We did  not


                                           40

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              calculate area weighted values for the other land use categories, as
              these categories were the same as those reported by Newell et al.
              (1992).

              We normalized the EMC data to the forest and wetlands EMCs
              and the normalized ratios are listed above.

       2.      Using land use from  Table 4.8 we calculated the  fraction of the
              area that was occupied by each land use for all the 18 subbasins in
              the watershed.

       3.      For each land use in a subbasin,  we then  multiplied the total
              nitrogen load that was estimated by Newell et al. (1992) (here after
              referred as "observed load") by the normalized EMC ratios and
              the  fraction  of each  land  use  found  in the subbasin. We thus
              obtained nitrogen loads from each land use that were normalized
              by the EMC ratios.

       4.      For each  subbasin  we then summed  the  load that was obtained
              from each  land use  to  obtain  the  "normalized"  loads.   A
              normalized  load ratio  of  the  individual land  use  load  to the
              normalized load was then obtained for all the subbasins.

       5.      We then  estimated nitrogen loads  for each  land  use in  all the
              subbasins by multiplying the observed load for the subbasin by the
              normalized load ratio for each land use in the subbasin. Thus the
              sum of the nitrogen  loads from each land use in the subbasin
              equalled  the total  observed nitrogen load, and  they reflected
              both the  relative EMCs and the  land  use  distribution in the
              subbasin. The calculated land  use  loads for all the subbasins are
              listed in Table  4.9.

The annual unit area loads (kg/ha/yr) were calculated for all  the land use categories using Table
4.8 and 4.9, and are  listed in Table 4.10. The average unit area load  from agriculture was
estimated as 6.31 kg/ha/yr.  This value is significantly lower than  the Chesapeake Bay estimates
derived by Donigian  et al. (1994), which were  reported to range from  16  to  34 kg/ha/yr.
However, these differences are not unexpected considering the differences  in  climate  (i.e.
rainfall, runoff), soils, topography, etc. between the two regions.
                                          41

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    Table 4.9    Total Edge-of-Field Nitrogen Loads from the Subbasins of the Galveston Bay Watershed (modified from Newell et
                 al., 1992)
.e-
K)
                 Subbasin
                 Name
Armand/Taylor
Bastrop/Austin*
Brays Bayou
Buffalo Bayou
Cedar Bayou
Chocolate Bayou
Clear Creek
Dickinson Bayou
East Bay*
Greens Bayou
North Bay*
San Jacinto River
Sims Bayou
South Bay*
Trinity Bay*
Trinity River
West Bay*
White Oak Bayou
                                       Land Use Total Nitrogen Loads (kg)
Urban

83717
34651
306631
732969
79193
24979
100167
33750
109274
316324
39505
57517
134666
79294
66760
109980
1 15038
284073
Open/
Pasture
51905
48882
57762
209797
74480
35443
103086
55751
99462
101058
20103
30712
67805
36048
89870
146929
127185
54710
Cropland

19151
76621
36723
288394
123113
108705
71530
26879
105610
34169
2077
14931
22663
11850
106301
163038
98860
22608
Wetlands

9171
19457
4885
38122
25382
15829
23680
12939
67580
14140
2210
7944
8769
10808
47967
90334
62586
3609
Forests

3057
1390
0
5718
18832
3044
2537
681
6075
31309
1105
14896
1096
0
45103
366720
1332
0
                   - Subbasins that drain directly into the Galveston Bay (Higgins, 1993, personal communications).

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The land use categories for which the nitrogen loads  were estimated consisted of urban,
open/pastures, agriculture, wetlands, and forests.  The following nitrogen inputs were assumed
for each land use:

       Land Use           Nitrogen Inputs on the Land Use

       Urban               Atmospheric deposition
       Open/Pastures       Atmospheric deposition
       Agriculture          Fertilizer application and atmospheric deposition
     . Wetlands            Atmospheric deposition
       Forest               Atmospheric deposition

Animal waste is usually applied to croplands and pastures.  However, we were told that there
are no animal waste data available for the Galveston Bay watershed (Hudson,  1993, personal
communications).  The areas occupied by various crops grown in the watershed were obtained
from the NOAA data base of Agricultural Pesticide Use  in Coastal Areas (1992).  According
to the NOAA data, the crops grown in the Galveston Bay watershed consisted of com, rice, rye
grass-hay, and soybeans. The percent area occupied by corn, rice, rye-grass, and  soybean as
reported by the NOAA data base is 16%, 36%, 25%, and 24% respectively. McCown (1993,
personal communications) from the local  extension service provided information on fertilizer
application amounts; he indicated an average of 120 kg/ha/yr of nitrogen fertilizer is applied on
the agricultural land  use in the watershed.

The point source data were obtained from Armstrong (1993, personal communications) and are
listed in Table 4.11.  There was only one NADP atmospheric deposition monitoring site in the
Galveston Bay watershed.  The site is located at Attwater Prairie Chicken NWR.  Observed
atmospheric deposition data for the period of 1983-1991 were available from this site. We used
an average deposition rate for our analysis, and the data  used are listed in Table 4.11.  There
were no dry atmospheric deposition data collected in the watershed, hence  in our analysis we
assumed that dry deposition equals wet atmospheric deposition. The travel time data from each
subbasin listed in Table 4.11 were provided by Newell (1993, personal communications).

After obtaining all the necessary inputs required for our analysis, we estimated the nitrogen loads
that are delivered to the Galveston Bay from the various nitrogen sources in the watershed. The
results obtained from the Lotus 123  spreadsheet analysis are presented in the section below.

4.2.2  Results Obtained from the Galveston Bav

The results obtained from the application of  the  project methodology  to the Galveston Bay
watershed are presented in Tables 4.12 and 4.13.  In Table 4.12 we present the annual nitrogen
loads in million kilograms from all the land use practices and all  the sources of nitrogen. The
percent of the total nitrogen load that is being contributed by the various sources of nitrogen is
presented in Table  4.13.
                                          43

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Table 4.10   Unit Area Annual Nitrogen Loads from the Various Land Use Categories in the
             Galveston Bay Watershed (kg/ha/yr)

                                Urban    Open/    Cropland     Wetlands    Forest
                                          Pasture
Armand/Taylor                    12.43      7.16      7.39      3.93         3.93
Bastrop/Austin**                    6.69      3.25      3.36      1.79         1.79
Brays Bayou                       14.62      8.58      8.86      4.71
Buffalo Bayou                     13.87      6.69      6.92      3.68         3.68
Cedar Bayou                       11.76      5.75      5.94      3.16         3.16
Chocolate Bayou                    8.77      4.28      4.42      2.35         2.35
Clear Creek                       10.74      5.94      6.14      3.27         3.27
Dickinson Bayou                    9.31      4.78      4.94      2.63         2.63
East Bay**                        10.82      5.33      5.51      2.93         2.93
Greens Bayou                     13.57      7.09      7.33      3.90         3.90
North Bay**                       13.87      7.76      8.02      4.27         4.27
SanJacinto                        13.88      6.98      7.21      3.83         3.83
Sims Bayou                       13.33      7.70      7.95      4.23         4.23
South Bay**                        9.88      6.33      6.54      3.48
Trinity Bay**                     10.31      5.03      5.20      2.76         2.76
Trinity River                       8.85      4.20      4.34      2.31         2.31
West Bay**                        8.38      4.68      4.83      2.57         2.57
White Oak Bayou                  15.23      8.45      8.73      4.64

Average                           11.46      6.11      6.31      3.36         3.17
Standard Deviation                  2.43      1.51      1.56      0.83         0.75
**
   - Subbasins that drain directly into Galveston Bay (Higgins, 1993, personal communications)
                                         44

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Table 4.11    Point Sources, Wet Atmospheric Deposition, and Travel Time for the Galveston
             Bay Watershed Subbasins
                          Point Sources
                          kg/yr
                               0
                          632602
                         1708663
                           30647
                             845
                          152939
                            3811
                          218997
                         1526219
                          403261
                         8176472
                          330078

                         6741559
Subbasin
Armand/Taylor
Brays Bayou
Buffalo Bayou
Cedar Bayou
Chocolate Bayou
Clear Creek
Dickinson Bayou
Greens Bayou
San Jacinto River
Sims Bayou
Trinity River
White Oak  Bayou

Direct Point
Source Loads to
the Galveston
Bay
Note: Point source data from N. Armstrong (1993, personal communications)
      Travel time estimates provided by Newell (1993, personal communications)
Atmospheric*
Deposition
kg/ha/yr

  12.89
  12.89
  12.89
  12.89
  12.89
  12.89
  12.89
  12.89
  12.89
  12.89
  12.89
  12.89
Travel
Time
   days

0.4
0.6
2.4
1.3
0.9
1.3
0.8
0.9
0.6
0.7
3.3
0.3
* _
Average of 1983-1991  annual wet atmospheric deposition at Attwater Prairie Chicken
NWR (Data from National Atmospheric Deposition Program)
The conclusions that can be drawn from Tables 4.11 and 4.12 are listed below:

       1.     A total of 36.20 million kilograms of nitrogen is delivered to the
             Galveston Bay annually.

       2.     Point sources deliver 17.54 million kilograms per year, which is
             a major portion of the total nitrogen load that reaches the Bay.

       3.     Direct  atmospheric  deposition  on  the water  surface  of the
             Galveston Bay accounts for 14.06 million kilograms of nitrogen
             per  year.
                                         45

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Table 4.12   Estimated Total Annual Nitrogen Loads  to the Galveston Bay  from Various
             Nitrogen Sources

                                                      Total Load (106 kg/yr)

             Point Sources (delivered through rivers)                     10.80

             Point Sources (direct loadings to Bay)                        6.74

             Direct Atmospheric Deposition to Bay                       12.90

             Direct Loadings to Bay from Watershed*
                    Fertilizer Application                                0.31
                    Atmospheric Deposition                              1.16

             Cropland
                    Fertilizer Application                                0.63
                    Atmospheric Deposition                              0.14

             Forest
                    Atmospheric Deposition                              0.36

             Open/Pastures
                    Atmospheric Deposition                              0.88

             Urban
                    Atmospheric Deposition                              2.03

             Wetland
                    Atmospheric Deposition                              0.22

             Total Load                                               36.20

* -   The estimates are for the subbasins that drain directly into the Galveston  Bay without
      undergoing any riverine losses.

Table 4.13 Percent Loadings  from Various Nitrogen Sources to the Galveston Bay

             Source                     Percent Contribution of Total Load

             Point Sources                                 48.5
             Atmospheric Deposition to the Bay              38.9
             Atmospheric Deposition from Watershed        10.0
             Fertilizer Application                           2.6


                                         46

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       4.     The land use categories in the watershed contribute 5.76 million
             kilograms of nitrogen annually to Galveston Bay.

       5.     The biggest contribution from the five land use categories is from
             the urban land.

       6.     Point sources and atmospheric deposition each account for 49% of
             the total nitrogen load that reaches the Galveston Bay.

To summarize the results from  the application of the methodology to the Galveston Bay, it can
be concluded that atmospheric nitrogen deposition is a major contributor of the nitrogen loadings
that are delivered to the Galveston Bay.

4.3 APPLICATION OF THE  METHODOLOGY TO TAMPA BAY WATERSHED

As stated above,  the Gulf of Mexico region ranks first among those of the U.S. in total estuarine
drainage area and in total water surface area (Rabalais, 1992).  The Tampa Bay watershed, our
second study site is also  an integral part of the area that drains into the Gulf of Mexico.

4.3.1   Available Data for Analysis

The data used in the analysis of the Tampa Bay watershed was  obtained initially from Greening
(1993, personal communications) and was subsequently published by Coastal Engineering Inc.,
(Zarbock et al, 1994)  who was conducting a nutrient balance study for the Tampa Bay watershed
as part of the Tampa Bay National Estuary Program (Greening, 1993, personal communications).
The watershed area that drains into the Tampa Bay occupies 0.60 million hectares and is divided
into 10 subbasins (Figure  4.3).  The water surface area occupied by the Tampa Bay is 0.1
million hectares (NOAA, 1990).

The nonpoint loading  data from  each of the land use categories in the ten subbasins was provided
by Greening (1993, personal communications) and later published by Zarbock et al. (1994). The
land uses in  the watershed  are comprised of urban, agriculture, wetlands, and forest.

The nonpoint nitrogen loading  data available consisted of bar graphs of total nitrogen  loads
produced from all the subbasins and associated land use categories. We used the bar graph to
estimate the nonpoint loadings  from the ten subbasins; these  loadings are presented in Table
4.14.
                                         47

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      MAJOR DRAINAGE BASINS OF TAMPA BAY
                     Modified from USGS
                                      Map Prepaiol by Coaml Ecvtronmcnul, Inc.

Figure 4.3  Tampa Bay Watershed with Subbasin Boundaries
                       48

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Table 4.14   Total Edge-of-Field Nitrogen Loads from the Subbasins of the Tampa Bay
             Watershed (Zarbock et ah, 1994)
       Subbasin
       Name
                   Total Nitrogen Loads (kg/yr)
         Urban    Cropland     Wetlands     Forests
       Alafia River               56127      256581        16036        85527
       Boca Ciega Bay            128290            0         5345            0
       Coastal Hillsborough Bay    53454        66818        10691        16036
       Coastal Lower Tampa Bay    10691        20313         2138        10691
       Coastal Middle Tampa Bay  90872         9622         2138         5345
       Coastal Old Tampa Bay     145396        20313        13898        42764
       Hillsborough River         160363      235199        48109      203127
       Little Manatee River        20847      198851        26727        69491
       Manatee River             48109      225578        64145        85527
       Terra Ciea Bay               5345         2673         2000            0
The land use categories  for which  the  nitrogen loads were estimated  consisted of urban,
agriculture, wetlands, and forests.  The following nitrogen inputs were assumed for each land
use:
       Land Use

       Urban
       Agriculture
       Wetlands
       Forest
Nitrogen Inputs on the Land Use

Atmospheric deposition
Fertilizer application and atmospheric deposition
Atmospheric deposition
Atmospheric deposition
 Animal waste is usually applied to croplands  and pastures.  However, no animal waste
 application data were available for the Tampa Bay watershed.  The area occupied by various
 crops grown in the watershed was obtained from the NOAA data base of Agricultural Pesticide
 Use in Coastal Areas (1992). According to the NOAA data, the crops grown in the Tampa Bay
 watershed consisted of rye grass-hay, citrus fruits, vegetables, and corn.  The percent area
 occupied by rye-grass, citrus fruits, vegetables, and corn as reported by the NOAA data base
 is 88%, 7%,  4%, and less than 1% respectively. Huff (1993, personal communications) from
 the local Soil Conservation Office provided  information  on fertilizer application amounts; she
 indicated an average of 120 kg/ha of nitrogen fertilizer is applied annually on the agricultural
 land use in the watershed.
                                          49

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The point source data were provided by Greening (1993, personal communications) and are
listed in Table 4.15.  There was only one NADP atmospheric deposition monitoring site in the
Tampa Bay watershed.   The site is located at Vema  Well Field.   Observed  atmospheric
deposition data for the period of 1984-1991 were available from this site. We used an average
deposition rate for our analysis, and the data used are listed in Table 4.15. There were no dry
atmospheric deposition data collected in  the watershed; hence in our analysis we assumed that
dry deposition equals wet atmospheric deposition.   The travel time data from each subbasin
listed in Table 4.15 was provided by Zarbock (1993, personal communications).

After obtaining all the necessary inputs required for our analysis, we estimated the nitrogen loads
that are delivered to the Tampa Bay from the various nitrogen sources in the watershed.  The
procedure adapted for routing the loads in this analysis is slightly different than the procedures
used in the Chesapeake  Bay or Galveston Bay analysis.  Here, we did not calculate retention
parameters or individual land use total nitrogen loads based on unit area loads as we did in the
analysis of Chesapeake Bay and Galveston Bay,  as information available on  the land use areas
was not available to us.  The input source ratios were calculated based on the nitrogen inputs
on each land use. These ratios were then used to calculate the source of nitrogen load from the
total nitrogen load that was provided to us by Greening, for all the land use categories in the ten
subbasins.   The procedure of estimating output source ratios and routing the nitrogen loads
through the river/stream channel network to the bay using travel time and first-order decay rate
was the same as described in the project methodology. The results obtained from the Lotus 123
spreadsheet analysis are presented in the section below.

4.3.2  Results Obtained from the Tampa Bay

The results obtained  from the application  of  the  project methodology to the Tampa  Bay
watershed are presented in  Tables 4.16 and 4.17.  In Table 4.16 we present the nitrogen loads
in million kilograms per year from all the land use practices and  all  the sources of nitrogen.
The percent of the total nitrogen load contributed by the various sources of nitrogen is presented
in Table 4.17.

The conclusions that can be drawn from Tables  4.16 and 4.17 are listed  below:

       1.     A total of 4.48 million kilograms of nitrogen per year is delivered
              to the Tampa Bay.

       2.     Point sources annually deliver 0.87 million kilograms of nitrogen
              to the Tampa Bay.

       3.     Direct atmospheric deposition on  the  water surface  of the Tampa
              Bay accounts for  1.80 million kilograms of  nitrogen per year,
              which is a major portion of the total nitrogen load that reaches the
              Bay.
                                           50

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Table 4.15    Point Sources, Wet Atmospheric Deposition, and Travel Time for the Tampa
             Bay Watershed Subbasins
       Subbasin
       Alafia River
       Boca Ciega Bay
       Coastal Hillsborough Bay"
       Coastal Lower Tampa Bay"
       Coastal Middle Tampa Bay
       Coastal Old Tampa Bay
       Hillsborough River
       Little Manatee River
       Manatee River
       Terra Ciea Bay
Point Sources    Atmospheric*
       kg          Deposition
                    kg/ha
   93282
   15909
  484800
   96960
   95454
   90909
  106818
    2273
   23636
    2727
10.04
10.04
10.04
10.04
10.04
10.04
10.04
10.04
10.04
10.04
Travel
 Time
  days

   5.0
   0.5
   1.0
   1.0
   1.0
   2.0
   6.0
   3.0
   5.0
   1.0
Note: Point source data from Greening (1993, personal communications)
      Travel time estimates provided by Zarbock (1993,  personal communications)

* -    Average of 1984-1991 annual wet atmospheric deposition at Verna Well Field
      (Data from National Atmospheric Deposition Program)

** -  Includes Fugitive Emissions from Fertilizer Production Plants as Point Source Loadings
      (Greening, 1993, personal communications)
                                        51

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Table 4.16   Estimated Total Nitrogen Loads to the Tampa Bay from Various Nitrogen
             Sources

                                                     Total Load (106 kg/yr)

             Point Sources (delivered through rivers)                      0.87

             Direct Atmospheric Deposition to Bay                       1.80

             Cropland
                   Fertilizer Application                                0.63
                   Atmospheric Deposition                              0.11

             Forest
                   Atmospheric Deposition                              0.14

             Urban
                   Atmospheric Deposition                              0.58

             Wetland
                   Atmospheric Deposition                              0.36

             Total Load          -                                     4.48


Table 4.17 Percent Loadings from Various Nitrogen Sources to the Tampa Bay

       Source                       Percent Contribution of Total Load

             Point Sources                               19.35
             Atmospheric Deposition to the Bay            40.17
             Atmospheric Deposition from Watershed       26.44
             Fertilizer Application                        14.04
                                         52

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       4.    The land use categories in the watershed contribute 1.82 million
             kilograms of nitrogen annually to Tampa Bay.

       5.    The biggest contribution from the five land use categories is from
             cropland.

       6.    Point sources and atmospheric deposition, respectively, account for
             19% and 67%  of the total nitrogen load that reaches the Tampa
             Bay each year.

To summarize the results from the application of the methodology to the Tampa Bay, it can be
concluded that atmospheric nitrogen deposition is the major contributor of nitrogen loadings that
are delivered to the Tampa Bay.  However,  the total bay loads are considerably less than the
loads delivered in the other two study sites. Also, the dominance of the atmospheric component
is likely due to the relatively large water surface area and relatively small watershed areas of the
Tampa Bay system, compared to the other study sites.

4.4    SENSITIVITY ANALYSIS

The results obtained from the application of the methodology to the study sites were presented
in the sections above. In this section we present results obtained from the sensitivity analysis
of three parameters that were used in the Lotus  123  spreadsheet to estimate nitrogen loadings
to aquatic systems.  The sensitivity analysis was performed for the Chesapeake Bay watershed
on selected parameters used in the assessment of the initial results, which we will call the base
scenario  presented in Section 4.1.  The sensitivity analysis was performed by varying one
parameter at a time in all the fourteen subbasins of the Chesapeake Bay watershed, while holding
the other parameters constant. The three parameters analyzed were:

       1.     Ratio of Wet to Dry Atmospheric Deposition;
       2.     First-order decay rate in rivers; and
       3.     Fertilizer application amounts (varying agronomic recommended rates).

One of the major assumptions of the sensitivity analysis was that the nitrogen retention parameter
for the various land use practices in the fourteen subbasins will  not be affected; these retention
parameters are listed in Table 4.18.
4.4.1 Sensitivity of Wet and Drv Atmospheric Deposition Ratios

As this  project deals jwith estimating  the percent of nitrogen contribution from  atmospheric
deposition with respect to total nitrogen loadings to the bay or watershed outlet from all sources,
nonpoint and point, it was very important to study the effect of the ratio between  wet and dry
atmospheric nitrogen deposition.  The major reasons to study the effect of the ratio between wet
and dry atmospheric deposition are:


                                          53

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       1.     Wet atmospheric deposition data are readily available  for 200
             stations in the contiguous United States.  The wet deposition data
             are collected by the staff of the National Atmospheric Deposition
             Program (NADP).

       2.     The dry atmospheric deposition data are very scarce and are not
             readily available.  Thus, the ratio of wet to dry deposition is often
             used to determine Total deposition.

       3.     In  the  Lotus  123   spreadsheet  developed under the current
             methodology, we estimate dry atmospheric deposition as a factor
             times the wet atmospheric deposition.

For the base scenario, 40% of the total nitrogen input on the Chesapeake watershed and water
surface was derived from atmospheric  deposition (Table 4.1).  Fisher et al.  (1988)  cited
literature where the ratios  of dry atmospheric deposition to total atmospheric deposition were
reported; these ratios and the literature source are listed below:

Literature Source                             Percent of Total Atm. Deposition as
                                             Dry Deposition

Levy and Moxim, 1987 and Ro et al., 1988     20 to 85 %
Logan, 1983                                  38 to 55%
Meyers and Sisterton, 1989                    30 to 45%

During the sensitivity analysis, the wet deposition data were not altered as these data represented
measured wet atmospheric deposition at the four sites in the Chesapeake Bay watershed. In our
analysis for the base case scenario,  we assumed that  dry deposition is 50% of the total
atmospheric deposition,  which meant that dry deposition was equal  to wet deposition and,
therefore,  total deposition is two times wet deposition.  The following scenarios were analyzed
during the sensitivity analysis:

       1.     Dry deposition  is 20% of Total  Deposition (this  means dry
             deposition is 25% of wet deposition);
       2.     Dry deposition  is 33% of Total  Deposition (this  means dry
             deposition is 50% of wet deposition); and
       3.     Dry deposition  is 60% of Total  Deposition (this  means dry
             deposition is 150% of wet deposition);

The results obtained from the sensitivity analysis are presented in Table 4.19 and Table 4.20.
In Table 4.19 we present the total nitrogen loads from all the land uses and  nitrogen sources,
and in Table 4.20 the percent of individual  nitrogen source  loads as they relate to the total
nitrogen load to the Chesapeake Bay are presented.  The conclusions drawn from the sensitivity
analysis are presented below:


                                          54

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Table 4.18 Retention Factors for the Base Case for the Chesapeake Bay Watershed

                                Cropland     Forest        Pasture      Urban
East Branch Susquehanna
West Branch Susquehanna
Juniata
Lower Susquehanna
Conowingo
Upper Potomac
Shenandoah
Lower Potomac
Rappahannock
Mattaponi
Pamunkey
James
Appomattox
Patuxent
                   0.88
                   0.80
                   0.87
                   0.83
                   0.87
                   0.90
                   0.88
                   0.90
                   0.90
                   0.85
                   0.85
                   0.89
                   0.85
                   0.90
0.63
0.68
0.47
0.64
0.30
0.63
0.68
0.64
0.65
0.92
0.90
0.87
0.90
0.85
0.73
0.79
0.70
0.54
0.34
0.75
0.94
0.58
0.74
0.70
0.68
0.40
0.62
0.84
0.24
0.21
0.16
0.20
0.23
0.12
0.18
0.15
0.14
0.50
0.59
0.26
0.37
0.16
      3.
The total atmospheric nitrogen input to the Chesapeake Bay was
63%, 75%, and  125% of the base case scenario input when dry
deposition rates were assumed to be 20%, 33%, and 60% of the
total atmospheric deposition respectively.

The total nitrogen load to the Chesapeake Bay was 80%, 89%, and
120% of the  base case scenario load when dry deposition rates
were assumed to be  20%, 33%, and 60% of total atmospheric
deposition, respectively.  This indicates that estimating the percent
of total atmospheric  deposition resulting  from dry deposition is
very important as it is one of the major sources of nitrogen input
to the water surface, and is deposited on all land uses of the
watershed.

Under  the base  scenario,  we estimated  that  43%  of the total
nitrogen load  resulted  from atmospheric deposition; however,
decreasing the percent of dry deposition to 20% and 33% of total
atmospheric deposition  resulted  in  32%  and  36%  of the total
nitrogen load  resulting from atmospheric deposition.  When  we
increased the  percent of dry deposition in the total atmospheric
deposition to  60%, we  estimated that 49% of the total nitrogen
load  to the Chesapeake Bay results from atmospheric deposition.
These results were obvious because decreasing  the percent of dry
                                         55

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             deposition as a percent of total atmospheric deposition resulted in
             a lower input of atmospheric deposition to the various land uses in
             the watershed, and  increasing the percent of  dry deposition
             resulted in higher inputs of atmospheric nitrogen.

      4.     The direct atmospheric deposition to the water  surface of  the
             Chesapeake  Bay was estimated  to be 11 % of the total nitrogen
             load.  Decreasing the dry atmospheric deposition to 20% and 33%
             of total atmospheric deposition  resulted in 8 and  10% of direct
             atmospheric deposition  to  the  water surface.   Increasing  the
             percent of dry deposition to 60% of total atmospheric deposition
             resulted in 12% of direct atmospheric deposition to the Bay water
             surface.

      5.     Under the base scenario, we estimated that 43% of total nitrogen
             load to the Chesapeake Bay resulted from atmospheric deposition.
             The sensitivity analysis indicates that changing the percent of  dry
             deposition in total atmospheric deposition to 20%, 33%, and 60%
             results in 74%,  84%,  and  113%, respectively, of the total
             atmospheric nitrogen deposition relative to the base case scenario.

From the  above analysis it can be concluded  that either better estimates of atmospheric dry
deposition or the total atmospheric deposition  are needed.  This is important because it has a
significant impact when calculating the percent contribution of atmospheric deposition to the total
nitrogen load delivered to the watershed outlet.

4.4.2 Sensitivity to Riverine Decay Parameter

Another important parameter that controls the  total amount of nitrogen that  is delivered to the
Chesapeake Bay or any watershed outlet of interest is the first-order riverine  decay rate used in
our analysis.  Under the base case scenario, we assumed a decay rate of 0.08/day, this rate is
a typical  rate of nitrogen loss/decay reported  in the literature (Barnwell, 1993, personal
communications). During  the sensitivity analysis the decay rate was varied as 50% (0.04/day),
75% (0.06/day), and 200% (0.16/day) of the base scenario rate.   The results obtained from the
sensitivity analysis are presented in Tables 4.21 and 4.22, and are discussed below:

       1.     The total nitrogen load to the Chesapeake Bay was 116%, 107%,
             and 78%  of the base case  scenario load  when decay rates were
             assumed to  be 50%,  75%, and 200%  of the base scenario  rate
             respectively. This indicates that decreasing the decay rate results
             in higher nitrogen loads to be delivered to the Chesapeake Bay  and
             vice versa.  A decrease of the decay rate by 50% of the base
             scenario decay rate resulted in 16% increase in total nitrogen load
             that was delivered to the Chesapeake Bay; however, an increase in

                                          56

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      Table 4.19   Sensitivity of Ratio of Dry  to Wet Atmospheric Deposition Rates to
                   Nitrogen Loadings to Chesapeake Bay

                                Base Scenario
Source

Point Sources (AFL)
Point Sources (BFL)
to the Bay

Cropland
      Fertilizer
      Atmospheric Dep
      Manure
Forest
      Atmospheric Dep.
Pasture
      Manure
      Atmospheric Dep.

Urban
      Atmospheric Dep.

Total Load

      Table 4.20


Source
Point Sources
Atmospheric Deposition
Fertilizer
Manure
Dry Dep. Dry Depo.
50% of 20% of
Total Total
Deposition Deposition
106 kg/yr 106 kg/yr
7.38 7.38
26.60 26.60
>eposition
12.90 8.06
23.00 23.00
>ep. 3.25 2.03
4.48 4.48
)ep. ^21.70 13.50
3.79 3.79
)ep. 4.50 2.81
)ep. 7.16 4.48
115.00 96.20
Dry Depo.
33% of
Total
Deposition
106 kg/yr
7.38
26.60
9.68
23.00
2.44
4.48
16.30
3.79
3.38
5.37
102.00
Dry Depo.
60% of
Total
Deposition
106 kg/yr
7.38
26.60
16.10
23.00
4.07
4.48
27.10
3.79
5.63
8.95
138.00
Percent Contribution to the Chesapeake Bay with Varying Ratios of Wet
to Dry Atmospheric Deposition Rates
Percent to the Bay
Base 20% of 33% of 60% of
Scenario Total
Depo. Depo.
29.6 35.4
on 43.1 32.1
20.1 23.9
7.2 8.6
Total
Depo.
33.2
36.3
22.5
8.1
Total
Depo.
26.7
48.7
18.1
6.5
                                        57

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             the decay rate to 200% of base scenario rate resulted in 22% less
             nitrogen loadings to the Bay.  We assumed that the nitrogen
             delivery to the Bay  from the subbasin river network to follow an
             exponential decay process; thus nitrogen loss was a function of the
             product of the decay rate and travel time in the rivers.  This
             analysis indicates that estimating nitrogen losses in rivers and
             streams is important as it controls the amount of nitrogen that will
             be delivered to the bay or watershed  outlet.

       2.     The change in riverine decay parameter only affected the total
             nitrogen load  to the Chesapeake Bay; the percent loadings from
             individual sources of nitrogen  as presented in Table 4.22 did not
             change significantly, as the nitrogen loadings from all the sources
             were influenced by  the change in decay rate.

4.4.3  Sensitivity  to Fertilizer Application Amounts

The percent of nitrogen input from fertilizer application under the base scenario was 35 % of the
total nitrogen input to the Chesapeake Bay watershed and water surface (Table 4.1).  Nitrogen
is an important nutrient for crop growth and is supplied to the crops from fertilizer and manure
application. Each crop needs a certain amount of nitrogen for attaining maximum growth and
production.  This nitrogen rate is normally determined by agricultural extension scientists based
on  field  experiments.   The rate is normally referred to  as  the  recommended agronomic
application rate.  It has been reported  that farmers do not apply fertilizers at recommended
agronomic rates;  they often apply excess fertilizer than what is needed by the crops.  This
excessive application results  in greater amounts of nitrogen being delivered to the streams and
rivers. During the sensitivity analysis we varied the fertilizer application under the base case
scenario to 50%, 150%, and 200% of base scenario fertilizer input.  The results obtained from
the sensitivity analysis are presented in Tables 4.23 and 4.24, and are discussed below:

       1.     The total nitrogen load to the  Chesapeake Bay was 90%, 110%,
             and  120% of the base case scenario load when fertilizer application
             rates were assumed to be 50%, 150%,  and 200% of the base
             scenario rate, respectively.   This indicates that  decreasing  the
             fertilizer application amount by 50% resulted in a 10% decrease
             in total load to the Chesapeake Bay as compared to the base case
             scenario. Increasing fertilizer application rates to 150% and 200%
             of the base scenario rate resulted in  a  10% and 20% increase in
             nitrogen loads to the Bay.

       2.     From Table 4.24, it can be seen that the percent contribution to
             the Chesapeake Bay was 20% from fertilizer application under the
             base case scenario.  Changing the application rates to 50%, 150%,
             and 200% of the base application rates resulted in 11%, 27%, and
             33% of loadings to  the Chesapeake Bay due to fertilizer


                                          58

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Table 4.21    Sensitivity of Riverine Decay Rates to Nitrogen Loadings to Chesapeake Bay
Source
Point Sources (AFL)
Point Sources (BFL)
Direct Atmospheric Deposition
to the Bay
Cropland
Fertilizer
Atmospheric Dep.
Manure
Forest
Atmospheric Dep.
Pasture
Manure
Atmospheric Dep.
Urban
Atmospheric Dep.
Total Load
Table 4.22 Percent Contribution
Rates
Source
Point Sources
Atmospheric Deposition
Fertilizer
Manure
Base
Scenario
10* kg/yr
7.38
26.60
12.90
23.00
3.25
4.48
21.70
3.79
4.50
7.16
115.00
50% Base
Scenario
106 kg/yr
9.14
26.60
12.90
28.33
4.11
5.67
27.90
4.54
5.26
8.86
133.00
75% Base
Scenario
10* kg/yr
8.20
26.60
12.90
25.50
3.65
5.03
24.60
4.14
4.86
7.95
123.00
200% Base
Scenario
106 kg/yr
5.02
26.60
12.90
16.00
2.12
2.87
13.60
2.72
3.45
4.92
90.20
to the Chesapeake Bay with Varying Riverine Decay
Percent to the Bay
Base 50% Base 75% Base 200% Base
Scenario Scenario Scenario Scenario
29.6
43.1
20.1
7.2
26.8
44.3
21.2
7.7
28.2
43.7
20.7
7.4
35.0
41.0
17.7
6.2
                                         59

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Table 4.23    Sensitivity of Fertilizer Application Rates to Nitrogen Loadings to Chesapeake
             Bay
Source

Point Sources (AFL)
Point Sources (BFL)

Direct Atmospheric Deposition
to the Bay

Cropland
      Fertilizer
      Atmospheric Dep.
      Manure

Forest
      Atmospheric Dep.

Pasture
      Manure
      Atmospheric Dep.

Urban
      Atmospheric Dep.            7.16         7.16         7.16         7.16
Total Load                       115.00       103.00       126.00        138.0
Base
Scenario
106 kg/yr
7.38
26.60
12.90
23.00
3.25
4.48
21.70
3.79
4.50
50% Base
Scenario
106 kg/yr
7.38
26.60
12.90
11.50
3.25
4.48
21.70
3.79
4.50
150% Base
Scenario
106 kg/yr
7.38
26.60
12.90
34.60
3.25
4.48
21.70
3.79
4.50
200% Bas
Scenario
10" kg/yr
7.38
26.60
12.90
46.10
3.25
4.48
21.70
3.79
4.50
Table 4.24   Percent Contribution to the Chesapeake Bay with Varying Base Scenario Fertilizer
             Rates
                                                   Percent to the Bay
Source                          Base         50% Base    150% Base   200% Base
                                Scenario     Scenario      Scenario     Scenario

Point Sources                      29.6         32.9         26.0         24.7
Atmospheric Deposition             43.1         47.9         39.2         35.9
Fertilizer                          20.1         11.2         27.4         33.4
Manure                            7.2          8.0           6.5          6.0


                                         60

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              application.  Inputs from fertilizer application accounted for 35%
              of the total nitrogen input to the Chesapeake Bay watershed.  As
              fertilizer application rates  increase, they account  for  a larger
              percent of the total  nitrogen load, and other sources of nitrogen
              becomes smaller percentages, and vice versa.
4.4.4  Conclusions from Sensitivity Analysis

The results obtained from the sensitivity analysis were presented in Section 4.4.1, 4.4.2, and
4.4.3 for three assessment parameters — dry deposition to wet deposition ratio, riverine decay
rate, and fertilizer application amounts.  Of the three parameters analyzed, dry deposition to wet
deposition ratio and fertilizer application amounts directly affect the total amount of nitrogen that
is input onto the watershed, or the water body, which has a direct impact on the total amount
of nitrogen load that is delivered to the watershed outlet.  A  similar magnitude of change in
these two parameters and its corresponding effect on total nitrogen load relative to base scenario
load is presented in Table 4.25.  It can be concluded from the table that the total nitrogen
loadings are more sensitive to atmospheric deposition than fertilizer application. The reason for
this sensitivity is atmospheric nitrogen deposition occurs on all  of the land use categories found
in the watershed along with the water surface occupied by the Bay.  The fertilizers are however,
only applied to the cropland in the watershed.

The nitrogen decay  in the riverine systems affects the total amount of nitrogen that will be
delivered by the rivers/streams to the watershed outlet.  A  decrease in decay rate by 50% and
75% of the base scenario decay rate resulted in  a 16% and 7% increase in nitrogen load that was
delivered to the Chesapeake Bay.  An increase in the decay rate  by 100% of the base scenario
rate resulted in a 22% decrease in nitrogen load, while a decrease by 50% resulted in a decrease
of total nitrogen load by 12%.

From the above analysis it can be concluded  that the output from the spreadsheet analysis is
sensitive to all  three parameters analyzed.  However, the ratio of  dry to wet deposition has a
greater impact on the total amount of nitrogen delivered to the  Chesapeake Bay as it directly
affects the total nitrogen input to the various land uses and water  surfaces of the Chesapeake
Bay.  The nitrogen decay in  streams is the second most sensitive parameter as it controls the
amount of nitrogen that will be delivered to the watershed outlet via the stream/river network.
Fertilizers are  predominantly applied  to cropland  in the  watershed.  However,  the  typical
variation in fertilizer (and manure) application rates is much  larger than the variation in the other
parameters, with  many farmers applying two to three times agronomic rates.  In a watershed
where  the majority of the land use is under cropland, then  the amount of fertilizer application
will be an important parameter, if not the most important parameter, in estimating nitrogen loads
to aquatic systems.
                                           61

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Table 4.25   Effect of Changing Atmospheric Deposition and Fertilizer Application of Total
             Nitrogen Load from the Chesapeake Bay Watershed

                                 Decrease by 37.5%              Increased by 50%
                          Atm. Depo1   Pert. Rate2          Atm. Depo1   Pert. Rate2

Percent Change in
Total Nitrogen
Load Relative             -20%               -8%          +20%        +10%
to Base Scenario
Load

Note:  A positive/negative sign represents an increase/decrease relative to base scenario.

1      -     Total atmospheric input was increased or decreased relative to the base
             scenario input by changing the dry to wet deposition ratio
2      -     Fertilizer rates were increased or decreased relative to base scenario
             fertilizer application
                                          62

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                                    SECTION 5.0

                     CONCLUSIONS AND RECOMMENDATIONS
The coastal waters of the eastern  United States are receiving large inputs of nutrients, in
particular nitrogen (Fisher et al., 1988).  Large inputs of nitrogen cause excessive growth of
algae, loss of oxygen and light to the water and the long term decline of marine life.  Fisher et
al.  (1988)  conducted a study to estimate the various  sources  of nitrogen that reach the
Chesapeake Bay.  The authors estimated the contributions to the Chesapeake Bay from various
nitrogen sources as they relate to total nitrogen load.  The reported percentages from various
nitrogen sources are: 24% from point sources, 39% from atmospheric deposition, 34%  from
fertilizers, and 4% from animal waste.

The overall objective of this project was to assess the applicability of the EOF approach and
associated findings to other parts of the country, in particular the Gulf Coast region.  The
approach consisted of reviewing and evaluating the EDF methodology, improving and refining
the EDF methodology and assumptions, and  then implementing  these refined procedures to
reproduce their results (in a Lotus 123  spreadsheet format) using commonly available databases.

The EDF methodology was reviewed and the following EDF assumptions were modified under
the current  methodology:

       1.     The nitrogen loadings from  the  watershed  were calculated by
             dividing  the watershed into smaller subbasins to better represent
             the variation in land use in the watershed, rather than lumping all
             the land uses together, as was done in the EDF analysis.

       2.     Fertilizer applications  were  based on  recommended  fertilizer
             application rates as  suggested  by local  and state government
             agencies.

       3.     The Lotus 123  spreadsheet developed for the methodology allows
             the user to vary the relationship between wet and dry atmospheric
             deposition, retention  parameters for various land uses, riverine
             decay rates, and fertilizer application rates.  This is in contrast to
             the constant rates that were used in the EDF study.

       4.     Riverine losses are calculated using travel time and nitrogen losses
             during travel to the watershed outlet, rather  than a constant loss of
             nitrogen from all the  rivers/streams in the watershed.


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The Lotus  123  spreadsheet developed under the new methodology was applied  to the
Chesapeake Bay, Galveston Bay, and Tampa Bay watersheds.  The following conclusions were
drawn from the three study sites analyzed:

       1.     The methodology developed for this project improved and refined
             the assumptions and procedures that were developed in the EDF
             study.

       2.     Our study results indicate that atmospheric nitrogen deposition is
             a major source of nitrogen that is delivered to the study sites,
             accounting for over 40% of the total nitrogen load that is delivered
             to the bays.

       3.     The majority of the atmospheric deposition load occurs as direct
             deposition onto the water surfaces.

       4.     For many East Coast and Gulf Coast watersheds, a majority of the
             nitrogen input may be derived from atmospheric deposition as it is
             deposited on all the land use categories found in the watershed.
                           •
       5.     The Lotus 123  spreadsheet developed under the methodology is
             sensitive to the nitrogen inputs  received from various nitrogen
             sources.

       6.     Application of the Lotus 123  spreadsheet provides reasonable
             estimates of nitrogen loads to the watershed outlet for an average
             climate year.

Based on the analysis that was performed there is a need to further investigate the assumptions
that were used in the  current methodology.   The following  is a list of research  issues and
recommendations that are  derived from the current study:

       1.     We need to  further analyze nitrogen loadings from various sources
             under transient conditions.

       2.     We need to  better characterize and  understand  total nitrogen
             deposition,  and the relationship between wet and dry atmospheric
             deposition.

       3.     We need to investigate possible changes in the first-order nitrogen
             loss/decay  rate with  the  travel  time  associated  with various
             river/streams found in the watershed.
                                           64

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4.     We need to study further how changing climate, crops, soil type,
       and topography affect the retention parameters for the various land
       use practices in the watershed.

5.     We need to better evaluate retention parameters for atmospheric
       nitrogen that is deposited on the watershed as opposed to other
       human nitrogen inputs, such as  fertilizers and animal waste.

6.     We need to investigate the possible impacts, if any, of biological
       nitrogen fixation on retention parameters, especially for crops and
       forests.

7.     We need to focus efforts on the effects of nitrogen deposition and
       its delivery from the watershed to the watershed outlet. We can
       account for the contribution of atmospheric nitrogen deposition on
       the water surface; however, there is limited  information regarding
       the amount  of atmospheric nitrogen  that is deposited on the
       watershed and is finally delivered to the aquatic system of interest.
                                    65

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                                   SECTION 6.0

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      Ecosystems: Foundations and Applications, pp. 261-278. Martinius Nighoff/Dr. W. Junk
      Publishers. The Hague.

U.S. E.P.A. Chesapeake Bay Program.  1983. Nutrients.  Chapter 3. In: Chesapeake Bay: A
      Framework for Action, Annapolis, MD.

Weller, D.E., W.T. Peterjohn, N.M. Goff, and D.L. Correll.  1986. Ion and Acid Budgets for
      a Forested Atlantic Coastal Plain Watershed and their Implications for the Impacts of
      Acid Deposition. In: Watershed Research Perspectives. Correll, D.L. (ed.). Smithsonian
      Institution Press, Washington, D.C., p. 392-421.
                                        67

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Zarbock, H., A. Janicki, D. Wade, D. Heimbuch, and H. Wilson. 1994.  Estimates of Total
      Nitrogen, Total Phosphorus,  and Total Suspended Solids Loadings to Tamps Bay,
      Florida.   Prepared for Tampa Bay  National Estuary  Program. Prepared by coastal
      Engineering, Inc. St.  Petersburg, FL.
                                         68

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                          APPENDIX A
              DESCRIPTION OF LOTUS 123* SPREADSHEET
A.I   INTRODUCTION
A.2   INPUTS TO LOTUS 123* SPREADSHEET
A.3   OUTPUTS FROM LOTUS 123* SPREADSHEET
A.4   NOTES ON USING LOTUS 123* SPREADSHEET
                              A-l

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                   DESCRIPTION OF LOTUS 123* SPREADSHEET
A.I  INTRODUCTION

The  project methodology was described in Section 3.0.   A  Lotus 123  spreadsheet  was
developed using the project methodology to estimate nitrogen loadings to aquatic systems.  The
inputs required for the Lotus 123  spreadsheet are explained below in Section A.2.  In Section
A.3 we provide instructions for obtaining the outputs from the spreadsheet, and Section A.4 we
provide notes on using the spreadsheet.

A.2  INPUTS TO LOTUS  123 SPREADSHEET

The  spreadsheet is developed  for a watershed with a twelve subbasins.   If the watershed of
concern has only six  subbasins then the user  can input data for  the  six subbasins in the
spreadsheet. This is because, the default nonpoint and point loads from all the twelve subbasins
are set to zero in the spreadsheet.  For watersheds with more than twelve subbasins, the  user
can copy the block of cells A9..AI42 which represents one subbasin,  to cell location A440 and
A475, etc.  for each additional  subbasin. The total nitrogen load at the watershed outlet is the
sum  of nitrogen loads  from identical nitrogen sources from  each land use category and point
sources from all  the subbasins  in  the watershed.   For watersheds with more than  twelve
subbasins, the user has to add  the nonpoint and point source contributions from any additional
subbasin to the total nitrogen load that is computed using only twelve  subbasins.  The formulas
in the following cells need to  be modified to include the contributions from the additional
subbasins: Point Sources - Cell AN15; Cropland - Fertilizer (Cell AN22), Atm.  Dep. (Cell
AN23), and Manure (Cell  AN24); Forest - Atm. Dep.  (Cell AN27), Pasture - Manure (Cell
AN30), Atm. Dep. (Cell AN31); Wetlands - Atm. Dep. (Cell AN34); and Urban -  Amt. Dep.
(Cell AN37).

The  inputs to the spreadsheet  include both nonpoint and point sources.  The nonpoint source
inputs are needed for the land  use categories of forest, cropland, wetlands, pastures, and urban
areas.  The point source inputs are from industrial waste discharges and sewage treatment plant
discharges.  The various nitrogen sources that  a user can input for  the land use categories is
listed in Table A.I.

As can be seen from Table A.I, the user does not input dry atmospheric deposition  because we
calculate dry atmospheric  deposition as a  factor of wet atmospheric deposition.  The dry
deposition  is computed in  the spreadsheet  by  multiplying the factor times wet atmospheric
deposition.  This factor is an  input to the spreadsheet.  If dry atmospheric deposition data is
available to the  user,  then  he/she can compute the factor  which when  multiplied  with wet


                                         A-2

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atmospheric deposition will yield dry atmospheric deposition.  A layout of the Lotus  123*
spreadsheet is shown in Table A.2.
Table A. 1  Land Use Categories and Nitrogen Sources

                                  Nitroen Sources
             Forests             Wet atmospheric deposition (kg/ha)
             Cropland           Wet atmospheric deposition (kg/ha), Fertilizer
                                 application (kg/ha), and Manure application (kg/ha)
             Wetlands           Wet atmospheric deposition (kg/ha)
             Pastures            Wet atmospheric deposition (kg/ha) and Manure
                                 Application (kg/ha)
             Urban              Wet atmospheric deposition (kg/ha)
The necessary inputs along with the cell locations in the spreadsheet are listed in Tables A.3 and
A.4.  The input cells are high lighted in the spreadsheet for the user, the user can only
input data in these cells as all the other cells are protected. The user does not have to input
any data elsewhere in the spreadsheet, except for water surface area of the bay (ha), direct
atmospheric deposition (kg/ha) and direct  point source loadings (kg) to the bay or
watershed outlet.

The spreadsheet computes the unit area inputs for each land use category by summing all the
nitrogen inputs  from the all the sources for the land use of interest. The total nonpoint source
unit area inputs (kg/ha) and total point source inputs (kg) are displayed in column P of the
spreadsheet.  In Column R, in the spreadsheet, we compute the Input Source Ratios using the
procedures described in Section 3.2.

The total nitrogen load from each land use is computed as a product  of the land use area and
either the retention factor or literature reported edge-of-field unit area load. If retention factors
are not available to the user, or if he/she intends to use edge-of-field  unit area  loads, then the
user should input 999  in the retention factor cells of the spreadsheet; this flag indicates that no
retention factor is used for that land use.  Table A.4 lists the last set of inputs that are required
to compute total nitrogen load from each land use category.  In Table A.5 we list a sample input
data set required for the spreadsheet.

After computing nitrogen loads from each land use category, we compute Output Source Ratios
in column AB of the spreadsheet, the procedure used in computing this are described in Section
3.4. Nonpoint loads  for each land use and the point sources are routed using  an exponential
nitrogen decay relationship which is based on travel time and first-order nitrogen decay rate (as
discussed  in Section 3.4 of the report);  these  computations are done in column AE of the


                                          A-3

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spreadsheet.

As can be seen from Table A.3 and A.4 there are no cells in the subbasin input section where
the user can input direct point source and atmospheric deposition to the bay or watershed outlet.
The direct loadings of point source and atmospheric deposition are not included in the subbasin
inputs as these loads are not routed using travel time and the first-order nitrogen  decay rate. In
cell B7, the user inputs direct  point source loadings to  the bay or watershed outlet in
kilograms, in cell C7 the user inputs the bay water surface area in hectares,  and in cell A7
the user inputs direct atmospheric deposition in kilograms/hectare, which is the sum of wet
and dry deposition in kilograms to the bay or watershed  outlet.

A.3  OUTPUTS FROM LOTUS  123 SPREADSHEET

The routed load then is multiplied by the Output  Source Ratios to determine the source of
nitrogen in the subbasin load;  the results from these computations are presented in column AI.
An example output from a subbasin is listed in Table A.6.

In Table A.7 and Table A.8 we present other forms of output that can be obtained by the user.
The user can print these tabular outputs by printing cells AK9..AN39  and cells  AP9..AR21,
respectively.  The output from the spreadsheet can also be obtained as a pie chart  for the entire
watershed. The pie chart is depicted in cells AT9..AX21, and is shown in Figure A.I. The
outputs listed in  Table A.7 are for a watershed with twelve subbasins. For watersheds with
more than twelve subbasins the user Jias to calculate the nitrogen load  at the watershed outlet
by summing nitrogen loads from identical nitrogen sources from each  land use  category and
point sources from all the subbasins in the watershed.

A.4  NOTES ON USING LOTUS  123* SPREADSHEET

The user should  follow the steps listed below to use the spreadsheet:

       1.     This spreadsheet is prepared  using WINDOWS* LOTUS  123*,
             Version 4.0.  The name of the spreadsheet is NITRLOAD.WK4.

       2.     The spreadsheet is set up  for  a  watershed  with up to twelve
             subbasins.

       3.     The nonpoint  and  point source inputs are  highlighted in the
             spreadsheet for each subbasin.  The cells in  which formulas are
             used for computations are protected.

       4.     The user should use Tables A.3 and A.4 as  a guideline to  input
             nonpoint and point source data for each subbasin.

       5.     After computing nitrogen loads from each subbasin, the  user can

                                         A-4

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       obtain tables (such as Table A.7 and A.8) listing the total nitrogen
       load from all the subbasins in the watershed by summing nitrogen
       loads from identical nitrogen sources for each land use category
       and point sources from all the subbasins in the watershed.

6.     After  computing  Table  A.8, the spreadsheet will  display the
       percent  loadings  from various  nitrogen sources to the bay or
       watershed outlet besides Table A.8.
                                   A-5

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Table A.2 Format of Lotus 123  Spreadsheet for a Watershed with One Subbasin

Cell Ranees                Spreadsheet Calculations
Cell A1..N435

Cell F16..N37'
Cell H40..N41*

Cell F52..N73*
CellH76..N77*

Cell F87..N108*
Cell HI 11..Ml 12*

Cell F123..N144*
Cell H147..N148'

Cell F159..N180*
Cell HI83..Nl84*

Cell F195..N216*
Cell H219..N220*

Cell F230..N251*
Cell H254..N255*

CellF266..N287*
CellH290..N291*

Cell F320..N323*
Cell H326..N327'

CellF338..N359*
Cell H362..N363*

Cell F373..N394*
Cell H397..N398*

Cell F409..N430*
Cell H433..N434*
Nonpoint and Point Source Data Input Sections

Nonpoint Source Data Input Range for Subbasin 1
Point Source Load Input Range for Subbasin 1 (kg)

Nonpoint Source Data Input Range for Subbasin 2
Point Source Load Input Range for Subbasin 2 (kg)

Nonpoint Source Data Input Range for Subbasin 3
Point Source Load Input Range for Subbasin 3 (kg)

Nonpoint Source Data Input Range for Subbasin 4
Point Source Load Input Range for Subbasin 4 (kg)

Nonpoint Source Data Input Range for Subbasin 5
Point Source Load Input Range for Subbasin 5 (kg)

Nonpoint Source Data Input Range for Subbasin 6
Point Source Load Input Range for Subbasin 6 (kg)

Nonpoint Source Data Input Range for Subbasin 7
Point Source Load Input Range for Subbasin 7 (kg)

Nonpoint Source Data Input Range for Subbasin 8
Point Source Load Input Range for Subbasin 8 (kg)

Nonpoint Source Data Input Range for Subbasin 9
Point Source Load Input Range for Subbasin 9 (kg)

Nonpoint Source Data Input Range for Subbasin 10
Point Source Load Input Range for Subbasin 10 (kg)

Nonpoint Source Data Input Range for Subbasin 11
Point Source Load Input Range for Subbasin 11 (kg)

Nonpoint Source Data Input Range for Subbasin 12
Point Source Load Input Range for Subbasin 12 (kg)
                                         A-6

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Table A.2  (contd.)

Cell Ranges

Cell A7*


Cell B7*


CellC?'

Cell O1..S435



Cell X1..AI435



Cell AK9..AN39

Cell AP9..AR21

Cell AT9..AX21
Spreadsheet Calculations

Input of Direct Atmospheric Deposition (wet+dry) to the Bay or
Watershed Outlet in kg/ha

Input of Direct Point Source Load to the Bay or Watershed Outlet
in kg

Bay Water Surface Area in ha

Calculation of Input Source Ratios and Nitrogen Loads from various
nitrogen sources for all the land use categories in the subbasin,
as described in Section 3.2 and 3.3 in the report.

Calculation of Output Source Ratios and Routing Nitrogen Load from
each land use to the watershed outlet, as described in Section 3.4 of
the report.

Summary of Nonpoint and Point Source Loads from the Watershed

Percent Contributions from Various Nitrogen Sources to Bay

Pie Chart depicting source of nitrogen loads to the Bay or Watershed
outlet
* - These are the only cells for the user to input data
                                         A-7

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Table A.3 Cell Locations in the Spreadsheet for Nonpoint and Point Source Inputs

                                      Fertilizer     Manure      Sewage       Industrial

                                      kg/ha        kg/ha        kg           kg

Subbasin 1
Wet         Factor*
Deposition
kg/ha
Nonpoint
Forest
Cropland
Wetland
Pastures
Urban
Point
Subbasin 2
Nonpoint
Forest
Cropland
Wetland
Pastures
Urban
Point
Subbasin 3
Nonpoint
Forest
Cropland
Wetland
Pastures
Urban

H16
H20
H26
H30
H35
—


H52
H56
H62
H66
H71
~


H87
H91
H97
H101
H106

F17
F21 G22
F27
F31
F36
..


F53
F57 G58
F63
F67
F72
_.


F88
F92 G93
F98
F102
F107

^_
H23
H32
—
—


..
H59
H68
~
—


....
H94
H103
—
                                                                H40
                                                                H41
                                                                H76
                                                                H77
Point
                                                    Hill
H112
                                         A-8

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Table A.3 (contd.)
Subbasin 4
Nonooint
Nonooint
Point
            Wet
            Deposition
            kg/ha
Factor"
Fertilizer    Manure

kg/ha       kg/ha
Sewage
Industrial

kg
Forest
Cropland
Wetland
Pastures
Urban
Point
Subbasin 5
H123
H127
H133
H137
H142
—

F124
F128
F134
F138
F143
—

                                     G129
            H130
                                                 H139
                                                             H147
                                    H148
Forest
Cropland
Wetland
Pastures
Urban
Point
Subbasin 6
Nonpoint
Forest
Cropland
Wetland
Pastures
Urban
H159
H163
H169
H173
H178
~


H195
H199
H205
H209
H214
F160
F164
F170
F174
F179
—


F196
F200
F206
F210
F215
                                     G165
            H166
                                                 H175
                                                             H183
                                    H184
                                    G201
            H202

            H211
                        H219
            H220
                                      A-9

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Table A.3  (contd.)
Subbasin 7

Nonpoint

Forest
Cropland
Wetland
Pastures
Urban

Point

Subbasin 8

Nonpoint
Point
            Wet
            Deposition
            kg/ha
Factor"
                        Fertilizer     Manure

                        kg/ha        kg/ha
                                     Sewage
                                     Industrial

                                     kg
H230
H234
H240
H244
H249
F231
F235
F241
F245
F250
G236
                        H237
            H246
                                                 H254
                                                 H254
Forest
Cropland
Wetland
Pastures
Urban
Point
Subbasin 9
Nonpoint
Forest
Cropland
Wetland
Pastures
Urban
H266
H270
H276
H280
H285
—


H302
H306
H312
H316
H321
F267
F271
F277
F281
F286
—


F303
F307
F313
F317
F322
                                     G272
                                     H273
                                                 H282
                                                             H290
                                                             H291
                                     G308
                                     H309

                                     H318
                                                 H326
                                                 H327
                                       A-10

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Table A.3  (contd.)

            Wet         Factor"      Fertilizer     Manure     Sewage      Industrial
            Deposition
            kg/ha                    kg/ha        kg/ha       kg           kg

Subbasin 10

Nonpoint

Forest       H338        F339
Cropland    H342        F343        G344        H345
Wetland     H348        F349
Pastures     H352        F353        -           H354
Urban       H357        F358

Point       --           --           --           »          H362        H363

Subbasin 11

Nonooint

            H373        F374
                                     G379        H380

                                                 H389


Point       -           -           -           --          H397        H398
Forest
Cropland
Wetland
Pastures
Urban
H373
H377
H383
H387
H392
F374
F378
F384
F388
F393
                                       A-ll

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Table A.3 (contd.)
             Wet          Factor*      Fertilizer     Manure      Sewage       Industrial
             Deposition
             kg/ha                    kg/ha        kg/ha        kg           kg
Subbasin 12

Nonpoint

Forest        H409        F410
Cropland     H413        F414        G415        H416
Wetland      H419        F420
Pastures      H423        F424        -           H425
Urban        H428        F429

Point        -            --           --           --            H433         H434

NOTE:       Total atmospheric deposition is the sum of wet and dry atmospheric
             deposition.  Observed dry atmospheric deposition is computed in cells H9, H13,
             H19, H23, H28 for the land use categories of forest, cropland, wetland,
             pastures, and urban areas, respectively.

*     -      The factor calculates dry atmospheric deposition as a function of wet deposition
             (Dry atmospheric deposition = Wet Atmospheric Deposition x Factor)
                                        A-12

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Table A.4
Input Cell Locations of Retention Factor, Edge-of-Field Load, Area, Travel Time,
and First-Order Nitrogen Decay Rate.
             Land Use
             Area
             (ha)
             Retention
             Factor"
Literature
Edge-of-Field
Nitrogen Load
             (fraction)    (kg/ha)
Travel
Time
(days)
First-Order
Nitrogen
In Stream
Decay Rate
(/day)
Nonpoint Subbasin 1
Forest
Cropland
Wetland
Pastures
Urban
J16
J20
J26
J30
J35
K16
K20
K26
K30
K35
L16
L20
L26
L30
L35
Nonpoint Subbasin 2
Nomxrint Subbasin 3
                                                          Ml 6
                                                         N16"
Forest
Cropland
Wetland
Pastures
Urban
J52
J56
J62
J66
J71
K52
K56
K62
K66
K71
L52
L56
L62
L66
L71
                                                          M52"
                                                         N52"
Forest
Cropland
Wetland
Pastures
Urban
J87
J91
J97
J101
J106
K87
K91
K97
K101
K106
L87
L91
L97
L101
L106
Nonooint Subbasin 4
                                                          M87"
                                                         N87"
Forest
Cropland
Wetland
Pastures
Urban
J123
J127
J133
J137
J142
K123
K127
K133
K137
K142
L123
L127
L133
LI 37
L142
                                                         M123'
                                                         N123*
                                        A-13

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Table A.4 (contd.)

             Land Use    Retention     Literature          Travel       First-Order
             Area        Factor*       Edge-of-Field       Time        Nitrogen
                                      Nitrogen Load                   In Stream
                                                                     Decay Rate
             (ha)         (fraction)     (kg/ha)            (days)       (/day)

Nonooint Subbasin 5

                                                        M159"      N159*
Forest
Cropland
Wetland
Pastures
Urban
J159
J163
J169
J173
J178
K159
K163
K169
K173
K178
L159
LI 63
L169
L173
LI 78
Nonpoint Subbasin 6

                                                         M195"      N195'
Forest
Cropland
Wetland
Pastures
Urban
J195
J199
J205
J209
J214
K195
K199
K205
K209
K214
LI 95
L199
L205
L209
L214
Nonpoint Subbasin 7

                                                         M230"      N230*
Forest
Cropland
Wetland
Pastures
Urban
J230
J234
J240
J244
J249
K230
K234
K240
K244
K249
L230
L234
L240
L244
L249
Nonpoint Subbasin 8

Forest       J266         K266        L266               M266"      N266*
Cropland     J270         K270        L270
Wetland      J276         K276        L276
Pastures      J280         K280        L280
Urban       J285         K285        L285
                                        A-14

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Table A.4  (contd.)

             Land Use    Retention     Literature          Travel        First-Order
             Area        Factor*       Edge-of-Field       Time         Nitrogen
                                       Nitrogen Load                    In Stream
                                                                        Decay Rate
             (ha)         (fraction)     (kg/ha)             (days)        (/day)

Nonpoint Subbasin 9

                                                          M302"       N302"
Forest
Cropland
Wetland
Pastures
Urban
J302
J306
J312
J316
J321
K302
K306
K312
K316
K321
L302
L306
L312
L316
L321
Nonpoint Subbasin 10

                                                           M338"       N338*
Forest
Cropland
Wetland
Pastures
Urban
J338
J342
J348
J352
J357
K338
K342
K348
K352
K357
L338
L342
L348
L352
L357
 Nonpoint Subbasin 11

                                                           M373"      N373*
Forest
Cropland
Wetland
Pastures
Urban
J373
J377
J383
J387
J392
K373
K377
K383
K387
K392
L373
L377
L383
L387
L392
 Nonpoint Subbasin 12

                                                           M409"      N409*
Forest
Cropland
Wetland
Pastures
Urban
J409
J413
J419
J423
J428
K409
K413
K419
K423
K428
L409
L413
L419
L423
L428
 *     -      Input 999 if the user wants to use edge-of-field nitrogen loads to compute
              total nitrogen load.
 **    -      Travel Time and First-Order Nitrogen Instream Decay Rate is only input once, the
              same travel time and first-order decay rate is used for all land use categories.


                                         A-15

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  Table  A.5   Example  Input  from Lotus  123 Spreadsheet  for  a  Subbasin
                   (Cell Range  A9..NA1)
SUBBASIN NAME

NONPCXNT SOURCES

          FORESTS
                   Aim. Wet Deposition kg/ha
                   Aim. Dry Deposition kg/Via

         CROPLANDS
                   Aim. Wet Deposition kg/ha
                   Aim. Dry Deposition kg/ha
                   Fertilizer kgAta
                   Manure kg/ha
         WETLANDS
                   Aim. Wet Deposition kg/ha
                   Aim. Dry Deposition kg/ha
         PASTURES
           URBAN
POINT SOURCES
                   Aim. Wet Deposition kg/ha
                   Aim. Dry Deposition kg/ha
                   Manure kg/ha
                   Aim. Wet Deposition kg/ha
                   Atm. Dry Deposition kg/ha
                   Sewage Treatment Plants kg
                   Industrial Discharge kg
                   INPUTS    INPUTS   INPUTS   INPUTS   INPUTS   INPUTS
                                                 NPS
                  Unit Area             Retention   Loads     Travel     River
                             Landuse    Factor   Literature    Time     Decay
        Pert. Rates             Areas    (fraction)    Values     days      /day
Factors    kg/ha      kg/ha       ha               kg/ha
    1.00
    1.00
    1.00
    1.00
    1.00
    1.00
                      7.74   1.73E+06    999.00
                      7.74
                                                   5.67
                                                             8.60
                                                        0.08
        7.74   6.03E+05    999.00     16.06      8.60      0.08
        7.74
99     99.00
       14.00
                      7.74   O.OOE+00    999.00      6.00      8.60      0.08
                      7.74
        7.74   2.58E+05    999.00
        7.74
       11.00
                                                    7.21
                                                             8.60      0.08
                      7.74   3.32E+05    999.00      11.80      8.60      0.08
                      7.74
                     15000
                     20000
                                                             8.60
                                                                      0.08
                                                    A-16

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Table A.6  Example Output from Louts 123 Spreadsheet for a Subbasin
          (Cell Range AG9..AI41)
         OUTPUTS
          To Bay
           from
         Subbasin
         FOREST
        Atm. Loads
OUTPUTS
 Subbasin
 Nitrogen
 Sources
   kg
 4.93E+06
        CROPLAND
        Atm. Loads

        Fertilizer
        Manure

        WETLANDS
        Atm. Loads
 5.86E+05

 3.75E+06
 5.30E+05
 O.OOE+00
        PASTURES
        Atm. Loads

        Manure

          URBAN
        Atm. Loads
 5.47E+05

 3.89E+05


 1.97E+06
        POINT SOURCES
                            1.76E+04
                        A-17

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Table A.7    Example Total Nitrogen Load Output from Lotus 123 Spreadsheet for the
            Watershed (Cell Range AK9..AN39)
                  Total Nitrogen Loads from Watershed
                  Point Sources-Watershed

                  Point Sources-Direct to Bay

                  Direct Atm. Depo. to Bay

                  Cropland
                        Fertilizer
                        Atmospheric Dep.
                        Manure
                  Forest
                        Atmospheric Dep.
                  Pasture
                        Atmospheric Dep.
                        Manure

                  Wetland
                        Atmospheric Dep.

                  Urban
                        Atmospheric Dep.

                  Total Load
Watershed
Results (kg)

1.76E+04

4.00E+05

1.29E+07
3.75E+06
5.86E+05
5.30E+05
4.93E+06
3.89E+05
5.47E+05
0.00+00


1.97E+06

2.60E+07
                                      A-18

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Table A.8    Example Output for Percent Nitrogen Contributions from Various Nitrogen Sources
             using Lotus 123* Spreadsheet for the Watershed (Cell Range AP9..AR21)
                                                  Percent
                                                  of Total
                                                  Load

                               Point Sources        1.60%
                               Atm. Bay          49.57%
                               Atm. Watershed     30.88%
                               Fertilizer           14.41%
                               Manure             3.53%
                                        A-19
U.S. Environmental Protection Agency
       '**"» 
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                  (49.6%)
              (30.9%)
                                   (1.6%)
                                   (3.5%)
                                 (14.4%)
    • Point Sources eg Atm. Bay
    ^Fertilizer     • Manure
lAtm. Watershe
Figure A.I   Percent Nitrogen Loadings  from Various Nitrogen
             Sources at Watershed Outlet  (Cell Range AT9..AX21)
                      A-20

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