&EPA
United States Environmental Office of Water EPA-822-R-02-025
Protection Agency Washington, DC 20460 March 2002
METHODS FOR EVALUATING WETLAND CONDITION
#17 Land-Use Characterization for
Nutrient and Sediment
RiskAssessment
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United States Environmental Office of Water EPA-822-R-02-025
Protection Agency Washington, DC 20460 March 2002
METHODS FOR EVALUATING WETLAND CONDITION
#17 Land-Use Characterization for
Nutrient and Sediment
RiskAssessment
Principal Contributor
Iowa State University
Arnold van derValk
Prepared jointly by:
The U.S. Environmental Protection Agency
Health and Ecological Criteria Division (Office of Science and Technology)
and
Wetlands Division (Office of Wetlands, Oceans, and Watersheds)
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NOTICE
The material in this document has been subjected to U.S. Environmental Protection Agency (EPA)
technical review and has been approved for publication as an EPA document. The information
contained herein is offered to the reader as a review of the "state of the science" concerning wetland
bioassessment and nutrient enrichment and is not intended to be prescriptive guidance or firm advice.
Mention of trade names, products or services does not convey, and should not be interpreted as
conveying official EPAapproval, endorsement, or recommendation.
APPROPRIATE CITATION
U.S. EPA. 2002. Methods for Evaluating Wetland Condition: Land-Use Characterization for
Nutrient and Sediment Risk Assessment. Office of Water, U.S. Environmental Protection Agency,
Washington, DC. EPA-822-R-02-025.
This entire document can be downloaded from the following U.S. EPA websites:
http://www.epa.gov/ost/standards
http://www.epa.gov/owow/wetlands/bawwg
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CONTENTS
FOREWORD v
LIST OF "METHODS FOR EVALUATING WETLAND
CONDITION" MODULES vi
SUMMARY 1
PURPOSE 1
INTRODUCTION 1
METHODS l
NUTRIENT LOADING INDEX 5
CASE STUDY 7
REFERENCES 10
LIST OF TABLES
TABLE l: POTENTIAL NUTRIENT Loss RATES
(KG/HA/YR) FOR DIFFERENT UPLAND
LAND-USE CLASSES 5
TABLE 2: LAND USES (HA) IN THE WATERSHEDS OF
WETLANDS W1, W2, AND W3 IN THE IOWA
GREAT LAKES REGION OF NW IOWA 7
TABLE 3: ESTIMATED CURRENT ANNUAL NITROGEN AND
PHOSPHORUS LOADINGS (TOTAL WATERSHED
Loss), PRESETTLEMENT LOADINGS (NATURAL
WATERSHED Loss), AND NUTRIENT LOADING
INDEX FOR THREE WETLANDS (Wl, W2, AND
W3) IN THE IOWA GREAT LAKES REGION 8
in
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TABLE 4: PERCENT OF THE WATERSHEDS FOR WETLANDS
Wl, W2, AND W3 CLASSIFIED AS AGRICULTURAL
LAND, PERCENT OF AGRICULTURAL LAND
CLASSIFIED AS HIGHLY ERODIBLE (HEL),
PERCENT OF WETLAND BOUNDARY ADJACENT
TO AGRICULTURAL LAND, AND PERCENT OF
WATERSHED DISTURBED BY RECENT LAND
CLEARING FOR THREE WATERSHEDS IN THE
IOWA GREAT LAKES REGION As WELL As THE
SEDIMENT RISK INDEX FOR EACH WETLAND
IV
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FOREWORD
In 1999, the U. S. Environmental Protection Agency (EPA) began work on this series of reports entitled
Methods for Evaluating Wetland Condition. The purpose of these reports is to help States and
Tribes develop methods to evaluate (1) the overall ecological condition of wetlands using biological
assessments and (2) nutrient enrichment of wetlands, which is one of the primary stressors damaging
wetlands in many parts of the country. This information is intended to serve as a starting point for States
and Tribes to eventually establish biological and nutrient water quality criteria specifically refined for
wetland waterbodies.
This purpose was to be accomplished by providing a series of "state of the science" modules concerning
wetland bioassessment as well as the nutrient enrichment of wetlands. The individual module format
was used instead of one large publication to facilitate the addition of other reports as wetland science
progresses and wetlands are further incorporated into water quality programs. Also, this modular
approach allows EPA to revise reports without having to reprint them all. A list of the inaugural set of
20 modules can be found at the end of this section.
This series of reports is the product of a collaborative effort between EPAs Health and Ecological
Criteria Division of the Office of Science and Technology (OST) and the Wetlands Division of the
Office of Wetlands, Oceans and Watersheds (OWOW). The reports were initiated with the support
and oversight of Thomas J. Danielson (OWOW), Amanda K. Parker and Susan K. Jackson (OST),
and seen to completion by Douglas G. Hoskins (OWOW) and Ifeyinwa F. Davis (OST). EPArelied
heavily on the input, recommendations, and energy of several panels of experts, which unfortunately
have too many members to list individually:
• Biological Assessment of Wetlands Workgroup
• Wetlands Nutrient Criteria Workgroup
More information about biological and nutrient criteria is available at the following EPA website:
http ://www. epa. gov/ost/standards
More information about wetland biological assessments is available at the following EPA website:
htto ://www.epa. gov/owow/wetlands/bawwg
V
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LIST OF "METHODS FOR EVALUATING WETLAND
CONDITION" MODULES
MODULE # MODULE TITLE
1 INTRODUCTION TO WETLAND BIOLOGICAL ASSESSMENT
2 INTRODUCTION TO WETLAND NUTRIENT ASSESSMENT
3 THE STATE OF WETLAND SCIENCE
4 STUDY DESIGN FOR MONITORING WETLANDS
5 ADMINISTRATIVE FRAMEWORK FOR THE IMPLEMENTATION OF A
WETLAND BIOASSESSMENT PROGRAM
6 DEVELOPING METRICS AND INDEXES OF BIOLOGICAL INTEGRITY
7 WETLANDS CLASSIFICATION
8 VOLUNTEERS AND WETLAND BIOMONITORING
9 DEVELOPING AN INVERTEBRATE INDEX OF BIOLOGICAL
INTEGRITY FOR WETLANDS
10 USING VEGETATION TO ASSESS ENVIRONMENTAL CONDITIONS
IN WETLANDS
11 USING ALGAE TO ASSESS ENVIRONMENTAL CONDITIONS IN
WETLANDS
12 USING AMPHIBIANS IN BlOASSESSMENTS OF WETLANDS
13 BIOLOGICAL ASSESSMENT METHODS FOR BIRDS
14 WETLAND BIOASSESSMENT CASE STUDIES
15 BIOASSESSMENT METHODS FOR FISH
16 VEGETATION-BASED INDICATORS OF WETLAND NUTRIENT
ENRICHMENT
17 LAND-USE CHARACTERIZATION FOR NUTRIENT AND SEDIMENT
RISK ASSESSMENT
18 BlOGEOCHEMICAL INDICATORS
19 NUTRIENT LOAD ESTIMATION
2O SUSTAINABLE NUTRIENT LOADING
VI
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SUMMARY
r IJ he condition of and potential threats to a
J. given wetland are often largely determined by
surrounding land use. Land use, especially the per-
cent of the watershed that has been cleared of natu-
ral vegetation, can affect the amount of water, sedi-
ment, pesticide, and nutrients entering a wetland as
well as the composition of its plant and animal com-
munities. A rapid and inexpensive assessment of
the land use around a wetland and its implications
for nutrient and sediment inputs can be made using
readily available maps and aerial photographs, in-
cluding county soil maps, USGS quadrangle maps,
land-use maps, and aerial photographs. These are
used to delimit the watershed or drainage basin
around a wetland and to classify land use within this
watershed. These land use data are used to calcu-
late an index of potential nutrient loading to the
wetland and a sediment risk index. Their intended
use is to identify quickly, easily, and cheaply those
wetlands that are at greatest risk from nutrient and/
or sediment inputs.
PURPOSE
/T"fhe purpose of this module is to suggest
J. procedures for assessing the relative risk to a
given wetland of nutrient and sediment loads from
its watershed.
Euliss 1998), and pesticide entering a wetland is
largely a function of land use in the watershed. In-
puts into wetlands, especially sediment from sur-
rounding areas, can affect the recruitment, growth,
and even survival of many plant and animal species
(Dieter 1991, Euliss and Mushet 1999, Jurik et al.
1994, Martin and Hartman 1987, Newcomb and
MacDonald 1991, Waters 1995).
The characterization of land use around a wet-
land is essential for evaluating wetlands for water
quality purposes. Data from maps and aerial pho-
tographs can be used to estimate the potential for
inputs of nutrients and sediments into wetlands.
Many factors determine the movement of nutrients
and sediments within most landscapes, including
vegetative cover (land use), soil type (credibility),
slope length, slope angle, and frequency and inten-
sity of rainfall. A variety of models and equations
exist for estimating the potential loads of nutrients
and sediments to a wetland from its surrounding
watershed. The data requirements of these models
generally make them unsuitable for preliminary
evaluations of the risk of nutrient or sediment inputs
to a wetland. Nevertheless, when reduced to its
simplest terms, the more a wetland's watershed has
been altered by human activities the more likely it is
at risk. Consequently, a quick-and-dirty assess-
ment of land use in the watershed should provide a
crude estimate of potential risks from nutrient and
sediment loadings to a wetland.
INTRODUCTION
METHODS
A11 wetlands are influenced by a number of
-/±. flow systems that bring materials into and out
of them. The most important flow systems are usu-
ally atmospheric flows, surface water flows, and
subsurface water flows. Many natural flow sys-
tems have been altered by human activities, espe-
cially surface and subsurface water flows into wet-
lands. The amount of water (Luoetal. 1994, Euliss
and Mushet 1996), nutrient (Omernik 1977), sedi-
ment (Martin and Hartman 1987, Gleason and
Geographic information systems (GIS) are
primary tools in analysis of landscapes, and
their use has significantly changed how spatial data
and related nonspatial data are collected, stored,
and analyzed. State, regional, and local GIS facilities
will often have on file in digital form all the information
needed to characterize landscapes around a given
wetland. Appropriate State, county, and municipal
agencies should be consulted to see if their GIS
facilities can be utilized for this purpose. If a GIS
1
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facility is not available, the work needed to
characterize the landscape in which a wetland is
found can be easily done by hand. The methods
outlined in this module presuppose that a GIS will
not be used. All of the proposed steps suggested
in this module, however, can be done utilizing existing
GIS software packages or simple modifications or
extensions of them. Although the proposed method
is simple, it will quickly become prohibitively time-
consuming if extended to a statewide assessment
of risks to wetlands. For large-scale projects, a
GIS should be used.
A very useful and practical manual that provides
detailed methodology for collecting the kinds of in-
formation needed to characterize lands use around
wetlands is Landscape Planning: Environmen-
tal Applications, 3rd Edition, by William M. Marsh.
Much of what follows is based on Marsh (1998).
Guidelines for selecting wetlands to sample are found
in Study Design for Wetland Monitoring.
There are many kinds of wetlands, including fringing
wetlands along lakeshores, rivers, and oceans, and
palustrine wetlands that cover their entire basin (see
module on Wetland Classification for details).
These wetlands vary greatly in size and shape and
in inputs and outputs of water. To simplify this mod-
ule, the methods described are applied to the evalu-
ation of the watershed of a palustrine wetland within
a well-defined watershed. This is an idealized situ-
ation. To apply this methodology to other wetland
types, suitable adjustment will need to be made. In
some cases, watershed boundaries will be difficult
or impossible to establish. In such cases, an arbi-
trary zone of influence around the wetland can be
substituted. This zone's width needs to take into
account known surface and subsurface flow pat-
terns into the wetland. For large wetland complexes,
e.g., riverine or estuarine wetlands, only portions of
the complex may be of interest. Again, suitable
adjustments will need to be made to determine the
zone of influence. In many cases, significant nutri-
ent and sediment inputs may come from other sec-
tions of the complex.
SOURCES OF MAPS, AERIAL
PHOTOGRAPHS, AND OTHER DATA
The Internet has made finding and acquiring rel-
evant maps and other data simpler and more effi-
cient than ever before. Increasingly, digitized maps
can be downloaded directly. Below is a list of some
major sources of maps and aerial photographs.
These Websites often have links to other sites with
relevant information. Only the URLs for their home
pages are listed because these are less likely to
change. See also the module on Wetland Classifi-
cation for additional sources of information.
Topographic maps are published by the U.S. Geo-
logical Survey (USGS) at a variety of scales. The
most useful topographic maps are the 7.5-minute
quadrangle maps. These maps contain information
on topographic relief, drainage systems, and some
land use features. The USGS Web site
(www.usgs.gov) provides a list of all topographic
maps and all other maps produced by the USGS.
It also describes several ways of ordering these
maps. Current and historic stream-flow data can
also be obtained from this site.
National Wetland Inventory (NWI) 7.5-minute
quad maps can be downloaded from their Website
(www.nwi.fws.gov) and are also available through
the USGS Earth Science Information Centers.
A list of all county soil maps in the United States
is available through the National Resources Con-
servation Service (NRCS) Website
(www.nrcs.usda.gov), as is a list of State NRCS
offices from which these county soil surveys can be
obtained. Information about regional soil erosion
patterns can also be obtained from this site.
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Information about aerial photographs that are part
of the National Aerial Photography Program
(NAPP) can be obtained from the USGS Website
(nsdi.usgs.gov/products/aerial.html). Aerial
photographs taken by State agencies and local gov-
ernments are also usually available. Recent aerial
photographs and land-use maps can usually be ob-
tained from State agencies such as the State Geo-
logical Survey and map and aerial photography col-
lections at maj or universities.
A National Land Cover Data Base (NLCD) based
on satellite imagery from the early 1990s is being
developed as part of the Federal Multi-Resolution
Landscape Characterization (MRLC) initiative. The
NLCD is a joint project of the USGS and the U.S.
Environmental Protection Agency (USEPA). Its
aim is to produce a consistent land cover data layer
for the conterminous U.S. based on 30-meter
Landsat thematic mapper (TM) data. Information
about available NLCD products can be obtained
from both USGS (edcwww.cr.usgs.gov/programs/
Iccp/nationallandcover.html) and EPA Websites
(www.epa.gov/mrlc/Regions.html).
Flood hazard maps from the Federal Emergency
Management Agency (www.fema.gov) can often
be used to delineate the boundaries of floodplain
wetlands along rivers and streams.
an additional limitation of USGS 7.5 quadrangle
maps, i.e., their vertical resolution. In short, for
small wetlands or wetlands in very flat landscapes,
maps with a better resolution than USGS quad-
rangle maps may be needed. Unfortunately, county
soils maps, which typically have a 1:20,000 scale,
have only a slightly better resolution than USGS
quadrangle maps. When no suitable maps are avail-
able, low-level aerial photographs may be the only
way to collect the data needed to characterize the
watershed in which the wetland is found.
WATERSHED OR DRAINAGE BASIN
DELINEATION
Establishing the boundaries of a watershed around
a given wetland is done by finding drainage divides
on a topographic map. In landscapes with well-
developed drainage systems, this process begins
with mapping the drainage network to establish the
order of its various branches. The results of this
exercise will largely be a function of the resolution
of the topographic map. One of the simplest ways
to identify drainage divides is to demarcate patterns
of overland flow with arrows drawn perpendicular
to contour lines. Where the arrows are divergent,
i.e., point in opposite directions, there is a drainage
divide. By inspection, first-order basins can be iden-
tified and aggregated into second-order watersheds
and so forth.
MAP LIMITATIONS
All maps have a minimum resolution level that is
determined by their scale. This establishes the small-
est area or unit that can be mapped. For USGS
7.5-minute quadrangle maps, whose scale is
1:24,000 or 1" = 2,000 ft, the smallest units that
can be mapped are 10 or more acres. Because
many wetlands, especially palustrine wetlands, are
often much less than 10 acres and their watersheds
are equivalency small, USGS quadrangle maps may
not have the resolution needed to delineate their
watershed boundaries. In flat landscapes, there is
Identifying watersheds, as described above, may
not be feasible in some landscapes. In flat, poorly
drained landscapes, drainage divides are often hard
to determine using topographic maps. In such situ-
ations, soils maps may be more useful for identify-
ing both wetland basins and intermittent, intercon-
necting drainage channels. In these landscapes,
groundwater inputs into wetlands are often very
important. When this is so, watershed delimitation
using surface flow patterns may greatly underesti-
mate the effective size of the area around a wetland
whose runoff enters the wetland. In both urban
and agricultural areas, the effective watershed of a
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wetland includes the area drained by storm sewers
or drainage networks, respectively, which discharge
into the wetland. The area covered by these storm
sewer or drainage networks generally is not con-
gruent with the wetland's surface runoff watershed.
Maps and other information about storm sewer and
drainage networks are often available from local
governments or from organized drainage districts.
These can be used to establish the effective water-
shed for wetlands in such altered landscapes, and
these effective watersheds should be used to esti-
mate potential loading rates of nutrients and pollut-
ants to the wetland.
LAND USE IN WATERSHED
Nutrient and sediment inputs into a wetland are
due to both point and nonpoint inputs. To a large
extent land use in the watershed will determine the
load of sediment and nutrients that a wetland will
receive. It will also determine how much of this
input is the result of point and nonpoint sources.
The more a watershed has been altered by human
activities, the greater the potential for the move-
ment of sediments into a wetland. The first step in
characterizing the watershed is to classify each rec-
ognizable area in the watershed by its predominant
land use:
• Natural vegetation (>75% forest and/or grass-
land)
• Mostly natural vegetation (50 to 75% forest
and/or grassland)
• Agriculture (>75% cropland)
• Mostly agriculture (50 to 75% cropland)
• Mostly urban (>40% developed)
• Mixed (doesn't fall into one of the previous
categories)
Because land use is changing constantly, up-to-
date information on current land use is needed. This
can most readily be obtained from recent aerial
photographs of the watershed. In some States, land
use maps may be available, but the classification
system used will undoubtedly be more sophisticated
than the simple system proposed above. Convert-
ing the classes on existing land-use maps to the land-
use classes proposed above is usually very easy
and may not be necessary at all if local estimates of
annual nutrient loss for different classes are avail-
able. In agricultural areas, USDA crop compli-
ance maps may be ready-made land-use maps. For
wetlands with large watersheds, the MRLC Na-
tional Land Cover Data Base may also be a useful
source of digital land-use data.
The area of the watershed in each land use class
then needs to be determined. This can be done in a
variety of ways with paper maps using a dot grid or
planimeter. If a digital version of the land use map
is available, a GIS can be used to calculate the area
of each land use class in the watershed.
Dot grids are transparent overlays with a regular
pattern of dots. With a dot grid, the number of dots
that fall on each land use class in the watershed is
counted. Each dot represents a certain area. To
determine the area represented by a dot, the fol-
lowing formula is used:
Area/dot = [area on map/(l linear unit)2]/
[number ofdots/(l linear unit)2]
For example, if 1 cm on the map is equivalent to
100 m, then 1 cm2 = 10,000 m2 or 1 ha.
If 1 cm2 on the dot grid contains 10 dots, then
Area/dot = (1 ha/1 cm2)/(10 dots/1 cm2)
= 0.1 ha/dot.
If 40 dots on the grid fell in areas that were cov-
ered with natural vegetation, the total area in natu-
ral vegetation is estimated to be 40 dots x 0.1 ha/
dot or 4 ha. To improve the precision of area esti-
mates using dot counts, several random drops of
the dot grid can be made on the watershed map
and the dots counted for each land-use class. The
average dot count per class is then used to calcu-
late the area of each land-use class.
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A planimeter is a device that is used to convert
the boundary or perimeter length of a polygon to its
area. Prolonged use of a planimeter is tedious, and
it takes skill and practice to trace the boundaries of
complex polygons. For occasional use, dot grids
are simpler to use and probably more reliable. If
large number of wetlands are going to be evalu-
ated, scanning land use maps to create digital ver-
sions and using a GIS to calculate total watershed
and land use class areas is strongly encouraged.
NUTRIENT LOADING INDEX
A simple index of potential nutrient loadings can
-/ibe used to characterize the nutrient environ-
ment of a wetland. This index is the ratio of the
potential loss of nitrogen (N) or phosphorus (P)
from the watershed with its current land use divided
by the potential losses if the entire watershed were
covered with permanent natural vegetation. Op-
erationally, potential nutrient loadings are calculated
by multiplying the area of each upland land use class
by the amount of N or P that is estimated to leave
this land use class (Table 1 or comparable local
data set) and then adding up the total annual esti-
mated output of N or P for the watershed. (See
also the Vegetation-Based Indicators of Wetland
Nutrient Enrichment section of this manual for ad-
ditional sources of information on nutrients in run-
off.) This approach to estimating total annual loads
of nutrients from watersheds into lakes has been
used for many years in the limnological literature
(Reckowetal. 1980). The total estimated annual
loss of P or N is then divided by the estimated out-
put of nutrient if the entire watershed were covered
in permanent natural vegetation (forests or grass-
lands).
TABLE 1: POTENTIAL NUTRIENT LOSS RATES (KG/HA/YR) FOR
DIFFERENT UPLAND LAND-USE CLASSES
LAND USE (I_U)
Natural vegetation
Mostly natural vegetation
Agricultural
Mostly agricultural
Mostly urban
Mixed
NUTRIENT LOSS RATE (NLR)
NITROGEN
0.44
0.45
0.98
0.63
0.79
0.55
PHOSPHORUS
0.0085
0.018
0.031
0.028
0.030
0.019
Source: Adapted from Omernik (1977) and Marsh (1998)
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(1) Nutrient Loading index = (Total
Watershed Loss)/(Natural Watershed
Loss)
where
(2) Total Watershed Loss = S(LU x NLRX)
LU;= area of watershed in upland land
use class i;
NLRX = nutrient loss rate for upland land
use class i for nutrient X (Table 1);
and
(3) Natural Watershed Loss = TUWAx
NLRNVX
TUWA= total upland watershed area;
and
NLRNVX = nutrient loss rate of nutrient
X for the natural vegetation land use class
(Table 1).
The higher the index value, the larger the potential
nutrient loading into the wetland. This index pro-
vides a measure of the relative threat to the wetland
from nutrient inputs from the current upland land-
scape.
The index does not give a realistic estimate of the
actual annual loadings of nutrients to a wetland. It
only provides a crude way of ranking wetlands based
on the likelihood that they are receiving nutrient
loadings from the surrounding watershed. Other
sections of this manual discuss methods for esti-
mating actual loadings of nutrients to wetlands.
If there are point sources of nutrients within the
watershed, the index needs to be adjusted appro-
priately. Annual inputs of nutrients from storm sew-
ers or agricultural drainage networks will be highly
site specific and can best be obtained from local
sources. In areas where septic tank fields are within
100 m of a wetland, the number of septic tank fields
in the watershed needs to be estimated. The input
load to the wetland needs to be adjusted by deter-
mining the potential inputs of nitrogen (11 kg per
drainage field per year) and phosphorus (0.28 kg
per drainage field per year) as suggested by Marsh
(1998). Many other point sources of nutrients may
be present in a watershed, e.g., feedlots, golf
courses, sewage treatment plants, etc. In short,
estimates of point sources are simply added to those
from nonpoint sources. The Nutrient Loading In-
dex is then calculated as above with estimated in-
puts of nutrients added to the numerator.
SEDIMENT RISK INDEX
The simplest method for assessing potential sedi-
ment-related impacts to wetlands is to do an ero-
sion risk assessment of the watershed. The maj or
factors that influence soil erosion rates are climate,
soil properties, topography, soil surface conditions,
and human activities. Within a given region, it is soil
surface conditions and human activity in close prox-
imity to the wetland that will largely determine the
potential sediment load. Potential risk of sediment
inputs increases with the amount of land in the wa-
tershed that is classified as agricultural. It further
increases if this agricultural land has steeper and
longer slopes, i.e., is classified as highly erodible
land (HEL), and/or if it is adj acent to the wetland.
The amount of the watershed classified as HEL can
be obtained by contacting the nearest NRCS Of-
fice. How much of the land that is classified as
agricultural land is adj acent to the wetland can be
determined with a planimeter. In many watersheds,
recent clearing of natural vegetation due to road
building, construction, mining, lumbering, etc., can
also cause significant short-term erosion problems.
The potential impact of these activities is assessed
by estimating the total percentage of the watershed
recently cleared of natural vegetation.
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Sediment Risk Index = ( Percent of agricultural
land classified as HEL x percent of agricultural
land)/100 + Percent of wetland boundary
adjacent to agricultural land + Percent of
watershed disturbed by land-clearing activities.
As with the Nutrient Loading Index, the Sediment
Risk Index is a quick-and-dirty way to identify
wetlands that have a greater risk of having high sedi-
ment loadings. There are many sophisticated meth-
ods for estimating sediment losses from watersheds
(see Boardman and Favis-Mortlock 1998, Mor-
gan 1986, Schmidt 2000). These should be used
to confirm that a given wetland is actually at risk
from sediment inputs.
source of nutrients and sediments, appropriate
changes in the methods for estimating nutrient and
sediment loadings will need to be made. Likewise,
better estimates of annual nutrient loss rates for ar-
eas with different land uses may be available lo-
cally. Because these rates are a function of local
soil types, fertilizer application rates, and precipita-
tion patterns, these should be substituted for those
in Table 1 when available. In short, this module
presents some simple methods for gauging the overall
risk to a wetland of various human impacts on wa-
tershed land use. Users are encouraged to adapt
the proposed methodology to the realities of local
landscapes in order to improve its reliability.
FINAL COMMENTS
In this module, overland flow and agricultural
drainage networks, if present, are assumed to be
the major sources of nutrients and sediments into a
wetland. For wetlands in watersheds where
stormwater sewers are a significant or predominant
CASE STUDY
and-use data adapted from data supplied by
the NRCS Office in Spirit Lake, IA, for three
watersheds around the Iowa Great Lakes are pre-
sented in Table 2. Land use classes in the NRCS
data have been converted into the land-use classes
TABLE 2: LAND USES (HA) IN THE WATERSHEDS OF WETLANDS W1, W2, AND W3
IN THE IOWA GREAT LAKES REGION OF NW IOWA
LAND USE
Natural vegetation
Mostly natural vegetation
Agriculture*
Mostly urban
Mixed
Total Upland Watershed
Area (TUWA)
Wetlands and aquatic
systems
Total watershed area
WATERSHED
OF Wl
87
88
132
0
12
319
80
399
WATERSHED
OF W2
69
0
43
67
26
206
87
293
WATERSHED
OF W3
349
44
2,797
0
57
3,247
276
3,523
Note: Data adapted from land-use data provided by the NRCS Office in Spirit Lake, LA.
"Agricultural land is all in row crops, mostly corn and soya beans.
7
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presented in Table 1. Wl and W3's watersheds
had land in the Conservation Reserve Program
(CRP). This CRP land was mostly planted to vari-
ous kinds of perennial prairie grasses. In Table 2
CRP land has been classified as mostly natural veg-
etation. Based on percent of the upland area of
each watershed in agricultural land (41%, 21%, and
86% for the watersheds for wetlands W1, W2, and
W3, respectively), wetland W3 has the highest risk
of inputs of nutrients and sediments from the sur-
rounding uplands, followed by Wl and W2.
The estimated annual loadings of nitrogen and
phosphorus to each wetland calculated using Equa-
tion 2 are presented in Table 3, as is the Nutrient
Loading Index for each wetland. Of the three wet-
lands, wetland W3 has the highest potential nutrient
loadings. This is largely because so much of its
watershed is in agricultural land. Wetlands in Wl
and W2 have identical Nutrient Loading Indices.
For Wl, however, the largest potential source of
nitrogen and phosphorus is agricultural land while
for W2 it is runoff from urban areas.
Data used to calculate the Sediment Risk Index
are given in Table 4, as is the Index. Wetland W2
has the highest Sediment Risk Index (56%), although
it is nearly identical to that for W3 (53%). Wetland
W2 is primarily at risk because so much of it (42%
of its shoreline) is adjacent to agricultural land and
because there is a lot of recently cleared land in its
watershed (10%). On the other hand, wetland W3
is at risk primarily because so much of the water-
shed is agricultural land (86%) and about 22% of
this land is classified as HEL
Collectively, the four indicators of the potential risk
of wetlands to nutrient and sediment inputs (per-
cent of agricultural land in the upland portions of
the watershed, Nutrient Loading Indices forN and
P, and Sediment Risk Index) suggest that wetland
W3 overall is at greatest risk, followed by W2 and
Wl.
TABLE 3: ESTIMATED CURRENT ANNUAL NITROGEN AND PHOSPHORUS
LOADINGS (TOTAL WATERSHED Loss), PRESETTLEMENT LOADINGS
(NATURAL WATERSHED Loss), AND NUTRIENT LOADING INDEX FOR
THREE WETLANDS (W 1, W2, AND W3) IN THE IOWA GREAT LAKES REGION
LAND USE
NITROGEN
PHOSPHORUS
W1
Natural vegetation
Mostly natural vegetation
Agricultural
Mostly urban
Mixed
Wetland and aquatic systems
Total watershed loss
Natural watershed loss
Nutrient loading index
38
40
129
0
7
0
214
140
1.5
W2
30
0
42
53
14
0
140
91
1.5
W3
154
20
2,741
0
31
0
2,946
1,429
2.1
W1
0.74
1.6
4.1
0.0
0.2
0.0
6.6
2.7
2.4
w2
0.59
0
1.3
2.0
0.5
0.0
4.4
1.8
2.4
w3
2.97
0.8
86.7
0.0
1.1
0.0
91.6
27.6
3.3
8
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TABLE 4: PERCENT OF THE WATERSHEDS FOR WETLANDS w l, W2, AND W3
CLASSIFIED AS AGRICULTURAL LAND, PERCENT OF AGRICULTURAL LAND
CLASSIFIED AS HIGHLY ERODIBLE (HEL), PERCENT OF WETLAND BOUNDARY
ADJACENT TO AGRICULTURAL LAND, AND PERCENT OF WATERSHED DISTURBED BY
RECENT LAND CLEARING FOR THREE WATERSHEDS IN THE IOWA GREAT LAKES
REGION AS WELL AS THE SEDIMENT RISK INDEX FOR EACH WETLAND
LANDSCAPE
CHARACTERISTICS
Percent in agricultural land
Percent of agricultural land that
is HEL
Percent of wetland boundary
adjacent to agricultural land
Percent of watershed disturbed
by land clearing
Sediment Risk Index
Wl
41%
9%
25%
0%
29%
W2
21%
18%
42%
10%
56%
W3
86%
22%
29%
5%
53%
9
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