EPA/600/R-13/085c | Revised September 2014 | www2.epa.gov/research
United States
Environmental Protection
Agency
National Stormwater Calculator
User's Guide
5-ipv-tlf- UndCnv LJU L
Office of Research and Development
National Risk Management Research Laboratory
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EPA/600/R-13/085C
Revised September 2014
NATIONAL STORMWATER CALCULATOR
USER'S GUIDE - VERSION 1.1
By
Lewis A. Rossman
Water Supply and Water Resources Division
National Risk Management Research Laboratory
Cincinnati, OH 45268
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OH 45268
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The information in this document has been funded wholly by the U.S. Environmental Protection Agency
(EPA). It has been subjected to the Agency's peer and administrative review, and has been approved for
publication as an EPA document. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
Although a reasonable effort has been made to assure that the results obtained are correct, the
computer programs described in this manual are experimental. Therefore the author and the U.S.
Environmental Protection Agency are not responsible and assume no liability whatsoever for any results
or any use made of the results obtained from these programs, nor for any damages or litigation that
result from the use of these programs for any purpose.
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Paul Duda, Paul Hummel, Jack Kittle, and John Imhoff of Aqua Terra Consultants developed the data
acquisition portions of the National Stormwater Calculator under Work Assignments 4-38 and 5-38 of
EPA Contract #EP-C-06-029. Jason Berner and Tamara Mittman, both in EPA's Office of Water (OW),
were the Work Assignment Managers for that effort. They, along with Alex Foraste (EPA/OW), provided
many useful ideas and feedback throughout the development of the calculator.
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ACRONYMS AND ABBREVIATIONS
ASCE = American Society of Civil Engineers
CMIP3 = Coupled Model Intercomparison Project Phase 3
GREAT = Climate Resilience Evaluation and Awareness Tool
EPA = United States Environmental Protection Agency
GCM = General Circulation Model
GEV = Generalized Extreme Value
Gl = Green Infrastructure
HSG = Hydrologic Soil Group
IMD = initial moisture deficit
IPCC = Intergovernmental Panel on Climate Change
Ksat = saturated hydraulic conductivity
LID = low impact development
NCDC = National Climatic Data Center
NRCS = Natural Resources Conservation Service
NWS = National Weather Service
OW = Office of Water
SWAT = Soil and Water Assessment Tool
SWMM = Storm Water Management Model
UDFCD = Urban Drainage and Flood Control District
US = United States
USDA = United States Department of Agriculture
WCRP = World Climate Research Programme
IV
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DISCLAIMER ii
ACKNOWLEDGEMENTS iii
ACRONYMS AND ABBREVIATIONS iv
TABLE OF CONTENTS v
LIST OF FIGURES vi
List of Tables vii
1. Introduction 8
2. How to Run the Calculator 10
Location 11
Soil Type 13
Soil Drainage 15
Topography 16
Precipitation 17
Evaporation 19
Climate Change 20
Land Cover 22
LID Controls 23
Results 24
3. Interpreting the Calculator's Results 27
Summary Results 27
Rainfall / Runoff Frequency 29
Rainfall Retention Frequency 32
Runoff by Rainfall Percentile 33
Extreme Event Rainfall/Runoff 35
Printing Output Results 35
4. Applying LID Controls 37
5. Example Application 41
Pre-Development Conditions 41
Post-Development Conditions 44
Post-Development with LID Practices 46
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Climate Change Impacts 53
6. Computational Methods 58
SWMM's Runoff Model 58
SWMM's LID Model 59
Site Model without LID Controls 61
Site Model with LID Controls 64
Precipitation Data 65
Evaporation Data 68
Climate Change Effects 69
Post-Processing 72
7. References 74
Figure 1. The calculator's main window 10
Figure 2. The calculator's Location page 12
Figure 3. Bird's eye map view with a bounding circle 13
Figure 4. The calculator's Soil Type page 14
Figure 5. The calculator's Soil Drainage page 16
Figure 6. The calculator's Topography page 17
Figure 7. The calculator's Precipitation page 18
Figure 8. The calculator's Evaporation page 19
Figure 9. The calculator's Climate Change page 21
Figure 10. The calculator's Land Cover page 22
Figure 11. The calculator's LID Controls page 24
Figure 12. The calculator's Results page 25
Figure 13. The calculator's Summary Results report 28
Figure 14. The calculator's Rainfall / Runoff Event report 30
Figure 15. The calculator's Rainfall / Runoff Frequency report 31
Figure 16. The calculator's Rainfall Retention Frequency report 32
Figure 17. The calculator's Runoff by Rainfall Percentile report 34
Figure 18. The calculator's Extreme Event Rainfall / Runoff report 36
Figure 19. Example of a LID Design dialog for a street planter 39
Figure 20. Runoff from different size storms for pre-development conditions on the example site 42
Figure 21. Rainfall retention frequency under pre-development conditions for the example site 43
Figure 22. Rainfall retention frequency for pre-development (Baseline) and post-development (Current)
conditions 45
VI
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Figure 23. Low Impact Development controls applied to the example site 47
Figure 24. Design parameters for Rain Harvesting and Rain Garden controls 48
Figure 25. Design parameters for the Infiltration Basin and Porous Pavement controls 49
Figure 26. Daily runoff frequency curves for pre-development (Baseline) and post-development with LID
controls (Current) conditions 51
Figure 27. Contribution to total runoff by different magnitude storms for pre-development (Baseline)
and post-development with LID controls (Current) conditions 52
Figure 28. Retention frequency plots under pre-development (Baseline) and post-development with LID
controls (Current) conditions 53
Figure 29. Climate change scenarios for the example site 54
Figure 30. Target event retention for the historical (Baseline) and Warm/Wet climate scenarios 56
Figure 31. Conceptual representation of a bio-retention cell 59
Figure 32. NWS rain gage locations included in the calculator 66
Figure 33. NRCS (SCS) 24-hour rainfall distributions (USDA, 1986) 67
Figure 34. Geographic boundaries for the different NRCS (SCS) rainfall distributions (USDA, 1986) 67
Figure 35. Locations with computed evaporation rates (Alaska and Hawaii not shown) 68
Figure 36. CMIP3 2060 projected changes in temperature and precipitation for Omaha, NE (EPA, 2012).
70
Table 1. Definitions of Hydrologic Soil Groups (USDA, 2010) 15
Table 2. Descriptions of LID practices included in the calculator 38
Table3. Editable LID parameters 40
Table 4. Void space values of LID media 40
Table 5. Summary results for pre-development conditions on the example site 42
Table 6. Land cover for the example site 44
Table 7. Comparison of runoff statistics for post-development (Current) and pre-development (Baseline)
conditions 45
Table 8. Runoff statistics for pre-development (Baseline) and post-development with LID controls
(Current) scenarios 50
Table 9. Summary results under a Warm/Wet (Current) climate change scenario compared to the
historical (Baseline) condition 55
Table 10. Depression storage depths for different land covers 62
Table 11. Roughness coefficients for different land covers 63
Table 12. Infiltration parameters for different soil types 64
VII
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1. Introduction
The National Stormwater Calculator is a simple to use tool for computing small site hydrology for any
location within the US. It estimates the amount of stormwater runoff generated from a site under
different development and control scenarios over a long term period of historical rainfall. The analysis
takes into account local soil conditions, slope, land cover and meteorology. Different types of low
impact development (LID) practices (also known as green infrastructure) can be employed to help
capture and retain rainfall on-site. Future climate change scenarios taken from internationally
recognized climate change projections can also be considered.
The calculator's primary focus is informing site developers and property owners on how well they can
meet a desired stormwater retention target. It can be used to answer such questions as:
• What is the largest daily rainfall amount that can be captured by a site in either its pre-
development, current, or post-development condition?
• To what degree will storms of different magnitudes be captured on site?
• What mix of LID controls can be deployed to meet a given stormwater retention target?
• How well will LID controls perform under future meteorological projections made by global
climate change models?
The calculator seamlessly accesses several national databases to provide local soil and meteorological
data for a site. The user supplies land cover information that reflects the state of development they
wish to analyze and selects a mix of LID controls to be applied. After this information is provided, the
site's hydrologic response to a long-term record of historical hourly precipitation, possibly modified by a
particular climate change scenario, is computed. This allows a full range of meteorological conditions to
be analyzed, rather than just a single design storm event. The resulting time series of rainfall and runoff
are aggregated into daily amounts that are then used to report various runoff and retention statistics. In
addition, the site's response to extreme rainfall events of different return periods is also analyzed.
The calculator uses the EPA Storm Water Management Model (SWMM) as its computational engine
(http://www.epa.gov/nrmrl/wswrd/wq/models/swmm/). SWMM is a well-established, EPA developed
model that has seen continuous use and periodic updates for 40 years. Its hydrology component uses
physically meaningful parameters making it especially well-suited for application on a nation-wide scale.
SWMM is set up and run in the background without requiring any involvement of the user.
The calculator is most appropriate for performing screening level analysis of small footprint sites up to
several dozen acres in size with uniform soil conditions. The hydrological processes simulated by the
calculator include evaporation of rainfall captured on vegetative surfaces or in surface depressions,
infiltration losses into the soil, and overland surface flow. No attempt is made to further account for the
fate of infiltrated water that might eventually transpire through vegetation or re-emerge as surface
water in drainage channels or streams.
The remaining sections of this guide discuss how to install the calculator, how to run it, and how to
interpret its output. An example application is presented showing how the calculator can be used to
8
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analyze questions related to stormwater runoff, retention, and control. Finally, a technical description is
given of how the calculator performs its computations and where it obtains the parameters needed to
do so.
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2. How to Run the Calculator
The Stormwater Calculator is a desktop application that runs on any version of Microsoft Windows with
version 4 or higher of the .Net Framework installed. An installation program for the calculator can be
downloaded from the following web page: http://www2.epa.gov/water-research/national-stormwater-
calculator. After running the installer, there will be a folder named "US EPA" added to your Windows
Start Menu. The folder contains a shortcut named "EPA Stormwater Calculator" that can be used to
launch the program.
NOTE: You must have an internet connection to run the Stormwater Calculator.
The main window of the calculator is displayed in Figure 1. It uses a series of tabbed pages to collect
information about the site being analyzed and to run and view hydrologic results. A Bing Maps display
allows you to view the site's location, its topography, selected soil properties and the locations of
nearby rain gages and weather stations.
B National Stormwater Calculator
Overv
^
'iert Location Soil Type Soil Drainage Topography Precipitation Evaporation Climate Change Land Cover LID Controls Results
Welcome to the EPA National
Stormwater Calculator
This calculator estimates the amount of
Stormwater runoff generated from a land
parcel under different development and
control scenarios over a long-term period
of historical rainfall.
The analysis-takes into account local soil
conditions, topography, land cover and
meteorology. Different types of low impact
development (LID) practices can be
employed to help capture and retain rainfall
en-site. Localized climate change scenarios
can also be analyzed.
Site information is provided to the
calculator using thetabbed pages listed
above. The Results page is where the site's
runoff is computed and displayed.
This program was produced by the U.S.
Environmental Protection Agency and was
subject to both internal and external
technical review. Please check with local
authorities about whether and how it can
be used to support local Stormwater
management goals and requirements.
Release 1.1,0,0
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Select the Location tab to begin analyzing a new site.
Analyze a New Site Save Current Site Exit
Figure 1. The calculator's main window.
The various pages of the calculator are used as follows:
1. Location page - establishes the site's location
2. Soil Type page - identifies the site's soil type
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3. Soil Drainage page - specifies how quickly the site's soil drains
4. Topography page - characterizes the site's surface topography
5. Precipitation page - selects a nearby rain gage to supply hourly rainfall data
6. Evaporation page - selects a nearby weather station to supply evaporation rates
7. Climate Change page - selects a climate change scenario to apply
8. Land Cover page - specifies the site's land cover for the scenario being analyzed
9. LID Controls page - selects a set of LID control options, along with their design features, to
deploy within the site
10. Results page - runs a long term hydrologic analysis and displays the results.
There are also three command options shown along the bottom status bar that can be selected at any
time:
1. Analyze a New Site: This command will discard all previously entered data and take you to the
Location page where you can begin selecting a new site to analyze. You will first be prompted to save
the data you entered for the current site.
2. Save Current Site: This command is used to save the information you have entered for the current site
to a disk file. This file can then be re-opened in a future session of the calculator by selecting the Open a
previously saved site command on the Location page.
3. Exit: This command closes down the calculator. You will be prompted to save the data you entered
for the current site.
You can move back and forth between the calculator's pages to modify your selections. Most of the
pages have a Help command that will display additional information about the page when selected.
After an analysis has been completed on the Results page, you can choose to designate it as a "baseline"
scenario, which means that its results will be displayed side-by-side with those of any additional
scenarios that you choose to analyze. Each of the calculator's pages will now be described in more
detail.
The Location page of the calculator is shown in Figure 2. You are asked to identify where in the U.S. the
site is located. This information is used to access national soils and meteorological databases. It has an
address lookup feature that allows you to easily navigate to the site's location. You can enter an address
or zip code in the Search box and either click on the Search icon, or press the Enter key to move the map
view to that location. You can also use the map's pan and zoom controls to hone in on a particular area.
Once the site has been located somewhere within the map's viewport, move the mouse pointer over
the site and then left-click the mouse to mark its exact location with a red square.
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Overview j Location | Soil Type | Soil Drainage | Topography | Precipitation | Evaporation | Climate Change | Land Cover | UP Controls | Results |
Site Name (Optional)
NERL - Athens
Search for an address or zip code:
900 College Station Rd., Athens, Gf |_j
Site Location (Latitude, Longitude)
33,93064811825752,-8335801094770431
Site Area (acres - Optional)
Open a previously saved site
Bring your site into view on the map
and then mark its exact location by
clicking the mouse pointer over it,
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Locate the site on the map.
Analyze a New Site Save Current Site Exit
Figure 2. The calculator's Location page.
The map display can be toggled between a standard road map view and a bird's eye aerial view. Figure
3 shows the site located in Figure 2 with a zoomed-in aerial view selected. You can also specify the area
of the site, which will result in a bounding red circle being drawn on the map. This is optional since the
calculator makes all of its computations on a per unit area basis.
You can also click on Open a previously saved site to read in data for a site that was previously saved to a
file to start working with that data once again. (Every time you begin analyzing a new site or exit the
program the calculator asks if you want to save the current site to a file.) Once you open a previously
saved site, the calculator will be populated with its data.
12
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Overview Location Soil Type | Soil Drainage [ Topography | Precipitation [ Evaporation | Climate Change [ Land Cover | LID Controls | Results |
Site Name (Optional)
NERL - Athens
Search for an address or zip code;
900 College Station Rd., Athens, Gt \ - ]
Site Location (Latitude, Longitude)
33,92618511455956,-83,35651719570158
Site Area (acres - Optional)
12.0
Open a previously saved site
Bring your site into view on the map
and then mark its exact location by
clicking the mouse pointer over it,
Locate the site on the map.
Analyze a New Site Save Current Site Exit
Figure 3. Bird's eye map view with a bounding circle.
Soil Type
Figure 4 shows the Soil Type page of the calculator, which is used to identify the type of soil present on
the site. Soil type is represented by its Hydrologic Soil Group (HSG). This is a classification used by soil
scientists to characterize the physical nature and runoff potential of a soil. The calculator uses a site's
soil group to infer its infiltration properties. Table 1 lists the definitions of the different soil groups.
You can select a soil type based on local knowledge or by retrieving a soil map overlay from the U.S.
Department of Agriculture's Natural Resources Conservation Service (NRCS) SSURGO database
(http://soils.usda.gov/survev/geography/ssurgo/). Simply check the View Soil Survey Data box at the top of the
page's left panel to retrieve SSURGO data. (There will be a slight delay the first time that the soil data is
retrieved and the color-coded overlay is drawn). Figure 4 displays the results from a SSURGO retrieval.
You can then select a soil type directly from the left panel or click on a color shaded region of the map.
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National Stormwater Calculator
Overview j Location | Soil Type Soil Drainage | Topography | Precipitation [ Evaporation | Climate Change [ Land Cover | LID Controls | Results |
What type of soil is on your site?
[3 View soil survey data
• © A - low runoff potential
H » B - moderately low
D C - moderately high
I D - high runoff potential
When soil survey data is displayed
you can select a soil type directly
from the map.
Help
« N * 0 © Road
Select a soil type for the site,
Analyze a New Site Save Current Site Exit
Figure 4. The calculator's Soil Type page.
The SSURGO database houses soil characterization data for most of the U.S. that have been collected
over the past forty years by federal, state, and local agencies participating in the National Cooperative
Soil Survey. The data are compiled by "map units" which are the boundaries that define a particular
recorded soil survey. These form the irregular shaped polygon areas that are displayed in the
calculator's map pane.
Soil survey data do not exist for all parts of the country, particularly in downtown core urban areas;
therefore, it is possible that no data will be available for your site. In this case you will have to rely on
local knowledge to designate a representative soil group.
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Table 1. Definitions of Hydrologic Soil Groups (USDA, 2010).
Group
Meaning
Saturated
Hydraulic
Conductivity
(in/hr)
Low runoff potential. Soils having high infiltration rates
even when thoroughly wetted and consisting chiefly of
deep, well to excessively drained sands or gravels.
>0.45
Soils having moderate infiltration rates when thoroughly
wetted and consisting chiefly of moderately deep to deep,
moderately well to well-drained soils with moderately
fine to moderately coarse textures. E.g., shallow loess,
sandy loam.
0.30-0.15
Soils having slow infiltration rates when thoroughly
wetted and consisting chiefly of soils with a layer that
impedes downward movement of water, or soils with
moderately fine to fine textures. E.g., clay loams, shallow
sandy loam.
0.15-0.05
High runoff potential. Soils having very slow infiltration
rates when thoroughly wetted and consisting chiefly of
clay soils with a high swelling potential, soils with a
permanent high water table, soils with a clay-pan or clay
layer at or near the surface, and shallow soils over nearly
impervious material.
0.05-0.00
Soil Drainage
The Soil Drainage page of the calculator (Figure 5) is used to identify how fast standing water drains into
the soil. This rate, known as the "saturated hydraulic conductivity," is arguably the most significant
parameter in determining how much rainfall can be infiltrated.
There are several options available for assigning a hydraulic conductivity value (in inches per hour) to
the site:
a) The edit box can be left blank, in which case, a default value based on the site's soil type will be
used (the default value is shown next to the edit box).
b) As with soil group, conductivity values from the SSURGO database can be displayed on the map
when the View Soil Survey Data checkbox is selected. Clicking the mouse on a colored region of
the map will make its conductivity value appear in the edit box.
c) If you have local knowledge of the site's soil conductivity you can simply enter it directly into the
edit box. This is preferred over the other two choices.
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How fast does standing water
drain from your site (inches/hour)?
0,108 (Default = 0.4)
\7\ View soil survey data
LH <- 0.01 inches/hour
I > 0.01 to <= 0.1 inches/hour
D > 0.1 to <= 1.0 inches/hour
I > 1 inches/hour
When soil survey data is displayed
you can select a value directly from
the map.
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tying
Enter the soil's drainage rate.
Analyze a New Site Save Current Site Exit
Figure 5. The calculator's Soil Drainage page.
It should be noted that the hydraulic conductivity values from the SSURGO database are derived from
soil texture and depth to groundwater and are not field measurements. As with soil type, there may not
be any soil conductivity data available for your particular location.
Topography
Figure 6 displays the Topography page of the calculator. Site topography, as measured by surface slope
(feet of drop per 100 feet of length), affects how fast excess stormwater runs off a site. Flatter slopes
results in slower runoff rates and provide more time for rainfall to infiltrate into the soil. Runoff rates
are less sensitive to moderate variations in slope. Therefore the calculator uses only four categories of
slope - flat (2%), moderately flat (5%), moderately steep (10%) and steep (above 15%). As with soil type
and drainage rate, any available SSURGO slope data will be displayed on the map if the View Soil Survey
checkbox is selected. You can use the resulting display as a guide or use local knowledge to describe the
site's topography.
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National Stonmwater Calculator
Overview | Location | Soil Type | Soil Drainage | Topography Precipitation j Evaporation | Climate Change | Land Cover | UP Controls | Results)
Describe your site's topography:
[V] View soil survey data
• G Flat (2% Slope)
• O Moderately Flat (5% Slope)
D '*' Moderately Steep (10% Slope)
• O Steep (above 15% Slope)
When soil survey data is displayed
you can select a slope category
directly from the map.
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Describe how steep the site is.
Analyze a New Site Save Current Site Exjt
Figure 6. The calculator's Topography page.
Precipitation
The Precipitation page of the calculator is shown in Figure 7. It is used to select a National Weather
Service rain gage that will supply rainfall data for the site. Rainfall is the principal driving force that
produces runoff. The calculator uses a long term continuous hourly rainfall record to make sure that it
can replicate the full scope of storm events that might occur. In addition, it identifies a set of 24-hour
extreme event storms associated with each rain gage location. These are a set of six intense storms
whose sizes are exceeded only once every 5, 10, 15, 30, 50 and 100 years, respectively.
17
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Overview | Location | Soil Type | Soil Drainage | Topography | Precipitation | Evaporation | Climate Change | Land Cover | UP Controls | Results)
Select a rain gage location to use as
a source of hourly rainfall data:
II - ATHENS BEN EPPS AIRPORT
(1970-2006)47.05"
2 - WINTERVILLE
[1998-2006)45.28"
3-WATKINSVILLEARS
(1970-2006)47.77"
4-UOFGAPLTSCIENCE
(1971-2006) 50.47"
5 - DANIELSVILLE
(1998-2006)46.38"
Save rainfall data for other uses
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Jefferson Nicholson
Arcade
Shiloh
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0
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Campton
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Comer
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Lexington
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Select a source of long-term hourly rainfall data.
Analyze a New Site Save Current Site Exit
Figure 7. The calculator's Precipitation page.
The calculator contains a catalog of over 8,000 rain gage locations from the National Weather Service's
(NWS) National Climatic Data Center (NCDC). Historical hourly rainfall data for each station have been
extracted from the NCDC's repository, screened for quality assurance, and stored on an EPA file server.
As shown in Figure 7', the calculator will automatically locate the five nearest gages to the site and list
their location, period of record and average annual rainfall amount. You can then choose what you
consider to be the most appropriate source of rainfall data for the site.
If the Save rainfall data ... command label is clicked, a Save As dialog window will appear allowing you to
save the rainfall data to a text file in case you want to use the data for some other application, such as
SWMM. Each line of the file will contain the recording station identification number, the year, month,
day, hour, and minute of the rainfall reading and the measured hourly rainfall intensity in inches/hour.
18
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Evaporation
The Evaporation page of the calculator is displayed in Figure 8. It is used to select a weather station that
will supply evaporation rates for the site. Evaporation determines how quickly the moisture retention
capacity of surfaces and depression storage consumed during one storm event will be restored before
the next event.
National Stormwater Calculator
Overview Location Soil Type Soil Drainage Topography , Precipitation | Evaporation Climate Change j Land Cover ] LID Controls | Results]
Select a weather station to use as
a source for evaporation rates:
- ATHENS BEN EPPS AIRPORT
(1970-2006) 0,20 inches/day
2 - WINTERVILLE
(2000-2006) 0.20 inches/day
3-UOFGAPLTSCIENCE
(1972-2006) 0,19 inches/day
4 - DANIELSVULE
(1999-2006) 0,17 inches/day
5-WINDER 4S
(1970-2004) 0,17 inches/day
Save evaporation data for ether uses
Help
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Jefferson Nicholson
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Auburn
Campion
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Mount Vemon
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Select a source of monthly average evaporation rates.
Analyze a New Site Save Current Site Exit
Figure 8. The calculator's Evaporation page.
Over 5,000 NWS weather station locations throughout the U.S. have had their daily temperature records
analyzed to produce estimates of monthly average evaporation rates (i.e., twelve values for each
station). These rates have been stored directly into the calculator. The calculator lists the five closest
locations that appear in the table along with their period of record and average daily evaporation rate
(the average of the twelve monthly rates). Note that these are "potential" evaporation rates, not
recorded values (there are only a few hundred stations across the U.S. with long term recorded
evaporation data). The rates have been estimated for bare soil using the Penman-Monteith equation;
and thus, transpiration or vegetative land cover is not explicitly represented. More details are provided
in the Computational Methods section of this document.
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As with rainfall, a Save evaporation data ... command is available in case you would like to save the data
to a file for use in another application. If this option is selected, the data will be written to a plain text
file of your choice with the twelve monthly average rates appearing on a single line.
The 2007 Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) states
that global warming is now unequivocal (IPCC, 2007). Some of the impacts that such warming can have
on the small scale hydrology addressed by the calculator include changes in seasonal precipitation
levels, more frequent occurrence of high intensity storm events, and changes in evaporation rates (Karl
et al., 2009). A climate change component has been included in the calculator to help you explore how
these impacts may affect the amount of stormwater runoff produced by a site and how it is managed.
Figure 9 displays the Climate Change page of the calculator. It is used to select a particular future
climate change scenario for the site. The scenarios were derived from a range of outcomes of the World
Climate Research Program's CMIP3 multi-model dataset (Meehl et al., 2007). This dataset contains
results of different global climate models run with future projections of population growth, economic
activity, and greenhouse gas emissions. The results have been downscaled to a regional grid that
encompasses each of the calculator's rain gage and weather station locations. Three different scenarios
are available that span the range of changes projected by the climate models: one is representative of
model outputs that produce hot/dry conditions, another represents changes that come close to the
median outcome from the different models, and a third represents model outcomes that produce
warm/wet conditions. Projections for each scenario are available for two different future time periods:
2035 and 2060.
Each choice of climate change scenario and projection year produces a different percent change in
monthly average rainfall, monthly average temperature, and annual maximum day precipitation for
each rain gage location and weather station in the calculator's database. The precipitation changes for
the current choice of rain gage are shown in the right hand panel of the Climate Change page. These
changes are used to adjust the historical meteorological records for the site as follows:
1. The changes in monthly average rainfall are applied as a multiplier to each historical hourly
rainfall reading that occurred in the particular month for each year of record.
2. The changes in monthly average temperatures are applied in similar fashion to the historical
daily temperature records used to calculate an average daily evaporation rate for each month of
the year.
3. The climate change influenced extreme event rainfalls are used in place of the historical ones.
The hot/dry, median, and warm/wet scenarios can be used to better understand the uncertainty
associated with future climate projections. For example, analyzing the two scenarios resulting in the
most severe increases and decreases in rainfall respectively, brackets the range of possible rainfall
conditions likely to occur. Alternately, if multiple scenarios are predicting increases in projected rainfall
it is more likely that larger rainfall events will occur. All three scenarios should be considered when
20
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bracketing future conditions, since the greatest projected change is not always associated with the
hot/dry or warm/wet scenarios and is different from one location to the next.
More details on the source of the climate change scenarios and how they are used to compute site
runoff are provided in the Computational Methods section of this users guide.
Overview Location Soil Type Soil Drainage Topography Precipitation ' Evaporation Climate Change Land Cover LID Controls Results
Select a future climate change scenario
to apply:
a No change
O Hot/Dry
O Median change
O Warm/Wet
Select the year to which the climate
change scenario is projected:
•*• 2035
2060
Help
30
20
11)
0
-10
-20
Percentage Change in Monthly Rainfall for 2035 Projections
— T— Hot.lDry — *•-- Median —&~ Warm/Wet
"»-,_.A.-
""A
.-• V
'
A
-•<*
H
Jan Fob Mar Apr May Jun Jul Aug Scp Get Nov Dec
Annual Max, Day Rainfall (inches) for 2035 Projections
-A--- Hot.'Dry —*~ Median --*?— Warm/Wet ~+~ Historical
15 30
Return Period (years)
50
100
Select a climate change scenario to use,
Analyze a New Site Save Current Site Exit
Figure 9. The calculator's Climate Change page.
Understanding regional climate impacts may help you select appropriate climate change scenarios.
Online resources highlighting regional climate change impacts for the contiguous U.S., Hawaii,
Alaska, and U.S. Territories are available at (http://www.epa.gov/climatechange/impacts-
adaptation/ (U.S. EPA, 2014) and at http://www.globalchange.gov/explore/ (USGRP, 2014)).
21
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Land Cover
Figure 10 displays the Land Cover page of the calculator. It is used to describe the different types of
pervious land cover on the site. Infiltration of rainfall into the soil can only occur through pervious
surfaces. Different types of pervious surfaces capture different amounts of rainfall on vegetation or in
natural depressions, and have different surface roughness. Rougher surfaces slow down runoff flow
providing more opportunity for infiltration. The remaining non-pervious site area is considered to be
"directly connected impervious surfaces" (roofs, sidewalks, streets, parking lots, etc. that drain directly
off-site). Disconnecting some of this area, to run onto lawns for example, is an LID option appearing on
the next page of the calculator.
You are asked to supply the percentage of the site covered by each of four different types of pervious
surfaces:
Overview j Location [ Soil Type | Soil Drainage Topography ) Precipitation [ Evaporation ] Climate Change Land Cover LJD Controls Results
Describe the site's land cover for the
development scenario being analyzed:
Hover the mouse over a cover category
to see a more detailed description.
An a I vze a New Site Save Current Site Exit
Figure 10. The calculator's Land Cover page.
22
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1. Forest - stands of trees with adequate brush and forested litter cover
2. Meadow - non-forested natural areas, scrub and shrub rural vegetation
3. Lawn -sod lawn, grass, and landscaped vegetation
4. Desert - undeveloped land in arid regions with saltbush, mesquite, and cactus vegetation
You should assign land cover categories to the site that reflects the specific condition you wish to
analyze: pre-development, current, or post-development. A pre-development land cover will most likely
contain some mix of forest, meadow, and perhaps desert. Local stormwater regulations might provide
guidance on how to select a pre-development land cover or you could use a nearby undeveloped area as
an example. Viewing the site map in bird's eye view, as shown in Figure 9, would help identify the land
cover for current conditions. Post-development land cover could be determined from a project's site
development plan map. Keep in mind that total runoff volume is highly dependent on the amount of
impervious area on the site while it is less sensitive to how the non-impervious area is divided between
the different land cover categories.
LID
The LID Controls page of the calculator is depicted in Figure 11. It is used to deploy low impact
development (LID) controls throughout the site. These are landscaping practices designed to capture
and retain stormwater generated from impervious surfaces that would otherwise run off the site. As
seen in Figure 11, there are seven different types of green infrastructure (Gl) LID controls available. You
can elect to apply any mix of these controls by simply telling the calculator what percentage of the
impervious area is treated by each type of control. Each control has been assigned a reasonable set of
design parameters, but these can be modified by clicking on the name of the control. This will also allow
you to automatically size the control to capture a 24-hour design storm that you specify. More details
on each type of control practice, its design parameters and sizing it to retain a given design storm are
provided in the LID Controls section of this users guide.
23
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National Stormwater Calculator
Overview | Location | Soil Type | Soil Drainage | Topography [ Precipitation j Evaporation | Climate Change | Land Cover | LID Controls | Results
What % of your site's impervious area
will be treated by the following LJD
practices?
Disconnection
Rain Harvesting
Rain Gardens
Green Roofs
Street Planters
Infiltration Basins
Porous Pavement
Design Storm for Sizing
(inches) (see Help]
Click a practice to customize its design,
0,00
Heli
Assign LID practices to capture runoff from impervious areas,
Analyze a New Site Save Current Site Exit
Figure 11. The calculator's LID Controls page.
Green infrastructure (Gl) is a relatively new and flexible term, and it has been used differently in
different contexts. However, for the purposes of EPA's efforts to implement the Gl Statement of
Intent, EPA intends the term Gl to generally refer to systems and practices that use or mimic
natural water flow processes and retain stormwater or runoff on the site where it is generated.
Gl can be used at a wide range of landscape scales in place of, or in addition to, more traditional
stormwater control elements to support the principles of LID.
Results
The final page of the calculator is where a hydrologic analysis of the site is run and its results are
displayed. As shown in Figure 12, by selecting the Site Description report option you can first review the
data that you entered for the site and go back to make changes if needed.
24
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National Stormwater Calculator
Overview j Location
Soil Type | Soil Drainage | Topography j Precipitation J Evaporation j Climate Change | Land Cover ] LID Controls Results
Years to Analyze
Event Threshold (inches) 0.10 [g|
Ignore Consecutive Days
Actions
Refresh Results
Use as Baseline Scenario
Remove Baseline Scenario
Print Results to PDF File
Reports
i® Site Description
1 _.• Summary Resu
11 Rainfall / Runol
O Rainfall / Runof
Rainfall Retenti
Runoff By Rainf
O Extreme Event f
Help
Is
f Events
: Frequency
)n Frequency
all Percentile
ainfall/ Runoff
Parameter Current Scenario Baseline Scenario
Site Area (acres) 120
Hydrologic Soil Group 8
Hydraulic Conductivity fin/hr) 0.108
Surface Slope (%) 10
Precip. Data Source ATHENS BEN EPPS AIR...
Evap. Data Source ATHENS BEN EPPS AIR...
Climate Change Scenario None
% Forest 18
% Meadow 8
%Lawn 25
% Desert 0
% Impervious 49
Disconnection 0
Rain Harvesting 0
Rain Gardens 0
Green Roofs 0
Street Planters 0
infiltration Basins 0
Porous Pavement 0
r
Help
Site data have changed - results need to be refreshed. Analyze a New Site Save Current Site Exit .:
Figure 12. The calculator's Results page.
The input controls on this page are grouped together in three sections: Options, Actions, and Reports.
The Options section allows you to control how the rainfall record is analyzed via the following settings:
1. The number of years of rainfall record to use (moving back from the most recent year on
record).
2. The event threshold, which is the minimum amount of rainfall (or runoff) that must occur over a
day for that day to be counted as having rainfall (or runoff). Rainfall (or runoff) above this
threshold is referred to as "observable" or "measureable".
3. The choice to ignore consecutive wet days when compiling runoff statistics (i.e., a day with
measurable rainfall must be preceded by at least two days with no rainfall for it to be counted).
The latter option appears in some state and local stormwater regulations as a way to exempt extreme
storm events, such as hurricanes, from any stormwater retention requirements. Normally, you would
not want to select this option as it will produce a less realistic representation of the site's hydrology.
Note that although results are presented as annual and daily values, they are generated by considering
the site's response to the full history of hourly rainfall amounts.
25
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The Actions section of the page contains commands that perform the following actions:
Refresh Results - runs a long term simulation of the site's hydrology and updates the output displays
with new results (it will be disabled if results are currently available and no changes have been made to
the site's data).
Use as Baseline Scenario - uses the current site data and its simulation results as a baseline against
which future runs will be compared in the calculator's output reports (this option is disabled if there are
no current simulation results available).
Remove Baseline Scenario - removes any previously designated baseline scenario from all output
reports.
Print Results to PDF File - writes the calculator's results for both the current and any baseline scenario
to a PDF file that can be viewed with a PDF reader at a future time.
The Reports section of the page allows you to choose how the rainfall / runoff results for the site should
be displayed. A complete description of each type of report available will be given in the next section of
this guide.
When the calculator first loads or begins to analyze a new site the following default values are used:
Soil Group:
Conductivity:
Surface Slope:
Rainfall Station:
Evaporation Station:
Climate Change Scenario:
Land Cover:
LID Controls:
Years to Analyze:
Event Threshold:
Ignore Consecutive Days:
B
0.4 inches/hour
5%
Nearest cataloged station
Nearest cataloged station
None
40% Lawn, 60% impervious
None
20
0.10 inches
No
26
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3,
The Results page of the calculator (Figure 12) contains a list of reports that can be generated from its
computed results. Before discussing what these reports contain it will be useful to briefly describe how
the calculator derives its results. After you select the Refresh Results command, the calculator internally
performs the following operations:
1. A SWMM input file is created for the site using the information you provided to the calculator.
2. The historical hourly rainfall record for the site is adjusted for any climate change scenario
selected.
3. SWMM is run to generate a continuous time series of rainfall and runoff from the site at 15-
minute intervals for the number of years specified.
4. The 15-minute time series of rainfall and runoff are accumulated into daily values by calendar
day (midnight to midnight).
5. Various statistics of the resulting daily rainfall and runoff values are computed.
6. The SWMM input file is modified and run once more to compute the runoff resulting from a set
of 24-hour extreme rainfall events associated with different return periods. The rainfall
magnitudes are derived from your choice of climate change scenario or from the historical
record if climate change is not being considered.
Thus for the continuous multi-year run, the rainfall / runoff output post-processed by the calculator are
the 24-hour totals for each calendar day of the period simulated. A number of different statistical
measures are derived from these data, some of which will be more relevant than others depending on
the context in which the calculator is being used.
The calculator's Summary Results report, an example of which is shown in Figure 13, contains the
following items:
• A pie chart showing the percentage of total rainfall that infiltrates, evaporates, and becomes
runoff. Note that because the calculator does not explicitly account for the loss of soil moisture
to vegetative transpiration, the latter quantity shows up as infiltration in this chart.
• Average Annual Rainfall: Total rainfall (in inches) that falls on the site divided by the number of
years simulated. It includes all precipitation amounts recorded by the station assigned to the site,
even those that fall below the Event Threshold.
• Average Annual Runoff: Total runoff (in inches) produced by the site divided by the number of
years simulated. It includes all runoff amounts, even those that fall below the Event Threshold.
• Days per Year with Rainfall: The number of days with measureable rainfall divided by the number
of years simulated, i.e., the average number of days per year with rainfall above the Event
Threshold.
27
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Current Scenario
Annual Rainfall = 45.22 inches
46%
48%
Runoff
Eva p.
Infil.
Statistic
Average Annual Rainfall (inches)
Average Annual Runoff (inches)
Days per Year With Rainfall
Days per Year with Runoff
Percent of Wet Days Retained
Smallest Rainfall w/ Runoff (inches)
Largest Rainfall w/o Runoff (inches)
Max. Rainfall Retained (inches)
Current Scenario
45,22
21.83
68.46
48.57
29.05
0.11
0.30
1.62
Baseline Scenario
Figure 13. The calculator's Summary Results report.
Days per Year with Runoff: The number of days with measureable runoff divided by the number
of years simulated, i.e., the average number of days per year with runoff above the Event
Threshold.
Percent of Wet Days Retained: The percentage of days with measureable rainfall that do not have
any measureable runoff generated. It is computed by first counting the number of days that have
rainfall above the Event Threshold but runoff below it. This number is then divided by the total
number of rainfall days above the threshold and multiplied by 100.
Smallest Rainfall w/Runoff: The smallest daily rainfall that produces measureable runoff. All
days with rainfall less than this amount have runoff below the threshold.
Largest Rainfall w/o Runoff: The largest daily rainfall that produces no runoff. All days with more
rainfall than this will have measureable runoff. Of the wet days that lie between this depth and
the smallest rainfall with runoff, some will have runoff and others will not.
28
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• Max. Retention Volume: The largest daily rainfall amount retained on site over the period of
record. This includes days that produce runoff from storms that are only partly captured.
Note that if the Ignore Consecutive Wet Days option is in effect then the retention statistics listed above
are computed by ignoring any subsequent back to back wet days for a period of 48 hours following an
initial wet day.
Direct interception of rainfall and transpiration by the tree canopy may be important processes
depending on the site you are modeling. While the SWC doesn't explicitly include these
processes, the model i-Tree Hydro can be used to determine the effect of trees on urban
hydrology for stormwater management at the catchment scale (USFS, 2014). For more
information about i-Tree Hydro visit: http://www.itreetools.org/hydro/index.php.
Rainfall / Runoff Events
The calculator's Rainfall/Runoff report contains a scatter plot of the daily runoff depth associated with
each daily rainfall event over the period of record analyzed. Only days with rainfall above the event
threshold (see page 25) are plotted. Events that are completely captured on site (i.e., have runoff below
the event threshold) show up as points that lie along the horizontal axis. There is not always a consistent
relationship between rainfall and runoff. Days with similar rainfall amounts can produce different
amounts of runoff depending on how that rainfall was distributed over the day and on how much rain
occurred in prior days.
29
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Rainfall / Runoff Events
5 -
4 --
M
i;
3
a
Q
2 --
i
3 4
Daily Rainfall (inches)
Figure 14. The calculator's Rainfall / Runoff Event report.
Rainfall / Runoff Frequency
An example of the calculator's Rainfall / Runoff Frequency report is seen in Figure 15. It shows how
many times per year, on average, a given daily rainfall depth or runoff depth will be exceeded. As an
example, from Figure 14 we see that there are three days per year where it rains more than two inches,
but only one day per year where there is more than this amount of runoff. Events with more than four
inches of rain occur only once every two years.
30
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Rainfall / Runoff Exceedance Frequency
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Figure 15. The calculator's Rainfall / Runoff Frequency report.
The rainfall frequency curve is generated by simply ordering the measureable daily rainfall results from
the long term simulation from lowest to highest and then counting how many days have rainfall higher
than a given value. The same procedure is used to generate the daily runoff frequency curve. Curves
like these are useful in comparing the complete range of rainfall / runoff results between different
development, control and climate change scenarios. Examples might include determining how close a
post-development condition comes to meeting pre-development hydrology or seeing what effect future
changes in precipitation due to climate change might have on LID control effectiveness.
On any of the calculator's line or bar charts you can make the numerical value of a plotted point
appear in a popup label by moving the mouse over the point on the line or bar you wish to
examine. You can also zoom in on any area of the chart by pressing the left mouse button while
dragging the mouse pointer across the area. To return to full view, you would right-click on the
chart and select Un-Zoom from the pop-up menu that appears.
31
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Rainfall Retention Frequency
Another type of report generated by the calculator is the Rainfall Retention Frequency plot as shown in
Figure 16. It graphs the frequency with which a given depth of rainfall will be retained on site for the
scenario being simulated. For a given daily rainfall depth X the corresponding percent of time it is
retained represents the fraction of storms below this depth that are completely captured plus the
fraction of storms above it where at least X inches are captured. A rainfall event is considered to be
completely captured if its corresponding runoff is below the user stipulated Event Threshold.
To make this concept clearer, consider a run of the calculator that resulted in 1,000 days of measureable
rainfall and associated runoff for a site. Suppose there were 300 days with rainfall below one inch that
had no measureable runoff and 100 days where it rained more than an inch but the runoff was less than
an inch. The retention frequency for a one inch rainfall would then be (300 + 100) / 1,000 or 40 percent.
Rainfall Retention Frequency
110
100 --
90 --
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40 --
30 --
20 -| i 1 1 1 1 i 1 1 i 1 1 1 1 1 1 i 1 1 1-
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Daily Rainfall (inches)
Figure 16. The calculator's Rainfall Retention Frequency report.
32
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The Rainfall Retention Frequency report is useful for determining how reliably a site can meet a required
stormwater retention standard. Looking at Figure 16, any retention standard above one inch would only
be met about 32 % of the time (i.e., only one in three wet days would meet the target). Note that any
rainfall events below the target depth that are completely captured are counted as having attained the
target (e.g., a day with only 0.3 inches of rainfall will be counted towards meeting a retention target of
1.0 inches if no runoff is produced). That is why the plot tails off to the right at a constant level of 29
percent, which happens to be the percent of all wet days fully retained for this example (refer to the
Percent of Wet Days Retained entry in the Summary Results report of Figure 13).
The Runoff by Rainfall Percentile report produced by the calculator is displayed in Figure 17. It shows
what percentage of total measureable runoff is attributable to different size rainfall events. The bottom
axis is divided into intervals of daily rainfall event percentiles. The top axis shows the rainfall depth
corresponding to each end-of-interval percentile. The bars indicate what percentage of total
measureable runoff is generated by the rainfall within each size interval. This provides a convenient way
of determining what rainfall depth corresponds to a given percentile (percentiles are listed along the
bottom of the horizontal axis while their corresponding depths are listed across the top of the axis.)
33
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Runoff Contribution by Rainfall Percentile
Daily Rainfall Depth (inches)
0.14 0.20 0.26 0.35 0.43 0.53 0.67 0.77 0.98 1 15 1.3fi 1.81 3.57
75
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Daily Rainfall Percentile
Figure 17. The calculator's Runoff by Rainfall Percentile report.
The X-th percentile storm is the daily rainfall amount that occurs at least X percent of the time,
i.e., X percent of all rainfall days will have rainfall amounts less than or equal to the percentile
value. It is found by first ordering all days with rainfall above the Event Threshold from smallest
to highest value. The X-th percentile is the X-th percent highest value (e.g., if there were 1000
days with observable rainfall the 85-th percentile would be 850-th value in the sorted listing of
rainfall amounts).
As an example of how to interpret this plot, look at the bar in Figure 17 associated with the 90th to 95th
percentile storm interval (daily rainfalls between 1.38 and 1.81 inches). Storms of this magnitude make
up 16 % of the total runoff (for this particular site and its land cover). Note that by definition the
number of events within this 5 percentile interval is 5 % of the total number of daily rainfall events.
34
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The final report produced by the calculator shows the rainfall and resulting runoff for a series of
extreme event (high intensity) storms that occur at different return periods. An example is shown in
Figure 18. Each stacked bar displays the annual max day rainfall that occurs with a given return period
and the runoff that results from it for the current set of site conditions. The max day rainfalls
correspond to those shown on the Climate Change page for the scenario you selected (or to the
historical value if no climate change option was chosen).
Note that the max day rainfalls at different return periods are a different statistic than the daily rainfall
percentiles that are shown in the Runoff by Rainfall Percentile report (see Figure 17). The latter
represents the frequency with which any daily rainfall amount is exceeded while the former estimates
how often the largest daily rainfall in a year will be exceeded (hence its designation as an extreme storm
event). Most stormwater retention standards are stated with respect to rainfall percentiles while
extreme event rainfalls are commonly used to define design storms that are used to size stormwater
control measures. The extreme event rainfall amounts are generated using a statistical extrapolation
technique (as described in the Computational Methods section) that allows one to estimate the once in
X year event when fewer than X years of observed rainfall data are available.
As mentioned previously, all of the information displayed on the Runoff pages of the calculator can be
written to a PDF file to provide a permanent record of the analysis made for a site. You simply select the
Print Results to PDF File command in the upper left panel of the Runoff page and then enter a name for
the file to which the results will be written.
35
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Extreme Event Rainfall / Runoff
f Depth (inches)
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Return Period on Max. Day Rainfall (years)
Figure 18. The calculator's Extreme Event Rainfall / Runoff report.
36
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LID controls are landscaping practices designed to capture and retain stormwater generated from
impervious surfaces that would otherwise run off the site. The Stormwater Calculator allows you to
apply a mix of seven different types of LID practices to a site. These are displayed in Table 2 along with
brief descriptions of each. This particular set of Gl practices was chosen because they can all be sized on
the basis of just area. Two other commonly used controls, vegetative swales and infiltration trenches,
are not included because their sizing depends on their actual location and length within the site,
information which is beyond the scope of the calculator.
Each LID practice is assigned a set of default design and sizing parameters, so to apply a particular
practice to a site, you only have to specify what percentage of the site's impervious area will be treated
by the practice (see Figure 10). You can, however, modify the default settings by clicking on the name of
the particular practice you wish to edit. For example, Figure 19 displays the resulting LID Design dialog
window that appears when the Street Planter LID is selected. All of the LID controls have similar LID
Design dialogs that contain a sketch and brief description of the LID control along with a set of edit
boxes for its design parameters. The Learn More ... link will open your web browser to a page that
provides more detailed information about the LID practice.
Table 3 lists the various parameters that can be edited with the LID Design dialogs along with their
default factory setting. Arguably the most important of these is the Capture Ratio parameter. This
determines the size of the control relative to the impervious area it treats. Note that because the
calculator does not require that the actual area of the site be specified, all sub-areas are stated on a
percentage basis. So, total impervious area is some percentage of the total site area, the area treated
by a particular LID control is some percentage of the total impervious area, and the area of the LID
control is some percentage of the area it treats.
Pressing the Size for Design Storm button on an LID Design form will make the calculator automatically
size the LID control to capture the Design Storm Depth that was entered on the LID Control page (see
Figure 10). This computes a Capture Ratio (area of LID relative to area being treated) for Rain Gardens,
Street Planters, Infiltration Basins, and Porous Pavement by taking the ratio of the design storm depth to
the depth of available storage in the LID unit. For Infiltration Basins it also determines the depth that
will completely drain the basin within 48 hours. For Rainwater Harvesting it calculates how many
cisterns of the user-supplied size will be needed to capture the design storm. Automatic sizing is not
available for Disconnection, since no storage volume is used with this practice, and for Green Roofs,
since the ratio is 100% by definition. The methods used to automatically size the LID controls are
described in the Computational Methods section of this users guide. Note that even when sized in this
fashion, a LID control might not fully capture the design storm because it may not have drained
completely prior to the start of the storm or the rainfall intensity during some portion of the storm
event may overwhelm its infiltration capacity. The calculator is able to capture such behavior because it
continuously simulates the full range of past precipitation events.
37
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Table 2. Descriptions of LID practices included in the calculator.
LID Practice
Description
Disconnection refers to the practice of directing runoff from impervious
areas, such as roofs or parking lots, onto pervious areas, such as lawns
or vegetative strips, instead of directly into storm drains.
Disconnection
Rain harvesting systems collect runoff from rooftops and convey it to a
cistern tank where it can be used for non-potable water uses and on-
site infiltration.
Rain Harvesting
Rain Gardens are shallow depressions filled with an engineered soil mix
that supports vegetative growth. They provide opportunity to store
and infiltrate captured runoff and retain water for plant uptake. They
are commonly used on individual home lots to capture roof runoff.
Rain Gardens
Green Roofs
Green roofs (also known as vegetated roofs) are bioretention systems
placed on roof surfaces that capture and temporarily store rainwater in
a soil medium. They consist of a layered system of roofing designed to
support plant growth and retain water for plant uptake while
preventing ponding on the roof surface.
Street Planters
Street Planters are typically placed along sidewalks or parking areas.
They consist of concrete boxes filled with an engineered soil that
supports vegetative growth. Beneath the soil is a gravel bed that
provides additional storage as the captured runoff infiltrates into the
existing soil below.
,—TJ-S* -
Infiltration basins are shallow depressions filled with grass or other
natural vegetation that capture runoff from adjoining areas and allow it
to infiltrate into the soil.
Infiltration Basins
Porous Pavement
Porous Pavement systems are excavated areas filled with gravel and
paved over with a porous concrete or asphalt mix or with modular
porous blocks. Normally all rainfall will immediately pass through the
pavement into the gravel storage layer below it where it can infiltrate at
natural rates into the site's native soil.
38
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Street Planter
Street Planters consist of concrete boxes filled with
an engineered soil that supports vegetative growth.
Beneath the soil is a gravel bed that provides
additional storage.
The walls of a planter extend 3 to 12 inches above
the soil bed to allow for ponding withing the unit.
The thickness of the soil growing medium ranges
from 6 to 24 inches while gravel beds are6 to 18
inches in depth.
The planter's Capture Ratio is the ratio of its area to v
Ponding Height (inches]
Soil Media Thickness (inches)
Soil Media Conductivity (in/hr)
Gravel Bed Thickness (inches)
% Capture Ratio
13
10.0
12
Learn more...
Size for Design Storm
Restore Defaults
Figure 19. Example of a LID Design dialog for a street planter.
There are some additional points to keep in mind when applying LID controls to a site:
1. The area devoted to Disconnection, Rain Gardens, and Infiltration Basins is assumed to come
from the site's collective amount of pervious land cover while the area occupied by Green Roofs,
Street Planters and Porous Pavement comes from the site's store of impervious area.
2. Underdrains (slotted pipes placed in the gravel beds of Street Planter and Porous Pavement
areas to prevent the unit from flooding) are not provided for. However since underdrains are
typically oversized and placed at the top of the unit's gravel bed, the effect on the amount of
excess runoff flow bypassed by the unit is the same whether it flows out of the underdrain or
simply runs off of a flooded surface.
3. The amount of void space in the soil, gravel, and pavement used in the LID controls are listed in
Table 4 below. They typically have a narrow range of acceptable values and results are not
terribly sensitive to variations within this range.
39
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Table 3. Editable LID parameters.
LID Type
Disconnection
Rain Harvesting
Rain Gardens
Green Roofs
Street Planters
Infiltration Basins
Porous Pavement
Parameter
Capture Ratio
Cistern Size
Cistern Emptying Rate
Number of Cisterns
Capture Ratio
Ponding Depth
Soil Media Thickness
Soil Media Conductivity
Soil Media Thickness
Soil Media Conductivity
Capture Ratio
Ponding Depth
Soil Media Thickness
Soil Media Conductivity
Gravel Bed Thickness
Capture Ratio
Basin Depth
Capture Ratio
Pavement Thickness
Gravel Bed Thickness
Default Value
100%
100 gal
50 gal/day
4 per 1,000 sq ft
5%
6 inches
12 inches
10 inches/hour
4 inches
10 inches/hour
6%
6 inches
18 inches
10 inches/hour
12 inches
5%
6 inches
100%
4 inches
18 inches
Table 4. Void space values of LID media.
Property
Soil Media Porosity
Gravel Bed Void Ratio
Pavement Void Ratio
LID Controls
Rain Gardens, Green Roofs and Street Planters
Street Planters and Porous Pavement
Porous Pavement
Default Value
45%
75%
12%
40
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5. Example Application
An example will now be presented to show how the calculator can be used to analyze small site
hydrology. The site shown earlier in Figure 3 will be used as our study area, although, because the
calculator is national in scope, we could have chosen any other location just as well. It is a 12 acre
environmental research facility. The baseline data for the site have already been obtained from Figures
4 through 8. These identified the site's hydrologic soil group as B, its hydraulic conductivity as 0.108
inches/hour, its topography as moderately steep, its closest rain gage as having an annual rainfall of
47.05 inches and its closest weather station averaging 0.2 inches per day of potential evaporation. We
will simulate three different development scenarios (pre-development, post-development, and post-
development with LID controls) to show how one can both derive and evaluate compliance with
different stormwater retention standards. After that we will see what effect a future climate change
scenario might have on the site's ability to comply with the standard.
Pre-Development Conditions
Pre-development hydrology is often cited as an ideal stormwater management goal to attain because it
maintains a sustainable and ecologically balanced condition within a watershed. It is also commonly
used to define specific stormwater retention standards, as will be discussed shortly. To simulate a pre-
development condition for our study area, we must identify the land cover that characterizes the site in
its natural pre-developed state. If you pan the site's map display to the left, you will observe an
adjacent natural area that suggests a pre-development land cover of 80 percent Forest and 20 percent
Meadow. These values are entered on the Land Cover page of the calculator (see Figure 7). For the
next page of the calculator no LID Controls are selected since we are analyzing a pre-development
scenario. On the final page of the calculator, we select to analyze the latest 20 years of rainfall data and
to not ignore back to back storm events.
Running the calculator for these conditions produces the Summary Results report listed in Table 5. It
shows that there is an average of 71 days per year with rainfall, but only 7 of these produce
measureable runoff. Of the 47 inches of rainfall per year, 91 percent is retained on site. The Runoff by
Rainfall Percentile plot for this run, shown in Figure 20, indicates that it is mainly storms above 1 inch
that produce almost all of the runoff.
Retention standards are developed by state and municipal governments and tailored to meet
stormwater control objectives unique to their jurisdiction. They stipulate the amount of rainfall
that must be "retained" on site and are used to determine the proper size of stormwater controls.
Standards are usually formulated in one of several ways including restoration of pre-development
conditions, rainfall depth retained, or percentile rainfall depth retained. Contact your local
government to learn more about the retention standards that apply in your area.
41
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Table 5. Summary results for pre-development conditions on the example site.
Statistic
Average Annual Rainfall finches)
Average Annual Runoff finches)
Days per Year With Rainfall
Days per Year with Runoff
Percent of Wet Days Retained
Smallest Rainfall w/ Runoff finches)
Largest Rainfall w/o Runoff finches)
Max. Rainfall Retained finches)
Current Scenario
47.01
4.C-S
73.00
6.3Q
9043
0.14
2.42
3.25
Baseline Scenario
Runoff Contribution by Rainfall Percentile
Daily Rainfall Depth (inches)
70 -,
60 -
1 50 -
3
~a
o '*° -
H
H-
0
WJ '"""
5
c
u
CL
10 ~
0 •
0.14 0.20 0.26 0.35 0.43 0.54 0.69 0.81 0.98 1.15
i
r-«
n ,
n
1 37 1.75 3.51
\
\
\
I
10 20 30 40 50 60 70 75 80 85
Daily Rainfall Percentile
1
t
| 1
'
• -
•
90 95 99
Figure 20. Runoff from different size storms for pre-development conditions on the example site.
42
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Now consider a stormwater retention standard that requires a site to capture all rainfall produced from
storms up to and including the 95th percentile daily rainfall event or the rainfall that would be retained
on the site in its natural pre-developed state, whichever is smaller. To identify the depth of runoff that
must be retained under this standard, we first need to know what the 95th percentile rainfall depth is.
This can be found from the aforementioned Runoff by Rainfall Percentile plot in Figure 20. The 95th
percentile storm corresponds to 1.75 inches. To determine the rainfall retained on the undeveloped
site, we can examine the calculator's Rainfall Retention Frequency report for this run shown in Figure
21. Because the standard attaches 95 % reliability to its target rainfall, we assume that the same would
hold for its retention target. From Figure 21, we see that a retention target of 1.3 inches could be met
95 % of the time (i.e., of the 71 days per year on average with measureable precipitation, for 67 of those
the site will retain either the entire rainfall or the first 1.3 inches, whichever is smaller). Because this is
less than the 1.75 inch, 95th percentile rainfall, the standard for this site would be to retain 1.3 inches.
Rainfall Retention Frequency
102
I I | I—I I I | I I I I | I I I I | I I I t | I I > I | I I I
0,0 0,5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Daily Rainfall (inches)
Figure 21. Rainfall retention frequency under pre-development conditions for the example site.
43
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Post-Development Conditions
Next the calculator will be used to analyze the example site's hydrology under post-development
conditions. Because we want to compare the results against those for the pre-development case, we
first select the Use as Baseline Scenario option on the Results page of the calculator to tell it to display
our pre-development results as a comparison baseline scenario in future runs. We then determine the
land cover for the site in its developed state. Table 6 shows the distribution of the different land cover
categories across the site. Impervious surfaces cover almost half of the total site area. Selecting the
Land Cover page of the calculator, we replace the pre-development land cover with this new one (refer
to Figure 10).
Table 6. Land cover for the example site in developed state.
Land Cover
Forest
Meadow
Lawn
Total Impervious Surfaces
Roofs
Parking
Roads & Sidewalks
% of Total Area
18
8
25
49
10
9
30
% of Impervious Area
-
-
-
100
20
20
60
We next return to the Results page and re-run the analysis. Table 7 contains the resulting comparison of
summary runoff statistics between the two conditions. Note how the developed site with no runoff
controls comes nowhere close to matching pre-development hydrology. Instead of only seven days per
year with measureable runoff, there are 51 and the total volume of runoff has increased more than
fivefold. As seen in the Rainfall Retention Frequency plot of Figure 22, the 1.3 inch retention target
identified earlier can only be met about 30% of the time (which consists primarily of those days where a
low amount of rainfall is entirely contained on site).
44
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Table 7. Comparison of runoff statistics for post-development (Current) and pre-development
(Baseline) conditions.
Statistic
Average Annual Rainfall finches)
Average Annual Runoff (inches)
Days per Year With Rainfall
Days per Year with Runoff
Percent of Wet Days Retained
Smallest Rainfall w/ Runoff (inches)
Largest Rainfall w/o Runoff (inches)
Max. Rainfall Retained (inches)
Current Scenario
47,01
22.73
71.00
50:77
28.50
an
0.34
1.70
Baseline Scenario
47.01
4.08
71.00
6,30
90,43
0.14
2.42
3.25
Rainfall Retention Frequency
— Current Scenario ^^— Baseline Scenario
120
1.0 1.5 2.0 2.5 3.0
Daily Rainfall (inches)
3.5
40
Figure 22. Rainfall retention frequency for pre-development (Baseline) and post-development
(Current) conditions.
45
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We will now add some LID practices to our example site to see how well they can make its post-
development hydrology more closely match that of pre-development. Returning the calculator to the
LID Controls page we see there are seven types of LID controls available to apply in any combination and
sizing to the impervious areas of the site. From Table 6, we see that roofs occupy 20 percent of the total
impervious area, parking lots another 20 percent, and the remaining 60 percent is roads and sidewalks.
Because the site houses a research facility, we assume that we can capture runoff from the roof of the
main building (15 percent of the impervious area) in Cisterns and use it for non-potable purposes within
the site. Runoff from the roofs, roads and parking areas on the north side of the site will be directed
into an Infiltration Basin. A portion of the south parking area will be replaced with Porous Pavement.
Finally, strategically placed Rain Gardens will be used to intercept runoff from the remaining roofs,
roads and sidewalks.
Figure 23 shows how the LID Controls practices page of the calculator was filled in to reflect these
choices. A design storm size of 1.75 inches, based on the 95th percentile storm, was chosen to
automatically size each LID control. Each LID's design dialog was launched to apply automatic sizing to
it. The results of this process are shown in Figures 24 and 25 (capture ratios for the infiltration basin,
rain gardens and porous pavement; number of cisterns / 1,000 square feet for rain harvesting).
46
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National Stormwater Calculator
Overview | Location | Soil Type | Soil Drainage | Topography [ Precipitation j Evaporation | Climate Change | Land Cover | LID Controls Results
What % of your site's impervious area
will be treated by the following LJD
practices?
Disconnection
Rain Harvesting
Rain Gardens
Green Roofs
Street Planters
Infiltration Basins
Porous Pavement
Design Storm for Sizing
(inches) (see Help]
Click a practice to customize its design,
!l.75
Hell
Assign LID practices to capture runoff from impervious areas,
Analyze a New Site Save Current Site Exit
Figure 23. Low Impact Development controls applied to the example site.
47
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Rain Harvesting
Rain harvesting systems collect runoff from rooftops
and convey it to a cistern tank where it can be used
for non-potable water uses and on-site infiltration.
The harvesting system is assumed to consist of a
given number of fixed-sized cisterns per 1000 square
feet of rooftop area captured.
The water from each cistern is withdrawn at a
constant rate and K assumed to be consumed or
infiltrated entirely on-site.
Cistern Size (gallons)
Emptying Rate (gallons/day)
Number per 1,000 sq ft
500
100
3.0
Learn more.
Rain Garden
Rain Gardens are shallow depressions filled with an
engineered soil mix that supports vegetative growth.
They are usually used on individual home lots to
capture roof runoff.
Typical soil depths range from 6 to 18 inches.
The Capture Ratio is the ratio of the rain garden's
area to the impervious area that drains onto it.
Ponding Height (inches)
Soil Media Thickness (inches)
Soil Media Conductivity (in/hr)
% Capture Ratio
Learn more.
Figure 24. Design parameters for Rain Harvesting and Rain Garden controls.
48
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Infiltration Basin
Basin Depth (inches)
% Capture Ratio
Infiltration basins are shallow depressions filled with
grass or other natural vegetation that capture runoff
from adjoining areas and allow it to infiltrate into
the soil,
The calculator assumes that the infiltration rate from
the basin is the same as for site's native soil.
The basin's Capture Ratio is the area of the basin
relative to the impervious area whose runoff it
captures.
41
Learn more...
Porous Pavement
Pavement Thickness (inches)
Gravel Layer Thickness (inches)
% Capture Ratio
Continuous Porous Pavement systems are excavated
areas filled with gravel and paved over with a porous
concrete or asphalt mix.
Modular Block systems are similar except that
permeable block pavers are used instead.
Normally all rainfall will immediately pass through
the pavement into the gravel storage layer below it
where it can infiltrate at natural rates into the site's
native soil.
Pavement layers are usually 4 to 6 inches in height
Learn more...
Figure 25. Design parameters for the Infiltration Basin and Porous Pavement controls.
49
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Re-running the calculator for the developed site with LID controls produces the summary results shown
in Table 8. The site now comes very close to matching the pre-development hydrology. It has only one
more day per year, on average, with runoff than does the pre-developed site and only one more inch of
annual runoff. Figure 26 shows that the runoff frequency of the controlled site is quite close to the pre-
developed site. Figure 27 shows an almost identical contribution of different size storms to runoff
between the two. Finally, from Figure 28 we see that with this extensive use of LID controls the site
could meets the 1.3 inch retention standard at the required 95% level of confidence.
Table 8. Runoff statistics for pre-development (Baseline) and post-development with LID controls
(Current) scenarios.
Statistic
Average Annual Rainfall (inches)
Average Annual Runoff (inches)
Days per Year With Rainfall
Days per Year with Runoff
Percent of Wet Days Retained
Smallest Rainfall w/Runoff (inches)
Largest Rainfall w/o Runoff (inches)
Max Rainfall Retained (inches)
Current Scenario
47.01
5.12
71.00
7.74
89.09
0.14
1.92
2.96
Baseline Scenario
47.01
4.08
71.00
6.80
90,43
0.14
2.42
3.25
50
-------
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c
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tainfall
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v—
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ff Exc<
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•
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;edanc
unoff
i
*
*
•--..
*v, .
;e Frequency
* . .
asclinc Rainfall
23456769
Depth (inches)
Figure 26. Daily runoff frequency curves for pre-development (Baseline) and post-
development with LID controls (Current) conditions.
51
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Runoff Contribution by Rainfall Percentile
Daily Rainfall Depth (inches)
Percentage of Total Runoff
-». W CO * tjl Q) —J
0 0 0 0 0 0 0 0
0,14 0,20 0.26 0,35 0,43 0.54 0.69 0.81 0.98 1 15 1 37
i i i i i
d
1
:• Ci
^3 B;
trrent
iselim
Scens
; Seen
no
ario
n
i
n rt
m
\
,-nl
1.75 3 51
10 20 30 40 50 60 70 75 80 85 90
Daily Rainfall Percentile
• 1
i
95 99
Figure 27. Contribution to total runoff by different magnitude storms for pre-development
(Baseline) and post-development with LID controls (Current) conditions.
52
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Rainfall Retention Frequency
— Current Scenario — Baseline Scenario
102
y
C 5
1.0 1.5 2.0 2,5 3,0
Daily Rainfall (inches)
3.5
j ;i
Figure 28. Retention frequency plots under pre-development (Baseline) and post-
development with LID controls (Current) conditions.
Climate Change Impacts
As a final step in our analysis of the example site we will calculate what impact a future change in local
climate might have on the ability of the LID practices we installed to control runoff. Figure 29 is the
Climate Change page for our site, showing how different scenarios projected to the year 2060 affect
monthly rainfall levels and extreme storm events. Observe that the Warm/Wet scenario results in
higher average rainfall while the Hot/Dry scenario produces slightly larger extreme storms. To provide
the largest climate change impact we will select the Warm/Wet scenario for this example.
Because we want to compare the effect that a future Warm/Wet rainfall pattern has on the developed
site with LID controls to the previous run that used the historical rainfall record, we return to the Results
page and remove the previous Baseline Scenario (the one for the pre-developed site) and replace it with
the most current set of results - the one for the developed site with LID controls analyzed for the
historical rainfall record. We then re-run the analysis, using our same set of LID designs but now subject
to changes in the rainfall record that reflect a Warm/Wet future climate condition.
53
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Select a future climate change scenario
to apply:
No change
O Hot/Dry
O Median change
•»• Warm/Wet
Select the year to which the climate
change scenario is projected:
2035 •» 2060
Helc
Percentage
--T
10
30
20 -
10 -
0
-10 -
-20 -
-30 -
-40
-50
^
- ^
A--
T -
Jan
Change in Monthly Rainfall for 2060 Projections
-- Hot/Dry — *•-- Median --•&-- Warm/Wet
i i i
•4"~ A
k, A ^ ••''' !;-~~
s--"'*-*.^ ""'*"" ..-*' \V
" X>-""^ /;: v-.'-T-
T - ' ^— ' *
Fob Mar Apr May Jun Jul Aug Sep Oct Nov
id
\
Dec
Annual Max. Day Rainfall (inches) for 2060 Projections
--A--- Hot.'Dry --•*-- Median --V--- Warm/Wet —0- Historical
13
M -
9 -
7 -
5
3
...-::
5
!
,.J
: ...-.:^:^
*-"9--'~--~~ --' '
--~~"" ---^'tf'~~--~' '
111 15 30 50 100
Return Period (years)
Select a climate change scenario to use,
Analyze a New Site Save Current Site Exit
Figure 29. Climate change scenarios for the example site.
The resulting Summary Results report for the adjusted rainfall record is shown in Table 9. Remember
that the Current Scenario results represent the site response under the future set of climatic conditions
while the Baseline Scenario results are for historical conditions. We observe that the climate change
impact on the long term performance of the site is quite modest. Although annual rainfall increases by
4 inches (8.5 %}, there is only 1.6 additional inches of runoff per year and only one more day per year
with measureable runoff.
From the Rainfall / Runoff Frequency plot of Figure 30 we see that the distribution of daily rainfall
events between the two climate scenarios is quite similar for the smaller size storms but that storms
above 3 inches will occur more frequently for the future Warm/Wet scenario. (E.g., daily rainfalls
exceeding 4 inches have historically occurred only once every 3 years but are predicted to occur once
every 18 months in the future.) Regarding the retention target of 1.3 inches, the Rainfall Retention
Frequency plot of Figure 31 shows that under the future Warm/Wet scenario there is a drop of only one
percentage point in the probability of meeting the target (from 95 to 94 %).
54
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Table 9. Summary results under a Warm/Wet (Current) climate change scenario compared to the
historical (Baseline) condition.
Statistic
Average Annual Rainfall (inches)
Average Annual Runoff (inches)
Days per Year With Rainfall
Days per Year with Runoff
Percent of Wet Days Retained
Smallest Rainfall w/Runoff (inches)
Largest Rainfall w/o Runoff (inches)
Max. Rainfall Retained (inches)
Current Scenario
51.04
6.72
72.9C
3.99
87,66
0.16
2,02
3.03
Baseline Scenario
47.01
5.12
71.00
7.74
89.09
0.14
1.92
2.96
Rainfall / Runoff Exceedance Frequency
..... Rainfall — Runoff
..... Baseline Rainfall — Baseline Runoff
mn n
•o
0)
"S '0:i
a
0
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L.
n
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a.
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234567:
Depth (inches)
1 9 10
FigureSO. Daily rainfall and runoff frequencies for the historical (Baseline) and Warm/Wet climate
scenarios.
55
-------
Rainfall Retention Frequency
— Current Scenario — Baseline Scenario
102
2 3
Daily Rainfall finches)
Figure 30. Target event retention for the historical (Baseline) and Warm/Wet climate scenarios.
Finally, we can examine how the site performs when faced with extreme, high intensity rainfall events
that are expected to occur only once every five or more years. Figure 32 shows the Extreme Event
Rainfall / Runoff report for the developed site subjected to the two climate scenarios. We observe that
there is only a minor increase in estimated rainfall amounts for all return periods under the Warm/Wet
scenario as compared to the baseline historical scenario. These amounts simply mirror the numbers
displayed on the Climate Change page of the calculator for this site (see Figure 29). None of these
extreme event storms can be completely captured by the LID controls deployed on the site. But this is
to be expected since the LID controls were only designed to capture up to 1.3 inches of rainfall. The
increase in the amount of bypassed rainfall under the future Warm/Wet scenario compared to the
historical record appears to be proportional to the difference in the amount of rainfall between the two.
56
-------
Extreme Event Rainfall / Runoff
I Runoff I Baseline Rainfall
Rainfall
Baseline Runoff
•I Z
g 10
i=
u
a.
a
6 --
££
i_
•D
TO i
a
55 10 10 15 ^5 30 30 50 50
Return Period on Max. Day Rainfall (years)
111!! '00
Figure 32. Extreme event rainfall and runoff for the Warm/Wet climate change scenario and the
historical record (Baseline).
57
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The National Stormwater Calculator uses SWMM 5 (EPA, 2010) as its computational engine. SWMM is a
comprehensive model that addresses surface runoff, infiltration, groundwater, snow melt, stormwater
detention, and full dynamic wave flow routing within any configuration of open and closed channels.
Only its runoff, infiltration, and LID sub-models are used by the calculator. This section describes how
SWMM carries out its hydrology calculations, how the calculator sets up a SWMM model for the site
being analyzed, how it populates the parameter values needed to run the model, and how it post-
processes the results produced by SWMM.
SWMM allows a study area to be subdivided into any number of irregularly shaped subcatchment areas
to best capture the effect that spatial variability in topography, drainage pathways, land cover, and soil
characteristics have on runoff generation. An idealized subcatchment is conceptualized as a rectangular
surface that has a uniform slope and drains to a single outlet point or channel or to another sub-
catchment. Each subcatchment can be further divided into three subareas: an impervious area with
depression (detention) storage, an impervious area without depression storage and a pervious area with
depression storage. Only the latter area allows for rainfall losses due to infiltration into the soil.
SWMM uses a nonlinear reservoir model to estimate surface runoff produced by rainfall over each sub-
area of a subcatchment (Chen and Shubinski 1971). From conservation of mass, the net change in depth
per unit of time of water stored on the land surface is simply the difference between inflow and outflow
rates over the subcatchment:
dd . f
- = l-e-f-q (1)
where d= depth of water on the land surface, /= rate of rainfall + any runon from upstream
subcatchments, e= evaporation rate, /= soil infiltration rate, q= runoff rate and t = time. Note that the
fluxes i, e, ^"and ^are expressed as flow rates per unit area. By assuming that the overland flow across
the sub-area's width is normal, the Manning equation can be used to express the runoff rate gas:
1A9WS1/2 , , , xc/T
q= An (d-ds)5/3 (2)
where W= width of the subcatchment's outflow face, S= subcatchment slope, n = roughness
coefficient, A = subcatchment area and ds = depression storage depth. The latter represents initial
rainfall abstractions such as surface ponding, interception by vegetation, and surface wetting. Note that
no runoff occurs when d\s below ds. How the calculator sets values for the parameters in this equation
is discussed later on in this section.
Substituting (2) into (1) produces an ordinary non-linear differential equation that can be solved
numerically for dover a sequence of discrete time steps given externally imposed rainfall and
58
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evaporation rates and a computed infiltration rate / By knowing d, (2) can be evaluated to determine
the runoff 17at each time step.
SWMM 5 offers a choice of three different methods for computing soil infiltration rates - the Morton,
Green-Ampt and Curve Number models. The Green-Ampt method was chosen for use in the calculator
because it is based on physical parameters that can be related to the site's soil type. SWMM uses the
well-known Mein-Larson form of this model (Mein and Larson, 1973):
(3)
where Ks= saturated hydraulic conductivity, (j)= soil porosity, 60 = initial soil moisture content, l//=
suction head at the wetting front, and F= cumulative infiltration volume. Equation (3) applies after a
sufficient time has elapsed to saturate the top layer of soil. During wet periods the moisture content of
the uppermost layer of soil increases at a rate of f/Lu where Lu is the layer depth equal to
(for Lu in inches and Ks in in/hr). During dry periods the moisture content decreases at a rate of
where the rate constant Aris estimated as ^Ks/75 . At the start of the next wet period do is set equal
to the current moisture content.
SWMM's LID Model
SWMM 5 has been extended to explicitly model several types of LID practices (Rossman, 2009).
Consider a typical bio-retention cell in the form of a street planter as shown in the left panel of Figure
33. Conceptually it can be represented by a series of three horizontal layers as depicted in the figure's
right panel.
Rainfall ET
Overflow f t
Run on
p
^
f /
Surface Layer
Soil Layer
Storage Layer
Infiltratic
Percolat
n
<>
Underdrain
on
Infiltration
Figure 31. Conceptual representation of a bio-retention cell.
59
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The surface layer receives both direct rainfall and run-on from other areas. It loses water through
infiltration into the soil layer below it, by evaporation of any water stored in depression storage and
vegetative capture, and by any surface runoff that might occur. The soil layer contains an amended soil
mix that can support vegetative growth. It receives infiltration from the surface layer and loses water
through evaporation and by percolation into the storage layer below it. The storage layer consists of
coarse crushed stone or gravel. It receives percolation from the soil zone above it and loses water by
either infiltration into the underlying natural soil or by outflow through a perforated pipe under drain
system.
The hydrologic performance of this LID unit can be modeled by solving the mass balance equations that
express the change in water volume in each layer over time as the difference between the inflow water
flux rate and the outflow flux rate. The equations for the surface layer, soil layer, and storage layer can
be written as
dd-\ . ,,
— =i + q0-ei-fi-qi (4)
f f
= A - «z - /z (5)
/2-/3-
-------
To analyze a site's hydrology without any LID controls, the calculator creates a single SWMM
subcatchment object and populates it with the following parameter values:
Site Area:
A nominal area of 10 acres is used. As mentioned earlier, because all results are expressed per unit of
area, there is no need to use an actual site area.
Width:
This is the width of the outflow face of a conceptual rectangular plane over which runoff flows. In most
SWMM models, it is initially set to the site area divided by the length of the overland flow path that
runoff follows, and is then refined by calibration against measured runoff hydrographs.
When assigning an overland flow path length, particularly for sites with natural land cover, one must
recognize that there is a maximum distance over which true sheet flow prevails. Beyond this, runoff
consolidates into rivulet flow with much faster travel times and less opportunity for infiltration.
There is no general agreement on what distance should be used as a maximum overland flow path
length. The NRCS recommends a maximum length of 100 ft (USDA, 2010), while Denver's Urban
Drainage and Flood Control District uses a maximum of 500 ft. (UDFCD, 2007). For the calculator, a
conservative value of 150 ft is used. The resulting width parameter for the SWMM input file is therefore
set to the nominal area (10 acres) divided by this length.
Slope:
A value of 2% is used for flat slopes, 5% for moderately flat slopes, 10% for moderately steep slopes, and
20% for steep slopes.
Percent Impervious:
SWMM only considers two types of land surfaces - impervious and pervious - each with its own
depression storage depth and surface roughness parameters. It does not explicitly consider the
different types of land covers that comprise these two categories and how their characteristics affect
depression storage and roughness. Impervious surfaces, such as roads, roofs, sidewalks, and parking
lots show minor variation in these parameters; it is therefore acceptable to treat them as a single
category.
To provide more refinement in characterizing pervious areas, the calculator allows the user to specify
the percentage of the site's area devoted to four different sub-categories of land surface cover: Forest,
Meadow, Lawn, and Desert. These sub-categories were chosen from a distillation of categories used in
the Western Washington Hydrology Model (Clear Creek Solutions, Inc, 2006) and the National Green
Values Calculator (Center for Neighborhood Technology, 2009). The remaining area is assigned as
Impervious Cover.
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Depression Storage Depth:
Depression storage corresponds to a depth that must be filled prior to the occurrence of any runoff. It
represents initial abstractions such as surface ponding, interception by flat roofs and vegetation, and
surface wetting. Separate values are supplied for the pervious and impervious areas of a catchment.
Depression storage for impervious surfaces is relatively small, ranging from 0.05 to 0.1 inches (ASCE,
1992). For the remaining pervious area, the calculator uses an area-weighted average of the storages
associated with each type of pervious land surface that covers the site. Table 10 contains depression
storage depths that have been suggested by different organizations for each land cover category. The
last column contains the value used in the calculator.
Table 10. Depression storage depths for different land covers.
Land Cover
Forest
Meadow
Lawn
Desert
Impervious
ASCE (1992)
0.3
0.2
0.1-0.2
0.05-0.1
UDFCD (2006)
0.4
0.35
0.05-0.1
USDA (2010)a
0.53
0.56
0.50
0.27
0.04
Calculator
0.40
0.30
0.20
0.25
0.05
a Set equal to the initial abstraction computed for the land cover's Curve Number and a Group D
soil (to minimize any contribution from infiltration).
Roughness Coefficient:
The roughness coefficient reflects the amount of resistance that overland flow encounters as it runs off
of the land surface. SWMM uses separate values for the impervious and pervious areas of a catchment.
Table 11 lists roughness coefficients published by several different sources for each land cover category,
along with those selected for use in the calculator. The value presented to SWMM, as representative of
the site's pervious area, is the area-weighted average of the roughness for each land cover category.
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Table 11. Roughness coefficients for different land covers.
Land Cover
Forest
Meadow
Lawn
Desert
Impervious
SWMa
0.4
0.2-0.35
0.01-0.014
Engmanb
0.01-0.32
0.3-0.63
0.01-0.013
Yenc
0.06-0.12
0.04-0.18
0.03-0.12
0.032-0.045
0.01-0.025
Calculator
0.40
0.20
0.30
0.04
0.01
a Stanford Watershed Model (Crawford and Linsley, 1966)
b Engman (1986)
c Yen (2001)
Percent of Impervious Area without Depression Storage:
This parameter accounts for immediate runoff that occurs at the beginning of rainfall before depression
storage is satisfied, caused by impervious areas immediately adjacent to storm drains. The calculator
assumes a value of 0 to give a maximum credit to the small amount of depression storage used for
impervious surfaces.
Infiltration Parameters:
There are three parameters required by the Green-Ampt infiltration model used in the calculator:
1. Saturated Hydraulic Conductivity (Ksat) - the rate at which water will infiltrate through a
completely saturated soil.
2. Suction Head (y) - capillary tension (force at which water is held within soil pores) at the
infiltration wetting front.
3. Initial Moisture Deficit (IMD) - the difference in moisture content between a completely wet
and completely dry (or drained) soil (i.e., the difference between the soil's porosity and its field
capacity)
Values for these parameters can be assigned based on soil group. Using the NRCS's definitions (USDA,
2010), an A soil is mostly sand, a 6 so/7 is typical of a sandy loam, a Csoil is like a clay loam, and a D soil is
mostly clay. Table 12 lists the average values of Ksat, y, and IMD for these four soil types from
measurements made from roughly 5,000 soils (of all types) across the U.S. (Rawls et al., 1983). Also
shown, are the values that were chosen for use in the calculator. Note that the calculator Ksat values
are defaults. The user can also use values extracted from the SSURGO data base or enter their own site-
specific numbers.
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Table 12. Infiltration parameters for different soil types.
Soil Type
Sand
Sandy Loam
Clay Loam
Clay
Rawls et al.
Ksat (in/h)
4.6
0.43
0.04
0.01
v|/(in)
1.95
4.33
8.22
12.45
IMD
0.38
0.26
0.15
0.10
Calculator
Ksat (in/h)
4.0
0.4
0.04
0.01
\|/(in)
2.0
4.3
8.2
12.5
IMD
0.38
0.26
0.15
0.10
Site Model with LID Controls
The basic SWMM model used by the calculator is extended when LID controls are applied to the site.
These extensions depend on the type of LID that is deployed.
Disconnection
A second subcatchment is added to the model when Disconnection is employed. Its impervious area
equals the fraction of the site's total impervious area that is disconnected, while its pervious area equals
the Capture Ratio times the latter area. Both of these areas are assigned the same parameters as the
original subcatchment, and the original subcatchment has its areas reduced to reflect the presence of
this second subcatchment. SWMM's option to internally route runoff from the impervious sub-area on
to the pervious sub-area is used with this subcatchment.
Infiltration Basin
An Infiltration Basin also adds an additional subcatchment to the model that contains the impervious
area treated by the basin plus a pervious area equal to the area of the basin. The impervious and
pervious areas of the original subcatchment are reduced accordingly. The impervious area in the new
subcatchment has the same parameter values as in the original subcatchment. However the pervious
area has its depression storage set equal to the Basin Depth as specified by the calculator user. Its
roughness coefficient is set to 0 which forces SWMM to treat any ponded water in excess of the Basin
Depth as immediate runoff. All runoff from the impervious sub-area is internally routed on to the
pervious (i.e., infiltration basin) sub-area. This setup is similar to that used for Disconnection, except
instead of allowing for sheet flow with infiltration across a pervious area it utilizes this area as an
infiltrating storage unit with overflow.
Rain Harvesting
This LID option is modeled by introducing an additional, completely impervious subcatchment whose
area is the portion of the original subcatchment impervious area that is captured by cisterns. This
amount of impervious area is subtracted from that of the original subcatchment. A new Storage Node
element is added into the SWMM model to represent the combined retention volume of the cisterns.
The added subcatchment sends its runoff to this storage node. The maximum depth of the storage node
64
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is set to a nominal height of 48 inches. Its surface area equals the area of its contributing subcatchment
times the number of cisterns per unit area (as supplied by the user) times the area per cistern. The
latter is found by dividing the user-supplied volume per cistern by the nominal depth. Note that any
nominal depth can be used since the area per cistern will adjust itself accordingly to maintain an equal
amount of total cistern storage volume. The rate at which the cisterns empty is converted into an
equivalent "infiltration" rate for the storage node, equal to the user-supplied emptying rate (in gal/day)
divided by the area per cistern. When the cisterns become full, any overflow shows up as node flooding
in SWMM, which gets added to the runoff from other portions of the site.
Other LID Controls
Rain Gardens, Green Roofs, Street Planters, and Porous Pavement do not require additional
subcatchments - they are all placed within the original subcatchment used to model the site. The
original pervious area of this subcatchment is reduced by the amount of area devoted to Rain Gardens,
while the original impervious area is reduced by the area taken up by any Green Roofs, Street Planters
and Porous Pavement.
LID Sizing
When the user supplies a design storm depth, the LID controls can be automatically sized to retain this
depth. For Rain Harvesting, the number of cisterns required per unit area is simply the design storm
depth divided by the volume of a cistern. For the other controls, the Capture Ratio (CR), which is the
ratio of the LID control area to the impervious area being treated, is computed as
^jj Dstorm
~~ Dlid-(Dstorm-0.5Ksat)
where Dstorm is the design storm depth (inches over 24 hours), Dlid is the storage depth (inches)
provided by the LID control, and Ksat\s the saturated hydraulic conductivity of the native soil
underneath the LID control (inches/day). The 0.5 factor accounts for the average amount of infiltration
occurring over the duration of the design storm. The LID storage depth ZV/c/consists of any ponding
depth plus the depths of any soil and gravel layers times their respective void fractions.
The SWMM model built by the calculator includes a single Rain Gage object that provides it with hourly
precipitation data. These data come from a nearby National Weather Service rain gage as selected by
the user. The calculator can access historical hourly rainfall data for 8,159 stations that are part of the
data holdings for EPA's BASINS system (http://water.epa.gov/scitech/datait/models/basins/index.cfm).
The data for each gage is contained in its own file on an EPA server, which is downloaded and made
available to the calculator. The national coverage provided by these gages is shown in Figure 34.
65
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Precipitation Locators
YrCount
SS-9
10-19
20*
States
Figure 32. NWS rain gage locations included in the calculator.
In addition to simulating a long term record of hourly rainfall, the calculator also computes the runoff
produced from a series of 24-hour rainfall events that represent extreme, high intensity storms with
different annual return periods. How the depths of these storms are estimated for each rain gage is
discussed in the Climate Change sub-section later on. To simulate each storm, the calculator uses the
NRCS (SCS) 24-hour distributions (USDA, 1986) to disaggregate the event's total rainfall depth into a
series of rainfall intensities (measured in inches per hour) at six minute intervals. Figure 35 shows the
different NRCS distributions and Figure 35 shows which distribution applies to each region of the US.
Each precipitation station is pre-assigned a distribution type (I, IA, II, or III) based on the region it falls in.
After the long term simulation is completed, the SWMM input file is modified as follows:
1. A time series object is added to the model which is the result of applying the appropriate SCS
distribution at a six minute interval to the total 24-hour rainfall amount being simulated.
2. The source of rainfall data for the model is set to the newly added time series.
3. The duration of the simulation is changed to three days starting on June 1.
4. After running the model, the only output recorded is the total runoff from the event.
These steps are repeated for each return period extreme event analyzed.
66
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1.0
2
-C
•*?
<*
0.0
II
I I I I I I I
III
3 6 9 12 15 18 21 24
Time (hours)
Figure 33. NRCS (SCS) 24-hour rainfall distributions (USDA, 1986).
TNH-
Type 1A
I I Type II
I I Type III
Figure 34. Geographic boundaries for the different NRCS (SCS) rainfall distributions (USDA, 1986).
67
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Evaporation Data
The BASINS holdings only include 329 stations with measured evaporation data more recent than
January 1, 2000 and at least a 5-year period of record. About 200 of the observed evaporation stations
appear to have missing data for some months of the year. Because of this sparseness of measured
evaporation, it was decided to generate evaporation values using daily temperature data from 5,236
weather stations across the U.S. which also measured hourly precipitation. The Penman-Monteith
algorithm was extracted from the SWAT model (Neitsch et al., 2005), and used to compute daily
potential evaporation from daily precipitation and min/max air temperature, along with generated solar
radiation, relative humidity, and wind speed. Additional details of this calculation can be found in the
Quality Assurance Report produced for this project by Aqua Terra Consultants (Aqua Terra Consultants,
2011). The locations for which evaporation rates were generated are displayed in Figure 37.
• =?•• •••i>:V:^^I;* ftftf $'• - :
^>ft"*£ffi'hTl:;:f:;y V
J'-V -T':;'jl.-,"*t V • / Ljf" • '
\-.' ^ •.••••::-.' i1 ' i^r1-—rt
Computed PMET
AnnualSum
D : - '-'-
1 I 30-40
B43- 50
50-60
60-70
70+
States
D
Figure 35. Locations with computed evaporation rates (Alaska and Hawaii not shown).
68
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The original result of these calculations was an average potential evaporation rate for each day of the
year (365 values) for each station. A sensitivity analysis was performed with the calculator to see what
effect there would be in using a monthly average value instead (12 values per station). Using the
monthly values produced annual runoff volumes that were only 2 to 5% different than those from the
daily values. It was therefore decided to use just the monthly average evaporation values for the
calculator. Each NWS station is identified by its latitude, longitude, and twelve monthly average
evaporation rates that are contained in a table built into the calculator. This table is used to supply
evaporation rates to the SWMM model constructed by the calculator.
The calculator obtains its climate change scenarios and their effect on local precipitation and
temperature directly from another EPA project called GREAT (Climate Resilience Evaluation and Analysis
Tool) (EPA, 2012). GREAT is a decision support tool to assist drinking water and wastewater utility
owners in understanding, evaluating and addressing climate change risks. It contains a database of
climate change effects across the US localized to a grid of 0.5 degrees in latitude and longitude (about
30 by 30 miles). These effects include changes in monthly average precipitation, monthly average
temperature, and extreme event 24-hour rainfall amounts for each of three different climate change
scenarios in two different future time periods.
GREAT uses statistically downscaled General Circulation Model (GCM) projections from the World
Climate Research Programme (WCRP) Coupled Model Intercomparison Project Phase 3 (CMIP3) archive
(Meehl et al., 2007) as the source of its climate change data. The CMIP3 archive was chosen by GREAT
because:
• it contains 112 runs from 16 internationally recognized models using several emission scenarios;
• it supported model-based analyses presented in the IPCC Fourth Assessment Report (IPCC,
2007);
• it facilitates the comparison and diagnosis of model outputs by standardizing many of the
assumptions and boundary conditions used;
• it is downscaled to appropriate spatial (regional, watershed) and temporal (monthly) scales
using a proven downscaling technique;
• it contains well-documented model output that is widely available to researchers;
• it has a high degree of scientific credibility and the archive encompasses a broad range of
assumptions concerning demography, economic integration, technological advance, energy use,
and greenhouse gas emissions.
GREAT limited its use of CMIP3 results to the nine GCM models that were most representative of US
climate conditions and used the IPCC's "middle of the road" projection of future economic growth. The
latter is characterized by (1) rapid economic growth, (2) global population that peaks in mid-century, (3)
the quick spread of new and efficient technologies, (4) the global convergence of income and ways of
life, and (5) a balance of both fossil fuel and non-fossil energy sources (IPCC, 2007).
69
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Each of the nine models produces a different set of results for each future year within each downscaled
7-i degree grid cell. To represent this type of uncertainty inherent in predicting future climate conditions,
GREAT defined three scenarios that span the range of results produced by the models for any given
projection year. The Warm/Wet scenario used the model that came closest to the 5th percentile of
annual temperature change and 95th percentile of annual rainfall change. The Median scenario selected
the model that was closest to the median temperature and rainfall changes. The Hot/Dry scenario used
the model that was closest to the 95th percentile temperature change and 5th percentile rainfall change.
Two different projection years were selected: 2035 and 2060.
15.4
12.7
10
7.3
4.7
0
CD
c
i 2
_c *•
0
CL
1
°- -0.7
-3.4
-6
-S.7
jjJBjMJI ||
P
f CCCM
— a j-
y
(jjRI
fiSpITl
H" i
^ECHO
11
^
4 3
3
9 4.2 4.5 4
Temp Change
5 5
(F)
2 5
5 5
B
Figure 36. CMIP3 2060 projected changes in temperature and precipitation for Omaha, NE (EPA, 2012).
An example of how the scenarios were defined is pictured in Figure 38 for the 2060 projections for the
grid cell containing Omaha, NE. In this figure, the square symbols are results from the nine different
climate models, the green circles are the target scenarios (5T/95P = warm/wet, 50T/50P = median, and
70
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95T/5P = hot/dry), and the three blue squares are the models selected for this particular location. Note
that the selection of which GCM model output goes with which scenario can change depending on grid
cell and projection year.
Once the model output to use for each scenario in each projection year in each grid cell was identified,
GREAT extracted its CMIP3 results to produce a database of percent changes in monthly average
precipitation and absolute changes in monthly average temperature for each scenario in each of the two
projection years in each grid cell across the US. For precipitation impacts, the stormwater calculator
used this data to construct a table for each combination of climate scenario and projection year (six in
total) containing the change in monthly (January - December) average precipitation for each of its 8,159
rain gages. When the calculator runs SWMM to evaluate the long-term rainfall / runoff for a site under
a particular climate change scenario, it first creates a new hourly rainfall file from the original one
downloaded from the EPA server. In this new file each historical hourly rainfall is adjusted by the
percent change (up or down) for the gage and month of the year contained in the appropriate climate
change scenario table.
Regarding temperature changes, the monthly changes in CREAT's database were used to generate new
sets of monthly average evaporation rates for the calculator. The same procedure described earlier,
using the SWAT model's Penman-Monteith procedure, was used to compute bare soil evaporation rates
for each day of temperature recorded at 5,236 different NWS weather stations. However now the daily
temperatures were first modified by applying the monthly temperature changes belonging to the
climate change scenario for the grid cell in which the weather station was located. The multi-year daily
evaporation values were then averaged into a set of twelve daily rates, one for each month of the year.
This process was repeated for each climate change scenario and projection year at each weather station
location. The result was another set of six tables, each containing a set of modified monthly evaporation
rates for all weather stations for a particular scenario and projection year.
It turned out that the climate change modified evaporation rates showed little variation between the
different scenarios for a given month at any particular location, with most differences being 0.02
inches/day or less. One possible reason for this is that climate change effects for the other variables
that influence the Penman-Monteith estimates, such as wind speed, relative humidity, and solar
radiation, were not considered. Even though the variations are slight, the tables were still constructed
and utilized for each of the climate scenarios as was done for monthly precipitation. The monthly
evaporation rates appearing in the table for the user's choice of climate change scenario are inserted
into the SWMM input file for a particular site instead of the rates based on historical temperatures.
The third climate-influenced outcome that the calculator considers is the change in the size and
frequency of intense precipitation events. GREAT considered this effect of climate change by fitting a
Generalized Extreme Value (GEV) probability distribution to the collection of annual maximum 24-hour
(midnight to midnight) rainfall amounts over a 30 year period simulated by the CMIP3 GCM used for
each scenario. Under the cumulative GEV distribution, the annual maximum daily rainfall amount x that
is exceeded only once every /years is:
71
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+Onl -)-* (8)
where u, is a location parameter, a is a scale parameter, and £ is a shape parameter. These GEV
parameters can be estimated from a series of annual data.
GREAT estimated GEV parameters for both the historical record and all six of the future climate
scenarios for each rain gage location in the calculator's database. From these parameters, values of the
annual maximum 24-hour rainfall depths for return periods of 5, 10, 15, 30, 50, and 100 years were
calculated using Eq. 8 and were placed in a set of seven tables, one for the historical record and six for
the future climate change scenarios (three different model outcomes in each of two future years). Each
set of extreme event storms corresponding to the six return periods for either the historical record or
for a future climate change scenario at a given rain gage location was simulated in SWMM using the
procedure described earlier in the Precipitation Data sub-section of this guide.
For the long-term continuous simulation of rainfall / runoff, the calculator runs its site model through
SWMM using a 5 minute computational time step over each year of the period of record selected by the
user, and requests that SWMM use a 15 minute reporting interval for its results. SWMM writes the
rainfall intensity and the runoff results it computes at this reporting interval to a binary output file. The
calculator then reads this output file and aggregates rainfall and runoff into daily totals, expressed as
inches, for each day of the simulation period. It also keeps track of how many previous days with no
measureable rainfall occur for each day with measureable rainfall. Measureable rainfall and runoff is
taken as any daily amount above the user-supplied threshold (whose default is 0.1 inches). For days
that have runoff but no rainfall, the runoff is added to that of the previous day. After the aggregation
process is complete, the long-term simulation results have been distilled down into a set of records
equal in number to the number of days with measureable rainfall; where each record contains a daily
rainfall, daily runoff, and number of antecedent dry days.
For extreme 24-hour storm events, SWMM makes a separate run for each event over a three day time
period to allow for LID storage to drain down. Each run has different values in its time series of rainfall
intensities reflecting the different total depth associated with each extreme event return period. For
these runs the only output recorded is the total runoff from the site.
The Summary Results report produced by the calculator (refer to Figure 13) comes from a direct
inspection of the long term daily rainfall/runoff record. The Maximum Retention Volume statistic is
simply the largest difference between daily rainfall and its corresponding runoff among all records.
The Rainfall / Runoff Event scatter plot (see Figure 14) is generated by plotting daily each daily rainfall
and its associated runoff for those days where rainfall exceeds the user-supplied threshold limit. For
wet days where the runoff is below the threshold value, the runoff value is set to zero (i.e., there is no
measureable runoff for those days).
72
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The Rainfall / Runoff Frequency report (see Figure 15) is generated by first sorting daily rainfall values
by size, ignoring consecutive rainfall days if the user selected that option. The days per year for which
each rainfall value is exceeded, is computed as (N -j)/Y, where N is the total number of rainfall values,
j is the rank order of the rainfall in the sorted list, and Y is the total years simulated. Then each rainfall -
exceedance frequency pair is plotted. The same set of operations is used to generate the runoff
exceedance frequency curve, except now N is the total number of runoff values and/ is the rank order of
a runoff value in the sorted list.
The Runoff by Rainfall Percentile report (see Figure 17) is generated as follows:
1. The daily measureable rainfall values are sorted by size and a set of different percentile values
are identified (the 10, 20, 30, 40, 50, 60, 70, 75, 80, 85, 90, 95, and 99-th percentiles).
2. The days with rainfall that fall within each percentile interval are identified, honoring the user's
choice to either include or exclude consecutive wet days.
3. The total runoff from events in each interval, as a percentage of the total runoff from all events,
is computed and plotted.
The Rainfall Retention Frequency report (see Figure 16) is generated by taking the same set of rainfall
percentiles used in the Runoff by Rainfall Percentile report, only referring to them as retention
volumes. For each retention volume, the percentage of daily rainfall events providing that amount of
retention is computed. This is done by examining each day with observable rainfall, ignoring back to
back wet days if that option was selected. If there was no measureable runoff for the day, then the
count of retained events for the retention volume being analyzed is incremented. Otherwise, if the
rainfall was at least as much as the target retention and the difference between rainfall and runoff was
also at least this much, then the count of retained events is also incremented. The retention provided
for the given retention target is simply the number of retained events divided by the total number of
daily events. This process is repeated for each of the thirteen pre-selected retention volumes and the
resulting pairs of retention volume - retention frequency values are plotted.
The Extreme Event Rainfall / Runoff report (see Figure 18) is generated by simply plotting the rainfall
and accompanying computed runoff in stacked fashion for each extreme event return period.
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7. References
American Society of Civil Engineers (ASCE) (1992). "Design and Construction of Urban Stormwater
Management Systems." American Society of Civil Engineers, New York, NY.
Aqua Terra Consultants (2011). "Quality Assurance Project Plan - Stormwater Calculator Technical
Support."EPA Contract #EP-C-06-029, Work Assignments #4-38 and 5-38.
Center for Neighborhood Technology (2009). "National Stormwater Management Calculator."
Chen, C.W. and Shubinski, R.P. (1971). "Computer Simulation of Urban Storm Water Runoff." Journal of
the Hydraulics Division. ASCE, 97(2):289-301.
Clear Creek Solutions, Inc. (2006). "Western Washington Hydrology Model Version 3.0 User Manual."
Crawford, N.H. and Linsley, R.K. (1966). "Digital Simulation in Hydrology: Stanford Watershed Model IV."
Technical Report No. 39, Civil Engineering Department, Stanford University, Palo Alto, CA.
Engman, E.T. (1986). "Roughness Coefficients for Routing Surface Runoff." Journal of Irrigation and
Drainage Engineering, ASCE, 112(l):39-53.
IPCC (Intergovernmental Panel on Climate Change) (2007). Climate Change 2007: Synthesis Report -
Summary for Policymakers. Available online at:
http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr.pdf
Karl, T.R., Melillo, J.M., and Peterson, T.C. (eds.) (2009). Global Climate Change Impacts in the United
States. Cambridge University Press.
Meehl, G. A., C. Covey, T. Delworth, M. Latif, B. McAvaney, J. F. B. Mitchell, R. J. Stouffer, and K. E.
Taylor. (2007). "The WCRP CMIP3 multi-model dataset: A new era in climate change research", Bulletin
of the American Meteorological Society, 88, 1383-1394.
Mein, R.G. and Larson, C.L. (1973). "Modeling infiltration during a steady rain." Water Resources
Research, 9(2):334-394.
Neitsch S.L, J.G. Arnold, J.R. Kiniry, and J.R. Williams (2005). "Soil and Water Assessment Tool
Theoretical Documentation." Version 2005, Agricultural Research Service and Texas Agricultural
Experiment Station, January 2005.
Rawls, W.J., Brakensiel, D.L, and Miller, N. (1983). "Green-Ampt Infiltration Parameters from Soils
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