EPA/600/R-13/085c | Revised September 2014 | www2.epa.gov/research

National Stormwater Calculator

User’s Guide

Before After

Office of Research and Development

National Risk Management Research Laboratory
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
DISCLAIMER

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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ACKNOWLEDGEMENTS

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

CREAT = Climate Resilience Evaluation and Awareness Tool

EPA = United States Environmental Protection Agency

GCM = General Circulation Model

GEV = Generalized Extreme Value

GI = 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

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TABLE OF CONTENTS

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

LIST OF FIGURES

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

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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

List of Tables

Table 1. Definitions of Hydrologic Soil Groups (USDA, 2010).....................................................................15

Table 2. Descriptions of LID practices included in the calculator. ..............................................................38

Table 3. 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

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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 predevelopment, 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 

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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-stormwatercalculator. 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. 

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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
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.

Location

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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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.
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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/survey/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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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.
Table 1. Definitions of Hydrologic Soil Groups (USDA, 2010).

Group Meaning

Saturated 

Hydraulic

Conductivity

(in/hr)

A 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

B 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

C 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

D 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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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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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. 
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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. 
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.

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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. 
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.

Climate Change

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 

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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.

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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/impactsadaptation/ (U.S. EPA, 2014) and at http://www.globalchange.gov/explore/ (USGRP, 2014)). 
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:

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Figure 10. The calculator's Land Cover page.
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 Controls

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 (GI) 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. 

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Figure 11. The calculator’s LID Controls page.

Green infrastructure (GI) 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 GI Statement of 

Intent, EPA intends the term GI 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. 

GI 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.

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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. 
25

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.
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: B

Conductivity: 0.4 inches/hour

Surface Slope: 5%

Rainfall Station: Nearest cataloged station 

Evaporation Station: Nearest cataloged station

Climate Change Scenario: None

Land Cover: 40% Lawn, 60% impervious

LID Controls: None

Years to Analyze: 20

Event Threshold: 0.10 inches

Ignore Consecutive Days: No

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3. Interpreting the Calculator’s Results

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.

Summary Results

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.

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28

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. 
• 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.

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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.
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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.
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.

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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.

32

Figure 16. The calculator’s Rainfall Retention Frequency report.
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).

Runoff by Rainfall Percentile

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
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). 

34

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.
Extreme Event Rainfall/Runoff

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.

Printing Output Results

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.

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36

Figure 18. The calculator’s Extreme Event Rainfall / Runoff report. 
4. Applying LID Controls

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 GI 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.

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Table 2. Descriptions of LID practices included in the calculator.

LID Practice Description

Disconnection

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.

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 onsite infiltration. 

Rain Gardens

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.

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.

Infiltration Basins

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.

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.

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39

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.
Table 3. Editable LID parameters. 

LID Type Parameter Default Value

Disconnection Capture Ratio 100 %

Rain Harvesting Cistern Size 100 gal

Cistern Emptying Rate 50 gal/day

Number of Cisterns 4 per 1,000 sq ft

Rain Gardens Capture Ratio 5 %

Ponding Depth 6 inches

Soil Media Thickness 12 inches

Soil Media Conductivity 10 inches/hour

Green Roofs Soil Media Thickness 4 inches

Soil Media Conductivity 10 inches/hour

Street Planters Capture Ratio 6 %

Ponding Depth 6 inches

Soil Media Thickness 18 inches

Soil Media Conductivity 10 inches/hour

Gravel Bed Thickness 12 inches

Infiltration Basins Capture Ratio 5 %

Basin Depth 6 inches

Porous Pavement Capture Ratio 100 %

Pavement Thickness 4 inches

Gravel Bed Thickness 18 inches

Table 4. Void space values of LID media. 

Property LID Controls Default Value

Soil Media Porosity Rain Gardens, Green Roofs and Street Planters 45 %

Gravel Bed Void Ratio Street Planters and Porous Pavement 75 %

Pavement Void Ratio Porous Pavement 12 %

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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 postdevelopment 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 predevelopment 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
Table 5. Summary results for pre-development conditions on the example site.

42

Figure 20. Runoff from different size storms for pre-development conditions on the example site.
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.

43

Figure 21. Rainfall retention frequency under pre-development conditions for the example site.
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 % of Total Area % of Impervious Area

Forest 18 -

Meadow 8 -

Lawn 25 -

Total Impervious Surfaces 49 100

 Roofs 10 20

 Parking 9 20

 Roads & Sidewalks 30 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
Table 7. Comparison of runoff statistics for post-development (Current) and pre-development 

(Baseline) conditions.

45

Figure 22. Rainfall retention frequency for pre-development (Baseline) and post-development 

(Current) conditions.
Post-Development with LID Practices

We will now add some LID practices to our example site to see how well they can make its postdevelopment 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
47

Figure 23. Low Impact Development controls applied to the example site.
48

Figure 24. Design parameters for Rain Harvesting and Rain Garden controls.
49

Figure 25. Design parameters for the Infiltration Basin and Porous Pavement controls. 
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 predeveloped 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. 

50
51

Figure 26. Daily runoff frequency curves for pre-development (Baseline) and postdevelopment with LID controls (Current) conditions. 
52

Figure 27. Contribution to total runoff by different magnitude storms for pre-development 

(Baseline) and post-development with LID controls (Current) conditions. 
53

Figure 28. Retention frequency plots under pre-development (Baseline) and postdevelopment 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.
54

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 %). 
Table 9. Summary results under a Warm/Wet (Current) climate change scenario compared to the 

historical (Baseline) condition. 

55

Figure30. Daily rainfall and runoff frequencies for the historical (Baseline) and Warm/Wet climate 

scenarios.
56

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. 
57

Figure 32. Extreme event rainfall and runoff for the Warm/Wet climate change scenario and the 

historical record (Baseline). 
6. Computational Methods

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 postprocesses the results produced by SWMM.

SWMM’s Runoff Model

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 subcatchment. 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 subarea 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:

∂∂∂∂

∂∂∂∂ = ii − ee − ff − qq (1) 

where d = depth of water on the land surface, i = rate of rainfall + any runon from upstream 

subcatchments, e = evaporation rate, f = soil infiltration rate, q = runoff rate and t = time. Note that the 

fluxes i, e, f, and q 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 q as:

qq = 11.444444SS11/22

AAAA (∂∂ − ∂∂ss)55/33 (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 is 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 d over a sequence of discrete time steps given externally imposed rainfall and 

58
evaporation rates and a computed infiltration rate f. By knowing d, (2) can be evaluated to determine 

the runoff q at each time step. 

SWMM 5 offers a choice of three different methods for computing soil infiltration rates – the Horton, 

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):

ff = KKss �11 + (φφ−θθ00)(∂∂+ψψ)

FF � (3) 

where Ks = saturated hydraulic conductivity, φ = soil porosity, θ0 = initial soil moisture content, ψ = 

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 ff/LLuu where Lu is the layer depth equal to 44/√KKss 

(for Lu in inches and Ks in in/hr). During dry periods the moisture content decreases at a rate of krθ0

where the rate constant kr is estimated as �KKss/77 . At the start of the next wet period θ0 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.

59

Figure 31. Conceptual representation of a bio-retention cell.
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 

∂∂∂∂11

∂∂∂∂ = ii + qq00 − ee11 − ff11 − qq11 

 

 

(4)

LL22

∂∂θθ22

∂∂∂∂ = ff11 − ee22 − ff22 (5)

φφ33

33

∂∂∂∂ = ff22 − ff33 − qq33

∂∂∂∂

(6) 

respectively, where d1 = depth of ponded surface water, θ2 = soil layer moisture content, d3 = depth of 

water in the storage layer, i = rainfall rate, q0 = upstream run-on rate, q1 = surface runoff flow rate, q3 = 

underdrain outflow rate, e1 = surface evaporation rate, e2 = soil zone evaporation rate, f1 = surface 

infiltration rate, f2 = soil percolation rate, f3 = native soil infiltration rate, L2 = depth of the soil layer, and 

φ3 = porosity of the storage layer.

The flux terms (q, e, and f ) in these equations are functions of the current water content in the various 

layers (d1, θ2, and d3) and specific site and soil characteristics. The surface and native infiltration rates 

are determined using the Green-Ampt model. The soil percolation rate decreases exponentially from Ks 

with decreasing soil moisture: ff22 = KKsseeeeee (−ρρ(φφ22 − θθ22)) where ρ is a percolation constant 

typically in the range of 5 to 15. Under drain outflow rate is modeled as a power function of head of 

water above the drain outlet: qq33 = αα( 33 − )ββ where α and β are constants and dd is the offset 

distance of the drain from the bottom of the unit.

This set of equations can be solved numerically at each runoff time step to determine how an inflow 

hydrograph to the LID unit is converted into some combination of runoff hydrograph, sub-surface 

storage, sub-surface drainage, and infiltration into the surrounding native soil. In addition to Street 

Planters and Green Roofs, the bio-retention model just described can be used to represent Rain Gardens 

by eliminating the storage layer and also Porous Pavement systems by replacing the soil layer with a 

pavement layer.

60
Site Model without LID Controls

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. 

61
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 ASCE (1992) UDFCD (2006) USDA (2010)a Calculator

Forest 0.3 0.53 0.40

Meadow 0.2 0.4 0.56 0.30

Lawn 0.1 – 0.2 0.35 0.50 0.20

Desert 0.27 0.25

Impervious 0.05 – 0.1 0.05 – 0.1 0.04 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.

62
Table 11. Roughness coefficients for different land covers.

Land Cover SWMa Engmanb Yenc Calculator

Forest 0.4 0.06 – 0.12 0.40

Meadow 0.01 – 0.32 0.04 – 0.18 0.20

Lawn 0.2 – 0.35 0.3 – 0.63 0.03 – 0.12 0.30

Desert 0.032 – 0.045 0.04

Impervious 0.01 -0.014 0.01 – 0.013 0.01 – 0.025 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 (ψ) - 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 B soil is typical of a sandy loam, a C soil is like a clay loam, and a D soil is 

mostly clay. Table 12 lists the average values of Ksat, ψ, 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 sitespecific numbers.

63
Table 12. Infiltration parameters for different soil types.

Rawls et al. Calculator

Soil Type Ksat (in/h) ψ (in) IMD Ksat (in/h) ψ (in) IMD

Sand 4.6 1.95 0.38 4.0 2.0 0.38

Sandy Loam 0.43 4.33 0.26 0.4 4.3 0.26

Clay Loam 0.04 8.22 0.15 0.04 8.2 0.15

Clay 0.01 12.45 0.10 0.01 12.5 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
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

CCCC = DDss∂∂DDDDDD

DDDDii∂∂−(DDss∂∂DDDDDD−00.55KKssss∂∂)

 (7)

where Dstorm is the design storm depth (inches over 24 hours), Dlid is the storage depth (inches) 

provided by the LID control, and Ksat is 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 Dlid consists of any ponding 

depth plus the depths of any soil and gravel layers times their respective void fractions. 

Precipitation Data

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
66

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.
Figure 33. NRCS (SCS) 24-hour rainfall distributions (USDA, 1986).

67

Figure 34. Geographic boundaries for the different NRCS (SCS) rainfall distributions (USDA, 1986).
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. 

68

Figure 35. Locations with computed evaporation rates (Alaska and Hawaii not shown).
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.

Climate Change Effects

The calculator obtains its climate change scenarios and their effect on local precipitation and 

temperature directly from another EPA project called CREAT (Climate Resilience Evaluation and Analysis 

Tool) (EPA, 2012). CREAT 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.

CREAT 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 CREAT 

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. 

CREAT 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
Each of the nine models produces a different set of results for each future year within each downscaled 

1⁄2 degree grid cell. To represent this type of uncertainty inherent in predicting future climate conditions, 

CREAT 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. 

 

70

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 
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, 

CREAT 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. CREAT 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 Y years is: 

71
xx = μμ − �σσ

ξξ

� �11 + (llll �11 − 11

YY

�)−ξξ� (8)

where μ is a location parameter, σ is a scale parameter, and ξξ is a shape parameter. These GEV 

parameters can be estimated from a series of annual data. 

CREAT 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.

Post-Processing

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).

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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 j 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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