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Calculating credits for stormwater and rainwater harvest and use/reuse

This page provides a discussion of how harvest and use/reuse practices can achieve stormwater credits. It is assumed that captured water is applied as irrigation and that all irrigation water infiltrates. To view the credit articles for other BMPs, see the Related pages section.

Recommended pollutant removal efficiencies, in percent, for infiltration BMPs. Sources.
TSS TP PP DP TN Metals Bacteria Hydrocarbons
NOTE: Pollutant removal is 100 percent for the volume that is captured and infiltrated. If captured water is routed to a non-infiltrating bmp, removal is determined by that bmp.
TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen

Green Infrastructure: Stormwater and rainwater harvest and use systems can improve or maintain watershed hydrology, reduce pollutant loading to receiving waters, increase water conservation, reduce stress on existing infrastructure, and reduce energy consumption

Information: This manual currently does not contain information on models and calculators developed specifically for rainwater/stormwater harvest and use/reuse systems. Once that information is developed it will be incorporated into this page.

Credit refers to the quantity of stormwater or pollutant reduction achieved either by an individual best management practice (BMP) or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in

Contents

Overview

This schematic shows Example Stormwater Harvesting and Use System Schematic

Schematic illustrating the basic concepts for a harvest and use-reuse system. Stormwater or rainwater is stored in a cistern or pond and delivered outdoors or indoors through a distribution system.

Stormwater and rainwater harvest and use/reuse systems capture and store runoff. The stored water is typically utilized for irrigation. This water is assumed to infiltrate. Credits for these BMPs are therefore similar to credits for other infiltration practices in that all water applied for irrigation and pollutants in that water are credited. The methodology differs, however, in that the water is captured instantaneously, but use of the water is dependent on the irrigation rate rather than the soil infiltration rate, as is the case with infiltration best management practice (BMPs). The period of use is also during the growing season, meaning the generated credits only apply at that time. If harvested water is used indoors, it may be discharged to a sewer system, to a septic drainfield, or to another stormwater BMP. Credits for these vary and are discussed below.

Pollutant removal mechanisms

Harvest and use/reuse practices reduce stormwater volume and pollutant loads through infiltration of the captured and stored stormwater runoff into the native soil. All pollutants in infiltrated water are considered to be removed from the stormwater conveyance system. Because infiltration typically occurs on turf or other vegetated media, a wide variety of stormwater pollutants will be retained through secondary removal mechanisms including filtration, biological uptake, and soil adsorption through plantings and soil media (WEF Design of Urban Stormwater Controls, 2012). See Other Pollutants, for a complete list of other pollutants retained by filtration practices.

Location in the treatment train

Stormwater treatment trains are comprised of multiple Best Management Practices that work together to minimize the volume of stormwater runoff, remove pollutants, and reduce the rate of stormwater runoff being discharged to Minnesota wetlands, lakes and streams. The position of a harvest and use/reuse system in a treatment train is a function of the surface from which the water is being collected. Rainwater harvest systems, which are designed to collect water from rooftops, will generally be located near the beginning of the treatment train, while systems that store water in ponds will be located near the end of treatment trains.

Methodology for calculating credits

This section describes the basic concepts and equations used to calculate credits for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Specific methods for calculating credits are discussed later in this article. If harvest water is being infiltrated, this practice is also effective at reducing concentrations of other pollutants including nitrogen, metals, bacteria, and hydrocarbons. This article does not provide information on calculating credits for pollutants other than TSS and TP, but references are provided that may be useful for calculating credits for other pollutants.

Assumptions and approach

In developing the credit calculations, it is assumed the harvest and use/reuse system is properly designed, constructed, and maintained in accordance with the Minnesota Stormwater Manual. If any of these assumptions is not valid, the BMP may not qualify for credits or credits should be reduced based on reduced ability of the BMP to achieve volume or pollutant reductions. For guidance on design, construction, and maintenance, see Stormwater and rainwater harvest and use/reuse

Warning: Pretreatment is required for all infiltration practices

In the following discussion, the Water Quality Volume (VWQ) is delivered as an instantaneous volume to the BMP. VWQ is stored in a cistern or a pond. VWQ can vary depending on the stormwater management objective(s). For construction stormwater, VWQ is 1 inch off new impervious surface. For MIDS, VWQ is 1.1 inches.

The approach in the following sections is based on the following general design considerations.

  • Credit calculations presented in this article are for both event and annual volume and pollutant load removals.
  • Stormwater volume credit equates to the volume of runoff that will ultimately be infiltrated into the soil.
  • TSS and TP credits are achieved for the volume of runoff that is infiltrated.

Volume Credit Calculations

Volume credits are calculated based on the capacity of the BMP and its ability to permanently remove stormwater runoff via infiltration into the underlying soil from the existing stormwater collection system. These credits are assumed to be instantaneous values. However, unlike other stormwater infiltration practices, for an irrigation system, the volume credit is a function of both the water available for storage, the rate at which water is applied, and the area over which the water is applied.

If we assume that on average there are 3 days between rain events, the volume reduction capacity of the BMP (V) that counts toward a performance goal is equal to either the storage capacity of the storage device or the amount of water that is used for irrigation and non-irrigation over a three day period, whichever value is lowest.

$^V=min[S; A_I * R_I * 1556 + V_{nonirrigation}]^$

Where: 

S is the storage volume of the storage container in ft3;
AI is the irrigation application area in acres;
RI is the calculated average achieved weekly irrigation rate May-August (inches per week);
1556 is a conversion factor; and
Vnonirrigation is the volume used for non-irrigation purposes, in cubic feet.

This credit can only be applied during the time of year when the irrigation system is in practice. To determine compliance with a performance goal throughout the year, we need to know the annual volume of runoff and the volume of water applied as irrigation. The annual volume captured and infiltrated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator and the Simple Method. Example values are shown below for a scenario using the MIDS calculator. For example, if a harvest and use/reuse system captures and uses 68 percent of the annual runoff volume on B soils, the system is capturing the equivalent of 0.5 inches of runoff annually, even though it may be capturing considerably more during the time of year when the system is operating.

Annual volume treated as a function of soil and water quality volume
Annual volume, expressed as a percent of annual runoff, treated by a BMP as a function of soil and Water Quality Volume1 
Soil Water quality volume (VWQ) (inches)
0.5 0.75 1.00 1.25 1.50
A (GW) 84 92 96 98 99
A (SP) 75 86 92 95 97
B (SM) 68 81 89 93 95
B (MH) 65 78 86 91 94
C 63 76 85 90 93
1Values were determined using the MIDS calculator. BMPs were sized to exactly meet the water quality volume for a 2 acre site with 1 acre of impervious, 1 acre of forested land, and annual rainfall of 31.9 inches. 

 The above calculations may include nonirrigated uses. The nonirrigated uses will need to be translated into the correct units.

Total phosphorus credit calculations

Pollutant removal for infiltrated water is assumed to be 100 percent. The mass of pollutant removed through infiltration (MTP,i), in pounds, is given by

$^M_{TP,i} = 0.0000624\ V_{inf,b}\ EMC_{TP} ^$

Where: 

Vinf,b is the volume of water infiltrated (cubic feet), and 
EMCTP is the event mean TP concentration in runoff water entering the BMP (milligrams per liter). 
 

The EMCTP entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs. For more information on TP emcs link here. The above calculation may be applied on an annual basis and is given by

$^M_{TP_f} = 2.72\ V_{annual}\ EMC_{TP}^$

Where: 

Vannual is the annual volume treated by the BMP (acre-feet).

Total suspended solid (TSS) calculations

Pollutant removal for infiltrated water is assumed to be 100 percent. The mass of pollutant removed through infiltration (MTSS, i) in pounds, is given by

$^M_{TSS,i} = 0.0000624\ V_{inf,b}\ EMC_{TSS} ^$

Where: 

Vinf,b is the volume of water infiltrated (cubic feet), and
EMCTSS is the event mean TSS concentration in runoff water entering the BMP (milligrams per liter).

The EMCTSS entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs. For more information on EMC values for TSS, link here. The above calculation may be applied on an annual basis and is given by

$^M_{TSS_f} = 2.72\ F\ V_{annual}\ EMC_{TSS}^$

Where: 

Vannual is the annual volume treated by the BMP, in acre-feet.

The annual volume captured and infiltrated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator.

Methods for calculating credits

This section provides specific information on generating and calculating credits from infiltration practices for volume, TSS and TP. Stormwater runoff volume and pollution reductions (“credits”) may be calculated using one of the following methods:

  1. Quantifying volume and pollution reductions based on accepted hydrologic/hydraulic models
  2. The Simple Method and MPCA Estimator
  3. MIDS Calculator
  4. Quantifying volume and pollution reductions based on values reported in literature
  5. Quantifying volume and pollution reductions based on field monitoring

Credits based on models

Users may opt to use a water quality model or calculator to compute volume, TSS and/or TP pollutant removal for the purpose of determining credits for infiltration practices. The available models described in the following sections are commonly used by water resource professionals, but are not explicitly endorsed or required by the Minnesota Pollution Control Agency. Furthermore, many of the models listed below cannot be used to determine compliance with the Construction Stormwater General permit since the permit requires the water quality volume to be calculated as an instantaneous volume.

Use of models or calculators for the purpose of computing pollutant removal credits should be supported by detailed documentation, including:

  1. Model name and version
  2. Date of analysis
  3. Person or organization conducting analysis
  4. Detailed summary of input data
  5. Calibration and verification information
  6. Detailed summary of output data

The following table lists water quantity and water quality models that are commonly used by water resource professionals to predict the hydrologic, hydraulic, and/or pollutant removal capabilities of a single or multiple stormwater BMPs. The table can be used to guide a user in selecting the most appropriate model for computing volume, TSS, and/or TP removal for biofiltration BMPs. Sort the table by Infiltrator BMPs to identify BMPs that may include infiltration practices.

Stormwater model and calculator comparisons‏‎
Comparison of assessment capabilities for select stormwater models and calculators.
Model Name BMP Category Assess TP removal? Assess TSS removal? Assess volume reduction? Comments
Constructed basin BMPs Filter BMPs Infiltrator BMPs Swale or strip BMPs Reuse MTDs1
Center for Neighborhood Technology Green Values National Stormwater Management Calculator X X X   X   No No Yes Does not compute volume reduction for some BMPs, including cisterns and tree trenches.
CivilStorm             Yes Yes Yes CivilStorm has an engineering library with many different types of BMPs to choose from. This list changes as new information becomes available.
EPA National Stormwater Calculator X   X   X   No No Yes Primary purpose is to assess reductions in stormwater volume.
EPA SWMM X   X   X   Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
HydroCAD X   X X     No No Yes Will assess hydraulics, volumes, and pollutant loading, but not pollutant reduction.
infoSWMM X   X   X   Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
infoWorks ICM X X X X     Yes Yes Yes  
i-Tree-Hydro     X       No No Yes Includes simple calculator for rain gardens.
i-Tree-Streets             No No Yes Computes volume reduction for trees, only.
LSPC X   X X     Yes Yes Yes Though developed for HSPF, the USEPA BMP Web Toolkit can be used with LSPC to model structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
MapShed X X X X     Yes Yes Yes Region-specific input data not available for Minnesota but user can create this data for any region.
MCWD/MWMO Stormwater Reuse Calculator         X   Yes No Yes Computes storage volume for stormwater reuse systems
Metropolitan Council Stormwater Reuse Guide Excel Spreadsheet         X   No No Yes Computes storage volume for stormwater reuse systems. Uses 30-year precipitation data specific to Twin Cites region of Minnesota.
MIDS Calculator X X X X X X Yes Yes Yes Includes user-defined feature that can be used for manufactured devices and other BMPs.
MIKE URBAN (SWMM or MOUSE) X   X   X   Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
P8 X   X X   X Yes Yes Yes  
PCSWMM X   X   X   Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
PLOAD X X X X   X Yes Yes No User-defined practices with user-specified removal percentages.
PondNet X           Yes No Yes Flow and phosphorus routing in pond networks.
PondPack X   [       No No Yes PondPack can calculate first-flush volume, but does not model pollutants. It can be used to calculate pond infiltration.
RECARGA     X       No No Yes  
SHSAM           X No Yes No Several flow-through structures including standard sumps, and proprietary systems such as CDS, Stormceptors, and Vortechs systems
SUSTAIN X X X X X   Yes Yes Yes Categorizes BMPs into Point BMPs, Linear BMPs, and Area BMPs
SWAT X X X       Yes Yes Yes Model offers many agricultural BMPs and practices, but limited urban BMPs at this time.
Virginia Runoff Reduction Method X X X X X X Yes No Yes Users input Event Mean Concentration (EMC) pollutant removal percentages for manufactured devices.
WARMF X X         Yes Yes Yes Includes agriculture BMP assessment tools. Compatible with USEPA Basins
WinHSPF X   X X     Yes Yes Yes USEPA BMP Web Toolkit available to assist with implementing structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
WinSLAMM X X X X     Yes Yes Yes  
XPSWMM X   X   X   Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
Additional information and descriptions for some of the models listed in this table can be found at this link. Note that the Construction Stormwater General Permit requires the water quality volume to be calculated as an instantaneous volume, meaning several of these models cannot be used to determine compliance with the permit. 
1Manufactured Treatment Devices (MTDs)
Access this table as a Microsoft Word document: File:Stormwater Model and Calculator Comparisons table.docx or in an Excel workbook containing this table. 

The Simple Method and MPCA Estimator

The Simple Method is a technique used for estimating storm pollutant export delivered from urban development sites. Pollutant loads are estimated as the product of event mean concentration and runoff depths over specified periods of time (usually annual or seasonal). The method was developed to provide an easy yet reasonably accurate means of predicting the change in pollutant loadings in response to development. Ohrel (2000) states: "In general, the Simple Method is most appropriate for small watersheds (<640 acres) and when quick and reasonable stormwater pollutant load estimates are required". Rainfall data, land use (runoff coefficients), land area, and pollutant concentration are needed to use the Simple Method. For more information on the Simple Method, see The Simple method to Calculate Urban Stormwater Loads or The Simple Method for estimating phosphorus export.

Some simple stormwater calculators utilize the Simple Method (EPA STEPL, Watershed Treatment Model). The MPCA developed a simple calculator for estimating load reductions for TSS, total phosphorus, and bacteria. Called the MPCA Estimator, this tool was developed specifically for complying with the MPCA Estimator, this tool was developed specifically for complying with the annual reporting requirement. The MPCA Estimator provides default values for pollutant concentration, runoff coefficients for different land uses, and precipitation, although the user can modify these and is encouraged to do so when local data exist. The user is required to enter area for different land uses and area treated by BMPs within each of the land uses. BMPs include infiltrators (e.g. bioinfiltration, infiltration basin, tree trench, permeable pavement, etc.), filters (biofiltration, sand filter, green roof), constructed ponds and wetlands, and swales/filters. The MPCA Estimator includes standard removal efficiencies for these BMPs, but the user can modify those values if better data are available. Output from the calculator is given as a load reduction (percent, mass, or number of bacteria) from the original estimated load.

Caution: The MPCA Estimator should not be used for modeling a stormwater system or selecting BMPs.

Because the MPCA Estimator does not consider BMPs in series, makes simplifying assumptions about runoff and pollutant removal processes, and uses generalized default information, it should only be used for estimating pollutant reductions from an estimated load. It is not intended as a decision-making tool.

Download MPCA Estimator here

MIDS Calculator

mids logo

Download the MIDS Calculator

The Minimal Impact Design Standards (MIDS) best management practice (BMP) calculator is a tool used to determine stormwater runoff volume and pollutant reduction capabilities of various low impact development (LID) BMPs. The MIDS calculator estimates the stormwater runoff volume reductions for various BMPs and annual pollutant load reductions for total phosphorus (including a breakdown between particulate and dissolved phosphorus) and total suspended solids (TSS). The calculator was intended for use on individual development sites, though capable modelers could modify its use for larger applications.

The MIDS calculator is designed in Microsoft Excel with a graphical user interface (GUI), packaged as a windows application, used to organize input parameters. The Excel spreadsheet conducts the calculations and stores parameters, while the GUI provides a platform that allows the user to enter data and presents results in a user-friendly manner.

Detailed guidance has been developed for all BMPs in the calculator, including infiltration practices. An overview of individual input parameters and workflows is presented in the MIDS Calculator User Documentation.

Credits Based on Reported Literature Values

A simplified approach to computing a credit would be to apply a reduction value found in literature to the pollutant mass load or concentration (EMC) of the pond or wetland device. A more detailed explanation of the differences between mass load reductions and concentration (EMC) reductions can be found on the pollutant removal page here. Designers may use the pollutant reduction values or may research values from other databases and published literature. Designers who opt for this approach should

  • select the median value from pollutant reduction databases that report a range of reductions, such as from the International BMP Database;
  • select a pollutant removal reduction from literature that studied a bioretention device with site characteristics and climate similar to the device being considered for credits;
  • review the article to determine that the design principles of the studied bioretention are close to the design recommendations for Minnesota, as described in this manual and/or by a local permitting agency; and
  • give preference to literature that has been published in a peer-reviewed publication.

The following references summarize pollutant reduction values from multiple studies or sources that could be used to determine credits. Users should note that there is a wide range of monitored pollutant removal effectiveness in the literature. Before selecting a literature value, users should compare the characteristics of the monitored site in the literature against the characteristics of the proposed stormwater pond, considering such conditions as watershed characteristics, pond sizing, and climate factors.

Credits based on field monitoring

Field monitoring may be made in lieu of desktop calculations or models/calculators as described. Careful planning is HIGHLY RECOMMENDED before commencing a program to monitor the performance of a BMP. The general steps involved in planning and implementing BMP monitoring include the following.

  1. Establish the objectives and goals of the monitoring. When monitoring BMP performance, typical objectives may include the following.
    1. Which pollutants will be measured?
    2. Will the monitoring study the performance of a single BMP or multiple BMPs?
    3. Are there any variables that will affect the BMP performance? Variables could include design approaches, maintenance activities, rainfall events, rainfall intensity, etc.
    4. Will the results be compared to other BMP performance studies?
    5. What should be the duration of the monitoring period? Is there a need to look at the annual performance vs the performance during a single rain event? Is there a need to assess the seasonal variation of BMP performance?
  2. Plan the field activities. Field considerations include
    1. equipment selection and placement;
    2. sampling protocols including selection, storage, and delivery to the laboratory;
    3. laboratory services;
    4. health and Safety plans for field personnel;
    5. record keeping protocols and forms; and
    6. quality control and quality assurance protocols
  3. Execute the field monitoring
  4. Analyze the results

This manual contains the following guidance for monitoring.

The following guidance manuals have been developed to assist BMP owners and operators on how to plan and implement BMP performance monitoring.

Urban Stormwater BMP Performance Monitoring

Geosyntec Consultants and Wright Water Engineers prepared this guide in 2009 with support from the USEPA, Water Environment Research Foundation, Federal Highway Administration, and the Environment and Water Resource Institute of the American Society of Civil Engineers. This guide was developed to improve and standardize the protocols for all BMP monitoring and to provide additional guidance for Low Impact Development (LID) BMP monitoring. Highlighted chapters in this manual include:

  • Chapter 2: Developing a monitoring plan. Describes a seven-step approach for developing a monitoring plan for collection of data to evaluate BMP effectiveness.
  • Chapter 3: Methods and Equipment for hydrologic and hydraulic monitoring
  • Chapter 4: Methods and equipment for water quality monitoring
  • Chapters 5 (Implementation) and 6 (Data Management, Evaluation and Reporting)
  • Chapter 7: BMP Performance Analysis
  • Chapters 8 (LID Monitoring), 9 (LID data interpretation]), and 10 (Case studies).

Evaluation of Best Management Practices for Highway Runoff Control (NCHRP Report 565)

AASHTO (American Association of State Highway and Transportation Officials) and the FHWA (Federal Highway Administration) sponsored this 2006 research report, which was authored by Oregon State University, Geosyntec Consultants, the University of Florida, and the Low Impact Development Center. The primary purpose of this report is to advise on the selection and design of BMPs that are best suited for highway runoff. The document includes chapters on performance monitoring that may be a useful reference for BMP performance monitoring, especially for the performance assessment of a highway BMP.

  • Chapter 4: Stormwater Characterization
    • 4.2: General Characteristics and Pollutant Sources
    • 4.3: Sources of Stormwater Quality data
  • Chapter 8: Performance Evaluation
    • 8.1: Methodology Options
    • 8.5: Evaluation of Quality Performance for Individual BMPs
    • 8.6: Overall Hydrologic and Water Quality Performance Evaluation
  • Chapter 10: Hydrologic Evaluation
    • 10.5: Performance Verification and Design Optimization

Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices

  • In 2014 the Water Environment Federation released this White Paper that investigates the feasibility of a national program for the testing of stormwater products and practices. The report does not include any specific guidance on the monitoring of a BMP, but it does include a summary of the existing technical evaluation programs that could be consulted for testing results for specific products (see Table 1 on page 8).

Caltrans Stormwater Monitoring Guidance Manual (Document No. CTSW-OT-13-999.43.01)

The most current version of this manual was released by the State of California, Department of Transportation in November 2013. As with the other monitoring manuals described, this manual does include guidance on planning a stormwater monitoring program. However, this manual is among the most thorough for field activities. Relevant chapters include.

  • Chapter 4: Monitoring Methods and Equipment
  • Chapter 5: Analytical Methods and Laboratory Selection
  • Chapter 6: Monitoring Site Selection
  • Chapter 8: Equipment Installation and Maintenance
  • Chapter 10: Pre-Storm Preparation
  • Chapter 11: Sample Collection and Handling
  • Chapter 12: Quality Assurance / Quality Control
  • Chapter 13: Laboratory Reports and Data Review
  • Chapter 15: Gross Solids Monitoring

Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance

This online manual was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory with funding provided by the Minnesota Pollution Control Agency. The manual advises on a four-level process to assess the performance of a Best Management Practice.

Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management credit. BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations. Level 4, Monitoring, is the method most frequently used for assessment of the performance of a BMP.

Use these links to obtain detailed information on the following topics related to BMP performance monitoring:

Other Pollutants

In addition to TSS and phosphorus, infiltration practices can reduce loading of other pollutants. According to the International Stormwater Database, studies have shown that infiltration practices are effective at reducing concentration of pollutants, including nutrients, metals, bacteria, cyanide, oils and grease, Volatile Organic Compounds (VOC), and Biological Oxygen Demand (BOD). A compilation of the pollutant removal capabilities from a review of literature are summarized below.

Relative pollutant reduction from infiltration systems for metals, nitrogen, bacteria, and organics.
Pollutant Category Constituent

Treatment Capabilities

(Low = < 30%; Medium = 30-65%;

High = 65 -100%)
Metals1 Cr, Cu, Zn High2
Ni, Pb
Nutrients Total Nitrogen, TKN Medium/High
Bacteria Fecal Coliform, E. coli High
Organics   High
1 Results are for total metals only
2 Treatment capabilities are based mainly on information from sources that referenced only metals as a category and did not provide individual efficiency for specific metals

References and suggested reading

Related pages