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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
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. For more information on using models to calculate credits see Methods and resources for calculating credits.

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). For specific tools and methods that can be used to calculate credits see Methods and resources for calculating credits. 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.

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