Rain water harvesting is the practice of collecting rain water from impermeable surfaces, such as rooftops, and storing for future use. There are a number of systems used for the collection, storage and distribution of rain water including rain barrels, cisterns, evaporative control systems, and irrigation.
1Assuming water is drained to a vegetated pervious area. Does not apply to volume of runoff that bypasses the system
Rain water harvesting can be accomplished using rain barrels and/or cisterns. Rain barrels are typically located at the downspout of a gutter system and are used to collect and store rainwater for watering landscapes and gardens.
The simplest method of delivering water is by the force of gravity. However, more complex systems can be designed to deliver the water from multiple barrels connected in a series with pumps and flow control devices.
Cisterns have a greater storage capacity than rain barrels and may be located above or below ground. Due to their size and storage capacity, these systems are typically used to irrigate landscapes and gardens on a regular basis reducing the strain on municipal water supplies during peak summer months. Again, cisterns may be used in series and water is typically delivered using a pump system.
The storage capacity of a rain barrel or cistern is a function of the catchment area, the depth of rainfall required to fill the system and the water losses. A general rule of thumb in sizing rain barrels or cisterns is that one inch of rainfall on a 1,000 square foot roof will yield approximately 600 gallons of runoff.
Stormwater harvesting and reuse is a practice of collecting and reusing stormwater for potable (for consumption) or non-potable applications. Outdoor irrigation is considered a non-potable water use. This discussion applies to irrigation of non-food crops, such as turf and landscaping. For the purposes of this document, stormwater is defined as runoff collected from roof and ground surfaces, including roadways, driveways, parking lots and other impervious areas. Rainwater is defined as runoff from roof surfaces only. Some of the literature sources reviewed as part of the development of this memorandum place emphasis on rainwater only, while others focus on stormwater for harvesting and reuse. Additionally, some of the documents and standards reviewed were originally developed for the reuse of reclaimed (treated wastewater).
The following overarching goals apply to the implementation of stormwater harvesting and reuse systems (EOR, 2011 (draft)):
Additionally, stormwater harvesting and reuse systems can be used to help achieve Leadership in Energy and Environmental Design (LEED) and other sustainable design credits related to stormwater quantity and quality as well as water efficiency.
The scale of stormwater harvesting and reuse systems can range from small residential systems to very large commercial systems. According to the U.S. EPA, when harvested rainwater is re-used, it generally is best for irrigation and non-potable uses of water closets, urinals and HVAC, as these uses require a lesser amount of on-site treatment than potable uses (EPA, 2008). Because of this, one of the most common reuse applications of stormwater and rainwater is urban irrigation (EOR, 2011 (draft)), which can include irrigation of athletic fields, golf courses, parks, landscaping, community gardens, and creation of water features (Metropolitan Council, 2011).
Nationally, outdoor water uses represent 58 percent of the domestic daily water uses while for hotels and office buildings, outdoor uses represent 10 to 38 percent of the daily water uses, respectively (EPA, 2008). In Minnesota during the summer, as much as 50 percent of potable water supply is used for outdoor, non-potable uses. During hot weather and extended periods of drought, Twin Cities’ property owners will use 45 to 120 gallons of treated drinking water per person per day for outdoor uses with peak usage on large lots and new turf reaching as much as 200 gallons per person per day (Metropolitan Council, 2011).
Although stormwater harvesting and reuse systems appear to be a viable alternative to help achieve required stormwater management standards as well as reducing the demand on the potable water supply, they are not without concerns. Concerns include human exposure to pathogens, cross-contamination of the potable water supply (EPA, 2008), exposure due to ingestion of crops potentially contaminated with pathogens, risk of toxic spills (within the stormwater reuse catchment area and potential for reuse of toxic/contaminated water), and concerns with mosquito breeding and contaminated pond sediments (EOR, 2011 (draft)). Additionally, there are often not well-defined operation and maintenance procedures for rainwater and stormwater harvesting and reuse programs (EOR, 2011 (draft)). These operation and maintenance programs help ensure stormwater reuse systems are functioning as designed and are meeting the required water quality to protect the public health.
In many areas, rainwater and stormwater harvesting is largely unaddressed by regulations and codes (EPA, 2008), although some cities and states have established stormwater harvesting and reuse requirements. Many of these requirements were originally developed for the reuse of reclaimed water (treated wastewater) rather than stormwater. However, the confusion about the different types of water to be reused (reclaimed, rainwater, stormwater, etc.) and the lack of national guidance for this topic has resulted in differing use and treatment guidelines/standards among state and local governments. Because of the lack of guidance for rainwater and stormwater reuse, these sources of reuse water are often regulated at the same level as reclaimed water, which typically has more clearly defined guidance and standards. Although the general guidance for the reuse of rainwater and stormwater would be similar to reclaimed and graywater, it may also differ because of lower levels of initial contamination and the potential uses (EPA, 2008). Often, the treatment requirements ultimately come down to the risk of exposure to pathogens determining the most stringent levels of treatment (EPA, 2008).
The level of treatment required by each municipality can influence the number of harvesting and reuse systems that are actually implemented. Simplifying the treatment requirements when public health is not at risk can lower the project cost for those entities intending to install stormwater harvesting and reuse systems and encourages broader adoption of the practices (EPA, 2008).
Because the main concern of stormwater reuse to human health is exposure to pathogenic bacteria, many jurisdictions evaluate stormwater reuse projects based on whether the application area has restricted or unrestricted public access. Restricted reuse applications are defined by areas where public access can be controlled such as irrigation of gated/private golf courses, cemeteries, and highway medians. Unrestricted use applications include areas where public access is not controlled which often includes irrigation in parks, playgrounds, school yards, and residences, and use in ornamental fountains and aesthetic impoundments. In order to limit the public health risk and exposure to pollutants in stormwater during reuse, reuse projects in unrestricted areas have more stringent water quality regulations than restricted areas (EOR, 2011 (draft)). Australia has implemented numerous water reuse projects throughout the country and the guidelines for managing the human health risk associated with stormwater reuse includes recommendations about signage and fencing around the irrigated areas to limit public exposure. However, if access cannot be controlled, then the guidelines recommended secondary treatment (which includes disinfection) (EOR, 2011 (draft)).
In addition, the scale of the stormwater reuse system may impact whether the system is regulated. For example, in Portland, Oregon, residential rainwater that is only used for outdoor irrigation is not covered by code and needs no treatment prior to use (EPA, 2008). Often, larger scale applications of reuse require treatment, but the extent of treatment is determined by the end use and it is up to the local jurisdiction to determine what treatment is required. Most systems are required to include some level of screening/filtration and most jurisdictions will require disinfection (UV or chlorination) (EPA, 2008). Some stormwater reuse systems primarily rely on the pollutant removal abilities of stormwater best management practices to treat stormwater (EOR, 2011 (draft)).
Cross-contamination of the potable water supply is another concern of water reuse systems and is often addressed in building codes. Cross-contamination concerns are usually most applicable when reuse water is brought inside for use within a building or if a potable water supply line is needed to make-up water in the reuse system if the harvested stormwater cannot meet the water demand, which is often the case for irrigation systems utilizing harvested stormwater. Codes will often require a backflow prevention device on the potable water supply lines, an air gap, or both along with a dual pipe system (purple pipes that indicate water reuse lines) and appropriate stenciling and signage (EPA, 2008).
Operation and maintenance of stormwater reuse systems are the responsibility of the property owner. However, there are often not well-defined operation and maintenance procedures for rainwater and stormwater harvesting and reuse programs (EOR, 2011 (draft)). Operation and maintenance should require regular maintenance to ensure the system is functioning as designed because of greater corrosion and clogging of pipes resulting from higher sediment and microbial loads in stormwater (EOR, 2011 (draft)). Maintenance of these systems can include backwashing or replacement of filters (depending on the system design), periodic flushing of pipes to remove sediment build-up and chlorination of pump heads or emitters to clear microbial scum.
Water testing to verify water quality is recommended as well as regular interval maintenance of the treatment system (replacement of filters, UV lights, etc.) (EPA, 2008). In Australia, officials have a major concern with lack of ongoing monitoring after construction which could lead to the potential risk of exceeding water quality guidelines. As a result, they recommends the biannual/quarterly monitoring of nutrients, sediments, pathogens to assess stormwater quality for irrigation (EOR, 2011 (draft)).
Many water reuse programs recommend municipal inspections occur during installation and annual inspections of backflow prevention systems. For example, the State of Florida requires filing of annual inspection reports and maintenance logs every two years. In North Carolina, the state requires inspection of the system (by owner/operator) within 24 hours of each rain event and on a monthly basis, keeping record of the operations and maintenance (EOR, 2011(draft)).
Because one of the environmental concerns related to stormwater reuse is the risk of toxic spills within the catchment area, guidelines in Australia require the incorporation of a 72-hour residence time into a stormwater pond prior to reuse. This provides a time buffer to stop the reuse of potentially contaminated stormwater (EOR, 2011 (draft)). However, this requirement of a 72-hour holding time is in conflict with suggestions for the control of mosquito breeding in stormwater management devices, which suggest that unless a storage system is completely sealed to prevent the entry of adult mosquitos, the water residence time should be less than 72 hours (CDHS, 2004).
Currently, the State of Minnesota does not have a state-specific code applicable to stormwater harvesting and reuse. The MPCA has developed guidelines for the use of reclaimed wastewater. In 2011, the Metropolitan Council developed the Stormwater Reuse Guide, which was developed based on review of water reuse programs and guidance from other states.
Draft water quality guidelines were compiled for stormwater harvesting and reuse systems used for irrigation in areas with public access. These guidelines were determined based on discussion during a meeting with staff from state agencies and a review of standards/guidelines available from other states. These draft guidelines are still considered preliminary to be used for discussion of these standards internally within each agency for additional comment and feedback. Additionally, the MDH would prefer to include treatment requirements along with the water quality outlined in these guidelines (similar to what is outlined in Tables R.3c.1 and R.3c.2 from the Metropolitan Council Stormwater Reuse Guide).
Summary of State of Minnesota water quality guidelines for stormwater harvesting and reuse systems for Irrigation
Link to this table
|Water Quality Parameter||Impact of Parameter 10||Water Quality Guideline – Public Access Areas||Water Quality Guideline – Restricted Access Areas||Water Quality Guideline – Irrigation of Food Crops||Comments|
|E. coli||Public Health||126 E. coli/100mL||Guidance to be determined at a future date||Guidance to be determined at a future date||2,3|
|Turbidity||Irrigation System Function||2-3 NTU||Guidance to be determined at a future date||Guidance to be determined at a future date||4,5|
|TSS||Irrigation System Function||5 mg/L||Guidance to be determined at a future date||Guidance to be determined at a future date||4,5,6|
|pH||Plant Health||6-9||Guidance to be determined at a future date||Guidance to be determined at a future date||4|
|Chloride||Plant Health; Corrosion of Metals||500 mg/L||Guidance to be determined at a future date||Guidance to be determined at a future date||7|
|Zinc||Plant Health||2 mg/L (longterm use); 10 mg/L (shortterm use)||Guidance to be determined at a future date||Guidance to be determined at a future date||8|
|Copper||Plant Health||0.2 mg/L (longterm use); 5 mg/L (shortterm use)||Guidance to be determined at a future date||Guidance to be determined at a future date||8|
|Temperature||Public Health||Guidance to be determined at a future date||Guidance to be determined at a future date||Guidance to be determined at a future date||9|
2 – MPCA Bacterial Impairment Standard: 126 E. coli/100mL (geometric mean of 5 samples in 30 day period); no individual samples greater than 1260 E. coli/100mL
3 – EPA 2012 Recreational Water Quality Criteria – Recommendation 1 (Estimated illness rate = 36/1000)
4 – Based on typical range/value for water reuse programs in other states
5 –Useful for distribution system design, but often used a general indicator parameter, too.
6 – TSS guidance provided by Cathy Tran, DLI
7 – Per Table R.3b.6 in Metropolitan Council Stormwater Reuse Guide
8 – Suggested by Bruce Wilson. Per Table R.3b.5 in Metropolitan Council Stormwater Reuse Guide
9 - Recommendation from Anita Anderson on 12/6/2012 email as temperature impacts bacterial growth
10 –Per Tables R.1a.1 and R.3b.5 in Metropolitan Council Stormwater Reuse Guide
In general, the State of Minnesota relies on the State of California Water Recycling Criteria (2000) as guidance for permitting of wastewater reuse and the MPCA has developed the Municipal Wastewater Reuse guidelines based on those requirements. In addition to the established water quality limits, this guidance requires the following to ensure protection of the public health and the environment.
MPCA municipal wastewater reuse water quality treatment limits (modified to only include irrigation-related uses)
Link to this table
|Types of Reuse||Reuse permit limits||Minimum Level of Treatment|
||Disinfected Tertiary – secondary, filtration, disinfection|
||23 total coliform/100mL||Disinfected Secondary 23 – secondary, disinfection|
|Fodder, Fiber, and Seed Crops
||200 total coliform/100mL||Disinfected Secondary 200 – secondary, disinfection (stabilized pond systems with 210 days of storage do not need a separate disinfection process)|
The Metropolitan Council Stormwater Reuse Guide summarizes water reuse and water quality standards from a variety of states, with the focus on the California water reuse regulations as these are currently the regulations that the State of Minnesota refers to for guidance (for reclaimed water). The following tables summarize some of the key tables included in the Stormwater Reuse Guide. These tables have been modified from the tables in the Stormwater Reuse Guide to focus primarily on stormwater reuse for irrigation purposes only.
|Type of Use||Total Coliform Limits (daily sampling is required)||Treatment Required||Comment (in relation to MIDS)|
|Irrigation of fodder, fiber, and seed crops; orchards/vineyardsa; processed food cropsb, non-food bearing trees; ornamental nursery stock/sod farmsc||No Limits Established||Oxidation||Most likely applies to non-food bearing trees; ornamental nursery stock/sod farms with restricted access|
|Irrigation of pasture for miling animals, landscape areas (controlled access); ornamental nursery stock and sod farms where public access not restricted); landscape impoundments||
||Oxidation Disinfection||This includes Cemeteries, freeway landscaping, restricted access golf courses, and other controlled access areas|
|Irrigation of food cropsa||
|Irrigation of food cropsd; Open access landscape areas; Decorative fountains||
||Oxidation Coagulatione Filtratione Disinfection||This includes parks, playgrounds, schoolyards, residential landscaping, unrestricted access golf courses, and other uncontrolled access areas; This may also apply to scenarios such as community gardens, etc.|
a No contact between reclaimed water and edible portion of crop
b Food crops that undergo commercial pathogen destruction
c No irrigation 14 days prior to harvesting, sale, or allowing public access
d Contact between water and edible portion of crop including edible roots
e Related to turbidity – See Metropolitan Council Reuse Manual for specific details
Summary of water reuse criteria for irrigation of parks, playgrounds, schoolyards, and similar areas from reuse programs from several statesa
Link to this table
|Water Quality Parameter||Water Quality Limits (Range based on information for all states included in table)||State of California Requirements||State of Minnesota – Limits Used by DLIb|
|Total Coliform||2.2 total coliform/100mL||2.2 total coliform/100mL||N/A|
|Fecal Coliform||No Detect/100mL – 100 fecal coliform/100mL (2.2 fecal coliform/100mL most common)||N/A||100 fecal coliform/100mL|
|E. coli||126 E. coli/100mL (only CO uses E. coli as a standard)||N/A||N/A|
|Turbidity||2 NTU – 3 NTU||2 NTU||N/A|
|TSS||5 mg/L – 30 mg/L (5 mg/L most common)||N/A||5 mg/L|
|BOD||5 mg/L – 30 mg/L (10 mg/L most common)||N/A||N/A|
|NH3||4 mg/L (only NC has NH3 as a standard)||N/A||N/A|
|Cl2 residual||0.5-1.0 mg/L||N/A||N/A|
a – Includes review of water reuse programs in AZ, CA, CO, FL, GA, HI, NV, NM, NC, OR, TX, UT, WA, and US EPA guidelines
b – Per 11/28/2012 conversation with Cathy Tran, DLI; General guidance
Summary of water reuse criteria for select nonpotable applications from reuse programs from several statesa
Link to this table
|Water Quality Parameter||Water Quality Limit – Food Crop Irrigation||Water Quality Limit – Restricted Access Irrigation||Water Quality Limit – Unrestricted Access Irrigation|
|Total Coliform||2.2 total coliform||23 total coliform/100mL||2.2 total coliform/100mL|
|Fecal Coliform||No Detect/100mL (some states prohibit usea)||200 fecal coliform/100mL||No Detect/100mL – 20 fecal coliform/100mL|
|E. coli||N/A||126 E. coli/100mL (only CO uses E. coli as a standard)||126 E. coli/100mL (only CO uses E. coli as a standard)|
|Turbidity||2 NTU||N/A||2 NTU – 3 NTU|
|TSS||N/A||20-30 mg/L||5 mg/L|
|BOD||10 mg/L BOD||20-30 mg/L||5-10 mg/L|
|CBOD||N/A||15-20 mg/L||5-20 mg/L|
Note: Many of these standards are based on water quality limits established for reclaimed water (treated wastewater), not stormwater specifically
a – Includes review of water reuse programs in AZ, CA, CO, FL, GA, HI, NV, NM, NC, OR, TX, UT, WA, and US EPA guidelines
Because exposure to pathogens, including bacteria, is one of the main concerns related to stormwater harvesting and reuse for irrigation, we have also summarized the fecal coliform standards used by the Minnesota Department of Health (MDH) for swimming beach closures as well as the Minnesota Pollution Control Agency (MPCA) E. coli standards for listing water bodies for bacterial impairments for reference (See Table 7).
The MDH tests public beaches for elevated levels of fecal coliform and/or E. coli and when high levels are found, beaches are closed to reduce the likelihood of disease. The MDH has established recommendations related to coliform levels to maintain healthy swimming beaches. The MDH will be changing the beach closing standard to reflect the new EPA guidelines (126 E. coli/100mL) and additional changes to that standard are likely based on improved testing methodologies and may included additional indicators.
Additionally, the MPCA has established numeric water quality standards for water bodies throughout the state to determine if the water quality in a water body would attain its intended use. Water bodies not attaining those standards are placed on the MPCA 303(d) list of impaired water bodies. The MPCA has established standards for E. coli within a water body, with those exceeding the standards being classified as having a bacterial impairment.
Summary of MDH “swimmable” standards for public beaches & MPCA standards for bacterial impairments
Link to this table
|Water Quality Parameter||Water Quality Limit||Source|
|Fecal Coliform||200 fecal coliform/100mL (average of 5 samples in a 30- day period should not exceed)||MN Dept of Health|
|1000 fecal coliform/100mL (no one sample should exceed)|
|E. coli||126 E. coli/100mL (Geometric mean based on 5 samples in a month)||MPCA Impaired Waters Criteria|
|1260 E. coli/100mL (maximum standard for one sample)|
Common pollutants in stormwater runoff include nutrients, sediments, heavy metals, salinity, pathogens, and hydrocarbons (EOR, 2011 (draft)). The Metropolitan Council Stormwater Reuse Guide includes several tables that summarize typical stormwater runoff quality information that are attached to the end of this memo and include the following:
However, the fact that the water quality in stormwater runoff is highly variable due to differences in land use and from event to event is extremely important to emphasize and this variability should be considered when evaluating a stormwater harvesting and reuse system and determining what treatment might be necessary.
Additionally, many irrigation systems propose using stormwater directly out of wet retention ponds on the landscape. Although appropriately designed ponds can provide significant particle settling and removal, there is some uncertainty as to the expected level of pathogens within a stormwater pond. There was not specific data within the sources reviewed as part of the development of this memo outlining typical bacteria concentrations within stormwater ponds and information related to this would be useful. However, the Minnesota Stormwater Manual does summarize the expected removal efficiencies of wet ponds and stormwater wetlands for some of the more common contaminants in stormwater. These removal efficiencies are summarized in the table below.
Summary of pollutant removal efficiencies in wet stormwater ponds/stormwater wetlands
Link to this table
|Parameter||Wet Pond Removal Efficiency (%)||Stormwater Wetland Removal Efficiency (%)|
In general, stormwater harvesting and reuse systems are typically comprised of several different components. These components include the collection and pretreatment system, the water storage units, the treatment devices, and the pumping and distribution system (Metropolitan Council, 2011).
The size of stormwater harvesting and reuse systems for irrigation can vary in scale ranging from small residential systems to large scale systems for irrigation.
Residential systems are typically simple systems that may only include primary screens to filter out debris and can have one to several rain barrels (often ranging from 50-100 gallons per barrel). Water collected in residential systems are often used to irrigate the landscaping or gardens on the residential parcel, either manually or via perforated hoses that slowly drain the barrels after each rain event, typically draining to pervious surfaces near the rain barrel system.
Larger scale irrigation systems are often designed to irrigate athletic fields, golf courses, parks, landscaping, community gardens, and supply water to various water features. These large scale irrigation systems are often much more complicated than residential systems used for irrigation.
The following sections discuss the typical components of a stormwater reuse system in more details.
Stormwater can be collected off of a variety of surfaces including roofs, driveways, sidewalks and trails, parking lots, and streets as well as any runoff generated on pervious surfaces in the catchment area. The collection system can include many of the components typically used in stormwater collection systems, such as gutters, catch basins, storm sewers, underground drains, and other stormwater BMPs. While rainwater (from rooftops) typically has less pollutants and particulates than stormwater, stormwater can be pretreated to improve water quality prior to entering the storage unit (EOR, 2011 (draft)). Therefore, the collection system may also include first flush diversions or other pretreatment systems such as screening or filtration to remove some of the larger particulate load from the runoff to reduce maintenance on the system, to reduce build-up of material in the storage unit, to protect downstream equipment, and to reduce the potential for odors and/or algal blooms.
The second component of a stormwater harvesting and reuse system is the storage unit used to collect and store the stormwater runoff prior to reuse. Stormwater storage can be above ground or below ground and can be constructed out of a variety of materials that typically include metal, polypropylene, polyethylene (and steel reinforced polyethylene), plastic, metal, fiberglass, and concrete. In addition to these engineered structures, stormwater may also be reused from stormwater retention ponds or stormwater wetlands that also provide some pretreatment (via settling and biological activity) of the stormwater runoff prior to reuse. When considering utilizing a pond for stormwater reuse, the pond should typically provide sufficient storage for sediment, the water quality treatment volume, the reuse volume, and the flood pool volume (Metropolitan Council, 2011). Alternatively, in some situations existing stormwater ponds are being utilized as sources of irrigation water, using a portion of the water quality volume as a source for irrigation water. Regardless of the type of storage selected, the ability to fully access the storage for maintenance is important (Contech, 2012).
The third component of a stormwater harvesting and reuse system is the treatment device. The type of treatment required is typically dependent on the source of the runoff (and the expected water quality), the end use of the harvested stormwater, and public/human access to the end use and application. In general, the goals of treatment include:
This treatment can include varying levels of filtration (for removal of debris, suspended and colloidal solids, residual suspended solids, residual colloidal solids, dissolved solids, residual and specific trace constituents) and disinfection (ultraviolet radiation, chlorination, and ozone). Many of the typical filtration methods can filter out particle sizes greater than 5 microns to 500 microns, depending on the type of filter selected and the level of treatment required (Contech, 2012). Common filtration methods include mechanical sand or disc filtration, in-pipe treatment filtration, cartridge and bag filters, filter screens, and sediment tanks. Ultraviolet (UV) disinfection is the most practical and commonly used disinfection techniques for small- to medium-sized systems, while chlorination can be more common in larger systems (Australia, 2009).
The final component of the stormwater harvesting and reuse system includes the pumping and distribution system. Typically, the pumping system is set-up in conjunction with the treatment system. Additionally, the type and requirements for the distribution system are dependent on the end use of the stormwater. Often when designing stormwater harvesting and reuse systems as a means of runoff reduction, there is need for a back-up supply of water for essential applications (Contech, 2012).
There are a variety of irrigation distribution systems. Each type of irrigation system has different potential for public exposure to the water used for irrigation and may impact the required water quality standards for the reused water to be utilized. Systems often used for urban irrigation include the following.
Preliminary modeling analysis was performed to begin understanding the potential impact of stormwater reuse for irrigation on annual volume and pollutant load reductions. This section includes a discussion of
Although the state of Minnesota has abundant water, there are times when the demand for water exceeds the supply (UMN Extension, 2009). As previously noted, as much as 50 percent of potable water supply is used for outdoor, non-potable uses in Minnesota during the summer. During hot weather and extended periods of drought, Twin Cities’ property owners will use 45 to 120 gallons of treated drinking water per person per day for outdoor uses, with peak usage on large lots and new turf reaching as much as 200 gallons per person per day (Metropolitan Council, 2011).
Exactly how much water is required for irrigation is dependent on many factors. These factors include the amount of precipitation and the potential evapotranspiration (PET). The PET is the amount of water that could be evaporated from the land, water, and plant sources if soil water were in unlimited supply. PET is a function of soil moisture, daily available sunlight, and air temperature (EOR, 2011 (draft)). The University of Wisconsin (UW) Agriculture Extension Service has estimated PET based on satellite derived measurements of solar radiation and air temperatures at regional airports using the UW Soil Science model.
Potential evapotranspiration for Minneapolis-St. Paul from 2001-2010
Link to this table
|Year||Mean Potential Evapotranspiration (in/d)||Maximum Potential Evapotranspiration (in/d)||Standard Deviation Potential Evapotranspiration (in/d)|
Source: EOR, 2011 (draft) based on Bland and Diak, 2011
Plots of monthly average rainfall and the average potential evapotranspiration can be used to identify periods when the potential evapotranspiration is greater than the average precipitation. These are periods of the year when irrigation may be necessary and they typically fall within the period from May through September. However, because of the geographic variation in climate, the expected irrigation periods vary across the state (UMN Extension, 2009).
The amount of water that needs to be applied during irrigation depends on the soil type and wetness of the soil. Typical irrigation depths for turf and landscaping in Minnesota range from 1 to 1.5 inches (minus any rainfall received) per week (UMN Extension, 2009). Additionally, the frequency of watering can be highly variable and is affected by the plant species, soil texture, climate, exposure, and intensity of use (UMN Extension, 2009). There is also variability based on the season. In the spring and summer, plants only require 40 to 60 percent of available water for evapotranspiration, while in mid-summer, turfgrass can require up to 100 percent (EOR, 2011 (draft)). Because of this variability in the supply and demand of water for irrigation, stormwater reuse systems for irrigation typically require a potable water back-up supply as well. Transpiration by urban trees can also vary by type of tree. Peters et al. (2010) found that water use by evergreens was greater than deciduous trees because they transpire more water and have a longer growing season. For example, transpiration from an evergreen can be on the order of 0.075 inches per per square meter of canopy, while for deciduous trees, the transpiration is on the order of 0.044 inches per day per square meter of canopy (EOR, 2011 (draft)).
When utilizing stormwater for irrigation as a means of managing stormwater for volume reduction or water quality improvements, the approach to the application of the irrigation water will likely vary from typical irrigation methods. The goal of the reuse is to maximize the saturation of the soil without limiting plant growth (EOR, 2011 (draft)). In addition to human health concerns related to the potential exposure to pathogens there may be concerns with the effect of stormwater pollutants on plants (e.g., nutrients, sediments, heavy metals, hydrocarbons, chlorides, and pathogens). Therefore, if possible, the plants selected for a stormwater irrigation system should have high tolerance to both water logging and pollutant concentrations.
P8 (Program for Predicting Polluting Particle Passage through Pits, Puddles and Ponds, IEP, Inc., 1990) is a computer model used for predicting the generation and transport of stormwater runoff and pollutants in urban watersheds. Barr used the P8 model, Version 3.4, in this analysis to simulate the stormwater runoff and phosphorus loads generated from hypothetical development sites with varying levels of imperviousness to represent variation in typical development density. The model requires user input for watershed characteristics, local precipitation and temperature, and other parameters relating to water quality and BMP pollutant removal performances. Barr then utilized a daily water balance spreadsheet model of the runoff water and pollutant loads generated by the P8 model to estimate the potential impact of stormwater reuse for irrigation on runoff volume reduction, and, ultimately, the pollutant load reduction.
The P8 analysis evaluated runoff from several hypothetical 10-acre development scenarios with varying levels of imperviousness. Twenty hypothetical watersheds were included in the P8 modeling nalysis, including
Watershed runoff volumes from pervious areas were computed in P8 using the SCS Curve Number method. Pervious curve numbers were selected for each hypothetical watershed based on soil type and an assumption that the pervious areas within the hypothetical development would be open space areas in fair to good condition. References on SCS curve numbers provide a range of curve numbers that would apply to pervious areas in fair to good condition. Pervious curve numbers of 39, 65, 74, and 80 were used for hydrologic soil groups A, B, C, and D, respectively.
Depression storage represents the initial loss caused by such things as surface ponding, surface wetting, and interception. As previously discussed, the P8 model utilizes the SCS Curve Number method to estimate runoff from pervious areas. For impervious areas, runoff begins once the cumulative storm rainfall exceeds the specified impervious depression storage, with the runoff rate equal to the rainfall intensity. An impervious depression storage value of 0.06 inches was used for the P8 simulation.
The P8 model requires hourly precipitation and daily temperature data; long-term data was used so that watersheds and BMPs can be evaluated for varying hydrologic conditions. The hourly precipitation and average daily temperature data were obtained from the National Weather Service site at the Minneapolis-St. Paul International Airport. The simulation period used for the P8 analysis was January 1, 1955 through December 31, 2004 (50 years).
For the P8 analysis, the 50-year hourly dataset was modified to exclude the July 23-24, 1987 “super storm” event, in which 10 inches of rainfall fell in 6 hours. This storm event was excluded because of its extreme nature and the resulting skew on the pollutant loading and removal predictions. Excluding the July 23-24, 1987 “super storm”, the average annual precipitation throughout the 50-year period used for the P8 modeling was 27.7 inches.
The NURP50.PAR particle file was used for the P8 model. The NURP 50 particle file represents typical concentrations and the distribution of particle settling velocities for a number of stormwater pollutants. The component concentrations in the NURP 50 file were calibrated to the 50th percentile (median) values compiled in the EPA’s Nationwide Urban Runoff Program (NURP).
There are numerous additional input parameters that can be adjusted in the P8 model. Several of the parameters related to simulation of snowmelt and runoff are summarized below.
A daily spreadsheet mass balance model was developed to estimate the expected annual stormwater runoff volume reduction due to reuse of stormwater for irrigation. We assumed that the annual pollutant reduction would be equivalent to the estimated annual volume reduction as the result of stormwater reuse for irrigation.
There are a variety of parameters that can influence the annual stormwater runoff reduction due to stormwater collection and reuse for irrigation. These parameters include:
To begin understanding the range in the potential impact of stormwater reuse for irrigation on stormwater runoff volume and pollutant removal, we ran a variety of reuse scenarios for each of the twenty watershed conditions. The daily mass balance modeling for the stormwater reuse was evaluated for the same period that was run in the P8 analysis, from January 1, 1955 through December 31, 2004 (50 years). The following is a summary of the assumptions used in the preliminary mass balance modeling analysis to develop the range in the potential impact of stormwater reuse for irrigation.
As previously mentioned, to generate the watershed runoff loads, the P8 model was used, evaluating 20 different 10-acre development scenarios, varying the imperviousness and soil types. A variety of potential application areas for irrigation were evaluated, ranging from 1 percent of the hypothetical watershed area (0.1 acre) to three (3) times the watershed area (30 acres). For the preliminary model runs, we assumed that the irrigation use rate was equivalent to 1 inch per week (or 0.14 inches per day) over the application area during the irrigation season and that the storage volume for stormwater (for irrigation) was equivalent to the one week demand for irrigation water. For the preliminary model runs, we assumed irrigation would begin in May and continue through the end of September and that irrigation would occur at a rate of 0.14 inches per day during that period, regardless of the amount of precipitation.
On a daily time step for the 50-year period that was evaluated in P8, the mass balance model tracked the available storage volume in the stormwater reuse system at the beginning of the day, the volume of watershed runoff (as generated by P8), the amount of watershed runoff that would bypass the system (due to storage not being available), the irrigation volume (if applicable and available), and the storage volume remaining in the stormwater reuse system at the end of the day. The estimated annual volume reduction (and equivalent pollutant reduction) was determined based on comparison of the average annual volume used for irrigation with the average annual watershed runoff volume.
The results of the long-term continuous simulation P8 and daily mass balance modeling analyses are presented below.
The table below summarizes the average annual watershed runoff volumes from the hypothetical 10-acre watersheds along with the range of average annual runoff volume reduction based on reuse for irrigation. There is a significant amount of variability in the estimated average annual removal efficiencies related to stormwater reuse for irrigation, based on the sizing of the reuse storage, application area, the irrigation rate and irrigation period. The modeled average annual removals are based on the assumption that the stormwater reuse storage volume is equivalent to the one week demand for irrigation water with an application rate of 1 inch per week over the application area for an irrigation period from May through September. Depending on the combinations of variables, the estimated annual volume reduction (and equivalent pollutant removal) can range from 1 to 98 percent.
Estimated impact of stormwater reuse for irrigation on average annual runoff reduction. The table shows average annual runoff volumes (acre-feet) for 10 acre wastersheds as a function of soil group and percent imperviousness. The range of average annual volume reduction from reuse is shown in the last two columns.
Link to this table
|Hydrologic Soil Group (HSG)||Watershed Area (acre)||Watershed Imperviousness (%)||Average Annual Runoff Volume1 (acre-ft)||Range of Average Annual Runoff Volume Reduction2 (%)|
1 - Based on 50-year P8 model run from January 1, 1955 through December 31, 2004 utilizing MSP climate data
2 - Assumes stormwater storage volume for reuse equivalent to the one week demand for irrigation water assuming an application rate of 1"/week over the application area & irrigation period from May through September
Curves from modeling analysis for the Twin Cities region show estimated average annual runoff volume reductions due to irrigation for a range of soil types (HSG A through D). The curves are based on a relationship between the estimated stormwater reuse storage volume and watershed area. However, exactly how stormwater reuse for irrigation can be incorporated into a site design or retrofitted into an existing system will vary. In some cases, the application area available may control the sizing of the reuse storage. In other situations, there is sufficient application area but a limited watershed area to generate the runoff for reuse resulting in a smaller storage volume for reuse water. Spatial constraints on the site may limit the amount of storage volume that could be included into the site design and layout. Additionally, depending on the geographic location within the state, the current climate, and/or the type of plant species that are being irrigated, the application rate may vary (0.5 to 2.0 inches per week) along with the irrigation period throughout the year. Currently, the set of curves developed from the preliminary modeling does not capture the potential variability in all these parameters (and resulting removal efficiencies).
Also, it is important to look at the relationship between all of these variables when designing a stormwater reuse system to optimize the sizing of the system for a specific site to maximize the cost-benefit of the system. For example, for a hypothetical 10-acre watershed at 50 percent impervious and B type soils, the average storm event runoff (based on the 50-year climatic period evaluated in P8) is 0.16 acre-feet. Assuming the stormwater reuse volume is sized to store the average event runoff from the watershed (and that there is sufficient application area to reuse the stormwater), this would result in an average annual reduction in stormwater runoff volumes of approximately 24 percent. Average annual TSS and total phosphorus reductions are also 24 percent for this example. Using a NURP pond after reuse leads to more than an 87 percent reduction in TSS and more than a 60 percent reduction in total phosphorus.
Examples of the estimated runoff volume reduction for a stormwater reuse storage system sized for the average storm event runoff volume
Link to this table
|Hydrologic Soil Group (HSG)||Watershed Area (acre)||Watershed Imperviousness (%)||Average Storm Event Runoff Volume (acre-ft)1||Average Annual Runoff Volume Reduction2|
1 - Based on 50-year P8 model run from January 1, 1955 through December 31, 2004 utilizing MSP climate data 2 - Assumes stormwater storage volume for reuse equivalent to the average storm event runoff volume and that the application area is sufficient to handle an irrigation application rate of 1"/week over the application area & irrigation period from May through September
Examples of the estimated pollutant load reductions for a stormwater reuse storage system sized for the average storm event runoff volume
Link to this table
|Hydrologic Soil Group (HSG)||Watershed Area (acre)||Watershed Imperviousness (%)||Average Annual Runoff Volume Reduction1,2(%)||Average Annual TSS Reduction Reuse Only3(%)||Average Annual TP Reduction Reuse Only3(%)||Average Annual TSS Reduction NURP & Reuse3,4(%)||Average Annual TP Reduction NURP & Reuse3,4(%)|
1 - Based on 50-year P8 model run from January 1, 1955 through December 31, 2004 utilizing MSP climate data
2 - Assumes stormwater storage volume for reuse equivalent to the average storm event runoff volume and that the application area is sufficient to handle an irrigation application rate of 1"/week over the application area & irrigation period from May through September
3 – Assumes pollutant removal equivalent to volume reduction for reuse only
4 - Assumes average annual NURP pond removal: TP=50%, TSS=84%.
When considering stormwater reuse only, it may be possible to increase the reuse storage volume for the same watershed and application area to maximize the expected annual removal efficiency. For example, to achieve an 80 percent average annual removal efficiency for the same hypothetical watershed, the stormwater reuse volume would need to be increased to 2.5 acre-feet (more than 15 times the average event runoff volume) and would also need sufficient application area to reuse the stormwater within a week at an application rate of one (1) inch per week. This increase in storage volume means that for most storm events, the reuse storage system will remain nearly empty. Additionally, this increase in storage volume typically comes with an associated cost that may be prohibitive, unless the volume is associated with an existing pond that can be utilized for storage and reuse. All of these factors must be weighed when evaluating and optimizing a stormwater reuse system in the context of a specific site.