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====Adjusting total phosphorus emcs for residential areas==== | ====Adjusting total phosphorus emcs for residential areas==== | ||
− | As discussed above, residential land uses are often classified based on density. Methods for classifying residential land uses are not consistent and we recommend avoiding these classifications in determining an appropriate emc for an area. | + | As discussed above, residential land uses are often classified based on density. Methods for classifying residential land uses are not consistent and we recommend avoiding these classifications in determining an appropriate emc for an area. |
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+ | Emcs can and should be adjusted when supporting data exist. Several factors affect emcs in residential areas. Below is a discussion of these factors and the effect on emcs. | ||
#Tree canopy coverage over impermeable surfaces. Janke et al. (2017) observed a strong correlation between total phosphorus and street canopy cover. The observed relationship was linear, with concentrations ranging from about 0.2 mg/L to 0.45 mg/L at 40% street canopy coverage. Waschbusch et al. (1999) observed a similar relationship in two Madison, Wisconsin neighborhoods, with total phosphorus ranging from about 0.1 mg/L with 5 percent street canopy coverage to 0.40 mg/L with 40 percent street canopy coverage. | #Tree canopy coverage over impermeable surfaces. Janke et al. (2017) observed a strong correlation between total phosphorus and street canopy cover. The observed relationship was linear, with concentrations ranging from about 0.2 mg/L to 0.45 mg/L at 40% street canopy coverage. Waschbusch et al. (1999) observed a similar relationship in two Madison, Wisconsin neighborhoods, with total phosphorus ranging from about 0.1 mg/L with 5 percent street canopy coverage to 0.40 mg/L with 40 percent street canopy coverage. | ||
#Season. Selbig (2016) measured monthly concentrations of total phosphorus in a residential area of Madison, Wisconsin having 17 percent street tree canopy coverage. Mean concentrations in spring were 0.67-0.74 mg/L, decreasing to 0.41-0.45 in summer, and rapidly increasing following Fall leaf drop, when concentrations increased to more than 2.5 mg/L. Janke et al. (2017) observed similar patterns in Minnesota. TP concentrations in spring (flowering) and fall (leaf drop) were correlated with street canopy coverage, while summer concentrations were not correlated with street canopy coverage. | #Season. Selbig (2016) measured monthly concentrations of total phosphorus in a residential area of Madison, Wisconsin having 17 percent street tree canopy coverage. Mean concentrations in spring were 0.67-0.74 mg/L, decreasing to 0.41-0.45 in summer, and rapidly increasing following Fall leaf drop, when concentrations increased to more than 2.5 mg/L. Janke et al. (2017) observed similar patterns in Minnesota. TP concentrations in spring (flowering) and fall (leaf drop) were correlated with street canopy coverage, while summer concentrations were not correlated with street canopy coverage. |
Land use | Recommended Total phosphorus event mean concentration (mg/L) |
Commercial | 0.20 |
Industrial | 0.235 |
Residential | 0.325 |
Freeways/transportation | 0.28 |
Mixed | 0.29 |
Open space | 0.19 |
Conventional roof | 0.03 |
|
This page provides information on event mean concentrations of total phosphorus and dissolved phosphorus in urban stormwater runoff. For a discussion of phosphorus in stormwater runoff, including information on sources, fate, and water quality impacts, see this page.
Event mean concentrations (emcs) are used in several models for predicting water quality impacts from stormwater runoff and stormwater treatment practices. This page provides summary information that can be used for selecting appropriate emcs. For a discussion of event mean concentrations, see Stormwater pollutant concentrations and event mean concentrations.
We conducted a review of literature to develop the emcs shown on this page. Nearly all studies provided summary information; we therefore did not analyze raw data with the exception of data from Capitol Region Watershed District (see discussion below). We compiled the summary information into a spreadsheet and conducted simple statistical analysis of the information. Data from the following studies were compiled into the spreadsheet.
In addition to the above sources, we compiled water quality monitoring data from 10 storm sewer outfalls in the Capitol Region Watershed in Minnesota. The data period for each outlet varied but generally spanned the period from about 2005 to 2019. The following information was compiled for each monitoring location.
We conducted simple statistical analyses of the data to identify characteristics and patterns of phosphorus in runoff and snowmelt.
Several factors affect emcs for total phosphorus. This section is divided into discussions of emcs based on factors affecting phosphorus concentrations in stormwater runoff.
Studies from the literature frequently provide concentrations for residential land use or occasionally for different types of residential land use, typically low-, medium-, or high-density residential. Most studies do not define criteria for dividing residential land use into these subcategories. Various definitions can be found in the literature. We use the following definitions.
Note that residential land uses can include other land uses, such as commercial and industrial. Many studies therefore classify land uses as mixed or urban, even though a specific land use may dominate a particular area.
Because of the variable and arbitrary manner in which residential land use is classified, we provide a single recommended value for event mean concentrations in residential land uses. We provide additional discussion below so that users can adjust this recommended value depending on local conditions. We used the following references for generating a recommended value for residential land use.
We chose these studies because they contained large amounts of data and they were located in humid and sub-humid areas of the U.S. The median of the above 7 values is 0.325 mg/L.
As discussed above, residential land uses are often classified based on density. Methods for classifying residential land uses are not consistent and we recommend avoiding these classifications in determining an appropriate emc for an area.
Emcs can and should be adjusted when supporting data exist. Several factors affect emcs in residential areas. Below is a discussion of these factors and the effect on emcs.
"Commercial land use is the use of land for commercial purposes including building offices, shops, resorts and restaurants as opposed to construction of a residential house" (Reference, accessed December 24, 2019). Commercial areas considered in this analysis do not include areas used for commercial crop production.
We used the following studies in our analysis.
The median concentration from these studies is 0.17 mg/L. This value appears to be relatively low compared to other studies for which ranges were reported.
Unlike residential areas, commercial areas in stormwater studies are typically well-defined and have relatively uniform land use. We recommend a value of 0.20 mg/L for stormwater runoff from commercial areas.
Unless specific local data exist, we do not recommend adjusting the default emc of 0.20 mg/L. However, reported loads from commercial areas often exceed loads from residential areas due to greater impervious and directly connected impervious surface.
Adjustments may therefore be needed depending on the specific model being used. See the discussion on modeling adjustments below.
We used the following studies in our analysis.
The median TP concentration from these studies is 0.235 mg/L. TP concentrations do not appear to vary much across different industrial land uses, with the primary sources likely being road salt and atmospheric deposition. However, the following may contribute to higher phosphorus loads in industrial areas.
Unless specific local data exist, we do not recommend adjusting the default emc of 0.235 mg/L. However, reported loads from industrial areas often exceed loads from residential areas due to greater impervious and directly connected impervious surface.
Adjustments may therefore be needed depending on the specific model being used. See the discussion on Adjusting curves numbers and runoff coefficients below.
Open space consists of land that is undeveloped. Typically it will not contain buildings or other built structures. Many open spaces are accessible to the public. Open space generally consists of green space (land that is partly or completely covered with grass, trees, shrubs, or other vegetation). Abandoned parcels lacking structures may be considered open space, but it is generally more accurate to include these areas in the land use that existed prior to the parcel being vacant, or including it in adjacent land use categories. The following references were used to generate a recommended value a TP emc for open space.
Parks and recreation areas are generally included in open space.
This land use includes major transportation corridors where the land use is exclusively transportation. These areas are typically highly impervious and may include only small vegetated areas consisting of swales or medians, and relatively small right-of-way areas. This land use does not include arterial streets in residential, commercial, and industrial areas. The following references were used to generate a recommended value a TP emc for open space.
The median value from these studies is 0.28 mg/L.
TP concentrations from transportation corridors are highly variable depending on inputs. The primary inputs include road salt, sediment, and vehicle-related wastes, including oil. The recommended value should be adjusted based on vehicle traffic and likely phosphorus sources and inputs.
If roof comprise a considerable portion of an area, it may be beneficial to include the contribution from roofs separate from other land uses. Note that the emcs for the above land uses generally consider the contribution from roofs.
Phosphorus concentrations from tradition (non-green) roofs is similar to concentrations in precipitation. Although concentrations vary, they are generally low and within the range of 0.01-0.05 mg/L MPCA literature review. A value of 0.03 mg/L is therefore considered appropriate.
In many cases, a specific land use will include multiple land uses. For these situations we recommend using the recommended value for mixed land uses (0.29 mg/L), adjusting this emc based on local data, or calculating the emc. An emc can be calculated if the total area of interest (Atotal), the area of each land use in the area of interest, and the emc for each land use in the area of interest are known.
Site emc = Σ1n ((AArea 1 * emcArea 1)/ (Atotal) + ... ((AArea n * emcArea n) / (Atotal)
where A = area in acres.
Example calculation
Overall emc = ((0.325 * 10)/31) + ((10 * 0.200)/31) + ((10 * 0.235)/31) + ((1 * 0.28)/31) = 0.254 mg/L
NOTE: To calculate loads for a mixed land use, a curve number or runoff coefficient must be calculated based on the impervious surface for each of the land uses.
Event mean concentrations for total phosphorus.
Link to this table
Land cover/land use | Range (mg/L) | Recommended value (mg/L) | Notes |
---|---|---|---|
Commercial | 0.20 - 0.34 | 0.200 | If applicable to models being used, adjust curve numbers/runoff coefficients when calculating loads |
Industrial | 0.23 - 0.55 | 0.235 |
|
Residential | 0.26 - 0.38 | 0.325 | Concentrations vary widely depending on local conditions |
High-density/Multi-family residential | 0.28 - 0.40 | Calculate1 |
|
Medium density residential | 0.18 - 0.40 | Calculate1 |
|
Low density residential | 0.24 - 0.40 | Calculate1 |
|
Freeways/transportation | 0.25 - 0.45 | 0.280 |
|
Mixed | 0.16 - 0.84 | 0.290 |
|
Parks and recreation | Use value for open space or calculate |
|
|
Open space | 0.12 - 0.31 | 0.190 | |
Conventional roof | 0.01 - 0.20 | 0.030 | |
Institutional | 0.14 - 0.422 | See note |
|
Forest/shrub/grassland | 0.03 - 0.45 | 0.090 | Concentrations are likely to vary with season in areas with fall leaf drop |
Open water and wetlands | see Notes (next column) |
|
|
Cropland (row crops) | 0.126-1.348 | 2 | Median from our review = 0.533 |
Pasture | 0.35-0.45 | 2 |
1The link takes you to information on calculating event mean concentrations for areas with multiple land uses.
2Our literature review was not extensive enough to warrant a specific recommend emc for this land use
Few studies examine seasonal differences in phosphorus concentrations. We analyzed data from Capitol Region Watershed District to assess the affect of season. We did not perform rigorous statistical analysis of the data, so the following represent observations.
Several studies show a first flush effect on total phosphorus concentrations, with concentrations being higher in the initial phase of runoff. However, this effect is dependent on several factors. General conclusions include the following.
The following table summarizes results from our literature review. There are insufficient data to support recommended event mean concentrations (emcs) for different land uses. The table provides a summary of data we felt is appropriate for selecting an emc for dissolved phosphorus. To see the full range of values compiled from the literature, open the Excel spreadsheet containing the data.
Summary of dissolved phosphorus event mean concentrations from various studies. There is inadequate information to provide recommended emcs for different land uses.
Link to this table
Study | Land cover/land use | Range (mg/L) | Mean | Median | Number of samples |
---|---|---|---|---|---|
Dallas-Fort Worth1 | Commercial | 0.01-0.47 | 0.09 | 0.06 | 42 |
Dallas-Fort Worth | Industrial | 0.03-0.45 | 0.14 | 0.09 | 63 |
Dallas-Fort Worth | Residential | 0.04-0.84 | 0.25 | 0.21 | 77 |
Forth Worth2 | Transportation | 0.11 | 28 | ||
Twin Cities3 | Mixed | 0.01-1.4 | 0.2 | 0.15 | 147 |
Madison4 | Medium density residential | 0.52 | 0.61 | 25 | |
Madison4 | Medium density residential | 0.4 | 0.14 | 25 | |
Madison4 | Medium density residential | 0.14 | 0.04 | 25 | |
Madison4 | Medium density residential | 0.05 | 0.03 | 25 | |
Madison4 | Medium density residential | 0.04 | 0.02 | 25 | |
Madison4 | Medium density residential | 0.03 | 0.02 | 25 | |
Madison4 | Medium density residential | 0.04 | 0.02 | 25 | |
Madison4 | Medium density residential | 1.54 | 0.81 | 25 | |
Madison4 | Medium density residential | 0.12 | 0.08 | 25 | |
Madison4 | Medium density residential | 0.11 | 0.07 | 25 | |
Madison4 | Medium density residential | 0.11 | 0.07 | 25 | |
US EPA Nurp Study5 | Residential | 0.143 | |||
US EPA Nurp Study5 | Mixed | 0.056 | |||
US EPA Nurp Study5 | Commercial | 0.08 | |||
US EPA Nurp Study5 | Open | 0.026 | |||
New York6 | Residential | 0.20 | 738 | ||
New York6 | Commercial | 0.18 | 323 | ||
New York6 | Industrial | 0.16 | 325 | ||
New York6 | Open | 0.16 | 44 | ||
Capitol Region Watershed District7 | Mixed | 0.020 - 0.888 | 0.073 | 0.052 | 89 |
Capitol Region Watershed District7 | Mixed | 0.020 - 0.565 | 0.108 | 0.087 | 120 |
Capitol Region Watershed District7 | Mixed | 0.020 - 0.506 | 0.074 | 0.059 | 112 |
Capitol Region Watershed District7 | Mixed | 0.020 - 0.361 | 0.073 | 0.053 | 121 |
Capitol Region Watershed District7 | Mixed | 0.005 -- 0.182 | 0.019 | 0.012 | 195 |
Capitol Region Watershed District7 | Mixed | 0.020 - 0.758 | 0.102 | 0.072 | 69 |
Capitol Region Watershed District7 | Mixed | 0.020 - 1.10 | 0.072 | 0.053 | 115 |
Capitol Region Watershed District7 | Mixed | 0.020 - 0.60 | 0.099 | 0.057 | 113 |
Capitol Region Watershed District7 | Mixed | 0.020 - 0.499 | 0.071 | 0.046 | 138 |
1Urban Stormwater Quality, Event-Mean Concentrations, and Estimates of Stormwater Pollutant Loads, Dallas-Fort Worth Area, Texas. 1992–93 Stanley Baldys III, T.H. Raines, B.L. Mansfield, and J.T. Sandlin U.S. Geological Survey Water-Resources Investigations Report 98–4158.
2Computed and Estimated Pollutant Loads, West Fork Trinity River, Fort Worth, Texas, 1997. United States Geological survey. Water Resources Investigations Report 01–4253
3Brezonik and stadelman. 2002. Analysis and predictive models of stormwater runoff volumes, loads, and pollutant concentrations from watersheds in the Twin Cities metropolitan area, Minnesota, USA. Water Research Volume 36, Issue 7, Pages 1743-1757
457.Waschbusch, R.J., W.R. Selbig, and R.T. Bannerman. 1999. Sources of phosphorus and street dirt from Two Urban Residential Basins in Madison, Wisconsin, 1994-95. USGS Water-Resources Investigation Report 99-4021
5U.S. EPA. Results of the Nationwide Urban Runoff Program. 1983. Volume I: Final Report. PB84-185552
6New York State Department of Environmental Conservation. August 2003. Stormwater Management Design Manual. Chapter 5 - Acceptable Stormwater Management Practices.
7Outfall monitoring data for Villa Park, Trout Brook East, Trout Brook West, Trout Brook Outlet, St. Anthony, Phalen Creek, Como 3, Como 7, and East Kittsendale
While trees contribute phosphorus to stormwater, resulting in high phosphorus concentrations during certain times of the year, trees also reduce total runoff. This occurs through interception by tree canopies and by reduced runoff from permeable surfaces. Adjusting TP emcs to account for inputs from leaves without adjusting runoff volumes will lead to overestimates of annual phosphorus loading.
Janke et al. (2017) observed that neither street canopy nor total vegetation were significant factors in nutrient loading. The researchers provide a discussion of the effect of street density, tree canopy coverage, and nutrient loading. Despite a relatively low sample size, they observed that nutrient loading decreased with increasing tree canopy at lower street densities, but as street density increased the opposite pattern occurred. They observed a threshold for TP loading at a street density of about 10 km/km2 (about 8-10 percent of an area consisting of streets, depending on street width). Below this threshold, reductions in runoff volume offset increased TP inputs from trees. Sanders (1986) modeled impacts of trees on runoff in Dayton, Ohio, and estimated under current conditions that tree canopy reduced runoff by 7 percent annually. Modest increases in canopy cover could increase this to 12 percent. Wang et al. (2008) developed a model to predict impacts of tree canopy on urban runoff in Maryland. Increasing the leaf area index from 3 to 6 increased tree interception by 2.7 percent, decreased runoff from pervious areas by 4.3 mm, and decreased runoff from directly connected impervious areas by 20.1 mm. Xiao and McPherson (2002) estimated interception accounted for 1.6 percent of annual precipitation, with seasonal differences and significantly greater interception by mature trees. Hathaway (2019) observed interception rates of 28-43 percent for three species in Tennessee.
Currently there is insufficient information to develop specific relationships between reduced loads associated with tree canopy
Using data from Janke et al. (2017), the recommended TP concentration of 0.325 mg/L corresponds with a street tree canopy coverage of about 20-25 percent. This is slightly below the national urban tree cover of 27.1 percent (Deeproot (2010). While residential areas are likely to have a higher percent canopy cover compared to commercial and industrial areas, it is also likely that much of the canopy coverage in residential areas is over pervious areas. Assuming an emc of 0.325 for a canopy street tree coverage of 20 percent, we recommend adjusting the emc by 0.06 mg/L for each 10 percent increase or decrease in street canopy coverage for canopy coverages between 0 and 40 percent. There is limited data for calculating adjustments at canopy coverages greater than 40 percent.
Example calculation: Tree canopy coverage in two adjacent area is 15 and 30 percent respectively. For the area with 15 percent canopy, the recommended emc is (0.325 - (0.06 * 0.5)) = 0.295 mg/L. For the area with 30 percent canopy coverage, the recommended emc is (0.325 + 0.06) = 0.385 mg/L.
if the model or calculator adjusts for volume decreases associated with canopy coverage and varying canopy coverages, no adjustment in volume is needed to calculate the phosphorus load. For example, the Minimal Impact Design Standards (MIDS) calculator automatically adjusts volumes for canopy coverage.
The above discussion primarily focuses on event mean concentrations for phosphorus. While estimating loads accurately requires
Some models may have curve number, runoff coefficient, or percent impervious as a model input. The MPCA Simple Estimator, for example, employs a default runoff coefficient of 0.8 for commercial areas, compared to 0.44 for residential areas. The tables below may be used to determine the proper curve number or runoff coefficient. Percent impervious can be converted to a curve number using the following formula.
\( Curve number = (Impervious * 98) + ((1 - impervious) * (open space curve number in good condition for the specific soil)) \)
where impervious is given as a fraction (not a percent).
For example, if an area on B soils is 50 percent impervious, the curve number is given as (0.5 * 98) + ((1 - 0.50)(61)) = 79.5.
Curve numbers for urban and agricultural areas. Source: [USDA Urban Hydrology for Small Watersheds - TR-55. USDA Urban Hydrology for Small Watersheds - TR-55].
Link to this table
Cover type and hydrologic condition | Soil Group A | Soil Group B | Soil Group C | Soil Group D |
---|---|---|---|---|
Open space poor condition (<50% cover) | 68 | 79 | 86 | 89 |
Open space average condition (50-75% cover) | 49 | 69 | 79 | 84 |
Open space good condition (>75% cover) | 39 | 61 | 74 | 80 |
Impervious surfaces | 98 | 98 | 98 | 98 |
Commercial (85% impervious) | 89 | 92 | 94 | 95 |
Industrial (72% impervious) | 81 | 88 | 91 | 93 |
Residential (65% impervious) | 77 | 85 | 90 | 92 |
Residential (30% impervious) | 57 | 72 | 81 | 86 |
Residential (12% impervious) | 46 | 65 | 77 | 82 |
Pervious, no vegetation (newly graded) | 77 | 86 | 91 | 94 |
Fallow with residue cover | 74-76 | 83-85 | 88-90 | 90-93 |
Row crop, no residue | 67-72 | 78-81 | 85-88 | 89-91 |
Row crop with residue | 64-71 | 75-80 | 82-87 | 85-90 |
Pasture, good condition | 39 | 61 | 74 | 80 |
Pasture, poor condition | 68 | 79 | 86 | 89 |
Meadow | 30 | 58 | 71 | 78 |
Woods, good condition | 32 | 58 | 72 | 79 |
Woods, poor condition | 57 | 73 | 82 | 86 |
Runoff coefficients for different soil groups and slopes. Coefficients are for recurrence intervals less than 25 years. Source: Hydrologic Analysis and Design (4th Edition) (McCuen, 2017).
Link to this table
Land use | Soil Group A | Soil Group B | Soil Group C | Soil Group D | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
0-2% | 2-6% | >6% | 0-2% | 2-6% | >6% | 0-2% | 2-6% | >6% | 0-2% | 2-6% | >6% | |
Residential (65% impervious) | 0.25 | 0.28 | 0.31 | 0.27 | 0.30 | 0.35 | 0.30 | 0.33 | 0.38 | 0.33 | 0.36 | 0.42 |
Residential (30% impervious) | 0.19 | 0.23 | 0.26 | 0.22 | 0.26 | 0.30 | 0.25 | 0.29 | 0.34 | 0.28 | 0.32 | 0.39 |
Residential (12% impervious) | 0.14 | 0.19 | 0.22 | 0.17 | 0.21 | 0.26 | 0.20 | 0.25 | 0.31 | 0.24 | 0.29 | 0.35 |
Commercial | 0.71 | 0.71 | 0.72 | 0.71 | 0.72 | 0.72 | 0.72 | 0.72 | 0.72 | 0.72 | 0.72 | 0.72 |
Industrial | 0.67 | 0.68 | 0.68 | 0.68 | 0.68 | 0.69 | 0.68 | 0.69 | 0.69 | 0.69 | 0.69 | 0.70 |
Streets | 0.70 | 0.71 | 0.72 | 0.71 | 0.72 | 0.74 | 0.72 | 0.73 | 0.76 | 0.73 | 0.75 | 0.78 |
Parking | 0.85 | 0.86 | 0.87 | 0.85 | 0.86 | 0.87 | 0.85 | 0.86 | 0.87 | 0.85 | 0.86 | 0.87 |
Open space | 0.05 | 0.10 | 0.14 | 0.08 | 0.13 | 0.19 | 0.12 | 0.17 | 0.24 | 0.16 | 0.21 | 0.28 |
Cultivated land | 0.08 | 0.13 | 0.16 | 0.11 | 0.15 | 0.21 | 0.14 | 0.19 | 0.26 | 0.18 | 0.23 | 0.31 |
Pasture | 0.12 | 0.20 | 0.30 | 0.18 | 0.28 | 0.37 | 0.24 | 0.34 | 0.44 | 0.30 | 0.40 | 0.50 |
Meadow | 0.10 | 0.16 | 0.25 | 0.14 | 0.22 | 0.30 | 0.20 | 0.28 | 0.36 | 0.24 | 0.30 | 0.40 |
Forest | 0.05 | 0.08 | 0.11 | 0.08 | 0.11 | 0.14 | 0.10 | 0.13 | 0.16 | 0.12 | 0.16 | 0.20 |