Event mean concentrations of total and dissolved phosphorus in stormwater runoff Revision as of 22:09, 26 December 2019 by Mtrojan (talk | contribs) (Created page with "This page is in development|alert-under-construction <div style="float:right"> <table class="infobox" style="border:3px; border-style:solid; border-color:#FF0000; t...")(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff) Event mean concentrations of total and dissolved phosphorus in stormwater runoff

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Summary information - phosphorus concentrations in stormwater runoff
Land use	Recommended emc TP (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
NOTE: For recommendations on adjusting these values or for land uses not included above, see the table Event mean concentrations for total phosphorus

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 Phosphorus.

Objective

Event mean concentration (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.

Information: Many factors affect phosphorus concentrations in stormwater. If you are unfamiliar with the concept of event mean concentrations, We recommend you first read Stormwater pollutant concentrations and event mean concentrations

Methodology

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 used to generate emcs for total phosphorus.

• National Stormwater Quality Database. This dataset provides data from several nationwide studies. We used only data from region 1, which includes Minnesota and states with similar rainfall patterns. Data were compiled for four land uses: Commercial (n=165), industrial (n=84), residential, (n=249), and open space (n=6).
• Lin (Review of Published Export Coefficient and Event Mean Concentration (EMCs) Data. This report includes summaries of multiple studies conducted in North America. Data existed for all land uses included in the table presenting recommended emcs.
• Washington District Department of the Environment - Selection of Event Mean Concentrations (EMCs). This study summarized data from studies in the Washington D.C. area. Land uses included commercial, roadway/highway, industrial, forest/open, and residential.
• U.S. Environmental Protection Agency, 1983, Results of the Nationwide Urban Runoff Program—Executive summary. Land uses included commercial and residential.
• Urban Stormwater Quality, Event-Mean Concentrations, and Estimates of Stormwater Pollutant Loads, Dallas-Fort Worth Area, Texas, 1992–93. Included commercial (n=42), residential (n=77), industrial (n=63) land uses.
• A review of sediment and nutrient concentration data from Australia for use in catchment water quality models . A compilation of multiple studies from Australia. Included forest (n=68) and mixed (n=36) land uses.
• Characterization of Urban Runoff Pollution between Dissolved and Particulate Phases . Study of five sites in China. Land uses included roof and transportation (roads).
• Quality Of Wisconsin Stormwater, 1989-94. Samples from mixed land use in Wisconsin (n=204).
• Analysis of Nonpoint Source Pollution Runoff from Urban Land Uses in South Korea. 23 samples in Korea from high density residential, medium desntiy residential, industrial, institutional land uses.
• Seasonal Performance Variations for Storm-Water Management Systems in Cold Climate Conditions. 15 samples from transportation land use in New Hampshire.
• Determination of event mean concentrations and first flush criteria in urban runoff. 31 samples from transportation land use in Los Angeles.
• Multiple linear regression models of urban runoff pollutant load and event mean concentration considering rainfall variables. 45 samples from commercial, industrial, and high density land uses.
• Stormwater runoff driven phosphorus transport in an urban residential catchment: Implications for protecting water quality in urban watersheds. 29 events from low density land use in Florida.
• Sources of phosphorus and street dirt from Two Urban Residential Basins in Madison, Wisconsin, 1994-95. 25 samples from medium density land use in Wisconsin.
• Nutrient Sources in Urban Areas – A Literature Review. Report summarizing multiple studies in Colorado. Land uses include residential, mixed, commercial, and open space.
• Contribution of surface runoff from forested areas to the chemistry of a through-flow lake. Forested land use in Poland.

Event mean concentrations for total phosphorus

Pollutant loads in stormwater runoff equal the pollutant concentration times the runoff volume. Thus, when calculating pollutant loading, it is necessary to consider not only the event mean concentration but factors affecting the volume of runoff. For most models and calculations, this requires adjusting curve numbers or runoff coefficients to account for differences in directly connected impervious surface between different land uses. There may be other adjustments to volume, such as accounting for interception by trees. See the discussion [Accounting for differences in pollutant loading].

The following table summarizes results from our literature review. The table includes a range of values observed in the literature. Note this range does not represent a statistically-derived range but instead is based on a combination of data analysis and best professional judgement. For example, we did analyze the data for outliers, but also omitted entire studies if we felt the data were not representative of conditions likely to be encountered in Minnesota. To see the full range of values compiled from the literature, open the Excel spreadsheet containing the data.

Residential land use

Studies from the literature frequently provide concentrations for residential land use or occasionally for different types of 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.

• Residential: land use where the current or intended use includes, but is not limited to, housing (single and multiple dwellings), educational facilities, day care, agricultural land, correctional facilities, custodial care or long term health care.
• High-density residential: More than 10 units per acre; can include multiple-occupant dwellings
• Medium-density residential: 1-10 dwellings per acre; can include multiple-occupant dwellings
• Low-density residential: one dwelling per 1-5 acres; can include multiple-occupant dwellings

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.

• Baldys et al., (1998). 77 data points from Dallas-Fort Worth area. Median concentration of total phosphorus = 0.33 mg/L
• Yand and Toor, (2018). 35 data points from Florida. Median concentration of total phosphorus = 0.29 mg/L
• Waschbusch et al. (1999). 25 data points from Madison Wisconsin. Median concentration of total phosphorus = 0.17 mg/L
• National Urban Runoff Program (1983). Summary of data from 28 studies in the U.S. Median concentration of total phosphorus = 0.38 mg/L
• Olson et al., (2017). Data from 254 samples in Colorado. Median concentration of total phosphorus = 0.45 mg/L
• Capital Region Watershed District. Outfall data from 9 catchments in St. Paul, Minnesota. Median concentration of total phosphorus = 0.325 mg/L
• National Stormwater Quality database. Data from 249 locations in Region 1 (including Minnesota). Median concentration of total phosphorus = 0.27 mg/L

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.

Information: The recommended event mean concentration for total phosphorus in residential areas is 0.325 mg/L

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. The following factors are important considerations in determining an emc for an area.

1. 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.
2. 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.
3. Time between dry periods. Typically, street solids tend to build up over time and reach a maximum value within one or two weeks after the previous rainfall event. Yang and Toor (2018) observed higher fractions of particulate phosphorus compred to dissolved phosphorus in runoff as the period between runoff events increased. Schiff et al. (2011) observed similar patterns in California, where dry periods can be lengthy.
4. Characteristics of permeable surfaces in the area of interest.

Commercial land use

"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.

• Baldys et al., (1998). 42 data points from Dallas-Fort Worth area. Median concentration of total phosphorus = 0.14 mg/L
• National Urban Runoff Program (1983). Summary of data from 28 studies in the U.S. Median concentration of total phosphorus = 0.20 mg/L
• Olson et al., (2017). Data from 254 samples in Colorado. Median concentration of total phosphorus = 0.22 mg/L
• National Stormwater Quality database. Data from 165 locations in Region 1 (including Minnesota). Median concentration of total phosphorus = 0.11 mg/L
• van Guerard and Weiss (1995). Data from 7 samples in Colorado Springs. Median concentration of total phosphorus = 0.12 mg/L
• Wang et al. (2013). Data from 20 samples in Chongqing, China. Median concentration of total phosphorus = 0.12 mg/L

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.

• Harper (1998) reported median concentrations of 0.15-0.43 mg/L in Florida
• Brezonik and Stadelmann (2001) reported concentrations of 0.22-0.77 mg/L in Minnesota

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.

Information: The recommended event mean concentration for total phosphorus in commercial areas is 0.200 mg/L

Model adjustments

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.

• Burton and Pitt (2002) reported export rates of 1.5 lb/ac/yr for commercial, 1.0 lb/ac/yr for high-density residential, 0.3 lb/ac/yr for medium density residential, and 0.03 lb/ac/yr for low density residential land use.
• Loehr et al. (1989) reported exported TP exports of 0.1-7.6 kg/ha/yr compared to 0.77-2.2 kg/ha/yr for residential land use.
• Baldys et al. (1998) reported TP export of 6.23 lbs/mi²/yr, compared to 4.96 lb/mi²/yr for residential land use in the Dallas-Fort Worth area

Adjustments may therefore be needed depending on the specific model being used. See the discussion on modeling adjustments below.

Industrial land use

We used the following studies in our analysis.

• Baldys et al., (1998). 63 data points from Dallas-Fort Worth area. Median concentration of total phosphorus = 0.21 mg/L
• Olson et al., (2017). Data from 254 samples in Colorado. Median concentration of total phosphorus = 0.25 mg/L
• National Stormwater Quality database. Data from 84 locations in Region 1 (including Minnesota). Median concentration of total phosphorus = 0.235 mg/L
• Rhee et al. (2012), data from 24 locations Bundang-gu, Gyeonggi provinces, South Korea. Median concentration of total phosphorus = 0.11 mg/L
• Capital Region Watershed District, St. Paul, Minnesota, Hidden Falls outlet (which drains an industrial area). Median concentration of total phosphorus = 0.29 mg/L

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.

• Cleaning and washing operations
• Heavy vehicle traffic
• Specific industries such as food processing plants, meat packing plants and lockers, metal finishing facilities, and industries that generate or handle animal waste (including human sources)

Information: The recommended event mean concentration for total phosphorus in industrial areas is 0.235 mg/L

Model adjustments

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.

• Burton and Pitt (2002) reported export rates of 1.3 lb/ac/yr for industrial, 1.0 lb/ac/yr for high-density residential, 0.3 lb/ac/yr for medium density residential, and 0.03 lb/ac/yr for low density residential land uses.
• Loehr et al. (1989) reported exported TP exports of 0.8-12.5 kg/ha/yr compared to 0.77-2.2 kg/ha/yr for residential land use.
• Baldys et al. (1998) reported TP export of 8.5 lbs/mi²/yr, compared to 4.96 lb/mi²/yr for residential land use in the Dallas-Fort Worth area.

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

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.

• National Urban Runoff Program, which consists of a nationwide study. Median concentration of total phosphorus = 0.121 mg/L
• The National Stormwater Quality Database (Region 1, which includes Minnesota). Median concentration of total phosphorus = 0.19 for 6 sites
• Olson et al., (2017). Data from 254 samples in Colorado. Median concentration of total phosphorus = 0.22 mg/L

Parks and recreation areas are generally included in open space.

Information: The recommended event mean concentration for total phosphorus in open space, urban parks, and urban recreations areas is 0.190 mg/L

Transportation corridors, highways, and freeways

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.

• Olson et al., (2017). Data from 254 samples in Colorado. Median concentration of total phosphorus = 0.28 mg/L
• Baldys et al., (1998). 28 data points from Dallas-Fort Worth area. Median concentration of total phosphorus = 0.21 mg/L
• Wei et al. (2013). 12 data points from Handan, China. Median concentration of total phosphorus = 0.625 mg/L
• Roseen et al. (2009). Data from 15 events in New Hampshire. Median concentration of total phosphorus = 0.125 mg/L
• Schiff and Tiefenthaler. Data from 31 events in California. Median concentration of total phosphorus = 0.41 mg/L

The median value from these studies is 0.28 mg/L.

Information: The recommended event mean concentration for total phosphorus in transportation areas is 0.280 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.

Roofs

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.

Information: The recommended event mean concentration for total phosphorus in runoff from conventional (non-green) roofs is 0.030 mg/L

Mixed land use

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 (A_total), 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 ((A_{Area 1} * emc_{Area 1})/ (A_total) + ... ((A_{Area n} * emc_{Area n}) / (A_total)

where A = area in acres.

Example calculation

• 10 acres of residential; emc = 0.325 mg/L
• 10 acres of commercial; emc = 0.200 mg/L
• 10 acres of industrial' emc = 0.235 mg/L
• 1 acre of transportation; emc = 0.28 mg/L

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	If applicable to models being used, adjust curve numbers/runoff coefficients when calculating loads Adjust upward if specific phosphorus sources exist
Residential	0.26 - 0.38	0.325	Concentrations vary widely depending on local conditions
High-density/Multi-family residential	0.28 - 0.40	Calculate1	Insufficient information to recommend a specific emc Concentrations vary widely depending on local conditions
Medium density residential	0.18 - 0.40	Calculate1	Insufficient information to recommend a specific emc Concentrations vary widely depending on local conditions
Low density residential	0.24 - 0.40	Calculate1	Insufficient information to recommend a specific emc Concentrations vary widely depending on local conditions
Freeways/transportation	0.25 - 0.45	0.280	Concentrations vary widely depending on inputs Adjust upward for areas receiving large inputs of road salt or sediment or having very heavy traffic loads Adjust downward for low traffic areas or areas with reduced inputs (e.g. little road salt application, limited truck traffic)
Mixed	0.16 - 0.84	0.290	Residential land use was the primary land use in most studies that cited values for mixed land use If the study area can be delineated into specific land uses and impervious area for each land use is know, we recommend calculating the emc
Parks and recreation		Use value for open space or calculate	emc will be a function of vegetative cover Adjust upward if street tree canopy cover is high or pervious areas are primarily grass on compacted soils
Open space	0.12 - 0.31	0.190
Conventional roof	0.01 - 0.20	0.030
Institutional	0.14 - 0.422	See note	Use low values in range (0.200 mg/L or less) for facilities such as campuses, where there is considerable pervious area Use high values in range (0.30 mg/L or greater) for areas with considerable impervious surface, such as sports facilities or facilities with large parking areas
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)	If data exist, use the phosphorus concentration for the water body of interest If data for a specific lake do not exist, use data from similar lakes in the area emcs for wetlands will typically be higher than for lakes in an area. Consider using a value equal to 2 times the value for lakes in an area.
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

Effects of season on total phosphorus emcs

Median total phosphorus (TP) concentrations (mg/L) by month for 9 monitoring stations in Capitol Region Watershed District.

Median ratios of dissolved phosphorus to total phosphorus for 9 monitoring stations in Capitol Region Watershed District.

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.

• Five of the nine monitoring stations showed statistically significant differences in monthly concentrations, while four stations showed no statistically significant differences. Overall, concentrations tended to be lowest in April and highest in February, and May through July. December concentrations were low but only three stations had data for December.
• Median concentrations across all nine stations were 0.325 mg/L for snowmelt runoff and 0.304 mg/L for storm runoff. For the five stations with data for January and February, median concentrations were 0.508 mg/L for snowmelt runoff and 0.304 mg/L for storm runoff. This suggests phosphorus in winter deicers may contribute to higher concentrations in winter. Analysis of chloride data in conjunction with phosphorus data would provide additional insight into this hypothesis. Winter phosphorus loads from snowmelt runoff are likely to be relatively low despite the higher concentrations, due to less runoff. Conversely, although concentrations tend to be lower in April, this may be due to dilution. Thus phosphorus loading in April may be high despite what appear to be lower concentrations.
• Seven of the nine monitoring stations showed statistically significant monthly differences in dissolved phosphorus to total phosphorus (DP:TP) differences. The greatest DP:TP ratios occurred from November through March. DP:TP ratios dropped sharply from March to April and remained relatively steady through September, when ratios began increasing. Median DP:TP ratios were 0.283 for snowmelt runoff and 0.224 for storm runoff. Overall, across all sites, the median DP:TP ratio was 0.273.

Effects of time between and during runoff events on total phosphorus emcs

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 first flush is more pronounced as the time between runoff events increases, due to buildup of solids.
• First flush effects are more pronounced in smaller watersheds with a high percentage of directly connected impervious surface.
• First flush effects vary with phosphorus sources in a watershed. For example, larger watersheds with significant phosphorus sources in the upper portions of the watershed do not typically show a first flush effect.
• Dissolved (soluble) phosphorus to total phosphorus ratios tend to increase during a runoff event.
• First flush is more evident with higher initial rainfall intensities.

Event mean concentrations for dissolved phosphorus

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

Accounting for differences in pollutant loading

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

Factors affecting event mean concentrations for phosphorus

Event mean concentrations (emcs) vary with several factors and therefore should be used with caution. For a discussion of emcs see the page titled Stormwater pollutant concentrations and event mean concentrations.

References

• Schiff, Kenneth C., and Liesl L. Tiefenthaler. 2011. Seasonal flushing of pollutant concentrations and loads in urban stormwater. Jour. Amer. Water Works Assoc. 47:1:136-143
• Yang, Yun-Ya, and Gurpal S. Toor. 2018. runoff driven phosphorus transport in an urban residential catchment: Implications for protecting water quality in urban watersheds. Scientific Reports. 8:1-10
• University of Wisconsin at Milwaukee. Phosphorus Speciation and Loads in Stormwater and CSOs of the MMSD Service Area (2000 – 2008) Final Report: February, 2011
• Lee-Hyung Kim. 2003. Determination of event mean concentrations and first flush criteria in urban runoff. Environ. Eng. Res. 8:4:163-176.
• Jae-Woon Jung, Ha-Na Park, Kwang-Sik Yoon, Dong-Ho Choi, Byung-Jin Lim. 2013. Event mean concentrations (EMCs) and first flush characteristics of runoff from a public park in Korea. Journal of the Korean Society for Applied Biological Chemistry. October 2013, Volume 56, Issue 5, pp 597–604.