Modeling Methodology for Annual Pollutant Loading Estimates

RUNOFF

Rainfall runoff volumes were estimated for each of the four subwatershed in the study as well as for the smaller contributing areas within each of the four subwatersheds. The estimation method is based on the 1993 U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) document Urban Hydrology for Small Watersheds (Technical Release 55). For each subwatershed, hydrologic soil groups were determined for the watershed from information available in the electronic Soil Survey of Kalamazoo County, MI (USDA, 1993). Soils were classified as group A, B, C or D, with A possessing the lowest runoff potential (highest infiltration capacity) and group D having the highest runoff characteristics. The soil survey information was overlapped with the land use/land cover layer within ArcView software to yield the specific areas in each subwatershed area possessing certain runoff characteristics.

A runoff curve number (CN) was assigned to each of the areas possessing unique runoff characteristics based on information in the USDA Technical Release 55. These curve numbers reflect the degree of infiltration expected in a given watershed area based on soil type and land cover. An area-weighted curve number, which accounts for all land cover/soil type combinations, was determined for each of the subwatersheds. Rainfall events of various frequencies were then modeled using regional rainfall data from the Rainfall Frequency Atlas of the Midwest (Huff and Angel, 1992). The rainfall volumes presented in each table represent estimates of rainfall runoff expected for the subwatersheds based on soil type, land cover and storm frequency.

Pollutant Load Estimates

Pollutant loads from stormwater runoff were calculated for the four project subwatersheds and the smaller contributing areas within each subwatershed using the Event Mean Concentration (EMC) method. EMCs are estimates of nonpoint source pollution determined in the USEPA Nationwide Urban Runoff Program (NURP). With this method, stormwater pollutant loadings are based on pollutant loading factors that vary by land use type and percent imperviousness (Wayne County, 1998). Loads can be computed using Equations 1 and 2 as follows:

M_L = EMC_L * R_L * K Eq. 1

Where:

M_L = Loading factor from land use L (lbs/ac/yr)

EMC_L = Event mean concentration of runoff from land use L (mg/L)

R_L = Total average annual surface runoff from land use L computed from Eq. 2

K = Unit conversion factor of 0.2266

Runoff Equation:

R_L = [ C_P + ( C_I - C_P ) IMP_L ] * I Eq. 2

Where:

R_L = Total average annual surface runoff from land use L (in/yr )

IMP_L = Fractional imperviousness of land use L

I = Long term average annual precipitation (in/yr)

C_P = Pervious area runoff coefficient (0.20)

C_I = Impervious area runoff coefficient (0.95)

Equation 1 shows that the loading factor (M_L) for land use L is the product of the event mean concentration for land use L, the annual runoff for land use L, and a unit conversion factor. The runoff calculation in Equation 2 provides the R_L value used in Equation 1 through the product of the annual rainfall depth (34" from Kalamazoo rainfall records) and the percent imperviousness of land use L, with the tuning coefficients C_P and C_I. The loading factor, M_L, is multiplied by the area of land use L to obtain a total annual loading for that land use. The pollutant load estimates for each of the four subwatersheds and the corresponding contributing areas are provided in the tables.

REFERENCES:

Huff, Floyd A. and Angel, James R. Rainfall Frequency Atlas of the Midwest. 1992. Illinois State Water Survey, Champaign, Bulletin 71.

State of Michigan Office of Regulatory Reform (MI-ORR). 2002. Part 30 - Water Quality Trading Rules.

U.S. Department of Agriculture, Natural Resources Conservation Service. 1993. Urban Hydrology for Small Watersheds (Technical Report 55). Washington, D.C.