The MPCA estimator worksheet presents an optional calculator to compute the pollutant load reduction for total phosphorus (TP), total suspended solids (TSS), E. coli (EC), and fecal coliform (FC). Results from the estimator can be used in the Cumulative reductions tab of the TMDL Annual Report form.
Download MPCA Estimator here: File:MPCA Estimator.xlsx
Gather BMP information
STEP 1: Define the watershed boundaries of each TMDL study area that intersects your jurisdiction. Steps 2-8 are completed for each TMDL calculation.
STEP 2: For each TMDL, determine which BMPs are reducing loads that can be quantified in the estimator.
Input data to estimator
STEP 3. Enter the TMDL Name in Row 2. This name will match a TMDL name from the BMP-Activities Completed tab.
STEP 4. Calculate the acreage for each applicable land use within the TMDL watershed (Column F, rows 6 through 17). Note default acreage of 0.001 is entered for each land use, since an acreage of 0 leads to calculation errors.
STEP 5. Determine the Annual Rainfall (Column G, rows 6 through 17). The default is 30.65 inches.
STEP 6. Determine if the default Event Mean Concentration (EMC) is appropriate for your situation (Columns B through E; rows 6 through 17).
Once acreages are entered for land uses, a total load is auto-calculated for the pollutant of concern. In the example below, this TMDL has 5 acres of commercial land and 5 acres of industrial land. Default values for rainfall and EMC are being used. Total loading is 16.15 lbs of TP, 10524 lbs of TSS, 1.76e7 EC organisms, and 2.03e7 FC organisms.
STEP 7. Choose the calculator that matches the pollutant for the TMDL you chose. The screenshot below shows the appropriate calculator for a Phosphorus TMDL. Enter the acreage treated by each BMP type for each appropriate land use within the appropriate reductions calculator. The acreage is a cumulative acreage for all BMPs of the same type.
Once acreages are entered, total pounds (for TP in this example) and percent load reduced are auto-calculated. In the example below, the permittee operates three acres of sand filters (could be multiple sand filters totaling 3 acres) in commercial areas and two acres of swales (could be multiple swales totaling 2 acres) in industrial areas. The total pounds of TP reduced are 3.088 and the total TP reduced is 19.12 percent. As a check, the worksheet will generate an error message if the acreages entered exceed total land use acreage.
STEP 8. Input the calculated reductions (choose pounds reduced or percent load reduced) to Category 1 in the Cumulative Reductions tab for that reporting year and TMDL.
