Tips for using the MIDS Calculator for tracking pollutant loading

The Minimal Impact Design Standards (MIDS) Calculator was initially designed to assess progress toward meeting a volume-based performance goal at individual sites and development projects. The default goal in the Calculator is 1.1 inches off impervious surface. The Calculator translates this into annual values for runoff volume, TSS, and total phosphorus retention. Specific information about the modeling used in developing the Calculator is found here.

The MIDS Calculator is being widely used for water quality purposes, such as tracking progress toward meeting total maximum daily loads (TMDLs). For this purpose, the MIDS Calculator has limitations, including but not limited to the following.

• default event mean concentrations for TSS and TP that are applied across the entire area being modeled
• default curve numbers for A, B, C, and D soils that are applied across the entire area being modeled
• default particulate and dissolved phosphorus fractions of 0.55 and 0.45, respectively, that are applied across the entire area being modeled
• default rain values that are linked to a zip code
• limited ability to model certain BMPs, such as proprietary devices, street sweeping, creation of natural space, improved maintenance, removal of illicit discharges, and aggressive pretreatment.

This page provides some tips for addressing these limitations to more accurately simulate pollutant loading using the MIDS Calculator.

NPDES permit considerations

Note that, where applicable, there are warnings for situations where manipulating the Calculator is not consistent with requirements in the Construction Stormwater Permit. An example would be creation of natural space for water retention or disconnection of impervious surfaces for water retention. These situations are identified with the following alert box.

Warning: This practice is not consistent with requirements of the Construction Stormwater General Permit

Adjusting event mean concentrations

The MIDS Calculator has default event mean concentrations (emc) of 0.3 and 54.5 mg/L for total phosphorous (TP) and total suspended solids (TSS), respectively. These values can be changed. Changing the values triggers a notification that the default value has been changed. Only a single emc can be assigned to an individual model run. Thus, if you are simulating pollutant loading for an area where emc varies, individual model runs are required for each modeled area.

This manual contains pages discussing appropriate emcs for different land uses. There is also discussing of modifying emc within a specific land use, such as modifying TP concentrations to account for differing tree canopy coverage. The following links provide this information.

• Event mean concentrations of total and dissolved phosphorus in stormwater runoff
• Event mean concentrations of total suspended solids in stormwater runoff

Comparison of pollutant loading between MIDS defaults emcs and land use-specific emcs
Condition	TP (lbs)	TSS (lbs)
Residential	2.09	451.3
Commercial	1.24	463.7
Industrial	1.45	587.3
TOTAL - adjusted emcs	4.78	1502.3
MIDS Default	5.56	1010.8

As an example, consider a development with the following land uses. Using the links above, the emcs for these land uses are as follows.

• Residential: TP emc = 0.325 mg/L; TSS emc = 73 mg/L
• Commercial: TP emc = 0.200 mg/L; TSS emc = 75 mg/L
• Industrial: TP emc = 0.235 mg/L; TSS emc = 95 mg/L

Assume a default annual rainfall of 31.9 inches and one acre of impervious surface for each land use, with no pervious surface. The pollutant loads with these adjusted emcs are shown in the adjacent table.

Differences in pollutant loading are significant, with 14 percent more phosphorus and 49 percent less TSS generated using default values. Note that no pervious surface was included in the loading. Thus, the runoff coefficients for the three land uses would be the same. Adjusting runoff coefficients to match typical values would further affect the results. This is discussed below.

Accounting for different fractions of particulate and dissolved phosphorus

Graph illustrating the correction factor to use for different fractions of particulate and dissolved phosphorus. To calculate the concentration of PP or DP for every one pound of total P in runoff, multiply the MIDS default fraction by the corrector. See the text for an example calculation.

Particulate phosphorus (PP) accounts for 55 percent of total phosphorus (TP) in the MIDS Calculator, with dissolved phosphorous (DP) accounting for the remaining 45 percent. For a discussion of phosphorus in stormwater, link here.

There is no way to simulate situations where the fraction of particulate or dissolved phosphorus differ. Results from the Calculator can be used outside the Calculator, such as in an Excel spreadsheet, to calculate removal of PP and DP if the fractions differ from the MIDS default values. The adjacent graph illustrates correction factors that can be used to determine the initial concentrations of PP and DP.

Example: Assume there is 1.8 pounds of total phosphorus in runoff at a site. The default values for PP and DP are 0.99 and 0.81 pounds, respectively. Assume the actual fraction of PP in runoff is 0.7, giving a value of 0.3 for DP. Using the adjacent graph, multiply the PP and DP amounts by the appropriate correction factor. The correction factors are 1.27 and 0.67 for PP and DP, respectively. Therefore, PP = 1.27*0.99 = 1.26 pounds and DP = 0.81*0.67 = 0.54 pounds.

For infiltration BMPs, all the phosphorus infiltrated is removed. Therefore, to calculate the amount of PP and DP removed by the BMP, simply calculate PP and DP in the runoff as described above, and then multiply by the removal percent for the BMP.

Example: Using the above example, assume an infiltration BMP removes 90 percent of the annual phosphorus. Multiply 0.9 by the PP and DP in the runoff to determine the amount removed. PP = 0.9*1.26 = 1.13 pounds and DP = 0.9*.54 = 0.49 pounds removed.

For non-volume BMPs, multiply the annual volume treated by the total pounds by the removal percent.

Removal percents for non-volume BMPs
BMP and condition	Particulate P	Dissolved P no amendment	Dissolved P w/ amendment)
Pond level 1	60	0	40
Pond level 2	84	8	40
Pond level 3	90	23	40
Example	55	0	NA
Wet swale	0	0	NA
Sand filter	41	0	NA
Sand filter	47	0	NA
Sand filter	50	0	NA
Sand filter	-	-	40
Sand filter	-	-	60