Frequently Asked Questions (FAQs) for the MPCA Simple Estimator Frequently Asked Questions (FAQs) for the MPCA Simple Estimator

Screen shot of Section 1 of the Estimator.

The MPCA Simple Estimator is a spreadsheet tool that calculates pollutant loading and load reductions associated with stormwater best management practices (BMPs). The spreadsheet utilizes the Simple Method.

This page provides answers to some questions about the Estimator. Like any calculation tool or model, there is a certain amount of user expertise and best professional judgement involved in calculating stormwater loads and load reductions. Consequently, some of the information on this page reflects recommendations for how to use the Estimator rather than specific answers and approaches.

Link to more information on the MPCA Simple Estimator

General questions

• How is acreage determined for Section 1 of the Estimator?
  • The Estimator requires the user to assign acreages to different land uses. Each land use has associated event mean concentrations and runoff coefficients, which affect the total pollutant load. It is therefore important to accurately portray the study area and assign correct acreages to each land use. We recommend the user initially develop a site map and delineate land uses on that map. Then determine acreages associated with each land use and enter into Section 1 of the spreadsheet.
• If the user makes changes to various parameters, are there places to enter notes in the spreadsheet for each and all of the changes?
  • The preferred place to enter notes is on the Notes tab. When adding notes, make sure to reference to which the note applies the applicable location within the spreadsheet.
• How are stormwater retention ponds tracked?
  • These are considered wet ponds
• Where would a user track alum treatments?
  • Alum treatments can be credited in Section 2 if they are for a stormwater structural practice, such as a wet pond or constructed wetland. Alum treatments for receiving waters cannot be credited unless the water from a receiving water discharges back into the stormwater conveyance system.

Loads

• Are the loads calculated in Sections 1 (unadjusted loads) and 2 (adjusted loads) for all runoff or just treated runoff?
  • The loads in these sections are for all stormwater runoff, which is a function of rainfall and the runoff coefficient. In Sections 3 and 4, the user specifies how much of the total annual runoff is treated.
• How does the baseline year in a TMDL factor into the calculations?
  • The baseline year is typically used by a permittee as the starting point for reducing loads; that is, TMDL loading reductions are from a baseline year. While the unadjusted load in Section 1 is considered a raw load with no BMPs, it may be useful for a permittee to design section 1 of the Estimator to reflect the baseline year. If this is done, then calculations in Sections 2-4 reflect changes from the baseline condition. Cities could also reflect the baseline year in Section 2, but would then need to track pollutant reductions associated with non-structural practices outside of the Estimator or using a second spreadsheet.
• How are loads tracked from year to year?
  • The Estimator does not track loads over time. The user should track this information separately.
• How do I track loads from a TMDL if they are written in units different from the units in the Estimator?
  • The user could track percent reduction in loads, which is not affected by units, or convert the TMDL units to the same units used in the Estimator.
• How are nonstructural practices such as street sweeping credited.
  • There is no specific approach for crediting non-structural practices. The user could adjust emcs or runoff coefficients based on data and professional judgement. If the user has data on actual load reduced with a practice, they could back-calculate to an emc or runoff coefficient that gives a mtach in pollutant loading to what they have observed. For example, if enhanced street sweeping results in 100 pounds of phosphorus removed annually, adjust the emc in the Estimator until you get a 100 pound reduction.

EMCs (event mean concentrations), runoff coefficients, pollutant removal

• Where can I find information on emcs?
  • Event mean concentrations of total and dissolved phosphorus in stormwater runoff
  • Event mean concentrations of total suspended solids in stormwater runoff
• Where can I find information on runoff coefficients?
  • Runoff coefficients for different soil groups and slopes
• What practices or conditions might result in a change in emc?
  • There are many practices and conditions that affect emc. Examples of site conditions that likely will increase an emc include increased tree canopy, greater connectivity of impervious surface, small watersheds with highly connected impervious surface, greater runoff from pervious surfaces (e.g. compacted soils), increased construction activity, and winter application of road salt and sand. We suggest reviewing information on the pages Event mean concentrations of total and dissolved phosphorus in stormwater runoff and Event mean concentrations of total suspended solids in stormwater runoff
• Does the MPCA recommend how to adjust emcs or runoff coefficients? Practices that affect emcs in runoff include a wide variety of non-structural practices, such as street sweeping, lawn yard debris collection, and removal of illicit discharges. Structural practices that return treated water to the runoff system also decrease emcs in runoff.
  • The MPCA provides information on adjusting emcs at the two links provided above. We do not provide guidance on adjusting runoff coefficients.
• What practices might result in a change in runoff coefficient?
  • The runoff coefficient should be modified for any practice that affects runoff volume. The default coefficients are based on B soils on flat slopes (0-2%). The coefficient should be increased for C and D soils and steeper slopes, and decreased for A soils. Other conditions that may affect runoff include, increased or decreased compaction of soils in pervious areas, disconnecting impervious surfaces, runoff collection (e.g. rain barrels), and increased or decreased tree canopy. See Runoff coefficients for different soil groups and slopes, Event mean concentrations of total and dissolved phosphorus in stormwater runoff and Event mean concentrations of total suspended solids in stormwater runoff.
• Is there information on pollutant removal for individual BMPs?
  • Yes. Click on the links to individual BMPs in the Estimator. You may also find the information at this link.
• When should I adjust pollutant removal efficiencies?
  • We recommend using the values in the Minnesota Stormwater Manual unless you have data to suggest a different value or a designed BMP to which values in the manual don't apply. For different designs, the user would have to make this determination. Adjustments to removal efficiencies are entered in Row 66 for TP and Row 91 for TSS. Examples of conditions where a removal efficiency could be changed include adding an amendment to a BMP to improve treatment (e.g. iron), using engineered media mixes that do not effectively retain phosphorus (e.g. mixes A, B, E, F), structurally enhanced practices (e.g. increased forebay size for a constructed pond), and increased pretreatement.
• What removal efficiency should I use for modified BMPs?
  • The pollutant removals in the Minnesota Stormwater Manual apply to the designs described in the manual. If the user has a modified design, they will have to determine the pollutant removal efficiency. There may be information in the literature, or contact us.
• What conditions apply when fraction treated or infiltrated changes?
  • The default treatment fraction is 0.90, which roughly corresponds with retention of 1 inch of runoff from impervious surfaces. With increased precipitation and increased intensity of precipitation, the default of 0.90 is likely to overestimate the fraction of annual runoff captured and treated. Consider adjusting the value down for areas with higher annual precipitation. Consider adjusting the value down as the contributing acreage from C and D soils increases. The value should be adjusted down for undersized bmps. The default infiltration values for filtration practices are based on infiltration into C soils. Adjust the value down in D soils. Certain designs, such as an upturned elbow in an underdrain, may increase the fraction of water infiltrating. Raising an underdrain may increase infiltration on C soils.

Land use

• How do I determine what land use to use?
  • See this section for definitions of land uses.
• Why is there only one residential land use option?
  • Users can create additional land uses, including different types of residential land use, such as low density residential. In the land use column of section 1, enter the desired land use and associated emc and runoff coefficient.

Drainage areas and subwatersheds

• How should I determine what sunwatersheds to use/create?
  • This would be up to the user to decide. We recommend the user initially develop a site map, including the location of structural BMPs. The user can then determine an appropriate way to model the area, including the possibility of making calculations based on drainage areas to structural practices. It is possible that more than one spreadsheet will be used if there are many subwatersheds and BMPs.
• Do the acreages in Sections 3 and 4 apply to drainage acreages to structural practices, and not the acreage of the practice?
  • Yes.
• How do I model a system with multiple BMPs and a regional pond?
  • We suggest modeling the area draining to the regional pond separately. Modeling for BMPs upstream of the pond depends on how the practices are connected in a treatment train. The user will have to decide when BMP drainage areas should be separated from each other.

Documentation and submittals

• What will MPCA look for in submittals?
  • This page provides some information about what MPCA's review might consider.
• What level of sub watershed is the MPCA expecting? The the area in the permittee identified in the TMDL/WLA, or a more detailed HUC subwatershed?
  • This is a decision the user needs to make. Use a drainage system that makes sense from a modeling standpoint. It may be that multiple Excel files would be needed for larger and more complex drainage systems.