Frequently Asked Questions (FAQs) for the MPCA Simple 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.
Contents
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.
EMCs (event mean concentrations), runoff coefficients, pollutant removal
- Where can I find information on emcs?
- Where can I find information on runoff coefficients?
- Does the MPCA recommend how to adjust emcs or runoff coefficients?
- The MPCA provides information on adjusting emcs at the two links provided above. We do not provide guidance on adjusting runoff coefficients.
- 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.
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.
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.