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.
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?
EMCs (event mean concentrations) and runoff coefficients
- Where can I find information on emcs?
- Where can I find information on runoff coefficients?
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
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.