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Overview of models used to meet MS4 TMDL permit requirements

The MS4 (Municipal Separate Storm Sewer System) permit requires permittees to demonstrate progress toward meeting applicable total maximum daily load (TMDL) wasteload allocations (WLAs). Permittees claiming to meet a WLA must document how they achieved the WLA. Whether a permittee is making progress toward or has achieved a WLA, they must quantify pollutant loads or load reductions.

Models are frequently used to calculate pollutant loading or load reductions associated with stormwater management practices. Many water quality models can be used to demonstrate permit compliance. Based on a survey of permittees, the following four models are commonly used for assessing or demonstrating TMDL permit compliance.

  • Program for Predicting Polluting Particle Passage thru Pits, Puddles, & Ponds (P8; Walker, 1990)
  • Minimal Impact Design Standards Calculator (MIDS Calculator; MPCA, 2017)
  • Minnesota Pollution Control Agency Simple Estimator (MPCA Simple Estimator; MPCA, 2015)
  • Source Loading and Management Model for Windows (WINSLAMM)

We developed guidance for using these four models to meet permit requirements. Model guidance for each of the four models is provided on separate pages (P8; WinSLAMM; MIDS calculator; MPCA Simple Estimator).

Individual MS4 permittees can use the summary information in the following tables to determine which model is most-suitable for demonstrating permit compliance based on circumstances unique to the MS4 and given WLA requirements.

  • TMDL model description and overview: A brief description of each model and its applicability for demonstrating permit compliance
  • TMDL model comparison matrix: A matrix comparing general model capabilities and relative complexity
  • Water quality BMP comparison matrix for TMDL models: A list of water quality best management practices (BMPs) which can be modeled in each program and a description of BMP types which are specifically included as a unique BMP type with unique BMP parameterization (i.e., “specific”) and which BMPs are not specifically included but can be modeled using general or user-defined model inputs (i.e., “general”).

Costs to develop the models should generally range between $5,000 and $20,000 for evaluating compliance with TSS or TP TMDL WLAs that involve approximately ten BMPs in ten subwatersheds for five or fewer outfalls draining to an impaired water body.

Models used to meet TMDL compliance and reporting requirements
Summary of models used to meet TMDL compliance and reporting requirements
Model Model description Applicability for Permit Compliance and Reporting
P8 P8 is a physically-based water quality model which simulates the generation and transport of sediment and associated pollutants from urban watersheds. The model is capable of predicting sediment particulate removal of five (5) particle sizes (including one soluble fraction) and associated pollutants at a variety of BMP types. P8 is an acceptable model for demonstrating compliance with TSS and TP WLAs.
MIDS calculator The MIDS Calculator is an Excel-based stormwater quality tool used to estimate runoff and pollutant removal at a variety of stormwater BMPs. The model was originally developed by the MPCA to assist designers and regulators evaluate conformance to MIDS performance goals for development-scale models. The MIDS Calculator is an empirical model which predicts pollutant removal based on correlation to P8 results and design-standard BMP removal rates from literature. MIDS calculator is an acceptable model for demonstrating compliance with annualized TSS and TP WLAs for simplified study areas. Specifically, the tool is limited in its ability to evaluate bypass from undersized BMPs and predict pollutant removal through non-volume reduction BMPs in series.
MPCA Simple Estimator The MPCA Simple Estimator is a spreadsheet-based tool that utilizes the Simple Method to estimate land use based pollutant loading from urban watersheds. The empirically-based model estimates pollutant removal from nine (9) BMP types based on design-standard BMP removal rates from literature. The MPCA Simple Estimator is an acceptable model for demonstrating compliance with annualized TSS and TP WLAs for simplified study areas. The tool is not capable of evaluating bypass from undersized BMPs or pollutant removal through BMPs in series.
WinSLAMM WinSLAMM is a water quality model originally developed for the USGS to evaluate nonpoint pollution in urban areas. The model predicts pollutant loading from a variety of land use and impervious area types and calculates pollutant reduction at a variety of control devices (BMPs). Pollutant reduction at control devices is based both on experimental field results (empirical) and tracking of particulate settling and filtration (physically-based). WinSLAMM is an acceptable model for demonstrating compliance with TSS and TP WLAs.
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TMDL model comparison matrix
TMDL model comparison matrix
Comparison Category Water quality model
  P8 MIDS calculator MPCA Simple Estimator WinSLAMM
Relative input complexity High Medium Low High
Public domain Yes Yes Yes No
TSS modeled Yes Yes Yes Yes
TP modeled Yes Yes Yes Yes
Volume reduction modeled Yes Yes Yes Yes
Model time step Event/continuous Annual Annual Event
Pollutant Loading Methodology EMC & Buildup/ Wash-off Simple method Simple method EMC & Buildup/ Wash-off
Pollutant removal methodology Physically based Empirical Empirical Physically-based / Empirical
Pollutant Removal Mechanisms Filtration, Sedimentation, Infiltration Empirically based Empirically based Filtration, Sedimentation, Infiltration
Capable of evaluating bypass from undersized BMPs? Yes Only for volume-reduction BMPs No Yes
Capable of modeling removal from BMPs in series? Yes Only for volume-reduction BMPs No Yes
GIS compatibility Low Low Low Medium
Model used for TMDL Development and Modeling Yes No No Yes
Costs - software None None None medium
Costs - model development Medium Low Low High
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Water quality BMP comparison matrix for TMDL models
Water quality BMP comparison matrix for TMDL models
Water quality BMP BMP included in model1
  P8 MIDS calculator MPCA Simple Estimator WinSLAMM
Bioretention, Infiltration / Filtration General Specific Specific Specific
Bioswale, Infiltration / Filtration Specific Specific Specific Specific
Cistern (e.g., Rain Barrel) General Specific   Specific
Constructed Wetland General Specific Specific General
Detention Basin, Dry Specific     Specific
Detention Basin, Wet Specific Specific Specific Specific
Grass Buffer / Filter Strip Specific Specific Specific Specific
Green roof General Specific Specific Specific
Hydrodynamic Separator / Grit Chamber2 General     Specific
Infiltration Basin Specific Specific Specific Specific
Other (i.e., user-defined BMP) General Specific Specific Specific
Permeable Pavement General Specific Specific Specific
Rain garden Specific Specific Specific Specific
Sand filter General Specific Specific Specific
Sand Filter, Iron-Enhanced General Specific General General
Street cleaning Specific     Specific
Tree Trench / Planter General Specific   General
Underground Infiltration / Filtration General Specific General General
1"Specific" = BMP type distinctly included in model; "General" = BMP can be modeled but is not distinctly included as a unique BMP type; and "--" = BMP cannot be modeled.
2Typically used as a pretreatment practice.
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