Many different digital models are used to evaluate the fate and transport of stormwater, estimate changes in pollutant loads to downstream receiving waters, and predict how surface waters respond to different conditions. These models are particularly useful for comparing scenarios and linking watershed loading to receiving water conditions. Surface water models have been widely applied to establish water quality-based effluent limitations (WQBELs), develop total maximum daily levels (TMDLs), and evaluate load reduction or restoration activities. For a comparison of surface water modelling tools, refer to EPA’s Assessment of Surface Water Model Maintenance and Support Status, which is contained in the Additional Resources below.
Different methods can be used to characterize water quality outcomes, including monitoring current conditions and estimating environmental response using predictive tools, such as Reasonable Assurance Analysis (RAA). This page provides more information about each of these topics.
Incorporating models into watershed planning
Many MS4 permits require a quantitative analysis to provide reasonable assurance that stormwater or watershed management programs can achieve pollutant load reductions or reduced stormwater impacts. The RAA process typically employs computer modeling or other quantitative techniques to demonstrate that a combination of specified best management practices (BMPs) or other control strategies will likely reduce pollutant loads or other stormwater impacts (e.g., peak flows) to achieve WQBELs (or TMDL waste load allocations expressed as WQBELs) within NPDES permit compliance schedules.
Although RAA is relatively new to MS4 permits, the concept has been integral to the watershed planning process for many years. In the early 1990s, EPA and many states recognized that models can assist in the watershed planning process, including targeting watersheds for management, developing goals and objectives, defining solutions, developing plans for management implementation, simulating storage and treatment effects of alternative management options, providing input to cost-benefit analyses, and tracking progress toward achieving goals.
Dozens of water quality modeling platforms are now available, ranging from simple to highly complex. The best model to choose may depend on the questions of interest and a suite of regulatory, technical, and resource considerations. These models require a significant amount of data on ambient water quality, land use, pollutant sources, rainfall and runoff relationships, and geography and soils. Some models can also incorporate BMPs to quantify the impact of various pollutant control strategies.
Using an RAA approach
Decision-makers should consider regulatory, planning, technical/analytical, and practical factors when selecting among a range of available RAA modeling and planning methods. Select a method that fully addresses long-term permit compliance and program planning and implementation needs. There is no “one-size-fits-all” approach for performing RAAs, and approaches can always to be tailored to meet local needs.
RAAs and other water quality models can be expensive to develop and maintain and are most effective in combination with a watershed master plan. RAA results can help refine BMP selection and placement within the watershed, show which BMPs should be constructed first with the highest return on investment, and help track progress in meeting water quality goals in receiving waterways. In addition, RAA results and associated long-term plans can support program financing plans to ensure that sufficient capital and operations and maintenance resources are available to fund the program.
Implementing an RAA
There are five key steps in implementing an RAA:
- Designate the area to be addressed.
- Characterize existing conditions and practices.
- Determine stormwater control needs and improvement goals.
- Demonstrate that proposed stormwater controls and management actions will attain goals.
- Document results to inform implementation, tracking, and success evaluation.
Resources
The publications listed below provide more information about watershed modeling.
| Article Description | Categories | categories_hfilter |
|---|---|---|
Watershed-Based National Pollutant Discharge Elimination System (NPDES) Permitting Implementation GuideAuthor: U.S. EPA | Developed/Updated on Date: December 2003 Web Link: https://www3.epa.gov/npdes/pubs/watershedpermitting_finalguidance.pdf The purpose of this guidance is to describe the concept of and the process for watershed-based permitting under the National Pollutant Discharge Elimination System (NPDES) permit program. Watershed-based NPDES permitting is an approach to developing NPDES permits for multiple point sources located within a defined geographic area (watershed boundaries) to meet water quality standards. This approach, aimed at achieving new efficiencies and environmental results, provides a process for considering all stressors within a hydrologically defined drainage basin or other geographic area, rather than addressing individual pollutant sources on a discharge-by-discharge basis. This guidance focuses on defining both the general approach and the process for watershed-based NPDES permitting. The guidance addresses issues related to program implementation, but it does not provide detailed technical information or address procedural and administrative actions related to permit issuance. Those will be covered in future guidance documents. | Water Quality Outcomes: Watershed-Based Models | watershed-based-models |
Local Watershed Management Planning in Virginia: A Community Water Quality ApproachAuthor: Virginia Watershed Advisory Committee | Developed/Updated on Date: Not Dated Web Link: https://www.dcr.virginia.gov/soil-and-water/document/wshedguideb2b.pdf The Virginia Watershed Advisory Committee (VWAC), a consortium of Virginia agencies, regional organizations, and local government representatives involved in watershed management and restoration, developed this guide to give communities tools to develop local, effective, community-based watershed management plans. These plans are a framework to improve management of Virginia’s 494 watersheds through strategies encompassing local solutions. VWAC recommends a watershed management plan with eight basic components to effectively capture local needs, while assisting with meeting state commitments. This guide provides background narratives for those eight components, as well as a related checklist. It also explores the relationship between local watershed management planning and larger state and federal water quality programs such as TMDLs and other Clean Water Act programs. | Water Quality Outcomes: Watershed-Based Models | watershed-based-models |
Handbook for Developing Watershed Plans to Restore and Protect Our WatersAuthor: U.S. EPA Office of Water | Developed/Updated on Date: 2008 This handbook provides information on developing and implementing watershed management plans that help to restore and protect water quality. A watershed is the area of land that contributes runoff to a lake, river, stream, wetland, estuary, or bay. A watershed management plan defines and addresses existing or future water quality problems from both point sources and nonpoint sources of pollutants. Experience over the past decade has shown that effective watershed management includes active participation from stakeholders, analysis and quantification of the specific causes and sources of water quality problems, identification of measurable water quality goals, and implementation of specific actions needed to solve those problems. | Water Quality Outcomes: Watershed-Based Models | watershed-based-models |
Developing Reasonable Assurance: A Guide to Performing Model-Based Analysis to Support Municipal Stormwater Program PlanningAuthor: U.S. EPA Region 9 (written by Paradigm Environmental) | Developed/Updated on Date: February 2017 This document is designed to assist municipal stormwater program managers, watershed stakeholders, consultants, and permitting authorities in understanding, selecting, and using model- based approaches to support development of rigorous, comprehensive municipal stormwater program management plans. Over the past 5 years, many municipal stormwater NPDES permits and associated local programs have shifted their planning focus to use robust analytical modeling tools to identify the specific stormwater management strategies and practices that will be necessary over the long term to attain specified water quality protection requirements. This general approach, based on what has been termed “reasonable assurance analysis” (RAA), has been developed as an alternative to traditional municipal stormwater permitting approaches that relied upon implementation of programmatic minimum stormwater management efforts and an iterative approach to stormwater control development that were often not grounded in rigorous analytical frameworks. This document is based on evaluation of several recent permits and local programs that are implementing this new RAA approach and is intended to provide a structured approach to selecting among alternative analytical tools and efficiently using the selected tools to support development of long-term stormwater management programs that will comply with NPDES permit requirements. | Water Quality Outcomes: Watershed-Based Models | watershed-based-models |
Assessment of Surface Water Model Maintenance and Support StatusAuthor: U.S. EPA | Developed/Updated on Date: September 2018 Web Link: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100VFM8.txt The U.S. EPA Water Modeling Workgroup (WMW) provides support for core surface water quality modeling tools used in Clean Water Act programs. The WMW identified a set of 18 separate surface water modeling applications that are currently applied to support development of Total Maximum Daily Load (TMDL) plans, assess permit conditions for National Pollutant Discharge Elimination System (NPDES) facilities, evaluate policy and water quality effects of potential actions, and conduct other water quality analyses. The WMW directed Tetra Tech to perform an assessment of the current maintenance and support status of each modeling tool to facilitate the identification of gaps in support or other activities that might warrant future WMW activity. This report summarizes the findings of Tetra Tech’s review of the maintenance and support status of the following modeling applications. | Water Quality Outcomes: Watershed-Based Models | watershed-based-models |
SUSTAIN—A Framework for Placement of Best Management Practices in Urban Watersheds to Protect Water QualityAuthor: U.S. EPA Office of Research and Development | Developed/Updated on Date: September 2009 Web Link: https://www.epa.gov/sites/production/files/2015-10/documents/sustain_complex_tools.pdf The U.S. EPA has been working since 2003 to develop such a decision-support system. The resulting modeling framework is called the System for Urban Stormwater Treatment and Analysis INtegration (SUSTAIN). The development of SUSTAIN represents an intensive effort by EPA to create a tool for evaluating, selecting, and placing BMPs in an urban watershed on the basis of user-defined cost and effectiveness criteria. SUSTAIN provides a public domain tool capable of evaluating the optimal location, type, and cost of stormwater BMPs needed to meet water quality goals. It is a tool designed to provide critically needed support to watershed practitioners at all levels in developing stormwater management evaluations and cost optimizations to meet their existing program needs. Due to the complexity of the integrated framework for watershed analysis and planning, users are expected to have a practical understanding of watershed and BMP modeling processes, and calibration and validation techniques. | Management of Post-Construction Site Runoff, Operations: Green Infrastructure, Water Quality Outcomes: Watershed-Based Models | management-of-post-construction-site-runoff green-infrastructure watershed-based-models |
Stormwater Best Management Practices (BMP) Performance AnalysisAuthor: U.S. EPA Region 1 | Developed/Updated on Date: March 2010 Web Link: https://www3.epa.gov/region1/npdes/stormwater/assets/pdfs/BMP-Performance-Analysis-Report.pdf The purpose of this project is to generate long-term cumulative performance information for several types of stormwater best management practices (BMPs). The information can be used to provide estimates of long-term cumulative efficiencies for several types of BMPs, according to their sizing. The curves reflect pollutant removal performance of BMPs designed and maintained in accordance with Massachusetts stormwater standards. Developing a BMP rating curve involved several major steps: (1) selecting an appropriate long-term precipitation record (data and location) that is representative of a major urbanized area within the New England region, (2) generating hydrograph and pollutant time series using a land-based hydrologic and water quality model, (3) simulating BMP hydraulic and treatment processes in BMP models, and (4) creating BMP performance curves on the basis of BMP model simulation results. | Management of Post-Construction Site Runoff, Operations: Green Infrastructure, Water Quality Outcomes: Watershed-Based Models | management-of-post-construction-site-runoff green-infrastructure watershed-based-models |
Plans and ReportsAuthor: City of San Diego, CA | Developed/Updated on Date: Various (2002–2020) Web Link: https://www.sandiego.gov/stormwater/plansreports The City collaborated with other jurisdictions and stakeholders throughout the San Diego Region to develop Water Quality Improvement Plans (WQIPs) for each of the watersheds within its jurisdiction. The goal of the WQIPs is to protect, preserve, enhance, and restore water quality of receiving water bodies. This goal will be accomplished through an adaptive planning and management process that identifies the highest-priority water quality conditions within a watershed and implements strategies to achieve improvements in the quality of discharges from the responsible agencies’ storm drain systems. | Program Goals and Management: Long-Term Planning, Water Quality Outcomes: Watershed-Based Models | long-term-planning watershed-based-models |
What is a Watershed?Author: Gills Creek Watershed Association | Developed/Updated on Date: 2020 Web Link: https://www.gillscreekwatershed.org/plans.html Gills Creek Watershed Association is dedicated to restoring the watershed through education, grass roots action, public and private partnerships, remediation projects, and well-managed development. The goal is to return the watershed to a living resource providing recreational opportunities, habitat for native wildlife and plants, and a national model for watershed planning and management. | Funding: Establishing a Plan, Water Quality Outcomes: Watershed-Based Models | establishing-a-plan watershed-based-models |
Memo on “Updating the EPA’s Water Quality Trading Policy to Promote Market-Based Mechanisms for Improving Water Quality”Author: U.S. EPA | Developed/Updated on Date: February, 2019 Web Link: https://www.epa.gov/sites/production/files/2019-02/documents/trading-policy-memo-2019.pdf Purposes of this memorandum: | Funding: Private Sector Opportunities, Water Quality Outcomes: Watershed-Based Models | private-sector-opportunities watershed-based-models |
Building Cities in the Rain: Watershed Prioritization for Stormwater RetrofitsAuthor: Washington State Department of Commerce (Heather Ballash) | Developed/Updated on Date: September 2016 Web Link: https://www.ezview.wa.gov/site/alias__1780/36739/watershed_prioritization_guidance.aspx Targeted watershed recovery and urban redevelopment and revitalization go hand in hand. This guidance describes an optional process for prioritizing watersheds for stormwater retrofits and the recovery of aquatic habitat in urban areas. It is intended to provide a tool for local governments to target investment in stormwater retrofits in a way that leverages opportunities for salmonid habitat restoration and facilitates redevelopment in urban centers. | Program Goals and Management: Long-Term Planning, Water Quality Outcomes: Watershed-Based Models | long-term-planning watershed-based-models |
BMP Performance Monitoring Data Compilation to Support Reasonable Assurance AnalysesAuthor: U.S. EPA | Developed/Updated on Date: Summer of 2002 Web Link: http://ftp.sccwrp.org/pub/download/ This project roughly doubled the existing data set for flow-through BMPs in California. Influent-effluent monitoring of 81 different flow-through structural BMPs was compiled, totaling 1,700 site-events (e.g., storm-BMP combinations). The most site-events were compiled for vegetated swales (22.4%), media filters (21.5%), and constructed wetlands (38.6%). However, the vast majority of constructed wetland site-events were from just two BMPs, hindering extrapolation and uncertainty analysis to other constructed wetlands. The dry pond (5.8%), wet pond (7.4%), permeable pavement (0.1%), and bioretention systems (4.2%) provided too few site-events for conducting uncertainty analysis. | Water Quality Outcomes: Stormwater Monitoring, Water Quality Outcomes: Watershed-Based Models | stormwater-monitoring watershed-based-models |
Water Quality Improvement PlansAuthor: City of San Diego, CA | Developed/Updated on Date: 2002–2020 Web Link: https://www.sandiego.gov/stormwater/plansreports The City of San Diego collaborated with other jurisdictions and stakeholders throughout the San Diego Region to develop Water Quality Improvement Plans (WQIPs) for each watershed within its jurisdiction. The goal of the WQIPs is to protect, preserve, enhance, and restore water quality of receiving water bodies. This goal will be accomplished through an adaptive planning and management process that identifies the highest-priority water quality conditions within a watershed and implements strategies to achieve improvements in the quality of discharges from the responsible agencies’ storm drain systems. | Funding: Establishing a Plan, Operations: Asset Management, Water Quality Outcomes: Watershed-Based Models | establishing-a-plan asset-management watershed-based-models |