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National Pollutant Discharge Elimination System (NPDES)

Climate and Extreme Weather Tools for the NPDES Program

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More intense precipitation, floods, droughts, increases in ambient water temperatures, and rising sea levels pose challenges for permitees and permit writers. Managing discharges to protect water quality under these changing conditions can be aided by the refinement of the methods, tools, and information used to develop and implement NPDES permits and programs.

EPA's Climate Change Adaptation Resource Center has a variety of information and tools.

Addressing Climate Change Impacts to Water Resources has water-related information and tools.

Flow and Critical Flow Statistics

Both high and low flows in streams in many parts of the United States are affected by weather, water withdrawals, changes in stormwater runoff due to changes in imperviousness, and other factors. This has highlighted the need for improving methods for calculating critical flow statistics, which are an integral element in developing water quality based effluent limits. Changes in receiving waters flow regimes affects other aspects of Clean Water Act implementation as well.

Low Flow Statistics Tools – A How-To Handbook for NPDES Permit Writers (2018) – Describes how to estimate low flow statistic values in a variety of situations using free publicly available tools.

Estimating Critical Flow Statistics

SWToolbox, created by USGS in collaboration with EPA, is designed to assist NPDES permit writers and other practitioners obtain better data and methods in estimating critical stream statistics such as the 7Q10. This downloadable tool combines and enhances functionalities of older statistical approaches (EPA’s DFLOW and USGS’ SWSTAT). The tool directly accesses current and historic flow data from USGS gaging stations, and also allows user to upload their own data. Enhancements include a batch run option and automated testing for trends, serial correlation, outliers, and unusually large skew. The tool also provides a method for estimating critical flows at ungaged locations.

Aquatic Life and Hydrologic Alteration

Protecting Aquatic Life from Effects of Hydrologic Alteration, an EPA-USGS technical report, describes the effects of flow alteration on aquatic life designated uses in streams, rivers, and other natural flowing water bodies; shows how Clean Water Act mechanisms address hydrology or flow alterations through state and tribal examples, and; provides a flexible, nonprescriptive framework to quantify flow targets to protect aquatic life from the effects associated with flow alteration.

Green Infrastructure

Green Infrastructure

Green Infrastructure. As different parts of the country become drier, wetter or hotter, green infrastructure can help improve community resiliency today and into the future. Learn more about how green infrastructure practices can help communities manage flooding, prepare for drought, reduce urban heat islands, lower building energy demands, spend less energy managing water, and protect coastal areas.

Green Infrastructure Wizard

EPA's Green Infrastructure Wizard, or GIWiz, provides access to tools and resources that can support and promote water management and community planning decisions, including preparing for extreme weather events.


Building Resiliency with Green Infrastructure. This 90-minute webcast highlights the new Green Infrastructure Decision Support Tool, followed by a presentation on the New York City Green Infrastructure Program’s efforts to incorporate resiliency into system planning.

Green Infrastructure for Localized Flood Management. In this 90-minute webcast practitioners will cover a range of practices that can help communities build flood resilience, from small scale interventions such as rain gardens and permeable pavement to coordinated open space and floodplain preservation.

Models and Tools

Creating Resilient Water Utilities
The CRWU initiative provides drinking water, wastewater and stormwater utilities with the practical tools, training, and technical assistance needed to increase resilience to extreme weather events. Through a comprehensive planning process, CRWU assists water sector utilities by promoting a clear understanding of potential long-term adaptation options.

Climate Resilience Evaluation and Awareness Tool (CREAT)
A risk assessment application, which helps utilities in adapting to extreme weather events through a better understanding of current and future climate conditions. CREAT provides:
  • Lists of drinking water and wastewater utility assets (such as water resources, treatment plants, and pump stations)
  • Possible related threats such as drought and water quality
  • Measures that utilities can implement to reduce potential impacts

The tool guides users through identifying regional threats and designing adaptation plans. After assessment, CREAT provides a series of risk reduction and cost reports to allow you to evaluate various adaptation options as part of long-term planning.

Hydrologic and Water Quality System (HAWQS)
A web-based interactive water quantity and quality modeling system to simulate the effect of management practices on water quality. HAWQS substantially enhances the usability of SWAT, its core modeling engine, to an extensive array of crops, soils, natural vegetation types, land uses, and scenarios for hydrology and the water quality parameters such as sediment, pathogens, nutrients, biological oxygen demand, dissolved oxygen, pesticides, and water temperature. HAWQS has interactive web interfaces and maps; pre-loaded input data; outputs that include tables, charts, and raw output data; a user guide, and online development, execution, and storage of a user's modeling projects.

National Stormwater Calculator
A desktop application that estimates the annual amount of stormwater runoff from a specific location anywhere in the United States. Estimates are based on local soil conditions, land cover, and historic rainfall records. The Calculator includes changes in seasonal precipitation levels, the effects of more frequent high-intensity storms, and changes in evaporation rates based on validated IPCC scenarios. Users can enter any U.S. location and select different scenarios to learn how specific management and design approaches can prevent pollution.

Storm Water Management Model (SWMM)
Used for planning, analysis and design related to stormwater runoff, combined and sanitary sewers, and other drainage systems in urban areas. There are many applications for drainage systems in non-urban areas as well. The Storm Water Management Model Climate Adjustment Tool (SWMM-CAT) is an add-in tool to SWMM. It is a simple to use software utility that allows future projections to be incorporated into the Storm Water Management Model. SWMM accepts monthly adjustment factors for time series that could represent the potential impact of future conditions and provides a set of location-specific adjustments.

U.S. Climate Resilience Toolkit
Provides a broad selection of models and tools.

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