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Green Infrastructure

Green Infrastructure Modeling Tools

Modeling tools support planning and design decisions on a range of scales from setting a green infrastructure target for an entire watershed to designing a green infrastructure practice for a particular site. Outputs that are particularly helpful include:

  • runoff volume,
  • runoff rate,
  • pollutant loading, and
  • cost.

Some models can predict the water quality and water quantity impacts of green infrastructure approaches. Learn more about those models, and others that address cost, air quality, and energy consumption on this page. Start with simpler, less resource-intensive models and advance into more complex models that require more time and expertise.

On this page:

Modeling Principles

You can use models to predict the environmental outcomes of different design and management approaches.

Why Model?

As a site designer, planner, or environmental manager, you can create a representation or simulation of a site or watershed and apply various environmental data to determine possible impacts.

At the site or neighborhood scale: Models can guide site designers in meeting mandatory or voluntary performance standards. You can link a site’s land cover and stormwater controls to the volume of stormwater discharged by the site and the pollutant loads exported by the discharges. The model compares the water quantity and quality outcomes associated with different design scenarios.

At the watershed scale: Models can guide planners and environmental managers in meeting mandatory or voluntary objectives for receiving waters. You can link land cover and stormwater controls implemented throughout a watershed to the hydrological, chemical, and ecological outcomes in receiving waters. All activities occurring in a watershed and the pattern of precipitation in a given year impact receiving waters, making models particularly valuable at the watershed scale. You can isolate the receiving water impacts associated with stormwater management approaches and compare the environmental outcomes of alternative management scenarios.

Choosing a Model

A variety of models are available for assessing the performance of green infrastructure practices in the urban environment. But, before you select a model, identify your needs and the resources at your disposal.

Define your objective: No model can accurately predict all environmental outcomes at all scales, but most models can predict a limited range of environmental outcomes within a limited range of scales. Identify:

  • the environmental parameters you want to include in your model (e.g., water quality, streamflow rates, groundwater recharge rates); and
  • the scale you want to simulate (e.g., a single site, a small headwater stream, a large lake).

Determine your data requirements: Simpler models require less data that might be retrieved from publicly available databases, while more complex models require more data to provide the necessary parameters and calibration. For each model you are considering, determine:

  • the amount of data it requires; and
  • the spatial and temporal resolution it requires.

Choose the simplest model that can meet your objective: Think about the level of accuracy that is required to meet your objective. Efficiently allocate staff and budget resources by weighing a simple model’s level of accuracy and cost against increasingly those of more sophisticated models. Is the incremental gain in accuracy of a particular model worth the incremental increase in cost?

For more guidance and EPA modeling tools visit the Green Infrastructure Modeling Toolkit.

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Sizing and Cost Spreadsheets

Spreadsheet tools allow you to quickly generate cost and/or performance estimates for multiple sets of site designs. These tools rely on a set of simplifying assumptions to generate their quick results. Be aware of these assumptions before applying the estimates generated by the models. Access these spreadsheet models by clicking their links.

Virginia Runoff Reduction Method(20 pp, 114 K, About PDF) Exit

  • Scale: Site
  • BMPs:
    • Green roof
    • Downspout disconnection
    • Permeable pavement
    • Grass channel
    • Dry swale
    • Bioretention
    • Infiltration
    • Extended detention pond
    • Sheetflow to filter
    • Wet swale
    • Constructed wetland
    • Wet pond
  • User Inputs: Annual precipitation, land cover distribution, soil type distribution, BMPs
  • Outputs: Runoff volume reduction (ft3 /design storm), phosphorus load reduction (lb/yr), nitrogen load reduction (lb/yr)

Water Environment Research Foundation (WERF) BMP and LID Whole Life Cost Models Exit

  • Scale: Site—Watershed
  • BMPs:
    • Green roofs
    • Planters
    • Permeable pavement
    • Rain gardens
    • Retention ponds
    • Swales
    • Cisterns
    • Bioretention
    • Extended detention basins
  • User Inputs: Drainage area, BMP characteristics, capital costs, maintenance costs
  • Outputs: Whole life costs, present value graphs

EPA's Green Long Term Control - EZ Template

  • Scale: City
  • BMPs:
    • Green roofs
    • Bioretention
    • Vegetated swales
    • Permeable pavement
    • Rain barrels and cisterns
  • User Inputs: Event precipitation, impervious area, BMPs, BMP parameters
  • Outputs: Runoff volume reduction, CSO volume, costs
  • Notes: Aids municipalities in integrating green infrastructure into long-term control plans for combined sewer overflows.

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Simple Models

You can use simple models—those that require relatively limited input data and technical expertise—as screening- and planning-level tools. Access these simple models by clicking their links.

EPA National Stormwater Calculator

A stormwater management model- (SWMM-) based desktop application that estimates the impact of land cover change and green infrastructure controls on stormwater runoff from a selected site. Estimates of runoff volume and frequency are based on local soil, topographic and climate data, and user-provided land cover and BMP data.

  • Scale: Site
  • BMPs:
    • Disconnection
    • Rainwater harvesting
    • Rain gardens
    • Green roofs
    • Street planters
    • Infiltration basins
    • Permeable pavement
  • User Inputs: Land cover, BMP characteristics (soil, topographic, and climate data from national databases)
  • Outputs: Runoff volume and frequency statistics

WERF BMP SELECT Model Exit

  • Scale: Site—Watershed
  • BMPs:
    • Extended detention
    • Bioretention
    • Wetlands
    • Swales
    • Permeable pavement
    • Filters
  • User Inputs: Hourly precipitation record, monthly evaporation rates, land use distribution, BMPs, BMP parameters
  • Outputs: Runoff volume, pollutant loads, costs
  • Notes: Cost calculations based on WERF Whole Life Cost Model

Center for Neighborhood Technology Green Values National Stormwater Management Calculator Exit

  • Scale: Site
  • BMPs:
    • Green roof
    • Planter boxes
    • Rain gardens
    • Cisterns
    • Native vegetation
    • Vegetation filter strips
    • Amended soil
    • Swales
    • Trees
    • Reduced street width
    • Permeable pavement
  • User Inputs: Annual or event precipitation, land cover distribution, soil type, runoff reduction goal, BMPs, BMP parameters
  • Outputs: Runoff volume reduction, costs, reduced air pollutants, carbon dioxide sequestration, value of trees, groundwater recharge, reduced energy use, reduced treatment benefits

North Carolina State University Rainwater Harvesting Model Exit

  • Scale: Site
  • BMPs: Rain cisterns
  • User Inputs: Hourly or daily rainfall record, BMP parameters, anticipated usage, water cost, sewer cost, cistern cost
  • Outputs: Runoff volume reduction, usage replaced, payback period

i-Tree Vue Exit

Uses freely available national land cover data maps to assess your community’s land cover, including tree canopy, and ecosystem services provided by your current urban forest. You also can model the effects of planting scenarios on future benefits.

  • Scale: City
  • BMPs: Urban forest
  • User Inputs: National Land Cover Database 2001 land cover, tree cover, and impervious cover datasets
  • Outputs: Carbon sequestration, air pollutant removal

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Complex Models

You can use complex models—those that require relatively extensive input data and technical expertise—to provide more accurate results. You must, however, define a larger number of physical parameters. Access these complex models by clicking their links.

EPA Stormwater Management Model (SWMM) with LID Controls

SWMM—a general-purpose urban hydrology and conveyance system hydraulics model—is used extensively throughout the nation. It explicitly models the hydrologic performance of specific types of low impact development (LID) controls (also known as green infrastructure controls), including:

  • porous pavement,
  • bioretention areas,
  • rain barrels,
  • infiltration trenches, and
  • vegetative swales.

Engineers and planners can use the model to accurately represent any combination of LID controls within a study area to determine their effectiveness in managing stormwater and combined sewer overflows.

EPA Hydrological Simulation Program – FORTRAN (HSPF)

Simulates watershed hydrology and water quality for conventional and toxic organic pollutants. Incorporates EPA's watershed-scale Agricultural Runoff Management Model (ARM) and Nonpoint Source Runoff Model (NPS) into a basin-scale analysis framework that includes fate and transport in one-dimensional stream channels. Allows integrated simulation of land and soil contaminant runoff processes with in-stream hydraulic and sediment-chemical interactions. Results in time histories of:

  • the runoff flow rate, sediment load, and nutrient and pesticide concentrations; and
  • water quantity and quality at any point in a watershed.

HSPF simulates sand, silt, and clay sediment types in addition to a single organic chemical and transformation products of that chemical.

i-Tree Eco Exit

Provides a broad picture of the entire urban forest. Uses field data from randomly located plots throughout a community along with local hourly air pollution and meteorological data to quantify urban forest structure, environmental effects, and value to communities.

  • Scale: Single tree—County.
  • BMPs: Urban forest.
  • User Inputs: Vegetation characteristics at sample locations.
  • Outputs: Carbon sequestration, air pollutant removal, energy effects from trees.
  • Notes: Data must be submitted through an online system. Results are returned the same day.

i-Tree Streets Exit

Focuses on the ecosystem services and structure of a municipality’s street tree population. Uses a sample or complete inventory to quantify and put a dollar value on trees’ annual environmental and aesthetic benefits, including:

  • energy conservation
    • air quality improvement,
    • energy conservation
    • carbon dioxide reduction
    • stormwater control, and
    • property value increases.
  • Scale: City
  • BMPs: Street trees
  • User Inputs: Street tree characteristics at sample locations, city information, cost information, benefit prices
  • Outputs: Energy conservation, runoff volume reduction, air pollutant reduction, carbon sequestration

i-Tree Hydro Exit

The first vegetation-specific urban hydrology model. Use to model the effects of changes in urban tree cover and impervious surfaces on hourly stream flows and water quality.

  • Scale: Watershed
  • BMPs: Urban forest
  • User Inputs: Digital elevation model for study watershed, land cover distribution, leaf area index, tree phenology
  • Outputs: Stream flow, pollutant load

RECARGA

  • Scale: Site
  • BMPs: Rain gardens
  • User Inputs: Hourly precipitation record or event precipitation, hourly evapotranspiration record, drainage area, impervious area, pervious area curve number, soil properties, rain garden properties
  • Outputs: Runoff volume reduction
  • Notes: Given a target retention volume, the required rain garden area can be calculated

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