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INNOVATIVE RESEARCH FOR A SUSTAINABLE FUTURE
National Stormwater Calculator
A desktop tool that helps users control runoff to
promote the natural movement of water
Stormwater discharges continue to cause impairment of our Nation's
waterbodies. Regulations that require the retention and/or treatment of the
frequent, small storms that dominate runoff volumes and pollutant loads are
becoming more common. EPA has developed a desktop application, the
National Stormwater Calculator (SWC), to help support local, state, and
national Stormwater management objectives in this regard. The primary
focus of the SWC is to inform site developers on how well they can meet a
desired Stormwater retention target, but it can also be used by landscapers
and homeowners.
The SWC, like any model, estimates an outcome based on available
information such as soil type, landscape and land-use information, and
historical weather. These estimates can be affected by limitations on site-
specific information and uncertainties about future climate. To better inform
decisions, it is recommended that the user develop a range of results with
various assumptions about model inputs such as percent of impervious
surface, soil type, and sizing of green infrastructure. An update to the SWC,
which will include the ability to link to several future climate scenarios, will be
released by the end of 2013. Climate projections indicate that heavy
precipitation events are very likely to become more frequent as the climate
changes. Green Infrastructure can increase the resiliency of Stormwater
management approaches to a changing climate, and this update will allow
users to consider how runoff may vary based both on historical weather and
potential future climate. Please check with local authorities about whether
and how use of these tools may support local Stormwater management goals
and requirements.
The SWC is comprised of nine different pages:
Page 1-Location. This step has an address lookup feature that allows the user
to easily navigate to a site selected anywhere within the United States.
Page 2-Soil Type. The type of soil present on the site is identified and is used
to infer infiltration properties. The soil type can be selected based on local
knowledge or by retrieving a soil map overlay from the online database.
Page 3-Soil Drainage. This step is used to identify how fast standing water
drains into the soil. The user can select the site's soil conductivity based on
local knowledge or retrieve it from the online database.
Page 4-Topography. For this step, the site's surface topography is
characterized, as measured by the surface slope. The user can rely on the
slope data display as a guide or can use local knowledge to describe the site's
topography.
Page 5-Precipitation. A National Weather Service rain gauge, which supplies
rainfall data for the site, is selected. A long-term rainfall record is used to
replicate the full scope of storm events that might occur.
Page 6-Evaporation. This step is used to select a nearby weather station that
will supply evaporation rates for the site.
Page 7-Land Cover. For this step, impervious land cover and the different
types of pervious land cover are assigned to the site for the condition the user
wants to analyze: pre-development, current, or post-development.
Page 8-Low Impact Development (LID) Controls. To help control runoff, seven
different LID controls can be selected for use throughout the site. These
controls are green infrastructure practices that help the user determine the
best way to mimic natural water flow processes to retain rainfall onsite.
Page 9-Runoff. A hydrologic analysis of the site is run and the results are
displayed for review.
Green Infrastructure
The SWC shows users how land use decisions and LID controls
affect the amount of Stormwater runoff produced. To help control
runoff from a chosen site, users can select from different green
infrastructure practices. Green infrastructure, such as the street
planter and porous pavers shown in the image above, promotes
the natural movement of water within an ecosystem or watershed,
instead of allowing it to wash into streets and down storm drains as
shown in the traditional infrastructure image.
These practices allow the Stormwater (rainfall) to be used as a
resource rather than a waste product. Having less water run off
into storm drains and roadways can help prevent contamination
of waterways, water infrastructure degradation, flooding, and
overwhelming of wastewater treatment plants.
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The tool and user's guide are available via the internet:
http://www.epa.gov/nrmrl/wswrd/wq/models/swc/
Contact: SWC@EPA.gov
Office of Research and Development
EPA/600/F-13/095 July 2013
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