This free desktop application estimates the annual amount of rainwater and frequency of runoff from a specific site anywhere in the U.S. The calculator uses EPA’s SWMM model and accesses several national databases that provide local topography, soil, historic rainfall, and evaporation information. It is most appropriate for performing screening level analysis of small sites of up to several dozen acres with uniform soil conditions. Required inputs are the site location and percentage of the site covered by impervious and four types of pervious surfaces. Users then input the percent of impervious area that will be treated by seven types of green infrastructure: disconnection, rain harvesting, rain gardens, green roofs, street planters, infiltration basins, and porous pavement. SWMM then runs a continuous time series of rainfall and runoff at 15-minute intervals. Outputs include average annual rainfall, runoff, and percent of all rainfall retained, as well as daily event statistics like days per year with runoff and smallest event with runoff. The calculator has an option that allows for a comparison of baseline and other scenarios. In January 2014, the calculator was updated so that users can elect to apply different future climate change scenarios that incorporate changes in seasonal precipitation levels, effects of more frequent high-intensity storms, and changes in evaporation rates based on validated Intergovernmental Panel on Climate Change climate change scenarios.
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