Sizing a stormwater Best Management Practice (BMP), Low Impact Development (LID) or Green Stormwater Infrastructure (GSI) facility can be an arduous process.  Water Quality Capture Optimization Statistical Model (WQ-COSM), an Excel-based program greatly simplifies the task of generating Water Quality Control Volumes (WQCVs). WS=COSM uses continuous simulation to process information about the catchment’s hydrologic parameters and the recorded precipitation data from the National Climatic Data Center (NCDC) (or other sources of precipitation data) to find runoff volumes which are then used to estimate a Water Quality Capture Volume (WQCV) (sometimes called Water Quality Volume).  It runs in batch mode to process one to ten catchment impervious conditions.  It also finds the optimized WQCV for each imperviousness (i.e., point of diminishing returns) for each.  WQ-COSM offer user the choice of three runoff models to calculate runoff volumes for each time increment of a long-tern rainfall data record.  Runoff methods available are (1) Rational Method, (2) modified Horton’s and (3) Green-Ampt.  The latter two are virtually identical to algorithms used in EPA’s SWMM software.

Urban Watersheds Research Institute (UWRI) offers this program to stormwater professionals free of charge. Only stipulation is that the user agrees to the terms of the DISCLAIMER.  Although this software was tested and found to provide reasonable and accurate results, UWRI cannot guarantee that its use for any specific site or location will do so.