Model My Watershed (MMW) is a free web application for modeling the influences of land use and best management practices on stormwater runoff and water quality. The public can access this tool at https://app.wikiwatershed.org/. One component of this tool is a function to define the model domain, or area of interest for analysis and modeling by interactively setting the outlet location and delineating the watershed draining to that location. This functionality has been developed using enhancements to the TauDEM hydrologic terrain analysis software ((http://hydrology.usu.edu/taudem) and includes a tool on the user interface and RESTFul Application Program Interface that accesses backend data generated from NHDPlus Version 2.1 gridded flow directions. The continental US was preprocessed into subwatersheds that include gridded flow directions and the polygon shapefile for the entire watershed draining to the subwatershed outlet. Thus when a point within the domain is input (clicked or entered to RESTFul API), the subwatershed that it falls in is first identified. It is then snapped to the stream by moving down to the first stream (NHDPlus medium resolution stream) encountered along the flow directions. Then the local watershed within the subwatershed is delineated based on subwatershed flow direction grid using an adaptation of the TauDEM gauge watershed function. This local subwatershed is then merged with shapefiles for any upstream watersheds to which it attaches. Small watersheds are delineated within a few seconds, with larger watersheds taking up to 40 s (entire Mississippi). The most time consuming step is the merging and generalization of shape information for display. The polygon that result from this process may be downloaded, and subject to size limitations also entered into the MMW analyze area function to summarize land use, hydrologic soils and other information of interest to hydrologic and water quality modeling within the delineated area. The resulting watershed polygon may also be entered into one of the stormwater or water quality models supported by MMW.

Presentation at 2018 AWRA Spring Specialty Conference: Geographic Information Systems (GIS) and Water Resources X, Orlando, Florida, April 23-25, http://awra.org/meetings/Orlando2018/.