Difference between revisions of "Building a Model:Delineating the Watershed"

Revision as of 17:57, 20 May 2008

The first step in building a GSSHA model is to delineate the watershed. The watershed is delineated from the DEM. DEM data of various resolutions can be obtained from the USGS and EPA Basins data bases, accessable through links provided by the EMRL geospatial data access website at BYU, http://www.emrl.byu.edu/gsda . 90m resolution data are available for all of the United States; 30m resolution data are available for most areas, and 10 m data are becoming available. While models are not routinely run with grid sizes finer than 90m, the 10m data has a much better vertical resolution, typically 0.1 m as opposed to 1.0 m for the 90m resolution data. Unless the GIS or GUI cannot digest the large number of data points, the 30m data will provide better watershed and stream delineations. These data are typically available as 7.5 minute quad sheets. When the watershed overlaps two or more maps, then the overlapping sheets need to be put together and any discrepancies between the different maps resolved. The WMS software has tools available to accomplish these tasks. With the DEM covering the watershed in hand, WMS can be used to delineate the watershed above any give point in the basin, basin outlet, based on calculations from TOPAZ (Martz and Garbrecht, 1992). The TOPAZ model also determines the stream network from the DEM data. This may or may not be useful. If a watershed boundary has been predetermined, the watershed polygon can be used to “cut out” the appropriate DEM data, or can be imported for use as the watershed boundary in WMS. If a watershed boundary is imposed on the DEM then it is likely that the DEM or resulting grid will have to be edited to force the cells in the predefined watershed to all drain toward the basin outlet.