The National Weather Service (NWS) Next Generation Weather Radar (NEXRAD) radar program generates a product called StageIII which offers gridded precipitation estimates spatially averaged over grid cells of approximately 16 km 2 and temporally averaged over 1 hour. Hydrologists need to consider how such distributed precipitation estimates may be translated into improved streamflow forecasts. Researchers at the U.S. Army Corps of Engineers Hydrologic Engineering Center (HEC) have proposed using a modified version of the Clark unit hydrograph method to incorporate NEXRAD rainfall data into their streamflow forecasts. The proposed method requires information about the area of each rainfall cell falling within each modeled subbasin and the average flow length from each rainfall cell to the corresponding subbasin outlet. A set of programs was written to obtain this information using Arc/Info GIS and USGS digital elevation models. Properly positioning NEXRAD rainfall cells relative to digital elevation model cells is an important issue. A fundamental problem is that NEXRAD estimates are referenced to a spherical earth datum while data sets describing the land surface (i.e. digital elevation models) are most commonly referenced to an ellipsoidal earth datum. A study of the equations required to transform NEXRAD cells and digital elevation model cells into a common ellipsoid-based map projection is presented.
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