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Clint Merrell

Mean Annual Precipitation in NSS Model

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Can someone help me out with computing the Mean Annual Precipitation values for a basin being modeled using the USGS NSS equations?

The particular basins I'm modeling are in California in the South Coast Region, so I need to compute the Drainage Area (DRNAREA) and Mean Annual Precipitation (PRECIP) values for the equations.

What type of GIS data do I need to have WMS compute and populate the PRECIP field? Gridded? Shapefile?

I can use the beta of the USGS NSS web app to compute the Avg Annual Precip but I'd rather do it in WMS, because the current USGS web app does not go as far as actually running the regression equation for the basin. I'm doing over 20 basins in the San Gabriel Mountains so if WMS can compute the precip and run the equations, that would be great.

Thanks

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Clint,

There is a way to bring an Arc/Info ASCII grid of average annual rainfall values into WMS and use it to compute the average annual precipitation for each sub-basin in your watershed, but WMS places the computed result into the HEC-1 rainfall field instead of into the average annual precip field used with the California NSS equations. You access this computation field using the "Compute GIS Attributes" command in the "Calculators" menu. You can also use the compute coverage overlay command if you have polygons representing different precipitation values to compute the % overlay of each precipitation polygon for each of your sub-basins. You would then have to do some hand calculations to area-weight each of your precipitation values for each sub-basin and enter the values by hand into the NSS input fields in WMS.

Hope this helps,

Chris

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