National Weather Hydrology Laboratory

             THRESHOLD RUNOFF IMPLEMENTATION CONCEPT

   The threshold runoff program is a development application tool for the RFCs that derives
gridded threshold runoff values used to compute gridded flash flood guidance.  The application
needs to be run only once for the entire RFC's forecast area.  When updated geo-data becomes
available or methods are modified, the application tool may be run again to update the gridded
threshold runoff values.
   HRL will derive a flow direction grid in projected space using projected versions of the
NOHRSC DEM and the NOHRSC edited RF1 files.  In addition, HRL will provide a database
containing parameters for regression equations used to compute 2-, 5-, and 10-year return period
flows.  HRL will provide the flow direction grid and the regression equation parameter database
for each RFC.
   With the database installed on the RFC's workstation and using the GIS ArcView, the RFC
can use the application tool to derive gridded threshold runoff values.  The final step is to load
the values into the Flash Flood Guidance System where they will be used to compute gridded
flash flood guidance.