National Weather Hydrology Laboratory

               THRESHOLD RUNOFF OPERATIONS CONCEPT

   The hydrologist will identify an area of interest by basin or window where gridded runoff
values are desired.  Most options will be set to default values but the user can select other choices
via buttons or keyboard.  Options will include minimum sub-basin size, duration, computation
method, and interpolation method.
   Internally, the application executes these steps to compute the gridded runoff values:
            1.  Delineates watersheds at stream junctions greater than or equal to a user-specified
      minimum area.
            2.  Computes hydrologic parameters at stream junctions.
            3.  Computes the peak flow using parameters from step 2.
            4.  Retrieves required parameters for the flow regression equations.
            5.  Computes the approximate bankfull flow using the regression equations.
            6.  Computes the threshold runoff for the sub-basins.
            7.  Interpolates sub-basin runoff values to the HRAP grid.
            8.  Outputs the gridded threshold runoff values to a file for the FFGS.
               Input, intermediate and output data can be viewed using ArcView,.  Input data includes 
DEM and  flow direction grids; intermediate includes sub-basin boundaries, hydrologic
characteristics, and runoff by sub-basins; output includes threshold runoff by sub-basins and by
grids.
   Gridded threshold runoff values are written to files used later to load the gridded threshold
runoff values into the Flash Flood Guidance System (FFGS).  The FFGS program ffguid
provides menus to load the runoff files, i.e. menu selections are Setup and Grids.