Williams, who co-developed the William’s Unit Hydrograph (WILHYD in Visual OTTHYMO) with Hann in 1973 later derived empirical relationships for both the K and TP variables in WILHYD. These relationships are:



The above relationships were derived for watersheds in the southern United States. Refer to the Theory Reference section of this manual for more information on the derivation of the WILHYD unit hydrograph.