Unit hydrographs for catchments of different sizes and dissimilar regions

A study was made of 96 flood events, larger than the mean annual flood, from 17 catchments situated in different regions of the Commonwealth of Pennsylvania, an area of widely differing geology, physiography and climate. The mean annual precipitation ranges from 90 to 125 cm, mean monthly temperatures vary from a minimum of −3° C in January to 24° C in July, and mean elevation differences within a catchment range from 220 to 875 m; catchment sizes are 10 to 450 km 2. Unit hydrographs, derived by the least-squares method on a digital computer, show some consistency of shape for one catchment but very considerable variations among catchments. Storm parameters seem to have small effects on the unit hydrographs but physiographic parameters cause the major inter-catchment variations in the shapes of the unit hydrographs. The baselength of the unit hydrograph is found to be closely related to the physiographic region while the time-to-peak and the peak discharge seem to be dependent on area and other associated physiographic parameters. It is possible within the ranges of catchment size and physiographic regions studied to develop unit hydrographs for ungaged watersheds, from which flood hydrographs and characteristic discharges may be derived.