UNIT HYDROGRAPH DERIVATION FOR UNGAUGED WATERSHEDS BY STREAM-ORDER LAWS

Hydrologists have attempted to relate the hydrologic response of watersheds as runoff production from rainfall to watershed topographic structures for many years. The recently proposed geomorphologic instantaneous unit hydrograph (GIUH) method is perhaps the most promising development in this direction; if successful, it would allow the derivation of the unit hydrograph (UH) for ungauged or inadequately gauged watersheds without the need of observed runoff and rainfall data. In this method, the geomorphic ratios of the Horton-Strahler stream-ordering laws are incorporated in the GIUH model for UH generation. In view of the variety of project requirements and the different levels of topographic detail available from maps or geographic information systems (GISs), five different levels of geomorphic data are allowed for incorporation with a kinematic-wave and stream-law based instantaneous UH model. Testing of the model on two hilly watersheds in the eastern United States and two relatively fiat-slope watersheds in Illinois are presented. Comparison between the simulated and observed hydrographs for a number of rainstorms indicate the potential of this model as a useful tool in watershed rainfall-runoff analysis.