The geomorphologic instantaneous unit hydrograph (GIUH) as a component of rainfall-runoff models directed to the determination of design hydrographs in ungaged basins is investigated. Specifically, we first performed a sensitivity analysis of the GIUH to errors in the basin lag estimated by commonly used empirical relationships involving basin area. Then, the details required in representing the geomorphologic features in the GIUH estimate for fixed basin lag, L, were examined. Real basins located in Central Italy were selected; they range in area from 12 km2 to 4,147 km2 and are characterized by a significant variability in the drainage channel density, D. It was found that given L a minimum detail was necessary in representing basin geomorphology. Further, the estimate of L through basin area led to large errors in computing design hydrographs for a few small basins. An explicit consideration of D is suggested in order to eliminate this shortcoming.

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