SOME REMARKS ON THE USE OF GIUH IN THE HYDROLOGICAL PRACTICE

The geomorphologic instantaneous unit hydrograph (GIUH) as a component of rainfall-runoff models directed to the determination of design hyd rographs in ungaged basins is investigated. Specifically, we first per formed a sensitivity analysis of the GIUH to errors in the basin lag e stimated by commonly used empirical relationships involving basin area . Then, the details required in representing the geomorphologic featur es in the GIUH estimate for fixed basin lag, L, were examined. Real ba sins located in Central Italy were selected; they range in area from 1 2 km(2) to 4,147 km(2) and are characterized by a significant variabil ity in the drainage channel density, D. It was found that given L a mi nimum detail was necessary in representing basin geomorphology. Furthe r, the estimate of L through basin area led to large errors in computi ng design hydrographs for a few small basins. An explicit consideratio n of D is suggested in order to eliminate this shortcoming.