Geomorphologic approach to synthesis of direct runoff hydrograph from the Upper Tiber River basin, Italy

The effective rainfall-direct runoff relationship was investigated for forty events on tour large basins by using a geomorphologic representation of the instantaneous unit hydrograph (IUH) proposed by Gupta et al. (1980). These basins are a part of the Upper Tiber River basin located in Central Italy and range in area from 934 km2 to 4,147 km2. For each event the volume of direct runoff was obtained by hydrograph separation. The effective rainfall hyetograph was then determined by using the two-term Philip infiltration equation in conjunction with a volume balance analysis. The geomorphologic parameters required by the IUH were extracted from a topographic map of each basin with the map scale of 1:200,000. It was found that the dimensionless form of the IUH remained practically Constant from one basin to another. By convoluting the geomorphologic IUH, derived for each basin, with the effective rainfall, the direct runoff hydrograph was synthesized for each event. The model results compare reasonably well with observations of each basin. The maximum and mean errors in computing peak flow were 33% and 15% respectively. Furthermore, the magnitudes of these errors did not depend upon the basin area. A sensitivity analysis of the model structure revealed that its order of geomorphologic representation could be reduced by at least one without a significant loss of accuracy. This small reduction in order amounted to a considerable reduction in geomorphologic complexity and computational effort. The model results were quite sensitive to basin lag and sorptivity parameter of the infiltration equation. A 10% variation in basin lag resulted in approximately 10% variation of computed peak discharge. However, a 10% variation in sorptivity produced an approximately 20% mean variation in computed peak discharge.