74-Assessment of the Relation between Rainfall and Flood Frequency Curves through Peak over Threshold Analysis.

Abstract

The analysis of the flood production mechanisms from rainfall is crucial for building flood frequency curves in ungauged basins. In this paper, the rainfall-runoff transformation under heterogeneous geomorphoclimatic conditions is investigated by comparing the shape of the flood and rainfall frequency curves in gauged basins. We propose an empirical comparison of the magnitude of equiprobable rain and flood peaks, at the basin scale and for different return periods. In order to obtain reliable estimates of the empirical frequency assigned to each peak we use a revised partial duration series procedure, named Filtered Peaks Over Threshold (FPOT), as an alternative to the customary annual maxima approach. The analysis is carried out on 33 time series of daily discharge and rainfall from basins located in North-Western Italy. Results of the analysis show that for the majority of basins the shape of the rainfall and runoff distributions is statistically indistinguishable. However, a relevant deviation from this rule is represented by basins where the dominating flood production mechanism is snow melting. For these cases, a simple theoretical interpretation of the difference between the shape of the frequency curves of rainfall and runoff is presented.