A Regional Methodology for Deriving Flood Frequency Curves (FFC) in Partially Gauged Catchments with Uncertain Knowledge of Soil Moisture Conditions

In this paper a Monte Carlo procedure for deriving frequency distributions of extreme discharges starting from a simplified description of rainfall and surface runoff processes and using regional data is presented. The procedure is based on two modules: a stochastic rainfall generator module and a catchment response module. In the rainfall generator module the rainfall storm, i.e. the maximum rainfall depth for a fixed duration, is assumed to follow the Two Components Extreme Value (TCEV) distribution whose parameters have been estimated at regional scale for Sicily. The catchment response has been modelled by using the Soil Conservation Service – Curve Number for the total-effective rainfall transformation and the classical rational formula for the flood routing. This method allows incorporation of information on soil type, land use, soil cover condition and Antecedent Soil Moisture (AMC). Furthermore, to take in account for the spatial variation of CN within the catchment, a semi-distributed approach of the rainfall-runoff model was implemented. Finally, the Generalised Likelihood Uncertainty Estimation (GLUE) procedure has been used to explore the estimation of the uncertain knowledge of AMC affecting derivation of FFC.