Incorporating Antecedent Moisture Conditions and Intraevent Variability of Rainfall on Flood Frequency Analysis in Poorly Gauged Basins

Abstract

[Abstract:] Rainfall-runoff models are frequently used for the prediction of flood discharges from extreme rainfall data in poorly gauged basins. However, the conventional methods used to estimate the frequency of flood discharges from precipitation data do not incorporate a realistic representation of the spatial and temporal structure of rainfall at the study site nor of the antecedent soil moisture content. Temporally and spatially idealized rainfall patterns that do not represent properly the intraevent variability of rainfall are in general assumed. In this paper, we propose a methodology for flood frequency analysis based on the hydrological simulation of potentially hazardous real storm events. The aim of the proposed method is to incorporate the impact of the spatial and temporal intraevent variability of rainfall on flood discharge estimates without making strong assumptions about the spatial and temporal patterns of storms. It also includes a calibrated relation between the antecedent rainfall depth and the soil infiltration capacity, in order to consider the combined effect of antecedent soil moisture content and rainfall intensity. The flood frequency estimates obtained with the proposed methodology are compared to those estimated with a conventional method based on the definition of synthetic design hyetographs from local intensity-duration-frequency curves. The analysis is presented in two watersheds of 84 and 353 km2 located in the NW of Spain. Results highlight the role of the antecedent soil moisture in flood frequency analysis and show that the conventional methodology tends to overestimate the flood discharges when compared to the proposed approach.