The importance of single events in arid zone rainfall-runoff modelling

Two high magnitude rainstorm floods are simulated by a distributed rainfall -runoff model in the 1400 km(2) arid catchment of Nahal Zin, Israel. Only t he processes dominating arid zone flood generation (generation and spatial concentration of Hortonian overland flow on the terrain and transmission lo sses into the dry channel alluvium) are described. No calibration with meas ured now data is performed. During one event (October 1991) almost the enti re catchment was covered by high intensity rainfall as detected by rainfall radar. During another (October 1979) only one ground station in the upperm ost headwaters recorded heavy precipitation, while the majority of the catc hment remained dry. For this event the distributed model serves as 'runoff- rainfall model' to reconstruct characteristics of the rainfall. Different e vent characteristics directly affect parameter; sensitivity and model uncer tainty. Maximum model uncertainty of the diminished October 1979 peak is go verned by transmission loss parameters and exceeds 300 %. During 1991 only 90 % is determined for this value and infiltration characteristics of the t errain are more relevant. Also a paleoenvironmental scenario on the hydrolo gical effects of widespread loess deposition is highly event dependent. It may be concluded that the separate analysis of single events is crucial for the understanding of high magnitude floods in arid catchments. (C) 2000 El sevier Science Ltd. All rights reserved.