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 the processes dominating arid zone flood generation (generation and spatial concentration of Hortonian overland flow on the terrain and transmission losses into the dry channel alluvium) are described. No calibration with measured now data is performed. During one event (October 1991) almost the entire catchment was covered by high intensity rainfall as detected by rainfall radar. During another (October 1979) only one ground station in the uppermost headwaters recorded heavy precipitation, while the majority of the catchment remained dry. For this event the distributed model serves as 'runoff-rainfall model' to reconstruct characteristics of the rainfall. Different event characteristics directly affect parameter; sensitivity and model uncertainty. Maximum model uncertainty of the diminished October 1979 peak is governed by transmission loss parameters and exceeds 300 %. During 1991 only 90 % is determined for this value and infiltration characteristics of the terrain are more relevant. Also a paleoenvironmental scenario on the hydrological 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 Elsevier Science Ltd. All rights reserved.