Modelling high magnitude events in large arid catchments - A field based approach in Nahal Zin, Israel

A physically-based distributed rainfall-runoff model was applied to the 1400 km² catchment of Nahal Zin, Israel, to study the major processes governing the development of big floods. Both runoff generation in the headwater surfaces and flow losses along the channel network were studied using spatial distribution. In this way physically-based input parameters can be incorporated into the model and no calibration is needed. The study concentrated on the rocky headwaters of the catchment and on one big flood using a GIS framework to combine map-derived information, images of rainfall radar, air photos and results of an extensive field survey. A modular model structure was chosen to facilitate the stepwise simulation of the dominant hydrological processes. In the upper rocky headwater catchment wtih its narrow channels simplifying assumptions were possible. The simulation yields results which compare reasonably with measured peak discharges. Multiple peaks by tributaries and the downstream decrease of floods can be described realistically. Sensitivity tests show the outstanding importance of a correctly calibrated rainfall radar as model input.