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 140 0 km(2) catchment of Nahal Zin, Israel, to study the major processes govern ing the development of big floods. Both runoff generation in the headwater surfaces and Bow losses along the channel network were studied using spatia l distribution. In this way physically-based input parameters can be incorp orated 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 f ramework 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 hydrologic al processes. In the upper rocky headwater catchment with its narrow channe ls simplifying assumptions were possible. The simulation yields results whi ch compare reasonably with measured peak discharges. Multiple peaks by trib utaries and the downstream decrease of floods can be described realisticall y. Sensitivity tests show the outstanding importance of a correctly calibra ted rainfall radar as model input.