Simulating mountain runoff with meso-scale weather model rainfall estimates: a New Zealand experience

Abstract

During October 1996 a series of intensive meteorological measurements were made along the 500 km length of the Southern Alps of New Zealand. These measurements were made to investigate the physical processes responsible for producing heavy rainfall and to evaluate the performance of a meso-scale weather forecast model. A by-product of the work was sets of simulated hourly rainfalls over the entire mountain range. Rainfall estimates were made every 24 h on a 20×20 km resolution grid covering the whole of New Zealand. Successive sets of 24-hourly values were combined to produce a continuous 29-day sequence of model generated rainfalls. The study region covers remote mountain river basins in which there are few rainfall data but over 20 continuously recording river flow monitoring stations. For many of the basins, the runoff rate is an order of magnitude larger than the potential evaporation rate. To a first approximation, and over time periods of days, the river catchments act like large rain gauges. For each basin a rainfall–runoff model was built using the Topmodel assumptions that saturated hydraulic conductivity decreases exponentially with depth from the ground surface, the hydraulic gradient of the saturated zone is equal to the topographic gradient, and subsurface recharge is uniform. As the water table rises in response to rainfall over each sub-basin, increasing amounts of the ground surface become saturated, and rainfall falling directly onto these saturated areas generates much of the storm runoff. Results are presented for basins ranging in area from 12 to 3830 km 2 and that lie on both the windward and leeward sides of the mountain range.