Impact of small-scale spatial rainfall variability on runoff modeling

Abstract

Rainfall and wind data obtained from a dense raingage network on a 4.4 ha semiarid catchment were used as input to a distributed rainfall-runoff model. It was shown that the wind direction and velocity have a relatively small impact on peak rate and runoff volume for this low relief watershed. However, even at this small scale, spatial variability of precipitation can translate into large variations in modeled runoff. When five model runs were conducted using input from one of five recording raingages, one at a time, the coefficient of variation for peak rate and runoff volume ranged from 9 to 76%, and from 2 to 65%, respectively, over eight observed storm events. By using four well distributed gages the variations in modeled runoff volume approach the sampling resolution of the raingages as well as the estimated accuracy of runoff volume and peak rate observations. The results of this study indicate that if distributed catchment modeling is to be conducted at the 5 ha scale in an environment dominated by convective air-mass thunderstorm rainfall, knowledge of the spatial rainfall variability on the same scale is required. A single raingage with the standard uniform rainfall assumption can lead to large uncertainties in runoff estimation.