A rainfall-runoff model for two small ungauged catchment using the water balance of a reservoir for calibration

In semi‐arid regions, small artificial surface reservoirs are important to meet the domestic and agricultural water requirements of smallholder farmers. The research objective of the study was to determine the rainfall‐runoff relation of two ungauged rivers using the measured water levels of the downstream reservoir. The study areas are the catchments of the upper Mnyabezi River (22 km2) and upper Bengu River (8 km2), which are tributaries of the Thuli River in southern Zimbabwe. The rainfallrunoff relationship is established with the SCS‐Method. The catchments were each divided into subareas by Thiessen‐polygons. For each polygon the amount of discharge was calculated based on the precipitation, land use, land treatment, antecedent moisture conditions and hydrological soil group. The summation of the calculated runoff per polygon formed the inflow variable for the reservoir models. The collected daily water level data from the period March ‐May 2007 were used to calibrate the rainfallrunoff model. The calibration process of the rainfall‐runoff model resulted in an initial abstraction of 7.6 % of the actual retention. It is concluded that the SCS‐method is a useful tool to simulate the rainfallrunoff relation of small ungauged catchments