Three monthly rainfall-runoff models have been applied to semiarid catchments within Namibia under the Southern African FRIEND (Flow Regimes from International Experimental and Network Data) programme. These are the Namrom model, developed specifically for Namibia, and two versions of the Pitman model, developed in South Africa and applied throughout the southern African region. The models were all calibrated on about 20 years of observed flow data and the calibrated parameters used to generate 68 years of simulated inflows to reservoirs located at the catchment outlets. A reservoir water balance simulation model was used with the three inflow series to assess the differences resulting from the application of the three models, with respect to estimates of the reliable yield. The calibration results for the three models were not very different and suggested that the yield estimates would be similar. However, the results based on the 68-year inflow records produced greater differences in yield which could have serious implications for the planning and management of water supply schemes.