A theoretical framework for model validation, based on the methodology originally proposed by Klemes <1985, 1986>, is presented. It includes a hierarchial validation testing scheme for model application to runoff prediction in gauged and ungauged catchments subject to stationary and nonstationary climate conditions. A case study on validation and intercomparison of three different models on three catchments in Zimbabwe is described. The three models represent a lumped conceptual modeling system (NAM), a distributed physically based system (MIKE SHE), and an intermediate approach (WATBAL). It is concluded that all models performed equally well when at least 1 year's data were available for calibration, while the distributed models performed marginally better for cases where no calibration was allowed. ¿ American Geophysical Union 1996