A semi-distributed, physically based hydrological modelling system, WATBAL, which accounts for the entire land phase of the hydrological cycle is described. As compared to the two alternative hydrological model types, i.e. the traditional lumped, conceptual rainfall runoff models (STANFORD model type) and the complex, fully distributed, physically based model (SHE model type) WATBAL represents an intermediate approach.

In the model, primary attention is given to the hydrological processes at the root zone level through a distributed, physically based approach whereas the groundwater processes are simulated in less details by use of a lumped, conceptual approach. This approach allows WATBAL to utilize spatially distributed input data to account for the spatial and temporal variability of meteorological conditions, vegetation and soil properties. Thus WATBAL can e.g. utilize digital satellite information as input data.

WATBAL has primarily been designed as a tool for predicting the runoff from ungauged catchments and for assessing the hydrological effects of land use changes. The capability of the model for simulating ungauged catchments is tested using results from a recent feasibility study for medium size dams in Zimbabwe.

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