Get reliable rangeland science

Four deterministic rainfall-runoff models are briefly described and applied to a small (0.18 km2) grassland catchment in the Eastern Cape Province of South Africa. The models vary in complexity, from a variable time step model with two soil layers and incorporating probability distribution principles for some of the parameters, through two daily time step models, to a relatively simple, but widely used, monthly time step model. The model parameters have largely been determined from measured physical characteristics (topography, soils, vegetation, etc.) of the catchment and not through calibration. As well as comparing the simulated runoff values, the model performances are compared using observed values of soil moisture measured over a period of some 29 months. Although the four models produce similar results, the variable time step model simulates the observed soil moisture variation most successfully and the monthly model least successfully. Most of the differences in the simulation results can be explained by either the differences in complexity of the modelling approach or the resolution of the input data.