Abstract
Increasing demand for timber products results in the expansion of commercial afforestation in South Africa. The conversion of indigenous seasonally dormant grassland to evergreen forests results in increased transpiration and ultimately a reduction in catchment runoff, creating a negative impact on the country's scarce water supplies. In order to assist managers in the decisionโmaking processes it is important to be able to accurately assess and predict hydrological processes, and the impact that land use change will have on water resources. The Soil and Water Assessment Tool (SWAT) provides a means of performing these assessments. One of the key strengths of the SWAT model lies in its ability to model the relative impacts of changes in management practices, climate and vegetation on water quantity and quality.
The aim of this study was to determine if the SWAT model could reasonably simulate hydrological processes in daily time steps from two small South African catchments. To verify the SWAT model a grassland (C VI~grass~) and Pinus patula afforested catchment (C II~pine~) were selected from the Cathedral Peak hydrological research station in the KwaZulu Natal Drakensberg mountains. These catchments were chosen because of the availability of detailed hydrological records and suitable land use.
Observed and simulated streamflow for C VI~grass~ and C II~pine~ were compared. When model fits of observed and simulated streamflow for C VI~grass~ were acceptable, this parameter set was then used in the configuration of C II~pine~. Results show that the model performs well for C VI~grass~ with reasonable agreement between modelled and observed data (R^2^ = 0ยท68). Comparisons for C II~pine~ show a total oversimulation of streamflow for the period 1950 to 1965, with deviations between observed and modelled data increasing from 1959 to 1965, due to the model not accounting for the increase in ET brought about by the maturing pine plantation. Copyright ยฉ 2005 John Wiley & Sons, Ltd.