A new approach to modelling streamflow reductions resulting from commercial afforestation in South Africa
The Department of Water Affairs and Forestry has, for some time, needed a comprehensive tool to incorporate the impacts of commercial afforestation on water resources into water use authorisation and allocation processes. Simulation modelling on a national scale, using the ACRU model, was identified as one possible solution. However, in order to establish confidence in the national simulation results, it was necessary to first verify model output against reliable observed data from process studies and long-term catchment afforestation experiments. In the verification phase, long term reductions in streamflow resulting from afforestation were satisfactorily simulated for five research catchments. Some problems were experienced with catchments in the Western Cape, with verifications on shorter duration experiments and the simulation of specific evaporative processes. Modelling of low flows was less successful than for total flows. In the second phase (the generation of the national database), simulations were first performed for the dominant Acocks (1988) veld type (i.e. 0% afforestation) within 843 Quaternary Catchments exhibiting afforestation potential (to determine baseline streamflow), followed by simulations of streamflow after 100% afforestation with eucalyptus, pine and wattle respectively. The difference between the unafforested and afforested simulations equated to streamflow reduction caused by afforestation. This culminated in the generation of maps and tables expressing reductions in streamflow per Quaternary Catchment, which represents a working solution for immediate application but may be improved upon with further work.