Modelling water quantity and quality using SWAT : a case study in the Limpopo River basin, South Africa

In the EAU4Food project the enormous challenges African agriculture is facing today are addressed: the agricultural productivity must increase in the coming years. At present the increase in food production cannot keep up with the population growth. In the coming years irrigation will gain importance, but at the same time the availability of fresh water and the sustainable use of soil resources is under increasing pressure. Hence, new approaches are required to increase food production in irrigated areas in Africa, while ensuring healthy and resilient environments. The need to use less water to produce crops requires innovative approaches. By using models the aim is to analyse feasible measures to improve water efficiency and to reduce negative impacts. The SWAT model has been applied in the Nsama sub-basin, which is situated within the Letaba basin in South Africa. SWAT is a conceptual, physically based hydrological model using daily time steps. In SWAT, a basin to be modelled is divided into multiple sub catchments, which are then further subdivided into Hydrologic Response Units (HRUs) that consist of a homogeneous land use, management, ground slope, and soil characteristics. Flow generation, sediment yield, and non-point-source loadings from each HRU in a sub catchment can be simulated. The purpose of this case study is to use the SWAT model to analyse the effects of changes to the hydrological system. Because of the lack of data, the model could not be calibrated, instead a sensitivity analysis was carried out. Measured discharges from the Letaba basin were scaled down to the Nsama in order to compare at that level measured and calculated discharges. As a test case two scenarios were modelled, being a change in land use and the effect of a DDT application. Based on the experience of this try-out with the SWAT model and the ArcSWAT user interface, the model will be used further for analysis of agricultural production changes and their effects on water quantity and quality.