Evaluation of integrated impacts of climate and land use change on the river flow regime in Wamkurumadzi River, Shire Basin in Malawi

Climate and land use change (CC and LUC hereafter) are interlinked factors that can lead to river flow regime changes, as well as affecting hydrological extremes such as floods and drought. There is now considerable evidence of CC and LUC in many catchments in Malawi but without corresponding evaluations of their impacts on river flow regimes. Therefore, this study assessed how both factors affect the flow regime of Wamkurumadzi River, a key tributary of the major Shire River in southern Malawi. Land use and hydroclimatic data for the basin were first analyzed for spatial-temporal trends in the historical period between the years 1984 and 2015. The Soil and Water Assessment Tool (SWAT) model was then applied with different LUC and CC scenarios in order to assess their sole and combined impacts on the river flow regime. The model was calibrated and validated using the split sample method from the year 1984 to 1999 and from the year 2000 to 2015. Model performance was acceptable according to the selected evaluation criteria, with the Nash-Sutcliffe (NSE) coefficient of 0.78 and coefficient of determination (R-2) of 0.96 during calibration and NSE of 0.93 and R-2 of 0.98 during validation. Results of the integrated impacts of LUC and CC suggest a slight increase in river discharge of 0.05 m(3)/s for the period between the 1980s and 2000s. During the 1980s-1990s, both CC through rainfall decreased and LUC resulted in decreases in the mean river discharges by 1.58 and 0.37 m(3)/s, respectively. The study also found that CC through increased rainfall in the 1990s-2000s decades saw an overall increase of 1.39 m(3)/s in mean river discharge, while LUC shows the increase of mean river discharge by 0.25 m(3)/s. However, the study observed that reforestation efforts in the basin were greatly responsible for the alteration of the river flow regime in the later period.