Separating anthropogenic and climate contributions to streamflow variations in Rietspruit sub-basin, South Africa

Abstract

Human activities and climate change continue to alter streamflow in many river basins. In this study, the influence of human activities and climate variability on streamflow changes in the Rietspruit sub-basin (RSB), South Africa, were investigated using precipitation, potential evapotranspiration, and discharge data for the period 1986 to 2018. Trends in the hydrometeorological variables were assessed using the Mann-Kendall test while the change point in the annual streamflow data was identified using the Pettitt test and the double mass curve technique. Results revealed no statistically significant change in annual precipitation, whereas annual streamflow showed a significant increasing trend while potential evapotranspiration showed a decreasing trend. It was identified that the change point occurred around 1999, thereby dividing the streamflow data into the pre-change (1986–1999) period with comparatively low urban cover (8.3%), and the post-change period (2000–2018) with urban land covering 16.6% of the study area. The results demonstrate that from 1986 to 2018, the elasticity of streamflow with respect to precipitation and potential evapotranspiration was 0.26 and −2.01, respectively. This implies that streamflow in the study area is more sensitive to potential evapotranspiration than precipitation. Nonetheless, changes in land use/land cover, particularly the urban settlement which increased by 101% between 1986 and 2018, were more influential than climate variability for the significant increase in streamflow in the RSB. The results provide necessary information for water resource management, land use planning and ecosystem development in the near future. This study emphasizes the necessity of addressing nonlinear and dynamic hydrological processes as a result of human impact and climate associated changes.