Understanding dominant hydrological processes and mechanisms of water flow in a semi-arid mountainous catchment of the Cape Fold Belt

Abstract

Improving our understanding of streamflow characteristics, water storage, and dominant flowpaths in mountainous regions is important as mountains play a vital role in delivering water to lowlands, particularly in semi-arid areas. This work characterized water sources, flowpaths, and streamflow characteristics in the semi-arid, mountainous Kromme catchment in Eastern Cape Province of South Africa. Precipitation, shallow and deep groundwater levels, and streamflow data were analysed to identify patterns that indicate the occurrence and/or dominance of certain processes, responses, and flowpaths. Results of the study demonstrated how the catchment responds to rainfall events across seasons and rainfall intensities. Steep and rocky areas that make up much of the catchment contributed to significant flood peaks after high-intensity storms. Quick and slow responses in flow after rainfall events indicated the dominance of both surface and subsurface flowpaths respectively. Furthermore, surface and subsurface flows were significant in recharging the floodplain alluvial aquifer as well as maintaining streamflow during dry periods. Average annual runoff coefficients were low (0.09), which implied large evapotranspiration (ET) withdrawals from dominant flowpaths and/or storage in inactive groundwater. The Kromme catchment has a sizeable floodplain with large alluvial aquifers, which make significant contributions to catchment storage and outflows. Overall, the catchment streamflow was sustained by baseflow (for ∼50% of the time). Recession patterns suggested that the channel receives flow from different storages with the alluvial and bedrock aquifers as main contributors. Flow contributions had different rates with maximum recession periods up to 22 days, indicative of interflow dominance and floodplain drainage. Throughout the monitoring period, the river system was gaining flow at the different monitored sites during both low and high flow conditions. The channel was also gaining from the mountain bedrock through tributary flows and from the alluvial aquifer. A conceptual model of flowpaths and processes at the catchment scale is presented to improve the understanding of catchment scale hydrological processes in a semi-arid meso-scale mountainous environment.