Spatial and temporal variations in baseflow across headwater streams in the Jonkershoek valley, South Africa.

An understanding of processes and factors accounting for spatiotemporal variations of baseflows is necessary for sustainable water resources management, particularly in semi-arid regions. Most of the southern part of the Western Cape Province of South Africa experiences a Mediterranean climate typified by long dry summers. Thus, baseflows of rivers draining areas with Table Mountain sandstones sustain water for humans and ecosystems. Processes and factors accounting for spatiotemporal variability of baseflows in these rivers are not adequately understood. This study aimed to determine the spatial and temporal variations of baseflows across streams in a mountainous region with Table Mountain Group. The study was done on three sub-catchments (1.0 to 2.5 km²) within the Jonkershoek Valley that drains the Hottentots Holland Mountains. A two-parameter recursive digital filtering method (RDF) was combined with the conductivity mass balance method (CMB) to separate baseflow from daily streamflows during the 1946-2019 period. The results of baseflow separation using the CMB method were used to calibrate the RDF parameter (BFI_max).  The results showed that annual baseflow contributions varied spatially and the temporal variations were minor. Baseflows contributed 38 to 86 % of the annual streamflows. This wide range was unexpected considering that the sub-catchments were adjacent to each other and with similar geology. The variations could be explained by rugged topography and spatial irregularity of fracturing.  The results also showed seasonality in baseflow contributions. All three sub-catchments had as expected higher contributions from baseflow in the dry summer compared to wet winter. For  Bosboukloof baseflows contribute 89% of summer flows and 82 % in winter, Tierkloof 73 % and 60 %, while these were 50% and 34 % for the Langrivier.  Differences in topography and vegetation were considered to account for the spatial variations of baseflow contributions. For instance, Bosboukloof has the lowest slopes and most scree material and had the highest proportion of baseflow. The decreasing trend in monthly baseflow was noted during the driest months (Oct-Dec). This indicates the vulnerability of baseflow. Again, this was particularly the case for Bosboukloof, the site most fully afforested with pines that may tap into the deeper water sources.