Hydrological connectivity in selected pristine catchments in the Kruger National Park.
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The understanding of interactions between hydrological processes is essential, especially in water limited ecosystems in semi-arid environments. It is through this understanding that informed planning and management decisions for ecosystem conservation are developed. Assessment of groundwater- surface water connectivity at catchment scale provides a holistic view of the abiotic template that sustains life systems within the catchment. Spatial differences in hydrological responses are thus understood since these are characterised by nonlinearities emanating from catchment heterogeneity across spatial and temporal scales. This study involved an assessment of groundwater-surface water interaction across incremental contributing areas which were based on stream orders. The study areas, Southern Granites and Southern Basalts, are located on the two dominant geologies in the Kruger National Park (KNP). At Southern Granites the 1st order, 2nd order and 3rd order contributing catchments have an area of 0.3km2, 0.9km2 and 1.5km2 respectively. At the Southern Basalts site the areas for similar incremental catchments were 15.4km2, 31.6km2 and 47.8km2 respectively. Both study sites had streamflow levelloggers installed at each outlet the 1st to 3rd order contributing areas. The assessment was done through a combination of hydrometric techniques, Electrical Resistivity Tomography (ERT) and tracer analysis methods. Monitoring of water levels and sampling in the stream, riparian boreholes and piezometers was conducted from September 2012 to May 2013. The monitoring network consisted of 28 piezometers and 19 boreholes at Southern Granites while 6 piezometers and 4 boreholes were installed at the Southern Basalts sites. Streambed hydraulic conductivities were determined using slug tests. Hydraulic gradients between the stream, piezometers and groundwater boreholes were calculated and used to determine direction of fluxes. Connectivity mechanisms were determined and contributions of different water sources to streamflow were quantified using two and three component tracer based hydrograph separations. Results showed that rainfall intensity was the major control to connectivity between surface and groundwater resources in these catchments. Contribution of event water to streamflow was estimated between 60% and 86% across the nested spatial scales for two monitored rainfall events (19 January and 20 February 2013) at Southern Granites study site. Although event water emerged as the dominant source at all scales, higher pre-event contributions were noted for lower order subcatchments at this site. A 2nd order stream channel, previously conceptualised as gaining was demonstrated through hydrometry and tracers to be increasingly losing subsequently behaving as an indirect recharge point at Southern Granites site. The study, therefore, revealed that lower order reaches on the granitic geology are important water sources that sustain baseflow at higher order perennial streams. At Southern Basalts study site limited subsurface contribution to streamflow was observed due to very low interfluvial gradients and low aquifer transmissivities that characterise the basalt geology. Assessment of groundwater-surface water interaction at this site was conducted only at the 3rd order catchment due to a limited network of groundwater boreholes. At this reach the contribution of event water was estimated between 51% and 64% for two monitored events (19 January and 20 February 2013). Groundwater contribution to streamflow through localised preferential fractured rock flow ranged between 36% and 49%.