Integrated hydrological model of the Central Kalahari Basin – optimal tool for assessment of sustainability of groundwater resources: powerpoint

Groundwater resources replenishment dynamics evaluation is critical for management purposes, especially in arid and semi-arid regions where it accounts for the majority of potable water demand. That evaluation, nowadays is done using distributed, integrated hydrological models(IHMs). Such models, inherently account for surface-groundwater interactions, relying on realistic hydrogeological conceptual models(HCMs)and reliable input data involving also surface hydrological fluxes like rainfall and potential evapotranspiration. In this study, the productive Central Kalahari Basin (CKB, ~200000km2) transboundary(Botswana and Namibia)Karoo Aquifer System IHM, was developed, to define its groundwater resources replenishment dynamics. An efficient data integration method for developing HCM of the CKB, was undertaken, applying 3-D geological modelling based on RockWorks code, in iterative combination with ArcGIS. Six hydrostratigraphic units(Kalahari Sand Unit, Stormberg Basalt Aquitard, Lebung Aquifer, Inter-Karoo Aquitard, Ecca Aquifer and Ghanzi Aquifer)were identified, their heads and related flow system interdependencies evaluated and hydraulic properties defined. The daily spatio-temporally variable IHM driving forces(rainfall and potential evapotranspiration)were defined using remote sensing technique. A fully 3-D, six-layer CKB IHM was developed using MODFLOW-NWTwithUZF1 package, accounting for variably saturated flow. The transient CKB IHM was calibrated throughout 13.5 years, using borehole hydraulic heads as state variables. Results of the model showed dominant water balance role of subsurface evapotranspiration restricting gross recharge to only few mm/year and typically negative, yearly net recharge (median -1.5mmy-1), varying from -3.6 (2013) to +3.0 (2006) mmy-1with respective rainfalls 287 and 664 mmy-1, explaining water table decline within the simulated period. Primary determinants of spatio-temporal distribution of net recharge in the CKB were amount and temporal distribution of rain, unsaturated zone thickness and vegetation type/density. The calibrated CKBIHM represents optimal tool for further assessment of impact of climate change upon the replenishment dynamics of groundwater resources of the CKB.