The Eastern Kalahari-Karoo Basin Transboundary Aquifer (EKK-TBA) extends from eastern Botswana into western Zimbabwe, is mainly located between latitudes 17° S and 22° S and longitudes 23° E and 29° E, and covers approximately 127 000 km2, of which 65% is in Botswana, and 35% is in Zimbabwe. The EKK-TBA system straddles two river basins: the Okavango and the Zambezi. The climate of the Basin is semi-arid, with rainfall occurring between October and April. Groundwater constitutes the primary source of water in the Basin. The 2020 population of the EKK-TBA was estimated at 600,000.

A hydrogeological assessment of the aquifer system, based on a review and analysis of publications and technical reports (mainly in the public domain), provided a conceptual understanding of the Basin with respect to groundwater flow, recharge, hydrochemistry, and groundwater abstractions. The first strategic objective of the EKK-TBA Strategic Action Plan (SAP) aimed to enhance knowledge and understanding of the transboundary aquifer for water and safety planning through catchment management. It is foreseen that the water availability situation within the Basin will worsen as a result of a growing population (projected to double by 2050), and this comes with increased demand for food, translating to increased irrigation, which in turn requires increased water, consequently imposing a higher demand for groundwater resources whose amount and availability are already being impacted by climate variability and change.

To sustainably manage the groundwater resources of the Basin, groundwater recharge and abstraction rates, including the impacts of climate variability and change, need to be quantified. SADC-GMI called for the construction of a regional Integrated Hydrogeology Model to quantify and evaluate groundwater dynamics in the Basin, building upon the 2020 hydrogeological assessment and conceptual model of the EKK-TBA.

The project aimed at developing a regional steady-state groundwater model of the EKK-TBA, which spans the border between Botswana and Zimbabwe, to facilitate the quantification of groundwater resources and the evaluation of groundwater dynamics in the Basin. This included quantifying and evaluating groundwater inflow (rainfall recharge and lateral inflow), groundwater flow through the Basin, and groundwater outflow (subsurface drainage, seepage, evapotranspiration, and abstractions). The 2020 hydrogeological assessment and conceptual model of the EKK-TBA formed the basis for the model’s construction. The objective of the model was to provide initial estimates of sustainable abstractions (e.g., setting a limit on abstraction), which are essential for joint sustainable groundwater management and help enhance water security in the Basin.

The USGS modular hydrologic model software, MODFLOW-6, was used to construct the EKK-TBA groundwater model, and USGS ModelMuse was utilized as the graphical user interface. Many PEST (a model-independent parameter optimiser) and manual calibration runs were conducted to simulate groundwater levels under steady-state conditions for the five model layers, and these were compared with observed and reconstructed groundwater levels before significant abstractions. Three scenarios were formulated to assess their impact on EKK-TBA groundwater levels, human population growth, climate change, and the combined effect of the two. A general decline in groundwater levels over time was observed, especially along the southern and southeastern fringes of the EKK-TBA. From a regional perspective, most of the decline can be attributed to climate change. A training course was held to build the capacity of groundwater professionals from government institutions in Botswana and Zimbabwe to set up and use the EKK-TBA groundwater model. The training entailed constructing the model, calibrating the model, and analyzing the scenarios