Recent advances with the integrated hydrological model of the Stampriet Transboundary Aquifer System (STAS)

The STAS is a large transboundary aquifer system (>120 000 km2) shared between Botswana, Namibia, and South Africa. It provides the only water resource in this arid region. As part of the GGRETA project, UNESCO and local stakeholders have been promoting the development of a groundwater model for the assessment and the sustainable management of this shared resource. In this communication, we retrace recent progress and lessons learnt from this modelling project. First, a detailed hydrostratigraphic study allowed us to refine the geometry of the STAS and in particular the position of its boundaries. This study also highlighted links between the STAS and the neighboring Central Kalahari Basin. To the south, a large complex of salt pans was identified as the regional outlet for the basin (Hakskeen, Koppieskraal, Uitsak pans). Second, although the isotope data for the basin were compiled in phase 1 of the GGRETA project, they had, to date, never been used as information for the STAS numerical model. Integration of environmental tracer data allowed the identification of key hydrological recharge, discharge, and aquifer exchange processes. In particular, the hydrochemical and isotopic synthesis highlighted the importance of land surface and groundwater interactions. In turn, this led us to select an integrated hydrological model capable of simulating interaction between land surface (UZF; Niswonger et al. 2006) and groundwater (MODFLOW). A feasibility study showed there would be great benefits moving from a former stand-alone model, which requires manual updating, to a state-of-the-art integrated modelling platform that can be shared by all stakeholders and updated automatically with remote sensing data.