Groundwater levels in the region are traditionally relatively shallow, particularly in the riparian zone and in the dry river beds during periods of low-flow.
The building of dams in the Limpopo River basin has lead to increased interaction between groundwater and surface water. This is because for long periods of the year the lower and middle reaches of the Limpopo River are dry (LBPTC 2010), with the water flowing instead in the alluvial aquifer of the river bed, as shown in the picture below.
Many of the rivers in the north of the basin are non-perennial, flowing at the surface only during and following heavy, episodic rainfall events, characteristic of the wet season in the region.
Confluence of the Shashe and Limpopo rivers.
Source: DiPerna 2009
( click to enlarge )
There is further evidence of observed river run-off reducing at Chokwé, with flows decreasing year after year since the installation of the flow gauging station (LBPTC 2010).
The lack of consistent, basin-wide information about groundwater/surface water interactions is one area of concern raised in the LBPTC Scoping Study (2010). This concern was raised because not only does abstraction of groundwater from an alluvial aquifer, or an aquifer located close to a river channel, have an influence on local water availability, it also has an impact on downstream water users (human and environmental; LBPTC 2010).
Aquifer Dependent Ecosystems
Aquifer Dependent Ecosystems (ADE) are plant (and animal) communities that survive in environments of lower water availability by tapping water from aquifers, rather than traditional atmospheric or surface water resources (Colvin et al. 2003; Colvin et al. 2007; Xu et al. 2003). The majority of ADE-related research in southern Africa has been conducted in South Africa, with various aquifers identified across the country providing water to ADEs. One such aquifer is the seasonal alluvial aquifer present in the Limpopo River bed, supporting plant and animal life in the riparian zone of the Limpopo River for parts of the year.
Mining and irrigation are thought to use approximately 10 Mm³/yr of water from the alluvial aquifers of the Limpopo River. As part of their licensing requirements, one of the mines in the region monitors water-stress in riverine tree communities to determine the impact these withdrawals have on ecosystems (Colvin et al. 2007).
As the Limpopo River alluvial aquifer runs through the Greater Mapungubwe Transfrontier Conservation Area, the maintenance of this aquifer to support biodiversity is critical to the region (Colvin et al. 2007).
Water Flowing out of the Basin
Water use efficiency is often now considered on a basin-scale, assuming that water lost through inefficiencies in one part of the basin will ultimately be available elsewhere in the basin. However, indications are that the Limpopo River basin is not a closed system (CGIAR 2003). Surface run-off percolating into the groundwater in the eastern basin, where run-off is highest, is thought to seep down to greater depths and out of the basin in a southeasterly direction (CGIAR 2003).
