Groundwater and surface water can be closely linked even when separated spatially. Each contributes to the other, with these interactions playing an important role in the hydrology of a region (USGS 2009).
Due to the hard-rock geology of most of southern Africa, groundwater-surface water interactions in the region generally occur only in narrow strips of alluvium along river channels (such as the Limpopo River), Cenozoic sand deposits fringing the coastline of the continent, or in the primary aquifers in Kalahari sand deposits (Scott and Le Maitre 1998).
Vegter and Pitman (1996) subdivided the interactions into three main categories:
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Influent or losing streams: where the water table is below the stream bed and water flows from the stream bed through a porous material to recharge the groundwater (below).
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Effluent or gaining streams: where the water table is above the stream and groundwater feeds into the stream (lower left).
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A fluctuating water table: where the water table shifts up and down, determining periodic influent or effluent flow (lower right).
Influent or losing stream.
Source: Commonwealth of Australia 2006
( click to enlarge )
 Effluent or gaining stream. Source: Commonwealth of Australia 2006 ( click to enlarge ) |
 A fluctuating water table. Source: Commonwealth of Australia 2006 ( click to enlarge ) |
Most commonly, groundwater contributes to a stream or rivers baseflow, and can be a significant contributor to surface water recharge, especially in higher rainfall regions. The interaction is often bidirectional, depending on the elevation of the groundwater level. If the groundwater level is low then groundwater may be recharged by the surface water body; alternatively if the groundwater level is high then it may be contributing to the surface water body. In shallow alluvial aquifers or riverbed sand aquifers, this normally depends on the seasonal cycle of rainfall and corresponding water level in the river banks (DWAF 2003).
