Abstract
Study regionMolototsi sand river, Limpopo, South Africa. Study focusEphemeral sand rivers are common throughout the world’s dryland regions, often providing a water source where more conventional sources are unavailable. However, these alluvial aquifers are poorly represented in the literature. Extensive field investigations allowed estimation of stored water volume and characterisation of an alluvial aquifer. New hydrological insights for the regionComputed alluvial aquifer properties included hydraulic conductivity of 20–300 m/d, porosity of 38–40%, and aquifer thickness of 0–6 m. Dykes and other subcrops commonly compartmentalise the aquifer though do not form barriers to flow. A hydraulic disconnect between deep groundwater (occurring in fractured metamorphic rocks) and the alluvial aquifer was revealed by groundwater levels and contrasting hydrochemistry and stable isotope signatures. The dominant recharge process of the alluvial aquifer is surface runoff occurring from torrential tributaries in the catchment’s upper reaches. A fraction of available storage is currently abstracted and there exists potential for greater exploitation for smallholder irrigation and other uses.
Highlights
Ephemeral sand choked rivers commonly occur in the world’s dryland regions
The Molototsi is very unlikely to be a losing stream or the sand river water table would not settle at the observed depth of ∼1 m and a response would likely be seen in the borehole groundwater levels to surface water flows
The groundwater levels are suggestive of a disconnected stream, where an unsaturated zone exists between the stream and deeper groundwater
Summary
Ephemeral sand choked rivers commonly occur in the world’s dryland regions. Such systems experience surface flows only following infrequent torrential rainfall (Tooth, 2000). Where the underlying geology is of low permeability, e.g. African crystalline basement, infrequent torrential flows fully recharge the alluvial aquifer creating an accessible water resource where unfavourable climate and geology create few alternatives. Drylands occupy 41.3% of the world’s surface and are home to 2.1 billion people (UN, 2017), many of whom obtain their water supplies from resources contained within “sand rivers” (Seely et al, 2003; Love et al, 2011). The relatively low stored water volumes can be sufficient for small-scale farming activities (Hussey, 2007; Love et al, 2007)
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