Abstract
Alluvial corridors of ephemeral river systems provide viable opportunities for natural water storage in dry lands. Whilst alluvial corridors are widely recognized as water buffers, particularly for areas experiencing constant water scarcity, little research has been undertaken in Sub-Saharan Africa to explore their hydrological variability and water resource potential as alternative water sources for nearby communities. This study investigated the water flow behavior and storage potential of an ephemeral river system in the Mara Basin of Kenya for purposes of supporting water resources development and ecological sustainability. The water flow processes - including the recharge rates and water loss processes - from existing sand storage systems were established through monitoring of ground and surface water levels. Water samples along the alluvial corridor were collected and analyzed for major ions and isotopic signatures required to establish the water storage dynamics. The storage potential was estimated through Probing and Electrical Resistivity Tomography techniques, augmented with in-situ measurements of hydraulic conductivities and channel bed porosities. The mean annual storage volume in the alluvium of the study reach was estimated at 1.1 Mm3, potentially capable of providing for the annual domestic and livestock water demands of the area. Transmission losses into the alluvium beneath the ephemeral channel-bed were noted to attenuate the flood peak discharges, depending on the level of saturation of the alluvial bed. However, water storage in the alluvium was subject to losses through evapotranspiration and seepage through fractured bedrocks. The study demonstrated the potential of alluvial corridors as water storage buffers providing alternative water sources to communities within the dry land regions with water scarcity, thereby to supporting ecosystem sustainability.
Highlights
Ephemeral river systems (ERS) are a common drainage feature in arid and semi-arid regions across the globe (Hussey, 2007; Camarasa-Belmonte, 2016)
We investigated the relationship between water flow dynamics, recharge into the alluvial corridors as a means of replenishing the natural water storage, and the natural processes that account for water losses in the alluvium
Even though observations in this research are based on only three storm events, limiting the extent to which concrete judgment on water flow behavior can be made, two critical observations are the time lag in the rising limbs and the similar behavior of the recession limbs
Summary
Ephemeral river systems (ERS) are a common drainage feature in arid and semi-arid regions across the globe (Hussey, 2007; Camarasa-Belmonte, 2016). The flows of ERS occur for short periods, usually days or even hours, and are a direct response to heavy storm events (Berardi et al, 2013; Dogramaci et al, 2015). A distinguishing feature of ephemeral rivers in semi-arid environments is the presence of a significant volume of water storage in the channel alluvium between runoff events (Mpala et al, 2016). The volume of flow events in such systems is influenced by transmission losses to groundwater storage beneath the channel bed (Abdulrazzak and Sorman, 2006; Shanafield and Cook, 2014; Pacheco-Guerrero et al, 2017). Studies have shown that water stored within the alluvium provides an alternative source for sustainable water supply and food production during dry periods (Arnold et al, 2016; Costigan et al, 2017; Koundouri et al, 2017; Olang et al, 2019)
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