22-27 September 2019
Trade Fairs and Congress Center (FYCMA)
Europe/Madrid timezone

Groundwater assessment in sand rivers in adaptation to climate variability and water scarcity: opportunities and challenges in semi-arid Africa

24 Sep 2019, 11:45
15m
Conference room 2.1 ()

Conference room 2.1

Oral Topic 1 - Groundwater assessment and management Parallel

Speaker

Dr Tibor Stigter (IHE Delft Institute for Water Education)

Description

Warm semi-arid regions, covering vast areas of the African continent, are frequently considered of low potential for agricultural development, due to a limited and highly irregular access to water. Notwithstanding, significant amounts of freshwater can be stored in thick sand beds of wide ephemeral rivers, formed as a consequence of pronounced dryland weathering and erosion. Additionally, the storage capacity of these so-called sand rivers can be enhanced through sand dams. Here we present the combined results from three studies on sand river storage potential, recharge, hydrodynamics and water quality, in Tigray Ethiopia, Shashane Zimbabwe and Limpopo Mozambique. Results were obtained through a combination of remote sensing, fieldwork, data analysis and numerical modelling. Fieldwork involved: i) data collection on climate, river discharge and groundwater levels; ii) drone surveying of sand river geometry and groundwater levels in large diameter wells; iii) field testing on aquifer hydraulic parameters; iv) geo-electrical surveying; v) manual borehole drilling and vi) sampling for hydrochemical and isotope analysis. Data obtained from the field and laboratory were used to improve the conceptual understanding of these sand river systems and subsequently build numerical models to simulate their behaviour under natural conditions and possible scenarios of sand river abstraction. All areas show very rapid recharge at moments of surface runoff, revealed by piezometry, major ion chemistry and stable isotope signatures. Recharge is so quick that it is believed to delay runoff significantly in its early stages. Hydraulic connection between the sand river and its banks appears generally limited, except where paleochannels are in connection with the main sand river. Drone imagery in combination with drilling did reveal influent and effluent reaches of the sand river in Tigray (Ethiopia), of a much smaller magnitude (20-30 m wide and 2-4 m thick) than those in the other two areas (200-500 m wide and 5-10 m thick). High water salinity forms a major challenge in Tigray, as the large hydraulic gradient within the sand river promotes continuous groundwater outflow at the two constructed sand dam sites, also revealed by isotope analysis. Pumping groundwater directly from the sand river, rather than using shallow wells dug in the river banks, could mitigate the salinization problem. Numerical models show that flow within the system is in the order of meters per day, that available storage is naturally depleted by evaporation, and that sand river abstraction results in lower evaporation losses, with the potential to irrigate up to several ha per km in the largest sand rivers. Seasonal depletion of storage will be rapidly restored upon river flow in the following wet season, but will also lead to a partial reduction in runoff, possibly affecting users downstream, including dependent ecosystems, which is a topic of further study.

Primary authors

Dr Tibor Stigter (IHE Delft Institute for Water Education) Mr Ahmed Walid Moulahoum Mr Angel Gonzalez-Carballo Mr Attoumane Abi (Université de Quebec à Chicoutimi) Mr Paulo Sérgio Lourenço Saveca (Instituto Superior Politecnico De Gaza (ISPG)) Sakhile Ndlovu

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