IWRA Proceedings

Semi-arid areas of sub-Saharan Africa are characterized by low and unreliable rainfall and high evaporation rates. Small water storage structures (SWSS) capture runoff during the rainy season and are commonly utilised in these areas to overcome the dry period. The principle of water storage and optimum site conditions differs with each SWSS, but since research is usually specific to a single structure, it becomes difficult for implementers to select the most feasible structure under a given set of conditions.

The current study assesses the feasibility and resilience of three SWSS, i.e. sand dams, micro-reservoirs and sand rivers, as a function of the hydro-climatic, geological and socio-economic contexts. Comprehensive literature analysis was performed on storage, sedimentation, water quality, costs and management impacts. The Tete district in Mozambique was used for mapping the potential sites for SWSS, comparison to actual sites and quantitative assessment of storage versus demand and costs. Mapping results showed that each SWSS has unique characteristics, which allows remote sensing and GIS tools to be used at the initial planning stages to map the potential of an area for developing each SWSS, based on geology, slope and stream order/width. Important mismatches were observed between siting results and existing locations of sand dams and micro-reservoirs, which could explain the poor functioning of several of these structures. SWSS have a relatively low storage potential, which makes them less effective in areas with a unimodal rainfall distribution. Sand dams have the smallest storage, but can still be of high local importance if well designed and sited. Micro reservoirs have larger storage but a higher volume is lost through evaporation.

Sand rivers can store the largest volumes of water if the river channel is wide, deep and is composed of coarse sediments on a flat riverbed. Human behaviour is responsible for sedimentation in SWSS through poor land management practices such as overgrazing,
deforestation and agricultural practices that contribute to erosion releasing silts into the reservoirs. Silts may carry agricultural pollutants, heavy metals and pathogens which affect the quality of water stored. Generally, much lower construction and maintenance costs are
incurred for sand dams since they are smaller compared to micro reservoirs. The way the SWSS is managed is also critical, as it will determine its success or failure. The implementing organization, with the help of extension workers, has to provide training to the water users to ensure the sustainability of the structures.