Sand dams are impermeable water harvesting structures built to collect and store water within the volume of sediments transported by ephemeral rivers. The artificial sandy aquifer created by the sand dam reduces evaporation losses relative to surface water storage in traditional dams. Recent years have seen a renaissance of studies on sand dams as an effective water scarcity adaptation strategy for drylands. However, many aspects of their functioning and effectiveness are still unclear. Literature reviews have pointed to a range of research gaps that need further scientific attention, such as river corridors and network dynamics, watershed-scale impacts, and interaction with social dynamics. However, the scattered and partially incomplete information across the different reviews would benefit from an integrated framework for directing future research efforts. This paper is a collaborative effort of different research groups active on sand dams and stems from the need to channel future research efforts on this topic in a thorough and coherent way. We synthesize the pivotal research gaps of a) unclear definition of “functioning” sand dams, b) lack of methodologies for watershed-scale analysis, c) neglect of social aspects in sand dam research, and d) underreported impacts of sand dams. We then propose framing future research to better target the synthesized gaps, including using the social-ecological systems framework to better capture the interconnected social and biophysical research gaps on sand dams, fully utilizing the potential of remote sensing in large-scale studies and collecting sand dam cases across the world to create an extensive database to advance evidence-based research on sand dams.

Alluvial aquifers in seasonal rivers are a yet underutilised resource in many (semi-)arid regions of Africa. These so-called sand river aquifers provide nature-based water storage within easy reach because they are shallow. They form a significant potential renewable source of water for irrigation development. Innovative approaches and solutions are needed to sustainably increase productive use of this resource to enhance rural livelihoods. The A4Labs action research explores the potential and pitfalls of introducing solutions designed for individual smallholder farmers. This entails innovation in three domains: the technology used (manually-installed shallow well-points in or next to a sand river combined with solar-powered water pumps), the arrangement (individual smallholder farmers), and the purpose (market-oriented farming). Pilots were established in the Limpopo river basin in Zimbabwe and Mozambique. Monitoring and assessment are ongoing, but preliminary findings indicate that successful adoption of the approach was not constrained by water availability. Despite the fact that these pilots were established during two subsequent drought years, there was no difficulty in accessing freshwater in sufficient quantity. Instead, successful adoption depends on previous farming experience, market access, and the possibility to grow adaptively in terms of technology, scale and financial risks. In addition, establishing an individual farm to grow cash crops requires acceptance and new skills, as irrigation for smallholder farmers in Africa has traditionally been framed as a communal activity in “collective” irrigation schemes with strong support by outside agencies, and with the well-known collective action challenges. This action research has also estimated that the potential for upscaling this innovation in the Limpopo river basin is significant. Our innovative solution for accessing water stored in shallow alluvial aquifers can start small, is within reach of smallholder farmers (initial investment being less than US$1,000/0.2 ha), and is scalable as farmers can gradually improve their system and expand. Moreover, the solution allows for the application of “adaptive development pathways” at the river-stretch scale.