Introduction
Water scarcity is one of the major threats to human wellbeing and a central challenge to achieving the milestones of the sustainable development goals (Gain et al., 2016) and ecological restoration (Fischer et al., 2021). Drylands are particularly vulnerable areas characterized by extreme water scarcity, variable rainfall and sparse vegetation, covering almost 40% of global land surface (Safriel et al., 2005). Because of the unstable and uncertain natural resources, drylands are often home to marginalised populations (Sietz et al., 2011), who have adapted to resources scarcity and uncertainty, thus developing resilience to environmental changes (Ifejika Speranza, 2010). However, climate change is expected to expand drylands by changing precipitation patterns and intensity (Feng and Fu, 2013), with serious consequences of increasing populations out-migrations and armed conflicts (Neumann et al., 2015; Sterzel et al., 2014). For example, Southern African drylands are expected to see a decrease in precipitation, runoff and soil moisture, which will likely affect the resilience of millions of agro-pastoralists (Piemontese et al., 2019).
Retaining water in the landscape has a critical role for increasing resilience of drylands to climate change “by avoiding major regime shifts away from stable environmental conditions, and in safeguarding life‐support systems for human wellbeing” (Rockström et al., 2014). Harvesting rainwater through small-scale and distributed interventions can be a crucial strategy to increase water retention in the landscape, increasing pasture and water availability for agro-pastoralists (Descheemaeker et al., 2010; Strohmeier et al., 2021).
Within the broad spectrum of water harvesting technologies, sand dams represent a particularly interesting and promising technology to harvest water in dry rivers. Sand dams are barriers built along ephemeral rivers, which trap coarse sediments (mostly sand), creating a small reservoir filled in sand (Ritchie et al., 2021). The sand trapped behind the dam serves as a water storage – up to 40% of the sand volume are pores that can be filled in water (Lasage et al., 2015). Sand dams have demonstrated successful in providing additional water points along ephemeral rivers and increasing communities wellbeing (Teel, 2019). In Kenya, where the majority of the research on sand dam is focused, sand dams have decreased the water collection time from more than 3 hours per day to as little as 15 minutes (Teel, 2019). Other benefits of sand dams include increased water use of about 3 times, increased irrigation water, thus increased food security (Ritchie et al., 2021; Villani et al., 2018).
Replicating and transferring water harvesting (WH) solutions like sand dams to other areas of the world could boost land and water restoration targets and contribute to strengthening social-ecological resilience of drylands (Ammar et al., 2016; Grum et al., 2016; Piemontese et al., 2020). However, especially in areas with no previous experience, planning the most appropriate location for implementation is a key step to ensure the usefulness and effectiveness of the technology. In fact, some 80% of the WH implementations fail because of wrong siting for both failed water retention and scarce involvement of local communities, who are the ones using and benefiting from the technologies (Ngugi et al., 2020).
This is the case of Angola drylands, where sand dams have not been documented in the scientific literature (Ritchie et al., 2021), but the successful experience of places with similar hydro-climatic conditions in Kenya and in the near Namibia (Hartley, 1997), makes it an appealing solution to water scarcity. However, especially in a new context, exploring the feasibility of such technology requires a deep understanding and careful inclusion local communities perceptions and knowledge, not only for selecting a suitable site, but also for ensuring a long-lasting beneficial use (Ziadat et al., 2012).
Conventionally, the siting of sand dams for large areas (regional to national) is approached as a top-down, data-driven analysis to highlight the areas, or river sections, with adequate biophysical conditions for sand dams construction (Forzieri et al., 2008). These large-scale biophysical studies have the advantage to effectively reduce the time and resources for a more detailed field inspection in large areas, but they require high resolution spatial data and often lack socio-economic information on technology viability and use. On the other hand, higher details of suitability analysis can be reached with local participatory approaches, which include a more thorough socio-economic enquire and community participation, but are limited in spatial scale (Ertsen et al., 2005). Although these two approaches seem polarized, and in fact they have been applied in silo, an integration is still possible and desirable to produce insightful large-scale assessments.
Here, we present a mixed-method approach to large scale siting considering both the biophysical requirements of a sand dam site and the needs, constraints and preferences of local communities. We apply the approach to Namibe, a semi-arid Province in Southwest Angola, by using a participative enquiry to assess the water security problems of the communities, analysed to incorporate the socio-economic conditions for the best siting of sand dams. The twofold contribution of this work tackle both the generic research gap on large-scale participatory planning of water harvesting technologies and contributes to expanding the geographical understanding of sand dams in a data-scarce and understudied region as the Angolan drylands.