Land degradation and water stress are reducing agricultural productivity and food security, with arid regions such as the Sahel particularly affected. In sub-Saharan Africa, growth in agricultural output has relied on expanding cultivation on increasingly marginal soils and shorter fallow periods, an approach that will be hard to sustain, especially with more frequent climate shocks (Glennerster and Suri, 2015). The lives and livelihoods of the estimated 135 million people of the Sahel, 80 percent of whom depend upon agriculture for their income, are affected by drought, land degradation, and desertification (World Bank, 2020). Drought affects up to 50 percent of arable land in any given year in the Sahel, and approximately 80 percent of agricultural land suffers from nutrient depletion and poor soil fertility (UNCCD, 2017).

On-farm rainwater-harvesting techniques can reverse land degradation, increase agricultural yields and revenues, sequester carbon in the soil, and combat desertification. Agronomic trials demonstrate that rainwater harvesting helps retain soil moisture and replenish soil nutrients while reducing the risk of crop failure, and it is particularly useful where irrigation is unfeasible and chemical input use is limited (Critchley and Siegert, 1991; Reij et al., 2009). Micro-catchments, including demi-lunes (half-circular bunds) and zai or tassa (soil pits), are often a particularly effective type of on-farm rainwater harvesting for smallholder farmers because they do not require specialized equipment and can be implemented after harvest when labor costs are lower (Reij et al., 2009). Demi-lunes are suited to sloped land with severely degraded soil, known as glacis, which covers an estimated 40 percent of degraded land in the agro-pastoral zone of Niger (Reij et al., 2009). Despite substantial investment in promoting demi-lunes in Niger, adoption remains low: only around 10 percent of farmers adopt demi-lunes on any part of their private land, even though most farmers are aware of the technique (Aker et al., 2020).

Simple training sessions cost-effectively increased the adoption of on-farm rainwater harvesting techniques in Niger. A one-time training conducted by the Ministry of Environment to teach farmers how to construct demi-lunes without special tools increased adoption by over 90 percentage points and increased field coverage by 20 demi-lunes per hectare (Aker et al., 2020). Demi-lune adoption increased farmers’ profits: per-farmer agricultural revenue among adopters increased by USD 34–37 per year, more than offsetting the costs of constructing demi-lunes (USD 30 in the first year and USD 4 in subsequent years) (Aker et al., 2020). Farmers were also able to bring previously unproductive land into cultivation—on average, an additional 0.3 hectares (Aker et al., 2020). The training had an average cost of USD 9 per participant and showed lasting impacts at least three years later, suggesting a high social cost–benefit ratio and a path to long-run sustainability with relatively small ongoing costs (Aker et al., 2020).

Knowledge from training on rainwater harvesting spreads among farmers and benefits those who were not trained directly. In Niger, households in villages where others received training were 18 percentage points more likely to have constructed demi-lunes three years after the training (Aker et al., 2020). Rainwater harvesting techniques also increase carbon sequestration in soils: demi-lunes in Niger could sequester approximately 0.32 tons of CO₂e per hectare per year, and zai or tassa soil pits could sequester approximately 0.19 tons of CO₂e per hectare per year (Lal, 2004; Reij et al., 2009). Training for rainwater harvesting techniques can be scaled through government delivery channels. The Ministry of the Environment in Niger has begun scaling the innovation, with plans to train more than 10,000 farmers in 400 villages, and many international organizations have experience training farmers in on-farm rainwater harvesting techniques and could serve as scaling partners (Aker et al., 2020).