Investment trends and evaluation gaps in rainwater harvesting in Sub-Saharan African drylands

Rainwater harvesting (RWH) can enhance agricultural productivity and strengthen climate resilience in the drylands of sub-Saharan Africa. However, the sustainability and effectiveness of RWH interventions remain inconsistent due to institutional, technical, and socio-economic constraints which are difficult to overcome without comprehensive reporting and evaluation. This review identified the commonalities driving these constraints across interventions, by systematically examining 34 government- and donor-funded RWH projects implemented across sub-Saharan Africa until November 2024. We found that while projects often aimed to improve crop yields and water availability, only a subset demonstrated sustained benefits. Successful projects embedded RWH into broader development frameworks, prioritized land tenure security, invested in post-construction maintenance, and emphasized community-led design. However, over 70% of projects reported insufficient detail on cost-effectiveness, long-term functionality, or social equity outcomes to holistically assess impacts. Further, we reviewed impact evaluation practices, which currently overemphasize yield metrics, neglecting socioeconomic issues and reducing their utility for adaptive planning. We recommend the adoption of quasi-experimental methods and longitudinal analyses to address these gaps and improve the policy relevance of RWH evaluations. Finally, we demonstrated the potential of Earth Observation (EO) data, including measurement of vegetation growth, evapotranspiration, and soil moisture, to complement traditional evaluations. EO enables scalable, cost-effective environmental monitoring and offers new opportunities to link biophysical changes to livelihoods. We conclude with a practical framework to guide RWH design, implementation, and evaluation of interventions for strengthening smallholder adaptation to climate variability.