Where rivers sleep

Abstract

This study presents a new geospatial framework for detecting and mapping ephemeral sand rivers (ESRs) across the West African Sahel, focusing on Burkina Faso, Mali, and Niger, where food security challenges are acute. ESRs, which remain dry most of the year, act as vital subsurface water reservoirs in arid environments. During the wet season, infiltrated streamflow is stored within sandy beds, minimizing evaporative losses and providing shallow groundwater with potential to support domestic, livestock, and agricultural needs during dry periods. The methodology integrates hydrological analyses, remote sensing, and machine learning. A high-resolution drainage network was derived from the 90 m MERIT DEM, based on national reference river networks and satellite-derived information. A Random Forest model predicted river flow intermittency and identified ephemeral rivers (flowing 1–4 months annually, catchment area ≥1000 km²), around which 500 m buffer zones were delineated for analysis. Two composite thresholds (CTs) combining NDESI–NDVI spectral indices achieved moderate accuracy: CT1 (42 %) and CT2 (72 %), with CT2 serving as a first-order tool for sandy riverbed detection. A multi-temporal supervised land use/land cover classification achieved high accuracy (92 %) and F1 scores >0.86, outperforming the spectral thresholds. Using vegetation presence as a proxy for shallow groundwater, 19 % of ESRs (402 km) were identified as areas of potentially accessible water storage near settlements representing about 3 million people (4.8 % of the population) across the three countries. These findings highlight the importance of ESRs for sustainable water management and climate-resilient livelihoods in the Sahel.