In semi-arid regions at the margins of the Sahel, large numbers of small reservoirs capture surface runoff during the rainy season, making water available during the dry season. For the local population, small reservoirs are important water sources which help them cope with droughts. The lack of knowledge of the number of existing reservoirs, their distribution, and their storage volumes hinders efficient water management and reservoir planning. The authors have developed a simple method that allows the estimation of reservoir storage volumes as a function of their surface areas. This function is based on an extensive bathymetrical survey that was conducted in the Upper East Region of Ghana. In combination with satellite imagery, this function can be used determine and monitor the storage volumes of large numbers of small reservoirs on a regional scale.