Distribution of small seasonal reservoirs in semi-arid regions and associated evaporative losses

To support the increasing demand of a growing population for freshwater, small-sized (<0.1 km(2)) water reservoirs are necessary in areas with limited infrastructure, especially in water-stressed regions having seasonal and variable precipitation. Seasonal storage in small reservoirs is often overlooked in present inventories. Accordingly, we assessed the current state in semi-arid regions using highly resolved Sentinel-2 satellite imagery. Globally, about 3 million small reservoirs are in operation in semi-arid regions having a total water surface area of 17 800 km(2) and seasonal storage of 37 km(3), supporting 15% of the world's population in semi-arid regions. Estimated upper-bound of evaporative losses represent 38% (14 km(3)) of storage during dry season. The study estimates the evaporation suppression using floating covers and lays foundation to assess the feasibility of this technical solution to increase water savings. Significance and originality statement This study provides new insights into the distribution of seasonal freshwater storage in water stressed regions that support large rural population. We provide the first baseline dataset of this distributed freshwater resource to better assess the sustainability of communities in semi-arid regions with chronic water shortages. Water storage in semi-arid regions is subject to potential evaporative losses of nearly 40% of the stored water. We analyzed conditions where evaporative losses could be significantly reduced using floating covers, a simple and scalable solution suitable for regions with underdeveloped economies. The study as well ranks regions with highest potential benefits from evaporation suppression measures.