Mapping one million small reservoirs in Brazil highlights widespread environmental and policy implications

Kylen Solvik , Yaffa Truelove, Jennifer Balch, Michael Lathuillière, Thiago Fontenelle, Andrea Castanho, Michael Coe, Christina Shintani, Carlos Souza, Marcia Macedo

Small dams and reservoirs disrupt hydrological connectivity and increase evaporative water loss across Brazil, yet most remain unmapped. Applying deep learning to Sentinel satellite data from 2021, we created a comprehensive map of 1.1 million small on-stream reservoirs (smaller than 50 ha). Their 7,597 km^2 cumulative surface area surpasses the combined area of Brazil’s three largest mega-dam impoundments. Especially concentrated in areas with high cattle herd densities and precipitation variability, 78% are found in headwater sub-basins, which are vital for downstream water quality and ecosystem health. The reservoirs lose an extra 4.2 km^3yr^-1 to surface evaporation—equivalent to 29% of total annual urban water use in Brazil. Our findings highlight the overlooked socio-environmental consequences of small reservoirs for land and water management.