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

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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.