Evolution of Drought Mitigation and Water Security Through 100 Years of Reservoir Expansion in Semi‐Arid Brazil

Abstract Brazil's Northeast region (BRN), especially the state of Ceará (CE), has dealt historically with severe drought events since the late 1800s, which commonly led to catastrophic impacts of mass migration and deaths of thousands of people. Throughout the last century, the “Droughts Polygon” region experienced an intense infrastructural development, with the expansion of a dense network of reservoirs. This paper presents a parsimonious hydrologic modeling approach to investigate the 100‐year (1920–2020) evolution of the hydrology of the 24,500 km 2 Upper Jaguaribe Basin, throughout the development of a dense reservoir network. We aimed at reproducing the hydrology at the basin scale and analyzed the outcomes of reservoir expansion in terms of water fluxes and water security. Our model's structure captured the growth in reservoir count and storage capacity, which was then confronted with an evolving water demand, allowing us to estimate how water security (i.e., proportion of demand being met) varied over the 100‐year period. Significant streamflow reduction at the basin's outlet and increase in evaporation losses, associated with a decrease in streamflow at varying exceedance frequencies were observed at the end of the study period. While reservoir expansion allowed for the transition from complete vulnerability to meteorological droughts to increased levels of water security, drought impacts had, in the meantime, disproportionally intensified, especially in reservoirs of medium to small capacities. Smaller reservoirs are suggested to have played the role of distributing water resources throughout the region, while larger reservoirs were more efficient as tools to promote water security.