The influence of a dense network of small reservoirs on drought evolution in a semiarid region

The rampant construction of reservoirs can cause a concentration of water in certain regions and scarcity in others. This gives rise to conflicts and social pressure to build more reservoirs, creating a vicious cycle. An example of this is Ceará, a  Brazilian state often affected by intense drought events. State-level water resources management is mainly focused on the administration of 155 public “strategic reservoirs”. There is also a Dense Network of (non-strategic) Reservoirs (DNR) formed by almost 107,000 dams that are informally constructed to unknown specifications, unmonitored, and their management is not aligned with the state and watershed level governance. The occurrence of a multi-year drought (2012-2020) revealed that a DNR can play a significant role in the evolution and intensification of drought events. We aimed to analyze this effect in the Banabuiu River basin from a socio-hydrological perspective. We modeled two scenarios through a mesoscale semi-distributed hydrological model for semi-arid regions (WASA): 1) simulating the current configuration of the hydrological system and 2) simulating a condition without the DNR. To explore the DNR effect on drought events, we used the novel Drought Cycle Analysis method (DCA) which combines the Standardized Precipitation Index (SPI) with the Volume Deficit information (VD, deviation of half of the total capacity of the monitored reservoirs). We also conducted interviews with smallholder farmers who are the main users of non-strategic reservoirs to understand the societal impact of these structures. Our results showed that the presence of the DNR advances the transition from meteorological towards hydrological drought and leads to slower recharge of the strategic reservoirs between and during drought events. On the other hand, farmers explained their water security has increased after having a non-strategic reservoir on their land. The interviews also revealed that non-strategic reservoirs are the primary water source for most farmers. Although there are other options for accessing water during multi-year drought events, these generally cannot meet the farmer's water demands, thus causing a reduction in their productive activity .