Estimating water flows and water availability in a poorly monitored semiarid small dam using reverse water balance and remote sensing.

Small communities of the semi-arid region suffer not only with water scarcity but also with data scarcity. The lack of data to perform water management at the local level is a common problem faced in developing countries. The absence of monitoring of water levels of small reservoirs across Ceará State (Northeast Brazil) results in inefficiencies in local water management and in difficulties to fully comprehend the extension of the impact of these changes in the natural streamflow and in the water supply for the state’s network of strategic dams. Small reservoirs construction is the individual private strategy mostly performed to cope with the local droughts. Recent studies shows a strong expansion of this strategy in the recent years, with the construction of more than 100.000 small reservoirs just in one of the state basins. This aspect reveals the necessity of strategies to obtain reliable information about these small reservoirs to perform a comprehensive estimation of these reservoirs impacts in both the local and state level of water management. In this study, local drone images were obtained for a small reservoir in the Ceará State. Remote sensing was used to obtain the reservoir depth-area-volume relationship and reverse water balance technique was used to estimate the different water fluxes and assess the local water availability. The results shows the inefficiency of these small reservoirs due to high evaporation losses, reflecting its high vulnerability to droughts. The proposed methodological scheme proves to be powerful in generating hydrological information by the intricate use of data, besides being replicable for other reservoirs in arid and semiarid regions.