Reservoir operation rule in semiarid areas: The quantity-quality approach

Abstract

The increase in water demand in association with the degradation of water quality can result in conflicts between users, especially in semi-arid regions where reduced flow is likely during long periods of drought. In these regions, reservoirs are normally operated to ration water, according to the level of water stored, in order to reserve this water to supply the demands of a future drought, i.e., using the Zone-based Guide Curve method. However, the quality of the water is also important because eutrophication in reservoirs limits the use and availability of water. The aim of this study was to use the quantity-quality approach to define the operating rule for four reservoirs in the semi-arid region of Brazil, with the aim of providing satisfactory supply to users while reducing water scarcity and eutrophication in the reservoirs. Scenarios of supplying the demands were simulated and optimized with the use of the Multiobjective Genetic Algorithm, NSGA-II. The results showed that the application of the Zone Hedging Rule (ZHR), considering only the quantitative approach, was able to guarantee the priority demands and reduce the water scarcity for non-priority requirements. However, by only using the quantitative approach the reservoirs did not meet the quality criteria required by Brazilian legislation. Applying the ZHR with a quanti-qualitative approach considering more favorable water quality conditions, the reservoirs were able to supply the demands with satisfactory quantity and quality; however, when the phosphorus concentration is high, a severe rationing of demands is triggered, which may not be viable, especially in reservoirs with greater demands in relation to the regularized flow and the average annual input flow. Therefore, actions are required to reduce nutrient concentrations in reservoirs, in addition to optimizing operating rules.