Planning the Optimal Operation of a Multioutlet Water Reservoir with Water Quality and Quantity Targets

The integration of quality and quantity issues in the management of water resources systems is key to meet society's long-term needs for freshwater while maintaining essential ecological services and economic benefits. Current water management practices are mostly targeted towards quantitative uses, and quality is usually addressed separately as an independent problem. One of the reasons for the lack of integration lies in the inadequacy of optimization techniques nowadays available to cope with the large, distributed, simulation models adopted to characterize the coupled ecological and biochemical processes in water bodies. In this paper we propose a novel approach based on the conjunctive use of a batch-mode Reinforcement Learning algorithm and a one-dimensional (1D) coupled hydrodynamic-ecological model to design the optimal operation of a multipurpose water reservoir accounting for both quantity and quality targets. We consider up to five operating objectives, including both in-reservoir and downstream water quality parameters, and design efficient operating policies conditioned upon not only the current storage but also water characteristics, such as temperature and total suspended solids at different depths. The approach is applied to a real world case study in Japan consisting of a water reservoir, Tono Dam, equipped with a selective withdrawal structure and used for flood protection, irrigation and recreational purposes. Results show that a potential control over in-reservoir and downstream water quality can be gained without impairing the hydraulic capacity of the reservoir by effectively exploiting-through the operating policy-the operational flexibility provided by the selective withdrawal structures.