Deriving Reservoir Cascade Operation Rules for Variable Streamflows by Optimizing Hydropower Generation and Irrigation Water Delivery

Abstract

Determining beneficial operation rules for complex reservoir cascades is challenging due to the variability of river inflows, multiple water users, and the computational burden imposed by optimization calculations. Water managers of Sri Lanka face the challenge of balancing water releases for hydropower and for irrigated agriculture in the Mahaweli project. This study derived Mahaweli reservoir operation policies—reservoir rule curves and water allocation rules—by optimizing hydropower generation and agricultural yield. We derived operation rules for three reservoirs and one water diversion structure, because these offer the main opportunities for controllability of hydropower and irrigation. We used a multiobjective evolutionary algorithm to optimize hydropower and agriculture yields for 1,000 synthetically generated reservoir inflows that capture the natural variability of the monsoon seasons. The optimization problem was simplified by focusing on the main components of the cascade and by carrying out the computation in two stages. Despite the simplification, the operation rules derived using multiobjective optimization enhanced hydropower and agriculture yield compared with the values from current rules. The solution set exposed the trade-off between energy and agricultural yield objectives. Risk-neutral and risk-averse optimized rules and trade-offs between the objectives provide information that can be used to adapt water management practices for different hydrological conditions, which can be beneficial to thousands of people. Water managers of similar reservoir cascades can use the method to select policies considering trade-offs among objectives.