Impacts of optimal and fuzzified rule curves on water deficits for nanhua reservoir

Southern Taiwan had suffered from water deficits due to uneven temporal and spatial distribution of rainfall. Reservoirs are important facilities to regulate fluctuating streamflow to offer stable water supplies in this region. Water releases from reservoirs are often guided by rule curves in Taiwan for assessing storage in various seasons. Water rationing is implemented to conserve water for future use in order to reduce possibilities of high deficits when water below certain level. However, finite rule curves leading to step-wise hedging rules as well as pre-defined rules curves insufficient to meet current hydrologic conditions are major reasons for low water-supply efficiency. To improve reservoir performance during droughts, two optimization models are considered in this study to construct optimal operating rules for Nanhua Reservoir located in southern Taiwan. Model I fuzzifies the current rules curves and Model II reconstructs the lower rule curve of the Nanhua Reservoir. A total of 8 water-shortage indices (maximum 10-day shortage ratio, maximum event duration, maximum event deficit, average event duration, average event deficit, risk, total shortage ratio, event frequency) are integrated into a single overall objective to derive the optimal operating rule using multi-criteria decision making. The results indicate that both optimization models have better performance than the current operation rule and Model II outperforms Model I.