Effects of Hedging Factors and Fuzziness on Shortage Characteristics During Droughts

Frequently suffering from water deficits induced by prolonged and severe droughts in Taiwan, hedging rules become an important component in reservoir operation to ensure stable water supplies during droughts. Conventional zone-based rule-curve operation is an easy rule to guide water releases since fixed rationing factors are assigned for various zones. A drawback for such hedging is abrupt changes of rationing factors from one zone to another. Fuzzified rule curves provide an alternative to create gradually varied rationing factors. In this study, impacts of hedging factors including rule curves, rationing factors, and fuzziness on water-deficit characteristics are explored. The method presented in this study is illustrated through an application to the Nanhua Reservoir located in southern Taiwan. The results reveal that different hedging factors have different impacts on shortage indices. More hedging factors involved in optimization models leads to more compromising hedging among conflicting shortage indices. According to the proposed overall index, which is a multi-criteria index based on normalized shortage indices, the current operation leads to the worst overall performance of 0.480. One-optimized-hedging-factor scenarios receive an improvement and range between 0.541 and 0.607, while the two-optimized-hedging-factor scenarios have a further improved overall index of 0.603−0.659. The best overall performance of 0.679 is achieved by optimizing three hedging factors and results in the most compromising alternative among all scenarios.