A dynamic corridor-searching algorithm to seek time-varying instream flow releases for optimal weir operation: Comparing three indices of overall hydrologic alteration

A full range of natural flow regime has been widely recognized as a primary driving force for sustaining the integrity of a riverine ecosystem. Existing instream flow methods strive to assure a constant minimum flow but not the natural flow variability. We present in this paper a dynamic corridor-searching algorithm to seek the optimal time-varying scheme for instream flow releases. A compromise programming (CP) is employed to search the optimal solution of an objective function aggregating the ecosystem and human needs objectives. The ecosystem need objective is represented by an overall index of hydrologic alteration, which integrates 32 indicators of hydrologic alteration (IHA) derived from the range of variability approach (RVA). The human need objectives are expressed by shortage ratios for the agricultural and municipal water supplies. The proposed method is applied to a weir operation in Taiwan. Three approaches to evaluating the overall degree of hydrologic alteration (i.e., the three-class, fuzzy-based, and overall-mean approaches) are compared here. The results show that the time-varying schemes improve the human need objective, but only slightly deteriorate the ecosystem need objective. Such advantages increase with the time-varying frequency. For the wet periods, smaller flow releases may be prescribed; for the dry periods, however, greater releases must be specified to secure a lower degree of overall hydrologic alteration. It is also revealed that use of the three-class approach to evaluate the overall hydrologic alteration facilitates to eliminate highly altered IHA and maintain those low-flow characteristics subtle to flow diversions. However, such outcomes are achieved at the cost of greater deficits for human water demands.