[1] The temporal variations of natural flows are essential elements for preserving the ecological health of a river which are addressed in this paper by the environmental flow schemes that incorporate the intra-annual and interannual variability of the natural flow regime. We present an optimization framework to find the Pareto-optimal solutions for various flow schemes. The proposed framework integrates (1) the range of variability approach for evaluating the hydrologic alterations; (2) the standardized precipitation index approach for establishing the variation criteria for the wet, normal, and dry years; (3) a weir operation model for simulating the system of flows; and (4) a multiobjective optimization genetic algorithm for search of the Pareto-optimal solutions. The proposed framework is applied to the Kaoping diversion weir in Taiwan. The results reveal that the time-varying schemes incorporating the intra-annual variability in the environmental flow prescriptions promote the ecosystem and human needs fitness. Incorporation of the interannual flow variability using different criteria established for three types of water year further promotes both fitnesses. The merit of incorporating the interannual variability may be superimposed on that of incorporating only the intra-annual flow variability. The Pareto-optimal solutions searched with a limited range of flows replicate satisfactorily those obtained with a full search range. The limited-range Pareto front may be used as a surrogate of the full-range one if feasible prescriptions are to be found among the regular flows.Citation: Shiau, J.-T., and F.-C. Wu (2007), Pareto-optimal solutions for environmental flow schemes incorporating the intra-annual and interannual variability of the natural flow regime, Water Resour. Res., 43, W06433,

The Range of Variability Approach (RVA) is used to investigate the hydrologic impacts of a diversion weir on Chou-Shui Creek, Taiwan. Thirty-two hydrologic parameters, called Indicators of Hydrologic Alteration (IHA), are used to evaluate the flow conditions before and after weir construction. One standard deviation from the mean for each of the pre-construction hydrologic parameters is set as the management target range. Under the prevailing diversion rules, large hydrologic alterations are observed, especially for low flows. The means of 19 hydrologic parameters presently fall outside of the targets and the average non-attainment rate for the 32 indicators is 73.2%. Increasing the instream flow release or reducing diversions could mitigate the hydrologic impacts of weir construction. Increasing the instream flow to 40 m 3 /s and reducing monthly water demands by variable percentages significantly improves the altered flow conditions. Under the proposed water release and diversion scheme, 29 hydrologic parameters will fall within the management targets and the average non-attainment rate will be reduced to 35.6%, much closer to the pre-construction value of 25.3%. Restoring the natural flow variability is expected to promote the natural stream biota.

In this paper we present a Histogram Matching Approach (HMA) for assessment of the flow regime alteration. The HMA uses the degree of histogram dissimilarity as a metric for impact assessment, which is based on the quadratic-form distance between the frequency vectors of the pre-and post-impact histograms weighted by a specified similarity matrix. The HMA is coupled with an aggregated multiobjective optimization genetic algorithm and applied to a case study on the Kaoping diversion weir (Taiwan) for determining the optimal environmental flow scheme that balances the ecosystem and human needs objectives. Two key issues are addressed in this study. First, we compare the performances of the HMA and existing Range of Variability Approach (RVA). Second, we employ three types of similarity function to investigate their effect on the outcomes of the HMA. The results reveal that the HMA consistently outperforms the RVA in preserving the natural flow variability regardless of what type of similarity function is used. No single type of similarity function can be found that would simultaneously best preserve the natural patterns of 32 Indicators of Hydrologic Alteration (IHA). For the situations where the water-supply reliability is of critical concern, the pulse similarity is recommended because it would assure the smallest water-supply deficit. If, however, minor degradation in the water-supply reliability may be overlooked, the linear similarity is suggested because it would generally result in the post-impact flows that most satisfactorily resemble to the natural flow regime.

A methodology based on the range of variability approach ͑RVA͒ is presented for determining the feasible combinations of flow diversion and instream flow release for a projected diversion weir. The RVA is designed to support efforts to manage water system operations in a manner that minimizes impacts on natural hydrologic variability, and thereby minimizes ecological impacts. This approach is used to evaluate the prediversion flows and establish the riverine management targets in terms of 32 hydrologic parameters called indicators of hydrologic alteration ͑IHAs͒. The goal is to make the postdiversion flows attain the target ranges at the same frequency as that which occurred in the prediversion flow regime. A weir-operation simulation approach is employed to compute the postdiversion flows. Based on the simulation results, the degree of hydrologic alteration under various combinations of flow diversion and release is evaluated and plotted as a contour diagram for each IHA. Overlapping the contour diagrams of the 32 IHAs, three overall hydrologicalteration regions are constructed. The feasible region, i.e., the overall low-alteration region, is defined by the combinations of flow diversion and instream flow release for which none of the 32 IHAs is significantly altered. The feasible combinations of flow diversion and release are further evaluated with their corresponding water-supply shortage indices. The proposed methodology allows for the incorporation of both water-supply and environmental protection concerns in water resources planning and management. The merits of this methodology are demonstrated with an application to the proposed Taitung diversion weir in Taiwan.

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Key Points Three operation scenarios to restore flow regime at multiple reaches Novel environmental flow approach to restore flow regime at 5 temporal scales Overall evaluation of operation scenarios and recommendations

[1] Environmental flow schemes may be implemented through active or restrictive strategies. The former may be applied via reservoir releases, and the latter can be executed by reducing water demands. We present a dual active-restrictive approach to devising the optimal reservoir operation rules that aim to secure off-stream water supplies while maximizing environmental benefits. For the active part, a multicomponent environmental flow target (including the minimum and monthly flows) is incorporated in the operation rules. For the restrictive counterpart, we use a novel demands partitioning and prioritizing (DPP) approach to reallocating the demands of various sectors. The DPP approach partitions the existing off-stream demand and newly incorporated environmental demand and reassembles the two as the first-and second-priority demands. Water is reallocated to each demand according to the ratios derived from the prioritized demands. The proposed approach is coupled with a multicriteria optimization framework to seek the optimal operation rules for the existing Feitsui Reservoir system (Taiwan) under various scenarios. The best overall performance is achieved by an optimal dual strategy whose operational parameters are all determined by optimization. The optimal environmental flow target may well be a top-priority constant base flow rather than the variable quantities. The active strategy would outperform the restrictive one. For the former, a top-priority base flow target is essential; for the latter, the off-stream demand can become vanishingly small in compensation for the eliminated base flow target, thus promoting the monthly flow target as nearly the top-priority demand. For either the active or restrictive strategy, a prioritized environmental flow demand would provide a path toward the optimal overall performance. A significantly improved overall performance over the existing operation rules is unlikely if the active and restrictive parameters are both favorable to the off-stream demand.Citation: Shiau, J.-T., and F.-C. Wu (2010), A dual active-restrictive approach to incorporating environmental flow targets into existing reservoir operation rules, Water Resour. Res., 46, W08515,

Deltas form over basements of various slope configurations. While the morphodynamics of prograding deltas over single‐slope basements have been studied previously, our understanding of delta progradation over segmented basements is still limited. Here we use experimental and analytical approaches to investigate the deltaic morphologies developing over two‐slope basements with unequal subaerial and subaqueous slopes. For each case considered, the scaled profiles of the evolving delta collapse to a single profile for constant water and sediment influxes, allowing us to use the analytical self‐similar profiles to investigate the individual effects of subaerial/subaqueous slopes. Individually varying the subaerial/subaqueous slopes exerts asymmetric effects on the morphologies. Increasing the subaerial slope advances the entire delta; increasing the subaqueous slope advances the upstream boundary of the topset yet causes the downstream boundary to retreat. The delta front exhibits a first‐retreat‐then‐advance migrating trend with increasing subaqueous slope. A decrease in subaerial topset length is always accompanied by an increase in subaqueous volume fraction, no matter which segment is steepened. Applications are presented for estimating shoreline retreat caused by steepening of basement slopes, and estimating subaqueous volume and delta front using the observed topset length. The results may have implications for real‐world delta systems subjected to upstream tectonic uplift and/or downstream subsidence. Both scenarios would exhibit reduced topset lengths, which are indicative of the accompanied increases in subaqueous volume and signal tectonic uplift and/or subsidence that are at play. We highlight herein the importance of geometric controls on partitioning of sediment between subaerial and subaqueous delta components.