Research On The Joint Operation Of Water Diversion And Water Supply Considering Ecological Objective

With the construction of reservoirs, more and more problems have arisen in river ecosystems, and ecological objective is being paid more and more attention in reservoir operation decision-making. To save the long distance water diversion costs, operation rules for water diversion and water supply both should be taken into account in the inter-basin water diversion condition. Reservoir operation becomes even more complicated when water diversion, water supply, ecological objective, etc. are considered together. A series of problems such as model optimization is easily trapped into the local optimum, and multi-objective decision making becomes more difficult arise consequently. Therefore, it is significant important to establish guidelines for model constructing, solving and decision-making of multi-objective optimization problem for the joint reservoir water diversion and water supply operation considering ecological objective.This paper takes Biliuhe reservoir as study object to research the determination of ecological objective and its computing method, then the solving and decision-making method for multi-objective optimization model and optimization solutions based on reservoir water diversion and water supply joint operation model were studied separately. The main results of this study are as follows:(1) Firstly, it introduces and checks the generality of study region, inflow data and water supply tasks of reservoir. The river hydrological situation under current reservoir operation are analyzed by the Range of Variability Approach-RVA method. Results showed that 19 of the total 33 evaluation indexes have high degree of change,11 indexes have middle and only 3 indexes have low degree of change, the total change of the hydrological situation is 83.71%. Finally, according to above hydrological situation variation results, the necessity for implementation of inter-basin water diversion project was proposed.(2) Under the maximal water diversion capacity condition, operation model taking the minimum environmental flow downstream of the dam as the ecological objective is built. Water diversion rules also be made based on inflow, water demand and water conveyance capacity of pipes for different time periods. Then solves the model using most diversion water to increase industrial and domestic water supply in addition to some environmental flow downstream of the dam, and finds that 17 of the total 33 evaluation indexes have high degree of change,9 indexes have middle and only 7 indexes have low degree of change, the total change of the hydrological situation is 79.95%, the variation degree remains high. The conclusion is that the downstream minimum environmental flow is insufficient to improve the hydrological situation of the river ecosystem.(3) Considering the response relationship between hydrological index and ecosystem, in combination with the problem that the descent of fish diversity downstream Biliuhe reservoir, a series of ecological indicators are selected as the ecological objective for reservoir operation. The ecological flow constraints method and setting ecological flow operation line method are used as two different ecological operation methods for the maximal water diversion capacity condition. The operation result shows that setting ecological flow operation line method can restrict the ecological water supply effectively at reservoir water shortage period, which can reduce the likelihood of disastrously water shortages for reservoir. By increasing the water supply which is important to ecological objective at reservoir water abundant period, the whole fitness of ecological objective could be improved. In addition, by the operation results comparison of different water demands distribution scenarios for different users, we determine that the amount of water supply for industry and urban life, agriculture and the ecological environment are 340 million m3,40 million m3 and 160 million m3.(4) To further cut water diversion cost, the amount of water diversion would be controlled by water diversion limit line according to different inflow and water demand conditions is proposed. The water diversion and water supply joint operation model which contains water diversion, ecological problems, etc. is built. The objectives include industry and urban life, agriculture water shortage index, ecosystem needs fitness and diverted water volume, and the decision variables include water levels on the industrial and domestic, agricultural, ecological water supply limit line and water levels on the upper and lower water diversion limit line, the total number of variables is 36+16+36 X 3=160.(5) Sobol's sensitivity analysis method is used to solve the water diversion and water supply joint multi-objective optimization model which has the problem of difficulty in finding its optimal solutions because of high dimensions. Sobol's method can pre-process the optimization problem to improve the search efficiency and quality. Firstly, sensitive water levels to different objectives would be selected from different operation lines by Sobol's method. Then, the sensitive water levels would be used for optimization to obtain the feasible solutions which will be used as initial solutions for going on global optimization. This method can significantly improve search efficiency, the quality of feasible solutions and run time compared to global optimization method without any pre-processing.(6) Visual analysis technique, marginal benefit analysis and fuzzy optimal method are used for optimal decision making of multi-objective operation schemes, which could combine graphics display with decision-maker's preference. Firstly, initial decision-making feasible solutions would be obtained by integrating the visual representation of Pareto solutions with decision-maker's subjective preference. Then, re-decision feasible solutions which considering the improvement benefit and relative degree of importance for different objectives would be obtained by marginal benefit analysis and fuzzy optimal method. Finally, the satisfying operation scheme is determined by comparative analysis of finite alternative schemes obtained under water diversion and water supply joint operation condition.