Evaluation of operational ranking effects in series and parallels reservoir dams on water supply (Case study: seven dams water resource system of Zohreh river)

Over the past decades, solving operation problems has been a challenge for water resource planners and managers. Holistic standpoint and integrated management have been applied as a replacement for traditional and partial insight in water resource development projects in recent years. Hence, one of the operator's key questions in multi-reservoirs systems is that which reservoirs should be depleted earlier for satisfying shared downstream demands. In addition, to refill the reservoirs, which ones should be filled earlier? Accordingly, multi-reservoir operation in water resource systems should be done to maximize the total water extracted. In this case, one of the important parameters affecting the system performance is ranking and prioritizing the reservoirs storages for supplying the shared downstream demands. In fact, refill and deplete order of cascade or parallel reservoirs are effective on the reservoir storage distribution. This approach can make also direct effects on the total losses including spill and evaporation of the reservoirs, reforming the total regulated water obtained of reservoir dams.
In this paper, a simulation model with a linear programming optimizing engine is utilized for operational ranking of the reservoirs during a long-term monthly inflow period. Simulation has been implemented for monthly time interval using inflow historical data. In this study, Acres Reservoir Simulation Program (ARSP) has been utilized including a linear programming for optimal water allocation and reservoir storage within each time interval. The main advantage of the ARSP is its inherent flexibility in defining the operating policies through a penalty structure. The model uses network flow optimization techniques in which total components of the system are made through nodes and arcs. Nodes are including reservoir dams, withdrawal and return flow locations, irrigation and water supply networks, and junction points for river reaches or streams. Arcs include different kinds of stream, canals, river reaches and different zone of reservoir dams. Nodes and arcs totally fulfill a closed network that characterizes final structure river system. There are arcs connecting reservoirs and demand nodes to the source and sink node. The source node supplies water to nodes within the network to simulate local inflow and the sink node receive flow from nodes within the network to represent consumptive use. In this state, certain number of arcs defines each stream and storage zone in reservoir. In fact, each arc represents an operating purpose, for which a comparative penalty representing relative priority of desired purpose can be assigned by the user. Based on this penalty for unit flow violation, ARSP imposes the operational strategy by allocating the available water in a manner that minimizes the total system penalties. In selecting these penalty values, three specific aspects should be considered: inter-reservoir zonal operation, relation between storage and flow violation, and inter-reservoir policies. In this process, the reservoirs are ranked according to priority relationships. The model first violates the storage in the lowest priority corresponding to the lowest zonal penalty, and then it proceeds to the other priorities.
As a case study, the seven reservoirs system of Zohreh water resource in southwest of Iran has been studied. A future planning horizon was selected for the study including seven reservoir dams, eleven irrigation networks, four public demands, two minimum flow channels, eleven junctions node, and some general channels. Target values for demands are also set based on the planned water demand for future horizon distributed as 71% for agriculture demands, 16% for the minimum flow, and 13% for public demands. To evaluate the long-term performance of reservoir operation, 48 years of monthly inflow is used, resulting in a total of 576 months. The other input data are reservoir properties, penalty values, evaporation from reservoir, and return flows. Here, different scenarios have been defined for the ranking of the reservoirs. This approach has been effective about 6% on decreasing the water losses including spillway and evaporation of reservoir. The comparison among different scenarios showed that reservoirs ranking affects in all modern indicators for the operation performance. Based on flexibility index, storage ranking in parallel reservoirs is more effective than cascade reservoirs. In this process, reliability and vulnerability indexes were improved for the selective scenario, however, resiliency index was reduced a little. Moreover, reservoirs refill order from upstream to downstream of the catchment, in comparison with opposite state, accomplished better performance as in this state the possibility for spill control in the next reservoir is better provided.