Model Type: Water Resources Application Simulation Model
Model Name: WRASIM (Water Resources Allocation Simulation Model)
Introduction
WRASIM is a product of the project “Development of Generalized Priority-Based Allocation Models for Regional Water Resources Management and Transfer”, undertaken by the Water Resources Planning Branch of Water Resources Agency from 2004 to 2006. This model was developed by the team led by Professor Frederick N.F. Chou at the department of hydraulic and ocean engineering of National Cheng Kung University. Development of the model was greatly inspired by the MODSIM originally developed by Colorado State University.
Methodology
WRASIM is based on minimum cost network flow programming. The formulation of which is to achieve the minimum cost flow within a network subject to the constraints of capacities of arcs and continuity throughout nodes. Three parameters are required for each arc, the first two of which are the upper and lower bounds of the arc flow to specify the transmitting capacity. The last of which is the cost coefficient of unit flow to account for the objective function of optimization and to drive water allocation according to the associated priorities. The real water resources system is conceptualized as a network. Nodes of which include reservoirs, inflows, confluences, treatment plants, demands, and water receiving bodies, while arcs could be channels, outlet works, canals, aqueducts, conduits, or pipes. Virtual arcs are added to impose daily inflow and initial storage into the physical system, as well as to receive water from demand consumptions, carryover storages, losses through allocation, and surplus into the terminal. Figure (Fig.) 1 shows the complete network with total circulation by the joining of physical and virtual components to symbolize a daily water allocation problem. Judicious assignments of cost coefficients on the virtual arcs of demand and storage enable simulation of the variety of comprehensive rules for water allocation (Chou and Wu, 2014).
Fig. 1 The network of a daily water allocation problem and its sequential routing
Function
Different modules are designed in WRASIM to allow simulating different aspects of water resources allocation encountered in Taiwan. Table 1 provides the summary of the primary modules.
Table 1 summary of all modules of WRASIM
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Module
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Simulation capabilities
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E
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The basic module which performs sequential daily water allocation. The allocation is based on the priorities of demands and reservoir storage designing to simulate water rights, operating rule curves and storage balancing functions of reservoirs, and other water allocation rules.
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Y
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The Y module evaluates the yield of the system for public water supply according to designated shortage criterion. The evaluation is carried by automatic iteration of the E module based on the nonlinear root finding algorithm.
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G
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The G module links WRASIM and the groundwater simulation model MODFLOW to allow conjunctive utilization of groundwater and surface water. The conjunctive simulation is driven by WRASIM to determine the artificial pump and recharge rate to the aquifer, MODLOW then simulates the response of groundwater body. It could also restrict groundwater withdrawal if the simulated groundwater table recedes below management threshold.
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D
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The D module performs dynamic water allocation to prescribe optimal strategy of spatiotemporal water allocation.
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I/O data
WRASIM provides seven types of nodes for the users to construct the water resources systems for their applications. The types of nodes include: reservoirs (operating either independently or jointly), demands (consumptive or non-consumptive), peak hydropower demands (considering the non-linear relationship with reservoir headwater), agricultural water demands accounting for return flows, inflows (either natural or simulated by hydrological models), river channel storage, pumping areas, water treatment plants, general confluences, diversions, and system terminals. Each node type requires specific input data to facilitate the simulation of water resource allocation and utilization.
Application scope and limitations
The model offers a graphical drag-and-drop interface for creating node and arrow networks for efficient constructions of diverse water resource systems.
With the imported hydrological series, reservoir capacities, water demands, and water allocation priorities, the model automatically converts the daily water allocation problem into the optimization formulation of network flow programming. The optimal feasible solution from the optimization algorithm ensures compliance with continuity and the capacity limits of arcs.
Appropriate settings of model parameters allow the simulation conforms to priorities of water rights and prioritizes the use of unregulated stream flows, allocate water according to operational rules, coordinate the storage and release of multiple reservoirs, and address preferences for various conveyance routes within the water resource system.