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Show Chapter 3 Model for Least Cost Groundwater Management The objective of the work reported on in this chapter was to develop a groundwater management model for the least cost operation of a system of wells to meet municipal water demands over a drought period, considering multiple water supply agencies and their associated water rights,- water quality maintenance, and limits on drawdown of piezometric head. A unit response formulation was used to develop a general purpose model to meet these objectives. This model can be used by itself with exogenously specified groundwater demands, or it can be embedded in a model for conjunctive management of ground and surface water resources. Decisions on annual pumpage for the drought period, for each water supply agency are made at each candidate and existing well location. The water rights and water quality maintenance constraints are specified by identifying appropriate geographical and physical boundaries, and by restricting average annual flow across these boundaries. The model developed is a distributed parameter model, and uses both simulation and optimization. Response of the aquifer ( in terms of boundary flows, and piezometric heads) to unit pumping at each well site is first simulated. The resulting response matrices are used to develop the coefficient matrix for the optimization model. Applications of the model were made to Salt Lake County. The USGS 3- dimensional finite difference flow model was used for aquifer simulations. The nonlinear optimization formulation was solved using the Successive Nonlinear Programming algorithm. Optimal pumpages were parametrically evaluated for various drought severities. Summary results illustrating optimal pumpage as a function of demand area, demand level, and drought severity are presented. The corresponding drawdown levels and average flow rates across boundaries of interest are also presented. The results suggest that the optimal groundwater use strategy in the county would lead to lower and less concentrated drawdowns in the aquifer with a significant reduction in the total pumping cost. It would be possible to restrict drawdowns, maintain water rights, and control aquifer water quality while more than doubling the groundwater use in the county. Unit pumping costs vary significantly across the county and an attractive groundwater use strategy may be the export of groundwater from some of the county's water supply agencies to others with higher pumping lifts and unit pumping costs. The following modeling needs were identified : 1) Determination of an optimal strategy for groundwater management in a basin by minimizing the cost of meeting specified aggregate groundwater demands for each agency for some time period. 2) The model developed should maintain a specified water rights structure, preserve water quality, meet a schedule of demands for each agency, restrain drawdowns, and recognize physical bounds on aquifer pumping and artificial recharge. 3) The model should have a structure that allows the investigation of the effects of varying demand levels ( degrees of drought), as well as natural variability of aquifer recharge and discharge on the optimal water use plan, water quality and the water rights hierarchy. 4) The model formulation and solution technique should be mathematically efficient and compact, to facilitate applications to field scale problems. 29 |