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Show Chapter 4 Model for Optimal Aquifer Yield Determination The need to consider relationships between the yield developed and aquifer drawdown, as well as water rights, water quality and demand constraints, complicates the assessment of firm yield for an aquifer. An optimization model that considers these factors is presented to aid the evaluation of an aquifer's firm yield. The objective of the model is to maximize the annual yield from a system of wells while meeting a set of constraints that include ( 1) meeting the demands of each water supply agency, ( 2) restricted steady state aquifer drawdowns, ( 3) maintenance of water quality, and ( 4) satisfaction of a prescribed water rights structure. The decision variables of the model are the annual pumping rates at each candidate well site, and the piezometric heads at each location of interest. An application of the model to the Salt Lake County aquifer system is presented. Parametric assessments of the impact of the above constraints on the maximal steady state yield of the aquifer system are performed. Todd ( 1959) defines the safe yield of an aquifer as an annual withdrawal rate that does not produce any undesirable results. Domenico ( 1972) notes that the undesirable results include the depletion or mining of the groundwater resource, the intrusion and transport of water of poor quality, infringements of prior water rights, and increased costs of pumping. He suggests the need to focus on the determination of an optimal yield, that recognizes these factors and selects a management scheme that is optimal in terms of some criteria. Such a determination may be made through the use of simulation and/ or optimization techniques. Since aquifers are usually developed for water supply through point discharges ( wells), constraints due to point drawdowns of piezometric head, water rights and degradation of water quality are likely to curtail the water supply that can be developed to levels well below those indicated by the average annual recharge. Well locations are restricted by the availability of land in urban areas, local aquifer characteristics ( hydraulic conductivity, presence of fines and turbidity, and the structural strength of the formation), proximity to areas of contamination, and to the geographical or physical boundaries of the water supply agency. Excessive drawdowns ( local or regional) in the aquifer can lead to problems with subsidence, increased pumping costs, and an increased potential for the spread of contamination. Often several users or agencies operate wells in an aquifer. In this case, the extent of pumping at some wells may be limited to avert water rights infractions among different users. Also, each user or agency has a target demand for groundwater that it seeks to satisfy. If wells are located near areas of poor groundwater quality, the pumping rates from these wells may also be restricted to prevent the migration of contaminated water into the wells. All the above factors can dictate the spatial pattern of pumping from the aquifer. This clearly has implications on the yield that can be developed from the aquifer. The demand for water and aquifer recharge rates are also not constant in time. Seasonal and long term variations in demand and recharge rates are observed. These may also affect the yield that can be developed, since they influence all the constraints listed above. The yield from the aquifer may be enhanced by considering artificial recharge of the aquifer with surface water at selected locations. If this is considered, an assessment of the increase in the aquifer yield as a function of the quantity of artificial recharge, is also of interest. 92 |