SECTAR: Model for economically optimal sustained groundwater yield planning

This paper presents an approach for computing economically optimal sustained yield ground-water extract ion strategies and supportable irrigated crop acreages. Computed regional strategies maximize the present value of net economic return_ They are useful for long-term agricultural planning because they are sustainable even beyond the planning period considered within the economic optimization. The hybrid approach uses only steady-state flow equations and iterative simulation/optimization to reduce optimization memory requirements below that required by conventional models utilizing both steady and unsteady equations. SECTAR, a quadratic optimization planning model assumes heads will evolve toward optimal steady-state but does initially not know the nonlinear rates of evolution in each cell. Rates of change in head are assumed, and optimization is performed. Subsequently, a linear or nonlinear simulation model computes actual time-varying rates of head change that would result from implementing the optimal strategy. Rates of change assumed in the optimization model are then corrected to correspond to simulated values. The process of assuming head-change rates, computing optimal pumping and eventual steady-state head values, and then simulating is repeated. Assumed and simulated heads are effectively the same within three or four cycles. Convergence occurs because of their common tendency to evolve to steady state. The procedure is applicable for situations of constant or time-varying transmissivity. Strategies developed for regions with initially stressed and unstressed potentiometric surfaces are compared. Evolution toward target steady-s,tate conditions and sensitivity of strategies to aquifer and economic parameters is presented