As the realization increases that groundwater and surface water are closely interconnected and need to be thought of as one hydrologic system, studies of their conjunctive use and management are also increasing. The increased interest in the subject led to an international symposium sponsored by the International Association of Hydrological Sciences in Budapest, Hungary, in 1986 [ Gorelick, 1986]. Optimization techniques have commonly been used in conjunctive use models where the economics of water development are of interest. For example, Danskin and Gorelick [1985] used constrained optimization to minimize the cost of allocating surface water in a setting near Livermore, California. The hydrogeologic setting included a multiaquifer system that included an unconfined aquifer connected to the river, and a surface water reservoir.
Lefkoff and Gorelick [1990] evaluated the possibility and economics of developing a water rental market for irrigation water for the Arkansas Valley, Colorado. The market becomes active during dry years when farmers without ground-water rights buy surface water from farmers that have ground-water rights. Results indicate that long-term profits increase for all participants, and that average short-term ground-water salinity decreases. Shaukat et al. [1987] developed a water resources planning model that they suggest could be used to screen, and put in sequence, proposed projects such that the present cost of meeting future water demands is minimized. Surface water and groundwater interactions are presented in the model by a set of constraints guaranteeing minimum surface-water flow that can be affected by diversions and ground-water pumping. Mueller and Male [1993] developed a linear programming model that was designed to assist regulatory agencies in Massachusetts in designing permits for groundwater use. The model links groundwater withdrawals with streamflow, considering consumptive use and interbasin transfers. The optimization minimizes the depletion of streamflow below a standard while honoring the statistical distribution of allowed withdrawals permitted each applicant. Morel-Seytoux [1987] compared four different applications of conjunctive management of surface water and groundwater for two rivers in the western United States and a river in Saudi Arabia. In each case the objectives were different, and they included enhancement of water use, financial gain, water rights, and prevention of water logging.