THE USE OF SIMULATION-OPTIMIZATION MODELING OF STREAM-AQUIFER SYSTEMS TO INFORM GROUND-WATER DEVELOPMENT DECISIONS, RHODE ISLAND

Over the past decade we have used simulation-optimization modeling of stream-aquifer systems in Rhode Island to assist State and local decisionmakers to determine ground-water development strategies that meet increased water-supply demands for human uses while simultaneously meeting minimum instream-flow requirements for aquatic habitats. These modeling techniques have been applied to a largely developed basin to improve management of the existing water-supply system and to a relatively undeveloped basin to identify possible locations and pumping rates of new wells. In both cases, the use of simulation-optimization modeling informed the decisionmaking process by providing a range of ground-water development options that are possible for alternative definitions of minimum instream-flow requirements. Minimum instream-flow requirements are difficult to define and have been a source of contention between water suppliers and the environmental community. For the analysis of the less-developed basin, we evaluated several alternative instream-flow requirements that have been considered for use in the State. The alternative requirements resulted in a range of optimal average-annual ground-water withdrawal rates from the basin of between 5 and 15 million gallons per day. Because of hydrologic and hydrogeologic conditions in the study area and the specific locations of wells and streamflow-constraint sites selected for the optimization model, the relation (tradeoff curve) between instream-flow criteria and optimal ground-water withdrawal rates was nonlinear. This nonlinearity resulted in relatively small changes in minimum instream-flow criteria leading to large changes in optimal withdrawal rates over part of the tradeoff curve, which would have been difficult to identify without the model. The model also provided scientists and water-resource decisionmakers with an improved understanding of hydrologic and hydrogeologic controls on water-resource management in the basin.