SUSTAINABLE GROUND-WATER DEVELOPMENT CONSTRAINED BY MINIMUM INSTREAM-FLOW REQUIREMENTS AND WATER-SUPPLY DEMANDS IN RHODE ISLAND, USA

As in many locations in the United States where streams and other surface-water features are in hydraulic connection with shallow, high-yielding aquifers, the State of Rhode Island is striving to balance sustainable development of ground-water resources to meet increased water-supply needs of its citizens and industries with minimum instream-flow requirements that support sustainable aquatic and riparian ecosystems. Over the past decade, we have developed conjunctive-management models that couple transient, numerical simulation with optimization techniques to help define hydrologic, hydrogeologic, and water-management controls on the sustainable limits of ground-water development in the Hunt and Big River Basins, as well as for hypothetical stream-aquifer systems representative of those of the northeastern United States. Simulation-optimization models are particularly useful in this process because they provide an effective tool to evaluate tradeoffs between alternative definitions of instream-flow requirements (which can be very difficult to determine) and total ground-water withdrawals.

The effects on water-supply development of up to 12 alternative definitions of minimum instream-flow requirements have been evaluated with the simulation-optimization models. These alternatives include requirements based on a minimum streamflow rate that is constant throughout the year (such as minimum flow rates established on the basis of discharge rate per unit drainage area) and those based on time-varying (seasonal or monthly) minimum streamflow rates. Other factors that have been evaluated with the models are temporally variable climatic conditions that affect recharge and streamflow rates, seasonality of water-supply demands, characteristics of the water-supply networks (number and configuration of wells, length of pumping periods, and minimum and maximum withdrawal rates), and the use of artificial recharge to enhance management options. Research to date indicates that small changes in the specified instream-flow requirement, the annual pattern of water-supply demands, and the effects of low-flow conditions brought on by droughts can lead to large changes in the amount of ground water that is available for withdrawal on a sustainable basis.