TOWARD THE SUSTAINABLE MANAGEMENT OF GROUND AND SURFACE WATER IN ARKANSAS

Conjunctive-use optimization modeling was done to assist water managers and planners by estimating the maximum amount of ground water that hypothetically could be withdrawn from wells within the Mississippi River Valley alluvial and Sparta aquifers without violating hydraulic-head or stream-discharge constraints. In 1997, nearly 6 million acre-feet of water were pumped from the aquifers by more than 45,000 wells, primarily for irrigation and for fish farming. Several large cones of depression have formed in the potentiometric surface, resulting in lower well yields and degraded water quality in some areas. MODFLOW-based ground-water flow models were developed for the north and south portions of the alluvial aquifer, primarily in eastern Arkansas, and for the Sparta aquifer in Arkansas, Louisiana, and Mississippi. MODMAN optimization models based on these flow models showed that continued pumping at 1997 rates are unsustainable without violating head constraints imposed as a part of Arkansas' Critical Ground-Water Area criteria. In addition, streamflow constraints were established based on minimum flow requirements for maintaining water quality and fish habitat. Preliminary results show that continuous pumping at 1997 rates indefinitely resulted in water levels dropping below the hydraulic-head constraints, making these rates unsustainable. Optimized sustainable pumping was obtained such that water levels were maintained at or above the hydraulic-head constraints, and streamflow was maintained at or above minimum levels. Optimized sustainable yields from streams were nearly 2 orders of magnitude greater than for ground water.