EFFECTS OF CONSTRAINT SELECTION ON SIMULATED SUSTAINABLE YIELD FROM THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER AND MAJOR RIVERS IN EASTERN ARKANSAS

The Mississippi River Valley alluvial aquifer is a water-bearing assemblage of gravels and sands that underlies most of eastern Arkansas and several adjacent States. Ground-water withdrawals have caused cones of depression to develop in the aquifer's water-level surface, some as much as 100 feet deep. Recharge to the alluvial aquifer from rivers becomes induced as ground-water levels decline. The expansion of the cones of depression and the consistent water-level declines indicate that ground-water withdrawals are occurring at a rate that is greater than the sustainable yield of the aquifer.

Optimization modeling was used to calculate the maximum sustainable yield from wells and rivers, while maintaining simulated water levels and stream¬flows at or above minimum specified limits. Sustainable yield of ground- water was compared for four different management scenarios involving different water-level constraints and river-withdrawal specifications. Scenario 1 is the baseline scenario in which river withdrawals were allowed from all river cells from 11 rivers specified in the model, while maintaining ground-water levels at or greater than half the saturated thickness of the aquifer. Scenario 1 includes withdrawals from two irrigation project areas that would remove water from either the Arkansas or White Rivers. Scenario 2 differs from Scenario 1 in that the water-level con¬straints were relaxed so that the aquifer must have at least 30 feet of saturated thickness everywhere. In Scenario 3, optimized surface-water withdrawal is removed from the model specifica¬tion in all 11 rivers; however, surface-water withdrawals are fixed at 2000 rates at select points, and no additional withdraw¬als are permitted. In addition, no withdrawals from either of two irrigation project areas that would remove water from either the Arkansas or White Rivers are specified in Scenario 3 (unlike Scenarios 1 and 2). Water-level constraints in the aquifer were set to half the saturated aquifer thickness. For Scenario 4, the same conditions as for Scenario 3 were specified, but water-level con¬straints were relaxed to have at least 30 feet of saturated aquifer thickness. Average differences in sustainable yield of ground water between baseline Scenario 1 and Scenarios 2, 3, and 4 show an increase of 6.7, 6.8, and 13.2 percent, respectively.