Using MODFLOW to compare management alternatives for a river alluvial aquifer

The ground water policies of Oklahoma and the Texas Panhandle Groundwater Conservation District (GCD) were compared to determine their effects on sustainability of an alluvial aquifer. The Oklahoma policy permits each well to withdraw water at a rate projected to allow not more than 50% of wells to go dry in 20 years. The GCD policy permits pumping at a rate that will consume no more than 50% of the water in 50 years, with reevaluation and readjustment of permits every five years. MODFLOW (Harbaugh and McDonald, 1996) simulations showed that at low development, Oklahoma's policy will limit the pumping rate more than the GCD policy. As development exceeds 65% of the area, more than half of aquifer storage will be depleted. The reevaluation required by the GCD policy will not permit aquifer storage to decline below 50%. Both Oklahoma and GCD policy will reduce baseflow of the river when development exceeds approximately 10%. Analysis showed that total aquifer storage was more sensitive to recharge rate and aquifer hydraulic conductivity than specific yield or streambed hydraulic conductivity. River leakage was most sensitive to aquifer hydraulic conductivity followed by specific yield and recharge. Streambed hydraulic conductivity was the least sensitive parameter overall. This research demonstrates that it is not only important to set a pumping rate based on the total storage in the aquifer, but it is important to consider the interchange with the river (baseflow and recharge).