A MODELING ASSESSMENT OF THE LONG-TERM SUSTAINABILITY OF GROUND-WATER PUMPING IN THE MIDDLE ARKANSAS RIVER SUBBASIN OF SOUTH-CENTRAL KANSAS

Ground-water levels have been declining during the last few decades in most of the High Plains aquifer in the Middle Arkansas River subbasin in south-central Kansas mainly as a result of irrigation pumping. These declines have decreased ground-water discharge to the Arkansas River, thereby decreasing river flow. A calibrated ground-water flow model was constructed to aid water-resources planning and management in the subbasin and, in particular, to assess the future impact of continued ground-water pumpage. The model was used to simulate historical conditions for the period of 1944-2004 and then assess a series of future scenarios for the 2005-2054 period. The average net pumping (pumpage minus irrigation return flow) increased from 0.17 x 10⁸ m³/yr for the first 30 years of the historical period (1944-1973) to 2.13 x 10⁸ m³/yr for the last 15 years (1990-2004). A substantial storage decline in the High Plains aquifer starting in the late 1970s was accompanied by a decrease in streamflow and a reduction in ground-water flow out of the subbasin. By 2004, the cumulative loss in aquifer storage reached 18.5 x 10⁸ m³. The net streamflow gain in the subbasin decreased from an annual average of 0.44 x 10⁸ m³/yr for 1944-1973 to 0.08 x 10⁸ m³/yr for 1990-2004, even though the average precipitation recharge for 1990-2004 (5.9 cm/yr) was greater than that for 1944-1973 (4.5 cm/yr). Two of the scenarios served to bound the range of possible future conditions in the subbasin. One scenario simulated a future in which pumping continued at current levels. This scenario indicated that ground-water levels continue to decline, causing further decreases in streamflow and lateral outflow of ground water. The cumulative loss in aquifer storage that began in the late 1970s sums to an additional 18.5 x 10⁸ m³ for 2005-2054. The other scenario assumed that all pumping ceased after 2004. This scenario showed that the long-term water-level declines in the aquifer that began in the late 1970s start to reverse within a few years after the wells are shut off. The change from streamflow loss to gain takes a few years longer due to the need to raise water levels enough to create substantial baseflow and reduce stream loss. Most of the aquifer storage lost from the late 1970s to 2004 is regained about 20 years after cessation of pumping.