Analysis and computer simulation of stream-aquifer hydrology, Arkansas River Valley, southwestern Kansas

Abstract

A study was made, in cooperation with the Division of Water Resources, Kansas State Board of Agriculture, to determine geohydrologic conditions underlying nearly 110,000 acres of the Arkansas River Valley between the Colorado-Kansas State line and the Bear Creek Fault zone in southwestern Kansas. The Arkansas River meanders atop and interacts hydraulically with the area's unconfined sand and gravel aquifer. Owing to decreasing recharge and increasing discharge during the 1970's, water levels declined an average of 4 feet during 1970-79. Average annual streamflow at Syracuse, Kansas, also decreased from 232 cubic feet per second during 1951-69 to 85 cubic feet per second during 1970-79. A digital-computer model was calibrated to simulate the trends of historic water levels and streamflow during 1970-79. Simulated 1975-79 conditions depict an annual recharge to the aquifer of 15,000 acre-ft (acre-feet) from river leakage, 9,000 acre-ft from boundary inflow, and 50,000 acre-ft from deep percolation. Simulated annual discharge consists of 12,000 acre-ft to boundary outflow across the Bear Creek Fault zone, 1,000 acre-ft as leakage to the Arkansas River, 11,000 acre-ft to groundwater evaporation, and 57,000 acre-ft to pumpage. Simulated annual recharge was 7,000 acre-ft less than simulated annual discharge of 81,000 acre-ft. Simulation indicates that: (1) The long-term effects of less recharge from smaller than average amounts of annual precipitation during the 1970's were offset by more recharge during brief, timely periods of much greater than the mean monthly amounts of precipitation, and (2) the effects of the increased pumpage were partly offset by increased recharge resulting from increased irrigation. Model results indicate that the water-level decline and streamflow shortage during 1970-79 were affected more directly by departures from historic (1951-69) rates of incoming streamflow than by either the smaller than average amounts of precipitation or the increased pumpage during the 1970's. Results also indicate that waterlevel declines and streamflow reduction would stabilize or reverse during 1980-82 if one of the following conditions prevailed: (1) Monthly precipitation increased to 25 percent greater than the normal for 3 years. (2) pumpage decreased to 50 percent of the 1979 rate or, (3) incoming streamflow increased to the 1951-69 rate.