The High Plains aquifer underlies 174,050 sq mi of eight states (Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming) and contains approximately 3.3 billion acre-ft of water in storage. Saturated thicknesses within the aquifer are as great as 600 ft near the southern border of southwest Kansas. The aquifer is replenished primarily by infiltration from precipitation. Average precipitation at the Garden City Experiment Station is 18.93 in/yr. Groundwater flow is generally from west to east under unconfined conditions. Hydraulic connection with subcropping consolidated aquifers allows ground water to flow vertically in minor quantities. The aquifer is depleted primarily by irrigation. Hydraulic conductivity estimates from 1,612 lithologic logs had an average value of 75 ft/day, with a standard deviation of 35 ft/day. Specific yields estimated from the same lithologic logs had a mean of 0.17 and a standard deviation of 0.047. Water from the High Plains aquifer in Kansas generally is suitable for human and animal consumption and irrigation of crops. Typically, it is a calcium bicarbonate type water, with concentrations of total dissolved solids ranging from 250 to 500 mg/L. The quality of water in the aquifer deteriorates toward the east due to mixing with recharge water containing dissolved minerals leached from the overlying soil and unsaturated zones and mineralized water from adjacent bedrock units. The simulated water budget for the steady state model of predevelopment (pre-1950) conditions in the High Plains aquifer in northwest Kansas showed that annual recharge to the aquifer from infiltration of precipitation was 87,000 acre-ft/yr and from boundary inflow, 21,000 acre-ft/yr. Annual discharge from the aquifer was 108,000 acre-ft/yr, including 81,000 acre-ft/yr from leakage to streams, 23,000 acre-ft from outflow at the boundaries of the aquifer, and 4,000 acre-ft from municipal and industrial pumpage. (Lantz-PTT)