High flow in the normally dry stream channels is uncommon, and significant recharge in these areas generally occurs only at intervals of many years. The average rate of recharge to the High Plains aquifer in Colorado and New Mexico was about 196,000 acre-feet per year before development of irrigated agriculture. About two-thirds of the recharge occurred in the larger area of the aquifer in Colorado. Recharge rates of about 0.07 inch per year were common in much of the area, but as much as 0.8 to 1.0 inch of water per year might have recharged the aquifer near some streams and between the South Platte and Republican Rivers, where flat-lying sandy soil and larger rates of annual precipitation are common. After agricultural development, infiltration of irrigation water applied to fields that overlie the aquifer became another source of recharge.
Most ground water is discharged from the aquifer by underflow, withdrawal, and evapotranspiration.
Ground-water withdrawal from wells is another major means of discharge. Evapotranspiration by phreatophytes in areas of shallow water table is a minor means of discharge in eastern Colorado and New Mexico. The altitude and configuration of the water table in the High Plains aquifer is most affected by the altitude and configuration of the underlying bedrock surface, the transmissivity of the aquifer, and the rate and distribution of recharge and discharge. In other parts of the aquifer, the water table is continuous and slopes eastward at gradients of about 10 to 40 feet per mile. The water-table altitude ranges from about 6,800 feet at the northern edge of the aquifer in Colorado to about 3,600 feet at the eastern boundary of Colorado and New Mexico.