The High Plains aquifer system underlies 174,000 square miles in parts of eight States. The aquifer is a water-table aquifer that is composed mainly of sand and gravel with some silt and clay deposits.
The Ogallala Formation, which underlies about 80 percent of the High Plains, is the principle geologic unit forming the aquifer. The maximum saturated thickness of the High Plains aquifer is about 1,000 feet, and the average saturated thickness of the aquifer is about 200 feet.
The Ogallala Formation was deposited by braided streams flowing eastward from the ancestral Rocky Mountains that deposited random (heterogeneous) sequences of gravel, sand, silt, and clay. Groundwater generally flows from west to east and discharges naturally to streams and springs, and by evapotranspiration in areas where the water table is near land surface. Pumping from more than 165,000 wells is another important mechanism of ground-water discharge.
Precipitation is the principle source of recharge to the aquifer. Estimates of recharge rates range from 0.024 inches per year in parts of Texas to 6 inches per year in areas of dune sand in Kansas and Nebraska.
Several major river systems cross the High Plains aquifer from west to east. These river systems include the Platte, Republican, Arkansas, Cimarron, and Canadian Rivers. The alluvial-aquifer system associated with each river is also an important local water resource.
The High Plains aquifer is in hydraulic connection (water can move from the High Plains aquifer to the alluvial sediments associated with the rivers and visa versa) with the major river systems crossing the aquifer (Weeks and others, 1988).
During low-flow periods, water in the rivers is almost entirely derived from ground-water discharge. Land use within the Study Unit primarily is agriculture and rangeland.
Regional variability of water-level changes in the High Plains aquifer results from large regional differences in climate, soils, land use, and ground-water withdrawals for irrigation. Substantial pumping of the High Plains aquifer for irrigation since the 1940's has resulted in water-level declines of nearly 150 feet in some parts of the aquifer.