The High Plains of Colorado includes all or part of 11 counties and has an area of about 9,500 square miles. The land surface slopes eastward and in most areas is gently rolling owing to erosion by ephemeral streams. The Ogallala Formation, of Pliocene age, is the principal aquifer. In pierces it is overlain by dune sand, alluvium, or loess. The Ogallala is semiconsolidated and consists of sand, gravel, silt, clay, and caliche.
The Ogallala Formation is recharged by precipitation at a rate of about 0.85 inch per year. The water table slopes generally eastward. The major use of ground water is for irrigation. About 72,500 acre-feet of water was pumped from 428 wells to irrigate about 56,600 acres in 1962. Estimates of consumptive use made by the Blaney-Criddle method show that the optimum amount of water is being used to irrigate parts of the High Plains. At the end of the 1963 irrigation season, 525 irrigation wells each pumped more than 300 gallons per minute. Water levels decline as much as 10 feet in some places during the irrigation season but return almost to normal at the completion of pumping. By prorating transmissibility on the basis of lithologic descriptions from well logs, the average permeability of the Ogallala Formation in various places can be estimated. Most water from the Ogallala Formation in the High Plains is a calcium bicarbonate solution having a dissolved-solids content ranging from 100 to 600 parts per million. The water is generally hard; its calcium carbonate hardness ranges from 100 to 350 parts per million. Except for some ground water in the area south of the Cheyenne-Kiowa County line, the ground water analyzed was suitable for all uses.