Ground-water withdrawals for irrigation increased from about 84,000 acre-feet to 430,000 acre-feet per year between 1960 and 1970 in the northern High Plains of Colorado, causing significant water-level declines in areas where high-capacity wells are concentrated. The number of wells increased from about 500 in 1960 to 2,560 in 1970. In 1968 the U.S. Geological Survey began a study in cooperation with the Colorado Water Conservation Board; the Colorado Division of Water Resources, Office of the State Engineer; and the Management Districts of the northern High Plains to provide information for administration and management of the vital ground-water reserves of the northern High Plains.

The northern High Plains of Colorado is formed from an erosional remanent of the Ogallala Formation of Pliocene Age that extends from the South Platte River on the north to the Arkansas River on the south, and from Limon, Colo. on the west across the Colorado State line on the east (fig. 1). This area includes all or part of 11 counties and contains about 9,500 square miles. General information describing the ground-water resources of the northern High Plains of Colorado is presented in table 1. Plate 1 shows the net changes in water level between 1964 and 1971. Other published water-level change maps are for the period from 1964-68 and 1964-69 (Boettcher and others, 1969). The map (pl. 1) is based on annual water-level measurements made in 1964 and 1971 in about 300 wells tapping the Ogallala aquifer. Areas with either a net decline or a net rise are shown. Large declines recorded near Yuma and Burlington correspond to relatively high concentrations (2 per sq mi) of high-capacity wells. Also included in this report are 11 hydrographs typical of the water-level changes during this period (figs. 2-4). The hydrographs show water-level changes from 1964, or as early as 1956 to 1971, and illustrate the consistency of water-level change rates. Because proper management of ground-water resources requires systematic and valid basic data, the annual water-level measurements should continue.