CHANGE IN WATER IN STORAGE IN THE HIGH PLAINS AQUIFER, PREDEVELOPMENT (ABOUT 1950) TO 2015

The High Plains aquifer underlies about 175,000 square miles in parts of eight States—Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming. The aquifer is generally unconfined, but can be confined locally. Depth to water generally varies from near surface to about 400 feet. The aquifer’s saturated thickness varies from less than 50 feet to about 1,100 feet, and area-weighted average specific yield of the aquifer is 15.1 percent. Quaternary to Tertiary-age geologic units comprise the aquifer, and these units are primarily composed of sand, gravel, loess, silt and clay.

The area overlying the aquifer is one of the major agricultural regions in the United States. More than 95 percent of the water withdrawn from the aquifer is used for irrigation. Withdrawals for irrigation that greatly exceed the aquifer’s recharge rate will result in water-level declines in that part of the aquifer. Groundwater withdrawals from the High Plains aquifer for irrigation began in the 1940’s and 1950’s in some parts of the aquifer area, including the southern part of the Texas panhandle, southwestern Kansas, and near the Platte River in central Nebraska. Water-level declines were documented in the early 1950’s in parts of Texas and in 1980 in parts of all the states that overly the High Plains aquifer, except South Dakota, where no groundwater level declines were observed at that time. In response to the declines observed by the early 1980’s, the U.S. Geological Survey, in cooperation with numerous Federal, State, and local water-resource agencies, began monitoring groundwater levels in the aquifer in 1987. Water levels are measured annually, primarily in irrigation wells, in winter to early spring when water levels generally have recovered from groundwater pumping for irrigation in the previous growing season. The water-level elevation for predevelopment (about 1950) conditions was determined using water-level measurements from more than 20,000 wells. The water-level elevation for 2015 was measured in about 8,360 wells; median depth to water in 2015 was 88.5 feet. Water levels were measured in 3,168 wells for both predevelopment and 2015. Water-level declines from predevelopment to 2015 were observed in about 69 percent of the wells, and 90 percent of the declines were less than 68.3 feet.