HYDROGEOLOGIC CHARACTERIZATION OF THE HIGH PLAINS AQUIFER, 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 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 is 15.1 percent. Quaternary to Tertiary-age geologic units comprise the aquifer; the aquifer is primarily composed of sand, gravel, sandstone, siltstone, 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 More than 95 percent of the water withdrawn from the aquifer is used for irrigation. In parts of the aquifer area, substantial withdrawals for groundwater irrigation generally began in the 1950’s. In parts of the aquifer area, groundwater declines occurred soon after the onset of groundwater irrigation and substantial declines had occurred by 1980. In 1987, Congress directed the U.S. Geological Survey to work with Federal, State, and local water-resource agencies and monitor groundwater levels in the aquifer and to report the results. Water levels are measured annually primarily in irrigation wells, when water levels generally have recovered from groundwater pumping for irrigation in the previous growing season. The water-level elevation for predevelopment conditions was determined using water-level measurements from more than 20,000 wells; the median measurement date for the predevelopment water levels was 1957. The water-level elevation for 2015 was measured in about 8,500 wells; median depth to water in 2015 was 85 feet. Water levels were measured in about 3,300 wells in both predevelopment and in 2015 and in about 7,500 wells in both 2013 and 2015. This presentation will discuss the results and interpretation of the water level compilation from predevelopment to 2015 and 2013 to 2015.