News Release, Kansas Geological Survey, Dec. 19, 2003
LAWRENCE--The amount of water that replenishes the Ogallala aquifer in western Kansas each year is extremely small, well under two inches per year, according to a recent study by researchers at the Kansas Geological Survey, based at the University of Kansas, and the U.S. Geological Survey.
The research results have important implications for an area that relies heavily on water from the Ogallala, one part of the High Plains aquifer, for irrigation. The aquifer underlies about the western third of the state.
The research, conducted in 2000 and 2001, was recently published in a report from the U.S. Geological Survey. The results were based on three specially instrumented wells in southwestern Kansas--two under irrigated farmland in southern Finney County, and one in rangeland in the Cimarron National Grasslands in Morton County.
The researchers found that recharge, or the movement of water from the surface down into the aquifer, was consistently higher under irrigated farmland, as opposed to rangeland. However, the water that returned to the aquifer beneath irrigated land was also higher in contaminants, such as herbicides and nitrates.
The wells, each about 150 feet deep, were equipped with sensors that could track the movement of water back underground to the water table. In addition, water samples were analyzed for several contaminants. The wells were measured and studied from May of 2000 through November 2001.
"In general, the amount of recharge we found is less than one percent of the annual precipitation," said Marios Sophocleous, a water scientist at the Kansas Geological Survey and one of the authors of the report. Average annual precipitation in this area is less than 20 inches per year.
Understanding the amount of recharge is crucial to managing the aquifer. Large amounts of water are pumped out of the aquifer, and water levels have been generally declining for the past several decades. Those declines range from a few feet to more than 100 feet in some parts of southwestern Kansas. Knowing the amount of water that replenishes the aquifer is crucial to water management in the area.
Researchers have previously estimated that recharge is less than an inch per year across much of the western third of Kansas. However, relatively few studies have attempted to precisely measure and analyze recharge in the field.
This study found that recharge under irrigated land ranged from 1.1 to 1.8 inches per year. Under the rangeland, it was 0.17 to 0.21 inches per year.
"In the rangeland, it appears that precipitation is captured by evaporation or by plants before it can get to the water table," said Sophocleous.
Higher amounts of contaminants were found in the water beneath irrigated land, however. Nitrate concentrations range from 20 to 24 parts per million in water beneath irrigated land. Atrazine, a commonly used herbicide, was found at levels slightly above 0.9 parts per billion.
"These results make it clear that levels of nitrate and herbicides are elevated in the water beneath land that is used for irrigated agriculture," said Sophocleous.
While recharge rates are higher under irrigated land than rangeland, irrigation is no solution to declining groundwater levels.
"Irrigation is now so widespread that it seems to contribute more to recharge than does precipitation," said Sophocleous. "But this recharge is coming at the expense of groundwater that was previously stored in the aquifer, and more irrigation is not the answer."
The study was part of the U.S.G.S.'s National Water-quality Assessment Program. The U.S. Geological Survey is a federal agency that deals with geology, natural resources, hazards, and the environment. The Kansas Survey is a research and service division of KU.