The Ogallala Aquifer's long history, uncertain future

Editor's Note: Caprock Chronicles is edited each week by Jack Becker, a librarian at Texas Tech University. This week’s essay, by Paul Carlson, emeritus professor of history at Tech, looks at the Ogallala Aquifer, the enormous underground water supply that under lies much of Great Plains.

The Ogallala Aquifer is a large underground reservoir that extends 800 miles through the western High Plains; from the southern edge of South Dakota to the Llano Estacado in Texas and New Mexico, eight states in all.

Its maximum width, which is in Nebraska and Wyoming, is 400 miles. Its thickness ranges from a thin sliver to nearly 1,000 feet. Early explorers commented on the springs, which emptied into water courses on top of the Caprock and that flowed out of the Llano’s eastern edge. Early settlers suspected the Caprock sat upon a large aquifer but did not know how large it was.

In the 1990s, the aquifer contained approximately 3 billion acre-feet of groundwater, but because humans withdraw water faster than natural forces replenish it, the aquifer’s water is disappearing, particularly in Texas and New Mexico.

Groundwater in the aquifer exists within voids (pore spaces) between sedimentary particles that compose the Ogallala Formation, an unconsolidated underground geologic remnant (or geologic zone) consisting of huge deposits of sand, silt, gravel, and other material washed eastward from the Rocky Mountains in huge ancient rivers millions of years ago.

At one time in the ancient past the depositional zone - an area in which the sand, silt, and gravel were deposited -  included much of the Great Plains. But over millions of years erosion on the eastern side of the Plains reduced the Ogallala Formation and its concomitant water supply—the Ogallala Aquifer.

On the western side of the Ogallala Formation (and thus the aquifer), beginning about one million years ago the Pecos River began to carve its crooked way north from its mouth at the Rio Grande just east of modern Langtry in Val Verde County.

Over time the Pecos lengthened, and its deepening trench and widening valley cut the head streams of the Running Water, Blackwater, Yellowhouse, and Red Rivers.

By the time the Pecos had reached its present extent and location, the Rocky Mountain run-off, robbed by the Pecos, could no longer replenish the Ogallala Aquifer.

In Texas and New Mexico, the aquifer underlies the flat but remarkable Llano Estacado, and through irrigation, which started in earnest after World War 2, it supplies most of the water needs of the Llano’s substantial agricultural economy, which includes dairying, corn and cotton growing, and livestock raising.

In Nebraska, where the Ogallala Aquifer is enormous, it underlies the state’s extensive Sand Hills, and as in Texas and New Mexico through irrigation it supplies most of the region’s agricultural water needs, particularly the new emphasis in the Sand Hills on corn growing for ethanol production.

In Texas in a general sense, the water table of the aquifer parallels the topographic surface of the Llano Estacado. That is, water in the aquifer mainly flows west to east, but there is also a definite north to south flow.

Today, as indicated, natural recharge is insufficient to make up for withdrawals from the Ogallala Aquifer. When the Ogallala Formation and its aquifer extended west to the Rockies during the Pleistocene epoch (1.6 million years ago to 10,000 years ago), natural recharge was large, and the water table remained high.

But erosion on the eastern edge and the Pecos River on the western side, and the onset of drier, warmer weather in the later Pleistocene (approximately 10,000 years ago) reduced the amount of recharge.

Irrigation in recent years has also impacted recharge. In 2019, for example, a farmer-rancher in eastern New Mexico reported that five of his seven center-pivot well and irrigation systems could no longer pump water; the aquifer was dry below each of the five big wells.

Conversely, a farmer-rancher in north-central Nebraska has not noticed a decline in the water table at any of his several center-pivot well and irrigation systems. Nonetheless, irrigation for agricultural purposes and pumping for urban needs continues to draw down the aquifer’s water supplies.

In a general sense, one might say that while a few places, such as in parts of the western Sand Hills of Nebraska, get a limited number of inches of recharge per year, the average recharge throughout the aquifer zone is less than one inch per year. The amount is not enough to sustain the aquifer.

In both the past and the present aquifer water has proved enormously beneficial for those people who live on the Caprock. Clearly, however, continued long-term use of the Ogallala Aquifer appears troublesome and in need of major reevaluation.