News Release, Kansas Geological Survey, Oct. 23, 1995
Survey geohydrologist Marios Sophocleous will discuss one water- management technique, called "safe yield," at the annual meeting of the Geological Society of America in New Orleans on Nov. 6.
Safe yield, a water-management technique used in parts of the Great Plains, allows water users to pump only the amount of groundwater that is replenished naturally through precipitation. That natural replenishment is known as "recharge." Unfortunately, says Sophocleous, the concept of safe yield depends on an artificial distinction between groundwater (underground water found in porous rock layers called aquifers) and surface water (lakes, springs, and streams).
Under natural conditions, recharge to an aquifer eventually results in an equal amount of water that is discharged into a stream, spring, or seep. When water is pumped from an aquifer, even in amounts equal to the recharge, water continues to leave the aquifer in the form of springs or seeps. As a result, water levels decline in the aquifer. Eventually those levels drop enough that discharge slows and less water flows into springs and streams.
"Safe yield does not take into account the impact that pumping has on surface water," said Sophocleous. "If you pump out the recharge, you will dry up streams, marshes, and springs. Even safe yield eventually causes a significant loss of groundwater reserves."
That's exactly what has happened over the past 50 years in several locations across the Great Plains. Probably the best known example is the Ogallala or High Plains aquifer. In parts of Texas, water tables have declined more than 200 feet, mostly because of pumping from the Ogallala for irrigation. Declines of more than 50 feet have occurred in eastern New Mexico, the Texas panhandle, and parts of western Kansas.
Maps comparing the perennial streams in Kansas in the 1960s to those of the 1990s show a marked decrease in miles of streamflow in the western third of the state.
Recharge in western Kansas is extremely low, probably less than an inch per year. But even in parts of Kansas where recharge rates are higher, safe yield may need to be redefined. In the Great Bend Prairie area of central Kansas, Sophocleous estimates that recharge is about two inches per year. Pumping has lowered water levels in the aquifer, lessening the discharge to streams.
"The aquifer is slowly being depleted because the amount of water that would ordinarily recharge the aquifer is being pumped away," said Sophocleous. "That means less water is available for natural discharge, and streamflows in the area have steadily declined since the mid 1970s."
As a result, Kansas water managers need to take a second look at safe yield.
"Concepts like safe yield, that assume you can pump a certain amount of water without having an impact in the water cycle, need to be re-examined," said Sophocleous. "You can use an amount of water equal to recharge, but you have to realize the effect on surface water."