Kansas Geological Survey, Public Information Circular (PIC) 18
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Usable water in the High Plains aquifer is in the pore spaces between particles of sand and gravel. This water (called ground water) accumulated slowly--in some of the deeper parts of the aquifer, over tens of thousands of years. In the subsurface, water in the aquifer generally moves slowly from west to east, usually at the rate of tens of feet per year.
Recharge is the natural movement of water into an aquifer, usually from precipitation. Natural recharge to the High Plains aquifer from precipitation is low, in part because much of the rain falls during the growing season, when plant roots intercept the soil moisture. In western Kansas, where precipitation is scant and the water table is relatively deep (several hundred feet) in many places, recharge occurs infrequently and the long-term average is less than an inch per year. In central Kansas, where the aquifer is closer to the earth's surface, where soils are sandier, and precipitation amounts greater, recharge can be significant, as much as 4 to 6 inches per year.
Water volumes and use are measured in various ways. One measure is an acre-foot, or the amount of water necessary to cover an acre of ground (a parcel about the size of a football field) with a foot of water. An acre-foot equals 325,851 gallons of water. In 1990, about 15.7 million acre-feet of ground water was removed from the High Plains aquifer eight-state region. In Kansas, the High Plains aquifer yielded 4.4 million acre-feet, of which 3.6 million acre-feet came from the Ogallala aquifer. Estimated average annual natural recharge to the Ogallala in Kansas is 0.72 million acre-feet.
Another measure of ground water is saturated thickness--the thickness of the sands, gravels, and other materials that are saturated with water. Saturated thickness is commonly measured in feet, but "feet of saturated thickness" is not the same as feet of actual water. Only about 10 to 25 percent of the aquifer volume is pore space that can yield extractable water. Therefore, in an aquifer with 17 percent pore space, removing 1 acre-foot of water causes the water table to drop by about 6 feet. In Kansas, saturated thickness in the High Plains aquifer is generally greatest in the southwestern part of the state (see fig. 4). There, saturated thicknesses of 300 feet were common before the onset of large-scale irrigation, a time that is often called "pre-development."
Figure 4--Predevelopment saturated thickness for the High Plains aquifer in Kansas.
Ground water can also be measured in terms of its availability: how much water can be removed by a well over short periods. Large volumes of water can be pumped rapidly (1,000 gallons or more per minute) from the High Plains aquifer in many locations. This contrasts with much of the rest of the state, where wells generally produce smaller amounts (less than 100 gallons per minute). By way of comparison, a good household well produces 5 to 10 gallons per minute, although many household wells produce less.
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Kansas Geological Survey, Public Outreach
1930 Constant Ave., Lawrence, KS 66047-3726
Phone: (785) 864-3965, Fax: (785) 864-5317
Web version October 2001