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 THE OGALLALA
AQUIFER
The Ogallala aquifer (pronounced OH-GA-LA-LA) is one of the
largest aquifer systems in the world. It stretches across all or portions of
eight states generally from north to south to include South Dakota, Nebraska,
Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Texas and underlies about
174,000 square miles. N.H. Darton is credited with describing and naming the
formation in 1899 after the town of Ogallala, Nebraska.
The Ogallala aquifer lies relatively near the land surface in most of the
above-described area with a maximum thickness of about 1,000 feet with a few
hundred feet more the norm. Even in those areas of only a few feet of thickness,
the aquifer can almost always be counted on to yield water to a well drilled
into it. Some wells yield only a few gallons of water per minute, while others
yield 1,000 gallons of water per minute or more. The Ogallala aquifer not only
includes the portion of the Ogallala that is saturated with water, but may also
include saturated portions of the overlying and underlying formations that are
hydraulically connected to the Ogallala. Water in the aquifer on the Southern
High Plains flows from northwest to southeast at about 150 feet per year under
natural conditions. This rate of movement can be altered by discharge from the
aquifer by pumping wells.
Deposition of the Ogallala Formation began 10 to 12 million years ago during
late Tertiary (Miocene/Pliocene) geologic time. Sand, gravel, silt, and clay
eroded from upland areas to the west and north were deposited over the erosional
land surface of the present-day High Plains by primarily eastward flowing
streams. The surface on which the sediments were deposited would have been
much like the present area located east of the High Plains escarpment
characterized by low hills, relatively shallow valleys, and meandering streams.
As a result of the burial of this land surface by predominantly Ogallala
sediments, the Ogallala Formation is thicker where these sediments filled the
old stream channels and thinner where hills or upland areas were buried. The
uppermost layer of the Ogallala Formation is typically a caliche layer described
as the “Caprock Caliche.” This layer locally varies in thickness and has been
reported to be as much as 60 feet in thickness in some areas. The caliche layer
formed about one million years ago after the land surface stabilized and soils
formed.
The water table is a term used to describe the uppermost surface of sediments
that are 100 percent saturated. Saturated thickness describes the thickness of
an aquifer. This interval is determined by subtracting the elevation of the base
of the aquifer from the elevation of the water table at a point of interest. In
some parts of Nebraska, the saturated thickness exceeds 1,000 feet and generally
thins to less than 20 feet in thickness in some areas of the Great Plains.
The amount of water that may be recovered in an aquifer, such as the Ogallala,
is dependent primarily
 on
the areal extent, the saturated thickness, and the specific yield of the
aquifer. Specific yield is a hydrologic parameter related to the volume of water
an aquifer will yield as a result of gravity drainage. As an example of this
parameter, 15 percent specific yield is an average value that is customarily
accepted for the Southern High Plains Ogallala aquifer. More specifically at 15
percent specific yield each cubic foot of water saturated aquifer volume will
yield 0.15 cubic-foot of water as a result of gravity drainage (See graphic
depicting specific yield at right).
In
1990, the Ogallala aquifer in the eight-state area of the Great Plains contained
3.270 billion acre-feet of water, of which about 65 percent was located under
Nebraska. Texas had about 12 percent of the water in storage or approximately
417 million acre-feet of water. Kansas has 10 percent of the water. About 4
percent was located under Colorado, with 3.5 percent located under Oklahoma.
Another 2 percent was under Wyoming. The remaining 1.5 percent of the water was
under New Mexico. A recent estimate of the volume of water in the eight-state
Great Plains area was just under 3 billion acre-feet.
Natural recharge to the Ogallala aquifer occurs primarily through the
percolation of precipitation through the soils and underlying sediments to the
water table. It is generally recognized that playa lakes are the primary points
of most natural recharge. The interplaya areas generally contribute a minimum of
the recharge, except for areas of exceptional accumulation of precipitation with
resultant extensive percolation of water to the water table in locations where
streambeds and dune areas are common. Various studies of natural recharge have
historically estimated various ranges of average recharge to the Ogallala
aquifer. Recent studies have estimated an average recharge rate for the entire
High Plains region of approximately 0.5 of an inch per year.
Before the development of irrigation, the discharge from the aquifer occurred
from both saline and fresh water like basins, from streams, and from seeps and
springs located primarily along the eastern escarpment. Some of these still flow
today; however, most seeps and springs have ceased to flow due primarily to
lowering of the water table as discharge has exceeded natural recharge.
Approximately 95 percent of the water pumped from the Ogallala is for
irrigation. The High Plains area represents 65 percent of the total irrigated
acreage in the United States. The quality of the water pumped from the aquifer
is suitable for irrigating; but in some places, the water does not meet U.S.
Environmental Protection Agency (USEPA) drinking water quality standards. For
example, some constituents identified above EPA standards include sulfate,
chloride, selenium, fluoride, nitrate, and total dissolved solids.
The Southern High Plains area is characterized as a semi-arid climate. Average
annual rainfall varies across the region. In the Lubbock, Texas area, the
average annual rainfall is about 18 inches per year. High evaporation rates are
common in the area. The average annual evaporation rate for the Lubbock area is
about 80 inches per year.
Monitoring of the depth-to-water in the aquifer’s Southern High Plains revealed
rapid declines in the water table in the early 1950s, 1960s, and the 1970s.
Declines of a foot or more per year were recorded throughout the 1940s; and
during the late 1950s at the peak of irrigation development, some monitoring
wells indicated as much as five feet of decline in a single year. The trend of
rapid decline started slowing in the mid-1970s. By 1985, the portion of the
Ogallala aquifer within the service area of the High Plains Underground Water
Conservation District No. 1 began to stabilize. In some limited or unique areas,
water level rises have been documented. A drought began in the area in
mid-1992, and continued until late 1996. Agricultural producers, out of
necessity, increased pumpage of water for irrigation to supplement
precipitation. The increased demand for crop water by irrigation resulted in an
increased rate of water level decline during this period. In subsequent years of
greater precipitation, decreased needs for pumpage have allowed water level
declines to decrease.
Early settlers believed the water supply that lay beneath them was
inexhaustible. In the 1930s, people had begun to realize the potential of the
vast water supply that lay beneath them. By 1949, about 2 million acres of the
Southern High Plains were irrigated. Pumpage for irrigation increased from about
4 million acre-feet in 1949 to nearly 18 million acre-feet in1980.
In
the early days of irrigation on the Texas High Plains, very little water
conservation equipment or technology was available. As a result, large amounts
of water were lost to evaporation and deep percolation. Open, unlined ditches
were used to transport the water from the well to the field being irrigated. It
was not uncommon to have water losses ranging from 10 to 30 percent per 1,000
feet of ditch. High pressure, hand-moved sprinklers had evaporation losses of up
to 50 percent.
Throughout the years, irrigation technology has evolved to allow agricultural
producers to apply water much more efficiently without waste. Irrigation water
escaping from fields into road ditches (“irrigation tailwater”) is not as common
as it once was in the 1960s and 1970s. Improved technologies, spurred on by
incentives such as the Environmental Quality Incentives Program (EQIP) and low
interest agricultural water conservation equipment loan programs, have helped
improve water use efficiencies. For example, average water use efficiency
within the High Plains Water District service area improved from about 50
percent in the mid 1970s to approximately 75 percent in 1990. Current
state-of-the-art low pressure, full dropline center pivot systems, used in
conjunction with furrow dikes, are about 95 percent efficient, while buried
subsurface drip irrigation lines approach 100 percent efficiency.
Producers are irrigating fewer acres. Land enrolled in the Conservation Reserve
Program (CRP), rising energy costs, declines in well yields, and low farm prices
also account for part of this reduction.
The rates of decline in the water levels in the aquifer continue to be somewhat
stable with noted annual decreases in decline resulting when annual
precipitation is above average and less ground water needs to be pumped.
Researchers continue to work on methods to increase natural recharge to the
aquifer and to improve water-use efficiency. The prospects for the future of
the Ogallala aquifer ultimately depend upon its management by each of its water
users.
Additional information is available by contacting Don
McReynolds, P.G., Technical Group Supervisor, at (806) 762-0181.
DOWNLOADS:
Map of the major aquifers
in Texas [PDF]
Map of the minor aquifers
in Texas [PDF]
This page last
updated Monday April 02, 2007.
This
page has been visited
times since July 22, 2005.
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