A)The vast grasslands of the High Plains in the central United States were settled by farmers and ranchersin the 1880's．This region has a semiarid climate, and for 50 years after its settlement.it supported a ow-intensity agricultural economy of cattle ranching and wheat farming．In the early twentieth century,however, it was discovered that much of the High Plains was underlain by a huge aquifer ( a rock layer ontaining large quantities of groundwater)．This aquifer was named the Ogallala aquifer after the Ogallala Sioux Indians, who once inhabited the region.
B)The Ogallala aquifer is a sandstone formation that underlies some 583,000 square kilometers of land extending from northwestern Texas to southern South Dakota．Water from rains and melting snows has been ccumulating in the Ogallala for the past 30,000 years．Estimates indicate that the aquifer contains nongh water to fill Lake Huron, but unfortunately, under the semiarid climatic conditions that presently xist in the region, rates of addition to the aquifer are minimal, amounting to about half a centimeter a ear.
C)The first wells were drilled into the Ogallala during the drought years of the early 1930's．The ensuing apid expansion of irrigation agriculture, especially from the 1950's onward, transformed the economy of he region．More than 100,000 wells now tap the Ogallala．Modern irrigation devices, each capable of praying 4.5 million liters of water a day, have produced a landscape dominated by geometric patterns of ircular green islands of crops．Ogallala water has enabled the High Plains region to supply significant mounts of the cotton, sorghum, wheat, and corn grown in the United States．In addition,40 percent of merican grain-fed beef cattle are fattened here.
D)This unprecedented development of a finite groundwater resource with an almost negligible natural recharge rate--that is, virtually no natural water source to replenish the water supply--has caused water
tables in the region to fall drastically．In the 1930's, wells encountered plentiful water at a depth of bout 15 meters; currently, they must be dug to depths of 45 to 60 meters or more．In places, the water able is declining at a rate of a meter a year, necessitating the periodic deepening of wells and the use of ver-more-powerful pumps．It is estimated that at current withdrawal rates, much of the aquifer will run ry within 40 years．The situation is most critical in Texas, where the climate is driest, the greatest mount of water is being pumped, and the aquifer contains the least water．It is projected that the remaining Ogallala water will, by the year 2030, support only 35 to 40 percent of the irrigated acreage in exas that is supported in 1980.
E) The reaction of farmers to the inevitable depletion of the Ogallala varies．Many have been attempting to onserve water by irrigating less frequently or by switching to crops that require less water．Other, however, have adopted the philosophy that it is best to use the water while it is still economically profitable
to do so and to concentrate on high-value crops such as cotton．The incentive of the farmers who wish to onserve water is reduced by their knowledge that many of their neighbors are profiting by using great mounts of water, and in the process are drawing down the entire region's water supplies.
F) In the face of the upcoming water supply crisis, a number of grandiose schemes have been developed to ransport vast quantities of water by canal or pipeline from the Mississippi, the Missouri, or the Arkansas rivers．Unfortunately, the cost of water obtained through any of these schemes would increase pum-
ping costs at least tenfold, making the cost of irrigated agricultural products from the region uneompetitive on the national and international markets．Somewhat more promising have been recent experiments or releasing capillary water (water in the soil) above the water table by injecting compressed air into he ground．Even if this process proves successful, however, it would almost triple water costs．Genetic ngineering also may provide a partial solution, as new strains of drought-resistant crops continue to be eveloped．Whatever the final answer to the water crisis may be, it is evident that within the High lains, irrigation water will never again be the abundant, inexpensive resource it as during the agricultural boom years of the mid-twentieth century.
G) The Ogallala Aquifer is a vast, shallow underground water table aquifer located beneath the Great Plains n the United States．One of the world's largest aquifers, it covers an area of approximately 174,000 m2(450,000 km2) in portions of eight states: (South Dakota, Nebraska,Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Texas).
H) It was named in 1898 by N．H．Darton from its type locality near the town of Ogallala, Nebraska．The aquifer is part of the High Plains Aquifer System, and rests on the Ogallala Formation, which is the principal geologic unit underlying 80% of the High Plains.
I) About 27 percent of the irrigated land in the United States overlies the aquifer, which yields about 30 percent of the ground water used for irrigation in the United States.
J) Since 1950, agricultural irrigation has reduced the saturated volume of the aquifer by an estimated 9%.
Depletion is accelerating, with 3% lost between 2001 and 2008 alone.
K) Certain aquifer zones are now empty; these areas will take over 100,000 years to replenish naturally through rainfall.
L) The aquifer system supplies drinking water to 82 percent of the 2.3 million people ( 1990 census) who live within the boundaries of the High Plains study area.
M) The deposition of aquifer material dates back 2 to 6 million years, from the late Miocene to early Plio-
cene ages when the southern Rocky Mountains were still tectonically active.
N ) From the uplands to the west, rivers and streams cut channels in a generally west to east or southeast direction．Erosion of the Rockies provided alluvial and aeolian sediment that filled the ancient channels and eventually covered the entire area of the present-day aquifer, forming the water-beating Ogallala Formation.
O) In that respect, the process is similar to those currently prevailing in other modern rivers of the area,such as the Kansas River and its tributaries．The major differences are time and depth.