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The Ogallala Aquifer is a shallow water table aquifer located beneath the Great Plains in the United States. One of the world's largest aquifers, it underlies an area of approximately in portions of eight states (South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Texas). It was named in 1898 by geologist 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.〔Darton, N.H. 1898. Preliminary report on the geology and water resources of Nebraska west of the one hundred and third meridian. In: Walcott, C.D. (ed), Nineteenth Annual Report of the United States Geological Survey, 1897-1898, Part IV, pp. 719-785.〕 About 27% of the irrigated land in the United States overlies the aquifer, which yields about 30% of the ground water used for irrigation in the United States. Since 1950, agricultural irrigation has reduced the saturated volume of the aquifer by an estimated 9%. Depletion is accelerating, with 2% lost between 2001 and 2009〔http://www.hpwd.com/aquifers/ogallala-aquifer〕 alone. Once depleted, the aquifer will take over 6,000 years to replenish naturally through rainfall. The aquifer system supplies drinking water to 82% of the 2.3 million people (1990 census) who live within the boundaries of the High Plains study area. ==General characteristics== The deposition of aquifer material dates back 2 to 6 million years, from the late Miocene to early Pliocene ages when the southern Rocky Mountains were still tectonically active. 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-bearing Ogallala Formation. 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. The depth of the Ogallala varies with the shape of then-prevailing surface, being deepest where it fills ancient valleys and channels. The Ogallala Formation consists mostly of coarse sedimentary rocks in its deeper sections, which transition upward into finer-grained material.〔(North Plains Groundwater Conservation District )〕 The water-saturated thickness of the Ogallala Formation ranges from a few feet to more than 1000 feet (300 m) and is generally greater in the Northern Plains.〔(High Plains Underground Water Conservation District #1 (Texas) ) retrieved April 9, 2007.〕 The depth of the water below the surface of the land ranges from almost in parts of the north to between throughout much of the south. Present-day recharge of the aquifer with fresh water occurs at an exceedingly slow rate, suggesting that much of the water in its pore spaces is paleowater, dating back to the most recent ice age and probably earlier. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ogallala Aquifer」の詳細全文を読む スポンサード リンク
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