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Sub-Surface Water – Western United States

“…Water supplies fall into two basic categories: surface water and groundwater. Surface water is the water that exists in streams, rivers, or lakes. When rain falls on the ground or when snow melts, much of this precipitation drains across the surface of the earth and collects in ravines, streams, and creeks. Gradually these smaller waterways join together and form rivers. A group of rivers and the streams that feed into them make up a river basin, also known as a watershed. A river basin denotes all land through which a particular river and its tributaries flow. With the exception of a few geographical areas, such as the Great Basin, the ultimate destination of surface water is an ocean. Most people think of surface water when they hear the term water resources, and the history section of this article primarily focuses on this type of water supply.

However, when precipitation falls to the ground, not all the water runs into rivers as surface flow. Some precipitation becomes groundwater after seeping down into the earth and collecting within underground reservoirs called aquifers. Some aquifers amass underground seepage during thousands, sometimes millions, of years and accumulate huge quantities of water. In many parts of the West, aquifers are important components of the regional water supply. This is especially true in the Great Plains east of the Rocky Mountains, where many farms pump water to use for irrigation from a vast underground reservoir known as the Ogallala Aquifer…” [From MSN Encarta, Water Policy in the American West]

The Ogallala Aquifer

The Qgallala Aquifer is a vast water-saturated layer of sand and gravel over 300 feet thick in places and covering over 174,000 square miles. It lies between 50 and 300 feet below the surface and contains over 3 billion acre feet or 9.78 trillion gallons of water (Opie, 2000). The Ogallala began forming 70 to 50 million years ago during the Laramide orogeny, a tectonic event causing uplift to the west , producing the present-day Rocky Mountains. Rivers from these newly formed mountains washed vast amounts of permeable sediments eastward overlaying impermeable limestone and shale deposited by earlier shallow inland seas. (Kuzelka and others 1993). Ancient rivers continued to fill the aquifer until glaciations, the most recent occuring between 25 to 10 thousand years ago, caused the rivers to flow in a more southerly direction. (Opie, 2000). Today there is very little recharge and most of the water withdrawn from wells tapping the Ogallala will never be replaced by precipitation infiltration down to the saturated layer.

The Ogallala underlies parts of seven states: South Dakota, Wyoming, Nebraska, Colorado, Kansas, Oklahoma, New Mexico, and Texas. (http://www.tribasinnrd.org/highplaindaquafir.html)

Nebraska has 65% of the Ogallala aquifer’s volume, Texas 12%, Kansas 10%. (Kromm and White. 1992). Early settlers were lured to Nebraska by the promise of abundant rain , but the homesteaders realized fairly soon that rain was not going to follow the plow and irrigation projects were organized. These early irrigation projects exploited surface sources of water. Conflicts and legal disputes were common. Well digging was a dirty and dangerous business and wells were limited to relatively shallow depths both because of the problems of drilling and the power needed to bring the water to the surface. The droughts of the 1930s created more pressure to irrigate drylands and rural electrification provided the power to exploit water deep underground. The numbers of wells in the state increased exponentially (http://snr.unl.edu/information/GroundwaterMaps.asp).

Nebraska is now second only to California in the number of irrigated acres of farmland. While declines in groundwater levels in Nebraska have not been as rapid as in some states, there still is significant depletion (http://snr5.unl.edu/csd-esic/GWMapArchives/2008GWMaps/Pred-2008.pdf). Besides declines in groundwater levels contamination by nitrates and agrochemicals is a concern. The state has established natural resource districts and wellhead protection areas to control use and degradation of both surface and groundwater. A number of watersheds are considered to be fully appropriated with no new permits for irrigation wells being issued. Farmers, ranchers, and feedlot operators continue to seek ways to make wise use of the Ogallala innovative technologies.