Ogallala Aquifer

The Ogallala Aquifer, also known as the High Plains Aquifer, is a shallow water table aquifer located beneath the Great Plains in the United States. One of the world's largest aquifers, it lies under about 174,000 mi² (450,000 km²) in portions of the eight states of South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Texas. It was named in 1899 by N.H. Darton from its type locality near the town of Ogallala, Nebraska.

General characteristics

The deposition of the aquifer material dates back to late Miocene to early Pliocene age when the Rocky Mountains were being uplifted. As the land to the west rose, rivers and streams cut channels in a generally west to east or southeast direction. Erosion of the Rockies provided alluvial and eolian sediment that filled the ancient channels and eventually covered the entire area of the present-day aquifer, forming the water-bearing Ogallala formation. The depth of the formation varies with the shape of the pre-Ogallala surface, being deepest where it fills ancient valleys and channels.

The water-permeated thickness of the Ogallala formation ranges from a few feet to more than 525 feet (160 m) and is generally greater in the northern plains. The depth of the water below the surface of the land ranges from almost 400 feet (122 m) in parts of the north to between 100 to 200 feet (30 to 61 m) throughout much of the south. Present-day recharge of the aquifer with fresh water occurs at a slow rate; this implies that much of the water in its pore spaces is paleowater, dating back to the last ice age.

Aquifer water balance

Any aquifer, and here specifically the High Plains Aquifer, is a storage reservoir in the water cycle. The USGS has performed several studies of the aquifer, to determine what is coming in (groundwater recharge from the surface) what is leaving (pumping and baseflow to streams) and what the net changes in storage are (have water levels risen, fallen or stayed the same — see figure above). Simply put, water in, less the water out, is equal to the change in water stored in the aquifer. This type of mass-balance "accounting" is how hydrologic budgets are performed; a crucial first step in sustainable management of any natural resource.

Groundwater recharge

The rate at which recharge water is currently entering the aquifer is limited by several factors. Much of the plains region is semi-arid with steady winds that hasten evaporation of surface water and precipitation. In many locations, the aquifer is overlain, in the vadose zone, with a shallow layer of caliche that is practically impermeable; this limits the amount of water able to recharge the aquifer from the land surface. Many of the playa lakes on the plains are lined with caliche or clay that likewise limits the amount of water that seeps from them into the aquifer. The caliche is partly due to the ready evaporation of soil moisture, due to the semi-arid climate; the aridity increases the amount of evaporation which increases the amount of caliche in the soil, both mechanisms reinforce the difficulty recharge has in reaching the water table.

Groundwater discharge

The regions overlying the Ogallala aquifer are some of the most productive regions for ranching livestock and growing corn, wheat and soybeans in the United States (often called the "breadbasket of America"); The success of large-scale farming in areas which do not have adequate precipitation and do not always have perennial surface water for diversion, depends heavily on pumping groundwater for irrigation.

The aquifer was first tapped for irrigation in 1911. Large scale use for irrigation began in the 1930s and continued through the 1950s, due to the availability of electric power to rural farming communities and the development of cheap and efficient electric turbine pumps. Because the rate of extraction exceeds the rate of recharge, water level elevations are decreasing. At some places the water table was measured to drop more than five feet (1.5 m) per year at the time of maximum extraction. In extreme cases, the deepening of wells was required reach the steadily falling water table; and it has even been drained (dewatered) in some places.

Several of the rivers in the region, such as the Platte, in places are below the water level of the aquifer and therefore the rivers actually receive groundwater flow (baseflow) rather than supply recharge to the aquifer.

Change in groundwater storage

Water conservation practices (terracing and crop rotation), more efficient irrigation methods (center pivot and drip), and simply reduced acreage under irrigation have helped to slow depletion of the aquifer, but levels are generally still dropping, particularly in the southern parts. See the figure above for an illustration of the places where large drops in water level have been observed (i.e., the red areas in Texas and southern Kansas). In the more humid areas water levels have actually risen since 1980 (i.e., eastern and central Nebraska).

See also

• Hydrogeology
• Aquifer and Water table
• Groundwater

External links

USGS High Plains Regional Groundwater Study (http://co.water.usgs.gov/nawqa/hpgw/HPGW_home.html)