The High Plains area of central USA stretches from the Rio Grand River on the south to the Canadian border on the north, and from the 'humid prairie plains' (Kromm and White, 1992, p. 1) on the east to the Rocky mountains on the west. The imprecise eastern boundary runs generally along the eastern portions of the tier of states extending from North Dakota to Texas on the south. It is generally a level, treeless, grassland surface, except along watercourses, with a windy and subhumid climate (Kromm and White, 1992, p. 1). The High Plains aquifer system, including the Ogallala and Equus Beds, is the 'largest underground reservoir in the country' (Kromm and White, 1992, p. 3; Sophocleous, 2005), contains approximately 4000 km3 of water and underlies parts of eight states. As shown in Fig. 14.1, the 570,000 km2 aquifer mostly underlies parts of three states: Nebraska has 65% of the aquifer's volume, Texas 12% and Kansas 10% (Kromm and White, 1992, p. 15). Yet, due to the varying thickness in the aquifer, only 37% of Nebraska overlies the aquifer. The saturated thickness can range from 365 m (Kromm and White, 1992, p. 16) to less than 1 m (Groundwater Atlas, 1995, p. 4). Recharge of the aquifer through precipitation is slight, ranging from a high of 15.25 cm annually to a low of 0.06 cm (Kromm and White, 1992, p. 16).
The total population in the High Plains aquifer region has hovered around 2 million since 1960, with small growth areas in some states, but with overall declines showing up in the past few years. With approximately 3.5 people/ km2, this region is very sparsely populated. The High Plains aquifer provides 30% of the groundwater used for irrigation in the USA, and approximately
©CAB International 2007. The Agricultural Groundwater Revolution: Opportunities and Threats to Development (M. Giordano and K.G. Villholth)
20% of the irrigated land in the USA is located in this region (Sophocleous, 2005, pp. 352-353). Agriculture dominates the economy, with virtually all land in some form of agricultural use and with the related agribusiness devoted to seed, fertilizer, pesticide, herbicide, machinery and credit (Kromm and White, 1992, pp. 17, 20). The main irrigated crops are: maize, wheat, sorghum and cotton; others include lucerne, potatoes, vegetables, soybeans and pinto beans. Centre pivot irrigation systems, typically covering circles of 52.6 ha on a square field of 64.8 ha, are common. Farm sizes typically range from 1300 to 2300 ha.
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