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<br /> <br />IRRIGATION SYSTEMS <br /> <br />Surface-water irrigation systems represent <br />some of the largest integrated engineering works <br />undertaken by humans. The number of these <br />systems greatly increased in the western United <br />States in the late 1840s. In addition to dams on <br />streams, surface-water irrigation systems include <br />(1) a complex network of canals of varying size <br />and carrying capacity that transport water, in <br />many cases for a considerable distance, from a <br />surface-water source to individual fields, and <br />(2) a drainage system to carry away water not <br />used by plants that may be as extensive and <br />complex as the supply system. The drainage <br />system may include underground tile drains. <br />Many irrigation systems that initially used only <br />surface water now also use ground water. The <br />pumped ground water commonly is used directly <br />as irrigation water, but in some cases the water is <br />distributed through the system of canals. <br />Average quantities of applied water range <br />from several inches to 20 or more inches of water <br />per year, depending on local conditions, over the <br /> <br /> <br />Gravity irrigation using surface water in <br />Nebraska. (Photograph by Les Sheffield.) <br /> <br />entire area of crops. In many irrigated areas, about <br />75 to 85 percent of the applied water is lost to <br />evapotranspiration and retained in the crops <br />(referred to as consumptive use). The remainder of <br />the water either infiltrates through the soil zone to <br />recharge ground water or it returns to a local <br />surface-water body through the drainage system <br />(referred to as irrigation return flow). The quantity <br />of irrigation water that recharges ground water <br />usually is large relative to recharge from precipita- <br />tion because large irrigation systems commonly <br />are in regions of low precipitation and low natural <br />recharge. As a result, this large volume of artificial <br />recharge can cause the water table to rise (see <br />Box N), possibly reaching the land surface <br />in some areas and waterlogging the fields. For this <br />reason, drainage systems that maintain the level of <br />the water table below the root zone of the crops, <br />generally 4 to 5 feet below the land surface, are an <br />essential component of some irrigation systems. <br />The permanent rise in the water table that is main- <br />tained by confinued recharge from irrigation <br />return flow commonly results in an increased <br />outflow of shallow ground water to surface-water <br />bodies downgradient from the irrigafed area. <br /> <br />57 <br />