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<br /> <br />Although early irrigation systems made use <br />of surface water, the development of large-scale <br />sprinkler systems in recent decades has greatly <br />increased the use of ground water for irrigation for <br />several reasons: (1) A system of supply canals is <br />not needed, (2) ground water may be more readily <br />available than surface water, and (3) many types of <br />sprinkler systems can be used on irregular land <br />surfaces; the fields do not have to be as flat as <br />they do for gravity-flow, surface-water irrigation. <br /> <br />,~ <br />B <br />~) <br /> <br />I!P <br />~ <br /> <br /> <br />Sprinkler irrigation using ground water in <br />Nebraska. (Photograph by Les Sheffield.) <br /> <br />Whether ground water or surface water was used <br />first to irrigate land, it was not long before water <br />managers recognized that development of either <br />water resource could affect the other. This is <br />particularly true in many alluvial aquifers in arid <br />regions where much of the irrigated land is in <br />valleys. <br />Significant changes in water quality accom- <br />pany the movement of water through agricultural <br />fields. The water lost to evapotranspiration is rela- <br />tively pure; therefore, the chemicals that are left <br />behind precipitate as salts and accumulate in the <br />soil zone. These continue to increase as irrigation <br />continues, resulting in the dissolved-solids <br />concentration in the irrigation return flows being <br />significantly higher in some areas than that in the <br />original irrigation water. To prevent excessive <br />buildup of salts in the soil, irrigation water in <br />excess of the needs of the crops is required to <br />dissolve and flush out the salts and transport <br />them to the ground-water system. Where these <br />dissolved solids reach high concentrations, the <br />artificial recharge from irrigation return flow <br />can result in degradation of the quality of <br />ground water and, ultimately, the surface <br />water into which the ground water discharges. <br /> <br />"Whether ground water or surface water <br />was used first to irrigate land, it was not <br />long before water managers recognized <br />that development of either water <br />resource could affect the other" <br /> <br />60 <br />