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12 <br />IRRIGATION <br />Irrigation from river <br />diversions has raised <br />the water table within <br />the irrigated areas <br />and near canals and <br />reservoirs. <br />Irrigation by wells has <br />tended to lower water <br />tables or at least <br />reduce the flow <br />toward the river. <br />Using surface water to irrigate contributes to the ground <br />water whereas using ground water to irrigate depletes the <br />ground water. Up to 70 percent of the surface water brought <br />in for irrigation can end up percolating down to the ground <br />water table through canal and ditch losses and deep <br />percolation. Conversely, using ground water to irrigate <br />lowers ground water levels by the amount of water <br />consumed by plants and evaporation. <br />Irrigation water in the Platte Valley comes from both <br />ground and surface water. Often, conjunctive use systems <br />supplement surface water with ground water. The <br />intermixing probably tends to stabilize the water table. <br />Ground water irrigation magnifies the normal pattern of <br />lower water tables in dry years and higher water tables in wet years. In dry years, there is <br />higher demand on ground water and, therefore, more pumpage; while in wet years, the <br />percentage of deep percolation is greater adding to the ground water recharge. Long- <br />term intensive pumping will lower the water table locally, but if pumping is discontinued <br />or reduced, the water table will usually recover over time. <br />Acreage irrigated from ground water in the Central Platte Natural Resource District has <br />increased each year since 1950 and has increased an average of one percent a year for the <br />last 10 years. Figure 6 tracks the increases in irrigation wells and acres under irrigation <br />for the Central Platte Natural Resources District (Woodward, 2000, personal <br />communication). At the same time, conservation methods may have reduced the amount <br />of ground water pumping. The State Natural Resources Commission home page lists <br />numerous ongoing programs designed to retard surface runoff and increase ground water <br />storage and states that "As a result of these efforts, over 160 irrigators are now pumping <br />4.2 billion gallons [nearly 13,000 acre -feet] less water ..." (Hinrichs, et al., 2000). <br />Conservation practices such as using low energy precision application sprinkler systems <br />and modified tillage methods decrease net depletion by 7 and 24 percent, respectively <br />(Boldt et al., 1998). These conservation practices are being implemented by farm <br />operators in response to environmental and economic pressures. <br />While no on -site data have been collected that would support a definitive analysis of how <br />effective the conservation practices have been, a total savings of one percent or more per <br />year over the last 10 years seems well within the realm of possibility. Therefore, even as <br />Ground Water and River Flow Analyses <br />I J <br />1 <br />Ll <br />FJ <br />