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Table 4 indicates groundwater comprises a significant proportion of the irrigation water <br />use nationwide. A moderate decreasing trend is evident in the total irrigation water use <br />and reliance on groundwater, which may be attributed in part to reduced demand during <br />this period of relatively normal to wet conditions. Although the estimated total irrigated <br />land increased from 54 to 58 million acres during 1975 to 1995, it appears that the annual <br />rates of groundwater extraction decreased during this period. <br />Looking back further, figure 1 indicates annual groundwater use rates peaked at around <br />1975. This coincides with the application of advanced center -pivot pumping technology <br />from the 1950's through the mid- 1970's (Green, 1992). The decline in usage since 1975 <br />could suggest physical or economic limitations to groundwater use resulting in increased <br />irrigation efficiency or a shift to surface water. However, these trends do not reveal the <br />net balance with natural recharge or potential impacts of extended drought. <br />Figure 1. Annual Groundwater Use in the United States, 1950 -95 <br />LL 100 <br />c 90 <br />° 80 <br />2 70 <br />a� 60 <br />50 <br />40 <br />3 30 <br />20 <br />10 <br />2 0 <br />C7 <br />1950 1960 1970 <br />1980 1990 <br />- �— Total Groundwater Withdrawn <br />- -- Groundwater Used for Irrigation <br />Source: USGS, 1973; 1977; 1981; 1988; 1993; 1998; and EPA, 1987. <br />This information illustrates how important groundwater resources are to support irrigated <br />agriculture, particularly in the High Plains region, where up to 50 percent of the irrigation <br />water is supplied by groundwater. Nationwide, groundwater provides 30 to 40 percent of <br />the irrigation water used. Moreover, nearly 80 percent of all irrigated lands are located in <br />the 17 Western States where alternative surface water sources are very limited and often <br />over - appropriated. Groundwater is clearly a critical resource for the national agricultural <br />production and the corresponding local and regional economies. <br />Recharge Processes <br />Natural recharge occurs when rainfall or surface water infiltrates down through surface <br />soils to reach the water table or deeper aquifer formations. Artificial recharge (planned <br />recharge) is a deliberate process that directs surface water into the ground by spreading <br />Program Summary Report Part 1— Overview, Results, and Findings 2.11 <br />