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' <br />� <br />' <br />' <br />' <br />� <br />' <br />II <br />1 <br />, <br />' <br />' <br />' <br />, <br />' <br />, <br />� <br />� <br />4.0 DATA CONCERNING EXTENT OF GROUND WATER RECHARGE <br />4.1 Chan�e In Unconfined Storage In Western Portion of Closed Basin <br />In the San Luis Valley, the largest example of a ground water aquifer being used as a reservoir is <br />located north of the Rio Grande in the intensively farmed area outlined on the map included as <br />Figure 4-1. Into this area, several hundred thousand acre feet of water from the Rio Grande is <br />diverted through major canal systems that were built during the 1880's and 1890's. Before <br />construction and use of the canals, depth to ground water near the center of the outlined area was <br />reported to be 50 to 100 feet (Powell, 1958, Pgs. 56-57). After several years of diversion of <br />irrigation water into this area and resulting seepage into the ground water, levels were raised in <br />most azeas to less than 15 feet below ground surface. Raising the ground water table is <br />essentially the same as filling a reservoir. <br />During the drought of the 1950's, many wells were drilled into this shallow unconfined aquifer. <br />Water was withdrawn with pumps and used to supplement river water for irrigation. This was <br />the first major withdrawal of water from this underground reservoir. This artificially filled <br />aquifer had become a functioning reservoir that could be filled with seepage from canals and <br />flood irrigation, then withdrawn through wells during periods of river water shortages. <br />To better understand the continuing change in ground water levels in the most active portion of <br />the Closed Basin, a calculation and charting of the approximate change in shallow ground water <br />or unconfined aquifer storage has been performed since about 1981 by the author. This process <br />included calculating the annual changes in ground water storage based on approximately 26 Rio <br />Grande Water Conservation District (RGWCD) monitoring wells located in the highly irrigated <br />area shown on Figure 4-1. The dark triangles on the map in Figure 4-1 represent locations of <br />monitoring wells in which depth to water level below ground surface has been measured. The <br />study area encompasses approximately 335,000 acres. The period of the calculating and charting <br />4-1 <br />� <br />