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' <br />' <br />� <br />' <br />' <br />, <br />, <br />' <br />' <br />� <br />' <br />' <br />, <br />' <br />' <br />, �J <br />' <br />� <br />begin in 1975 at which time the RGWCD constructed and began monitoring a sufficient number <br />of wells to make the process meaningful. <br />The method of computing the change in ground water storage was in accordance with the <br />Thiessen mean method whereby a polygon is constructed around each observation well, with the <br />assumption that the change in water level throughout the area of the polygon is the same as the <br />change in the well within the polygon. This method was used in Geological Survey <br />Water-Supply Paper 1379 by W. J. Powell. The chart of the change in unconfined aquifer <br />storage is included as Figure 4-2. The change in storage was plotted as zero at the beginning of <br />the study period in 1975, with monthly changes plotted thereafter. <br />A combination of drought and lack of attention to river water diverted into the area resulted in <br />significant depletion of the underground reservoir in the late 1970's, with the apparent low point <br />in 1978. When storage in this reservoir reached these very low levels and well pumps began <br />sucking air rather than water, the need to better manage diverted river water became apparent. <br />This better management included filling or recharge of the underground reservoir at historical <br />locations and times. Since 1977, knowledge and understanding of how filling of the reservoir or <br />recharge can be accomplished has improved significantly. During 1996, a severe drought <br />occurred and declines in the ground water levels occurred, but it does not appear that levels <br />similar to those experienced in the late 1970's aze likely. <br />4.2 ESTIMATES OF RECHARGE THROUGH CANAL AND DITCH COMPANIES <br />Since the west central portion of the Closed Basin, as outlined in Figure 4.1, is the most <br />utilized portion of the unconfined aquifer within the District, this area has been the focus of <br />estimates of recharge through canal and ditch companies. The primary sources of water to this <br />area is water diverted from the Rio Grande, carried by canals into the area, and natural <br />inflow. <br />The major canals and ditches that carry water into the subject area included Rio Grande, San <br />4-2 <br />' <br />