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' <br />' <br />ll <br />� <br />1 <br />' <br />, <br />' <br />J <br />' <br />L_J <br />��i <br />I' � <br />� <br />' <br />' <br />' <br />' <br />Basin from the west, in addition to recharge of the unconfined aquifer, portions are lost to <br />evaporation and used to recharge the deeper confined aquifer and directly irrigate crops. <br />Through extensive investigations of canal and ditch company records by Harlan W. Erker in <br />1978, he concluded that recharge to the unconfined aquifer in the Closed Basin under the San <br />Luis Valley, Rio Grande, San Luis Valley Canals and the Prairie and Billings Ditches averaged <br />140,000 acre feet per year. <br />Since the vast majority of water for irrigated acres in the referenced study area is obtained from <br />wells and applied through center-pivot sprinklers, an estimate of irrigation demand for these <br />sprinklers will be approximately equal to recharge needed to sustain to the shallow aquifer. This <br />assumes that there is no long term change in ground water storage. To estimate total irrigation <br />demand or consumptive use for these sprinklers, it is necessary to assume an average crop <br />consumptive use and obtain a sprinkler count together with average area irrigated per sprinkler . <br />A crop consumptive use of 1.3 acre feet of water per irrigated acre was used in the referenced <br />report by Erker and subsequent information support this value. From a map of sprinkler <br />locations prepared by Rio Grande Water Conservation District, the number of sprinklers in the <br />study area is estimated at 1283. Estimated area irrigated per sprinkler is 130 acres. The estimate <br />of required ground water recharge in the study area is (1283 sprinklers x 130 acres/sprinkler x 1.3 <br />acre feet/ acre) = 216,827 acre feet per year. By comparing the crop consumptive use in the <br />study area of 216,827 acre feet per year to Erker's recharge estimate of 140,000 acre feet per <br />year, it appears that either Erker's estimate is quite low or recharge enters the area from other <br />unmeasured sources. <br />An effort to measure total recharge in the study area from ditches, canals and recharge structures <br />was made, however it became apparent that the accuracy of such a measurement would be <br />questionable. Although numerous canal/ditch loss measurements were made, it became apparent <br />that loss values were highly variable. As a minimum, water loss through seepage from <br />ditch/canals vary due to the following factors: <br />4-4 <br />' <br />