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<br />Studies have been made to determine streamflow <br />depletion and consumptive use of water for. the <br />pedod 1925-1935. The southwestern area of the <br />valley was chosen fm these studies by the Bureau <br />of Agricultural Engineering. <br />Generally, the area includes that beginning at <br />Del Norte and embracing the portion south and west <br />of the Rio Grande, extending virtually to the New <br />Mexico state line, the gwss area being aboutAOO,OOO <br />acres and the used area about 229,000 acres. <br />The streamflow depletion indicated 2.6 acre-feet <br />per acre irdgated in the wet year 1932, and 1.33 <br />acre-feet per acre in the extremely dry year 1934. <br />The average fm the period was 1.92 acre-feet per <br />acre irrigated. <br />The average consumpti ve use per acre irrigated <br />dudng the ll-year pedod 1925-1936 was 3.02 acre- . <br />feet per acre, and the average consumptive use per <br />acre of the entire tract was 1.66 acre-feet. <br />The range of the latter values is from 2.11 acre- <br />feet per acre in 1929 to LOS acre-feet per acre in <br />1934, a vel)' dl)' year. <br />Some studies of the entire valley have repmted <br />the consumptive use in acre-feet per acre irrigated <br />per year as 3.17, and the streamflow depletion per <br />acre irrigated per year as 2.31 acre-feet. <br />H is a pretty well established fact that little <br />change in the overall inigated acreage has taken <br />place in the San Luis Valley in the past 50 years, <br />as a study of the water commissioners' reports <br />will verify. <br />Tliere is strong evidence indicating that the San <br />Luis Valley as a whole is a "closed basin," as <br />would be confinned by the statements made here <br />relative to the confined artesian basins, the only <br />known outlet being the deep cut thmugh a solid wck <br />formation near the New Mexico state line. The latter <br />also is the outlet for the Rio Grande. <br />Within the valley is a tract of land known as the <br />'(Closed Basin," which lies north and east of a low <br />dike or impervious barrier which runs almost parallel <br />with and north of the Rio Grande from the Contin- <br />ental Divide to Mount Blanca, thus being effective <br />acwss the entire valley. <br />Water diverted to it from the Rio Grande flows <br />east toward the San Luis Lakes or the tlSump Area." <br />The Closed Basin comprises a total of approxi- <br />mately 350,000 acres with Some 200,000 acres ir- <br />rigated and fumishes the majm portion of land used <br />and adapted to subirrigation, as previously mentioned. <br />In the Rio Grande Compact the Closed Basin is <br />refened to as follows in Mticle 1 (d): "The 'Closed <br />Basin' means that part of the Rio Grande Basin in <br />Colorado where the streams drain into the San Luis <br />Lakes and adjacent territory, and do not normally <br />contfibute to the flow of the Rio Gfande." <br />Many studies have been made since 1910, by <br /> <br />12 <br /> <br />numerous engineers, to determine the feasibility <br />and pwbable water-yield costs of the "Sump Drain" <br />Project, designed to recover water from the Closed <br />Basin and to deli ver it to the river some miles below <br />Alamosa, such water to accrue to the credit of Colo- <br />rado, subject to its quality regaroing salinity. <br />Thus, a large part of the irdgated area of the <br />valley, the HClosed Basin," does not contribute <br />to the "return flow." <br /> <br />A study by the National Resources Committee <br />showed that the retum flow to the Rio Grande in <br />the San Luis Valley for the 3-year pefiod, 1934- <br />1936, averaged 17 percent of the total Rio Grande <br />Diversion or 36 percent of those diversions which <br />contribute return flow to the river (excluding diver- <br />sions to its Closed Basin). <br /> <br />A retum of 44 percent of diversions was indicated <br />on the Conejos River by data available for 1936. <br />The all-time, continuous,5-year dry pedod pre- <br />viously refened to, i. e., 1950-1954, has indicated <br />a very much lower percentage of return flow, which <br />has contdbuted largely to Colorado's current debit <br />under the Rio Grande Compact. <br />Reference has been made to the uniformity of the <br />topography of the valley flom, pwviding ideal natu- <br />ral irrigation conditions. However, the water users <br />have leamed fwm long experience that for most eco- <br />nomical use of water and greater crop yields, ac- <br />curate land leveling is justified. To indicate .Some- <br />thing of the extensiveness of land leveling in the <br />valley, the Soil Conservation Service reports that <br />about 130,000 acres have been leveled at an aver- <br />age cost of approximately $ 46.00 per acre, which <br />is a cost of 13 cents per cubic yard. <br /> <br />The growing season is comparatively short in--the <br />San Luis Valley owing to a mean altitude of about <br />7,500 feet; in Some years light fwsts have occuned <br />in every month. The average growing season is 9S <br />to 110 days; thus, only quick-maturing crops are <br />practical. <br /> <br />The principal crops produced in the valley are <br />small grains, alfalfa, native hay, sugar beets, po- <br />tatoes, and almost all garden truck, such as head <br />lettuce, spinach, cauliflower, cabbage, carrots, <br />garden peas, etc. In most cases yields are very <br />high and the quality excellent. <br /> <br />The gradual change over the last 50 years from <br />cereal crops to potatoes and alfalfa has increased <br />the demand per acre for water for irrigation. <br />The duty of water for the valley generally fm <br />various crops, it may be said, is very high, re- <br />quiring about 2 acre-feet per acre per year. Over <br />large areas, as has been mentioned, land leveling <br />has contributed to this economical use of water. <br />In the San Luis Valley the extreme dry pedod of <br />the last S years has compelled the farmers to re- <br />sort to use of the underground water by pumping <br />and has resulted in high cost of cwp pwduction. <br /> <br />~,: <br /> <br />-.;.. <br /> <br />:';: <br /> <br />~ <br /> <br />" <br /> <br />... <br /> <br /><:-: <br />e-' <br />;:;< <br />f1;; <br /> <br />/,!.,. <br />~ :-' <br />f~'~ <br /> <br />:"'J <br />i':~l <br /> <br />\~ <br />t!?~ <br /> <br />"n" <br /> <br />lJ0~:t;.8 <br />