<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.
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