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", .r r., ! ,,'... ::5 !";.: ) FRYINGP AN -ARKANSAS PROJECT 23 " after set forth-will not impair the future economic growth of the western slope, harm present water users, or create a risk in meeting the Lee Ferry obligation of the Colorado RivenYompact, IRRIGATION ~, 18, The main agricultural part of the eastern slope project area is in the semiarid zone of 11 to 16 inches of annual precipitation, Seventy to eighty-six percent falls during the April to October growing season. Dry farming i$ Dnd probably will continue to be practiced extensively. Livestock grazing on the ranges and in the forests is also an extensive €nterprise, However, both types of agriculture require large land areas, and dry farming particularly depends upon the vagaries of the weather, General cultivated agriculture and specialty high-value crops, many of which are required to stabilize the agricultural economy of the area, require more water than typical dryland crops. Irrigation is the only mell,ns of providing a dependable supply, 19. Early irrigation in the Arkansas Valley coincided with available stream runoff.' As ready markets developed, irrigation farming was expanded and It demand developed for late season water which could not be supplied by unregulated streamflow, Consequently, between 1890 and 1910, 3 reservoirs in the headwaters area and 11 off-stream reservoirs below Pueblo were constructed, In 1949 the John Martin Reservoir on the Arkansas River was completed by ,the Corps of Engineers for conservation storage and flood controt It also has an irrigation storage space of 420,000 acre-feet. The 3 headwaters reservoirs have a capacity of 84,400 acre-feet, The 11 off-stream reservoirs have a present capacity of 300,000 acre-feet which represents about 75 percent of the original capacity as a consequence of sedimen- tation. Eight privately owned transmountain diversion systems import about 48,000 acre-feet annually, 20, More th/Ln 40 canals and ditches supply irrigation water to lands in the valley between Canon City and the Colorado-Kansas iboul.1dary. SeQ.imentdeposition in canals. and ditches has become a major irrigation probtein in the Pueblo-Las Arrnnas reach, Insome instances long reservoir feeder canals have lost 50 percent of their capacity because of sedimentation, 21. The amo~nt of irrigation water available for the 322,000 acres of irrigated land in the project area varies considerably from year to year. Seldom is the supply adequate for maximum crop production. Irrigation water shortages as high as 78 percent of crop requirements have occurred, The estimated average canal headgate diversion re- quirement is 3.19 acre-feet an acre, Allowing for tolerable shortages, that headgate requirement can be reduced to 3.10 acre-feet. The average amount of seasonal irrigation water historically available between Pueblo and the Kansas State line has ranged from 0,9 acre- feet an acre in 1934 to 2.7 acre-feet in 1942. The base flow of every stream in the valley is overappropriated, Enhancement of the irn- gation water supply depends upon regulation of existing supplies for more efficient use, additional storage capacity for the conservation of excess flood flows, reservoir space for holdover storage, and new water supplies for whioh the only apparent source is transmountain diversion from the Colorado River drainage. o ~_ i.. ..; \'" _,.' ,v.~s. . ", " ...: ~..t:. . _~~r~' ''-'--; . ~'-~' .~':., ',;':' """"",,',r.;""~:~"{.''''>;~'~/,a ~,? .;:.~.~;,,;~;.:~;:,<:':<>..- .,/ ,: - -,,--,:~.":;. ...'....,. ",' ',e,'., _'.'" , .&;.... ~ J};