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I -, n .....{ CJ) "~ ~ C'J y~, C:.l i,; (,~~~\ ~a ';~ ~;i U ji;~) ~ I r 5; ~ ~~J ~ t " r. "'. t I j [) ~ t f I I ~. ;~ , , :~ l .,' ~' V . "-~"__- -,_ "", ,._ _. ; c.'."':'. - ; A check was nade of the type map in the report at various intervals on the ground above M::mte Vista against randomly selected aerial photos. 'Ihe napping done then is still ac=ate as related to the area of cottonwood - called Trees, bosque in their report.) Reliable figures on water use of cottonwoods are difficult to establish. There is a wide variation in infonnation available. Fran a study by the u. S. Geological Survey 1/ - Stands of 100 percent crown cover of cottonwoods and willeMs in the Safford Valley of Arizona along the Gila River were found to use fran 7.64 to 8.1 acre-feet per acre of water annually. Assuming the crown cover is about 50 percent, and the use of water is about 40 percent of that found in the Safford Valley, the con- sunptive use of water by cottonwoods in the San Luis Valley is about 38,500 a=e-feet annually. This arrounts to about 1. 54 acre-feet per acre annual average consunptive use of water by these cottonwood stands. In a preliminmy report of water use by phreatophytes in selected portions of the Rio Grande, Conejos, and San Antonio Rivers by M. W. Bittinger and Clayton Enix 2/ it was estimated that dense stands of phreatophytes use 23.7 a=e-inches per acre per year and light stands of phreatophytes use 15.8 a=e-inches per acre. The results of reducing dense stands to light stands would be about eight a=e-inches per acre. For the 25,000 acres of cottonwoods, the maximum savings would be about 16,500 a=e-feet annually. There is little validi t:y to the assunption that eliminating cottonwoods, regardless of a=eage, would increase the total water yield in the river, since other vegetation would naturally replace any vegetation rerroved. Along the Gila River in the Safford Valley of Arizona, cottonwoods and willavs were bull- dozed out between 1935 and 1936. Within three or four years salt cedar took over the entire river bottan and became a nore serious problem. Representatives of the Colorado State Forest Service and SCS estimated plots to cut from zero to 7,000 board feet per acre. Assuming an average of 2,500 board feet per a=e, there should be a total of about 62-1/2 million board feet of usable logs, over 12 inches in diameter, breast high. Because of the age and size classes available, it appears that about 50 percent of the volmre could be rerroved fran nost of these cottonwood stands, 1/ See reference 22 Y See reference 16 17