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I? <br />-- cover type influences quantity of runoff available (i.e., grass vs. deciduous trees) <br />-- evapotranspiration decrease varies with cover type <br />-- forest harvest location, elevation, and its distance from the outlet impact quantity and <br />quality of water yield <br />-- regional variation in geomorphology and hydrology impacts water yield <br />• Historic research suggests that removal of coniferous forests, deciduous hardwoods, brush <br />and grass cover have, respectively, a decreasing influence on water yield (Bosch and <br />Hewlett, 1982). <br />• Generally, changes in water yield from basins with less than 20% forest cover reduction <br />cannot be determined. Thus, approximately 20% of the watershed cover must be harvested <br />for a measurable increase in annual water yield (Stednick, 1996). However, this projection <br />guideline might not be applicable to watersheds larger than 3-5 mi2. <br />• Ponderosa Pine -- Brown et al. (1974) noted increases in streamflow ranging from <br />approximately 2 to more than 14 cm. after clearcutting and thinning experiments in <br />ponderosa pine forests near Beaver Creek, Arizona. In addition, water yield increases of 1 to <br />2 inches per year were found to be a result of various forms of timber harvest. Hibbert <br />(1979) estimated that within 6 million acres of managed ponderosa pine forest, there was a <br />potential to increase annual water yield by 1-3 inches in the Lower Colorado Basin. <br />Research in the Black Hills (Orr, 1975, Leaf, 1975) indicates that patchcutting and thinning <br />of ponderosa pine stands produces increases in water yields, though actual amounts are <br />unknown. <br />• Mixed Conifer -- Hibbert (1979) ascertained that if a11391,000 acres of mixed conifer were <br />converted to grass, approximately 100,000 acre-feet of additional water would be made <br />available to the Lower Colorado River Basin. In another study, Hibbert (1981) found that <br />of the 2.4 million acres of mixed conifer forest in the Southwest (Arizona, New Mexico, and <br />parts of Colorado, Texas, Utah, Nevada, and California), the potential water yield increase <br />was 3-4 inches (reflecting multiple use considerations). Schmidt and Solomon (1981) <br />estimated that mixed conifer offered an increase of only 7,000 acre-feet in additional water <br />yield due to limited extent of available (non-wilderness) and opperable lands. <br />• Chaparral -- In studies conducted in Arizona, water yield increased after chaparral was <br />converted to grass through rootplowing, chemical treatment, and controlled burns (Hibbert <br />et al., 1974). When combined, the catchments averaged a 28% increase* (inches?), or <br />approximately an extra 1/3 inch of water annually. The response of these sites was heavily <br />dependent on soil type, natural vegetation and precipitation. In another study, Hibbert