Estimating Additional Water Yield from Changes in Management

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?harvest, Hoover and Leaf (1967) suggested that snow intercepted in the adjacent canopy was redistributed into clear-cut blocks by wind, with little or no evaporation loss. Hoover and Leaf (1967), Leaf and Brink (1973) and later Troendle and Leaf (1980) assumed redistribution of snow, rather than reduced interception loss, was credited for the increase in snow pack in forest openings. But, with 18 years of additional record, Troendle and King (1985) found there was an overall increase of 9 percent in peak water equivalent on the Fool Creek watershed (significant at the one percent level). This finding supported the original explanation (Wilm and Dunford 1948) that reducing interception loss is an important factor in manipularing snowmelt runoff by harvesting timber. The 9 percent increase at the watershed level supported the argument that the average 30 percent increase in the openings reflected an interception reduction rather than redistriburion from adjacent forest (Troendle and King 1985). Subsequent efforts to further quantify the roles of interception loss and wind redistribution on snow pack accumulation include a study by Wheeler (1987) comparing snowfall in a clearing with snowfall in the surrounding forest. Wheeler (1987) frequently measured accumulation on a grid of snowboards and determined that most of the increase in accumularion in the opening occurred during storms (confirming earlier findings of Troendle and Meiman 1984, 1986). Between storms, observarions indicated little snow was added to the clearing, from the adjacent forest, by wind (Wheeler 1987). The magnitude of the forest-to-clearing difference in accumulation decreased, however, with increasing wind speed during storms indicating that interception and subsequent evaporation losses are reduced with increased wind speed during the event. In summarizing the effects of partial cutting on snow pack accumulation, Troendle (19$7) developed a linear relarionship between increases in snow pack accumulation and basal area (reduction) of the stand. On average, maximum interceprion loss is reached at a basal area approaching 100 ft2 acre''. Based on the results of eight separate experiments spanning nearly 50 years of study, and including observations in the watershed experiments mentioned earlier,,Troendle (1987) also developed a relationship between percent basal area removed and percent increase in snow pack accumulation. This general relationship has since been partitioned into responses specific to north, south, or east and west slopes (see Schmidt and Troendle 1989; ? .. . . " Schmidt et al. 1998; Troendle et al. 1991). 12