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t <br />h of i r h <br />In the snow zone o the Central Rockies, forest removal as been shown to <br />reduce canopy interception losses in the winter months, resulting in greater <br />snow pack accumulation (Wilm and Dunford 1948; Dietrich and Meiman <br />1974; Gary and Troendle 1982; Troendle and Kleiman 1984; Potts, 1984; <br />Gary and Watkins 1985; Troendle and King 1987; Meiman 1987; Schmidt <br />and Troendle 1989; and Troendle and Reuss 1997). A similar reduction in <br />interception loss (E), as well as reduced transpiration (T), occurs during the <br />growing season following harvest (Wilm and Dunford 1948; Troendle <br />1987a; Troendle and Reuss 1997). The reduction in summer ET results in <br />less soil -water depletion onsite, but it is only at the hillslope level that these <br />wetter soils have, heretofore, been demonstrated to result in an increase in <br />either late season base flow, or summer storm response (Troendle and Reuss <br />1997). This lack of demonstrated, late- season stream flow response to <br />timber harvest is a reflection of the limited precipitation, _causing the sub <br />alpine forest to be water - limited in the late summer. The elevated soil- <br />moisture levels in the harvested area, although not generally a demonstrated <br />factor in influencing current season runoff, do play a significant role in <br />response during the next snowmelt period. At that time, less melt -water is <br />needed onsite to recharge the soil and excess melt -water becomes available <br />for stream flow sooner (Troendle and King 1987; Troendle 1987b). As a <br />result, changes in flow resulting from forest disturbance in the snow zone <br />have always occurred on the rising side of the hydrograph, or early in the <br />runoff season. In all snow zone studies, monthly flow change has been <br />observed to consistently occur only in May and sometimes in June during <br />snowmelt runoff (Troendle et al. 1998) with no detectable change during the <br />balance of the runoff season. In addition, the largest increases in seasonal <br />flow, following timber harvest, occur during the wettest years while the <br />smallest increases in seasonal flow are usually associated with the drier <br />years (Troendle and Leaf 1980; Troendle and King 1985, 1987; Troendle et <br />al. 1998). These two factors mandate that adequate storage be available to <br />!!� make the increases in yield available when needed such as during periods of <br />Y gP <br />low flow. In contrast, the slow growth rate of sub alpine vegetation makes <br />hydrologic recovery following timber harvest, or the return to pre - harvest <br />flow levels, quite slow (Troendle and King 1985; Shepperd et al. 1991) and <br />makes the efficiency and cost effectiveness of water yield augmentation <br />seem quite attractive. <br />The first "paired" watershed study that looked at the effect of timber harvest <br />on water yield occurred on the headwaters of the Rio Grande River at <br />Wagon Wheel Gap, CO (Bates and Henry 1928). Stream flow from two <br />3 <br />