Estimating Additional Water Yield from Changes in Management

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In the snow zone of the Central Rockies, forest removal has been shown to reduce canopy interceptior.i losses in the winter months, resulting in greater - snow pack accumulation (Wi:(m and Dunford 1948; Dietrich and Nleiman 1974; Gary and Troendle 1982; Troendle and Meiman 1984; Potts, 1984; Gary and Watkins 1985; Troendle and King 1987; Meiman 1987; Schmidt and Troendle 1989; and Tr•oeridle and Reuss 1997). A similar reduc.tion in interception loss (E), as we;ll a.s reduced transpiration (T), occurs during the growing season following, harvest (Wilm and Dunford 1948; Tr-oendle 1987a; Troendle and Reuss 1997). The reduction in summer ET results in less soil-water depletion or.isite:, but it is only at the hillslope level that these wetter soils have, heretofo:re, lbeen demonstrated to result in an increase in either late season base flow, or'summer storm response (Troendle and. Reuss 1997). This lack of derrnon"trated, late-season stream flow respanse to timber harvest is a reflection of the limited precipitation, causing t:he sub alpine forest to be water-limited in the late summer. The elevated soil- moisture levels in the harve;sted area, although not generally a demonstrated factor in influencing curre,nt season runoff, do play a significant role in response during the next snoAfmelt period. At that time, less melt-water is needed onsite to recharge the :>oil and excess melt-water becomes available for stream flow sooner (Troen.dle and King 1987; Troendle 1987b). As a result, changes in flow res'ulting from forest disturbance in the snow zone -- have always occurred on tlze rzsing side of the hydrograph, or early in the runoff season. In all snoiv zone studies, monthly flow change has been observed to consistently oc;cur only in May and sometimes in June during snowmelt runoff (Troendle et at. 1998) with no detectable change duri.ng the balance of the runoff season. In addition, the lar est increases in seasonal flow, following rimber hai-ves,t, occur during the wettest years while the srr.?allest increases in seasonal flow are usuall associated with the drier _years (Troendle and Leaf 19M; roendle and King 1985, 1987; Troend t al. 1998). T hese two factors r.nan date t hat a dequate storage be avai lab le to make the increases in yield ava.ilable when needed such as during periods of low flow. In contrast, the slow- growth rate of sub alpine vegetation makes hydrologic recovery following timber harvest, or the return to pre-riarvest flow levels, quite slow (Traenclle and King 1985; Shepperd et al. 1991) and makes the efficiency and cos1; effectiveness of water yield augmeritation seem quite attractive. The first "paired" watersheci sriidy that looked at the effect of timber haivest on water yield occurred on the headwaters of the Rio Grande River at Wagon Wheel Gap, CO (Bates and Henry 1928). Stream flow from two -- 3