Estimating Additional Water Yield from Changes in Management

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In addition, the effect af forest density changes on summer evapotranspiration has also t>een better defined than in earlier r.nodels (Troendle 1987; Kaufinann et` al. 1987). Partial cutting or thinnirig can result in reduced transpiration in the summer; however, the efficie:ncy in delivering an increase in flc?w to the stream channel is a function of seasonal precipitation. In average or miet years, reduced basal area resulted in less soil water depletion and an i:ncrease in water available for stream flow; however, in below average pre:cipitation years, the residual vegetation used the available water. In all c;ase;s, water use per unit of residual basal area or leaf area index increased cirar.natically; as a result, total water use by the remaining vegetation increaseci. Observations at the plot level support and help interpret the responses observed in the watershed experiments. Since the early 1980's, process studies have documented much more about the interaction between forest canopy and snow pack accumulation. Small patch clear-cuts are no longer considered the only long-term harvesting practice for water yield augmentation. This improved understandimg has been incorporated in, a revision of the sub alpine water balance model WATBAL so that we m.ay now simulate evapotranspiration changes, summer and winter, and pi•oj ect water yields as a function of any forested £ ' 1A condition or silvicultural activi t y. In put data for the revised WA.TBAL - model include slope, elevation, aspect, average precipitation, and basal area by species for the forested are;a being modeled (Shepperd, et al. 1991 and Troendle 1991). As Leaf (1999) noted, the technology does exist to increase watex yield through timber harvest and, as he infers, the corollary exists that inci•easing forest density to the level of cc?mplete hydrologic utilizarion for the site will result in flow reduction (Troe:ndle and Leaf 1980). Shepperd et al. (1991) simulated that in a"no Yiarvest" alternative of mature stands of either lodgepole pine or spruce-:fir that represented stands already at complete hydrologic utilization, loilg-term water yield projections were largely unaltered because forest succession caused species composition and structure to change with nzinimal change in either stand density or water yield. Assuming the acreage c?f the landscape that is in forest is not altered, one would not expect to obse:rve changes in flow at the streamgage as a result of already mature forf;st stands succeeding to their climax state (Shepperd et al. 1991). .. ? :13