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t <br />1 <br />1 <br />In addition, the effect of forest density changes on summer <br />evapotranspiration has also been better defined than in earlier models <br />(Troendle 1987; Kaufmann et al. 1987). Partial cutting or thinning can <br />result in reduced transpiration in the summer; however, the efficiency in <br />delivering an increase in flow to the stream channel is a function of seasonal <br />precipitation. In average or wet years, reduced basal area resulted in less <br />soil water depletion and an increase in water available for stream flow; <br />however, in below average precipitation years, the residual vegetation used <br />the available water. In all cases, water use per unit of residual basal area or <br />leaf area index increased dramatically; as a result, total water use by the <br />remaining vegetation increased. Observations at the plot level support and <br />help interpret the responses observed in the watershed experiments. <br />Since the early 1980's, process studies have documented much more about <br />the interaction between forest canopy and snow pack accumulation. Small <br />patch clear -cuts are no longer considered the only long -term harvesting <br />practice for water yield augmentation. This improved understanding has <br />been incorporated in a revision of the sub alpine water balance model <br />WATBAL so that we may now simulate evapotranspiration changes, <br />summer and winter, and project water yields as a function of any forested <br />condition or silvicultural activity. Input data for the revised WATBAL <br />model include slope, elevation, aspect, average precipitation, and basal area <br />by species for the forested area being modeled (Shepperd, et al. 1991 and <br />Troendle 1991). <br />As Leaf (1999) noted, the technology does exist to increase water yield <br />through timber harvest and as he infers, the corollary exists that increasing <br />forest density to the level of complete hydrologic utilization for the site will <br />result in flow reduction (Troendle and Leaf 1980). Shepperd et al. (1991) <br />simulated that in a "no harvest" alternative of mature stands of either <br />lodgepole pine or spruce -fir that represented stands already at complete <br />hydrologic utilization, long -term water yield projections were largely <br />unaltered because forest succession caused species composition and <br />structure to change with minimal change in either stand density or water <br />yield. Assuming the acreage of the landscape that is in forest is not altered, <br />one would not expect to observe changes in flow at the streamgage as a <br />result of already mature forest stands succeeding to their climax state <br />(Shepperd et al. 1991). <br />13 <br />