Management and Disturbance Effects on Water Yeild

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V .a f occurred on a shorter rotation on the PSI National Forest than for the other two Forests. This did not seem to be a problem because the basic agelbasal area data representative of the PSI National Forest meshed so well with data for the other Forests; however, it does imply that what are considered fully stocked, hydrologically mature stands on the PSI National Forest are generally younger and less dense than on the other two Forests. Since energy is greater and precipitation is less on the PSI National Forest than on the other two Forests, the conclusion that younger, less dense, stands utilize all the precipitation (complete hydrologic utilization) appears reasonable. Of greater significance is the fact that the difference between the average age, and therefore basal area, of pole stands (stands averaging between 4.9 and 9.0 inches DBH) and sawtimber stands (stands averaging over 9.0 inches DBH) is quite small relative to the other two Forests-only about 10 years. Using a 20-year increment resulted in many stands on the PSI National Forest to initiate in the sawtimber class and not be classed as sawtimber again during the 1860 to 1997 period. This is an artifact of using a 20-year increment rather than a 5- or 10-year increment. The basal area estimates calculated for each time increment are the actual metric used to drive the hydrologic model, and the basal area estimates are continuous and do not reflect this bias. As observed in earlier analyses, more monitoring data are needed to improve definition of the relationship between size class, stand age, and basal area and to characterize current forest conditions more robustly. Along with improved GIS capability, the capability to type map specie composition, size class, and other individual or unique vegetation characteristics is improving rapidly. However, the development of adequate inventory data, or the necessary ground truthing, that is critical to characterizing what is contained on the individual mapped polygons has not kept pace with technology. This linkage is still inadequate. The ability to characterize current (or mapped) forest conditions is critical to studies like this as well as any landscape assessment. That said, the inventory data used in developing the age/basal area prediction equations appear to be quite consistent across all Forests and represents the best information available. Additional age/basal area data were provided using the newer Forest Inventory and Assessment (FIA) database, where available. Unfortunately, no apparent relationship between age and basal area seemed to exist in the FIA data. Also, basal area estimates derived from the FIA data, particularly for older stands, were quite low relative to any of the R2 VEG data; so combining data to increase sample size was not possible. FIA data were not used because a different protocol was used and the data were inconsistent with those used in this study. We strongly recommend that the disparity between the two data sets be evaluated further. Specie specific age/basal area relationships were developed for bristlecone pine, aspen (cottonwood was aggregated with aspen), Douglas fir, limber pine, lodgepole pine, ponderosa pine, Rocky Mountain juniper, and spruce-fir. As in earlier efforts (Troendle and Nankervis 2000; Troendle et al. 2003), the initial size class designation was used to estimate the reference water yield (1997) and average basal area of all stands using the age/basal area relationship. For each 20-year interval from 2000 (ca 1997) to 1860, 20 was subtracted from what was the current age of the stand and the age/basal area relationship was used to calculate the basal area for that stand 20-years earlier. The process was reiterated in 20-year increments back to 1860. If the calculated age of an existing stand was discounted to age zero or non-stocked during that 140-year 7 2/2/2007