Management and Disturbance Effects on Water Yeild

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effort. All non-stocked, seedling, and sapling stands were combined into the youngest age group, and this grouping consisted of all stands with vegetation less than 4.9 inches in diameter at breast height (DBH). The second size class was poles and consisted of stands where the average DBH ranged from 4.9 to 9.0 inches. The third size class consisted of sawtimber stands with an average DBH greater than 9 inches. The GIS was used to overlay each original stand polygon with topographic aspect and precipitation. This overlay procedure created a new, composite, GIS map depicting all the logical combinations of stand polygons, aspect, and precipitation. The definition of aspect consisted of assigning one of the four cardinal directions to each resulting polygon. In a later lumping process, polygons with either an east- or west-aspect designation were considered similar in the modeling process. Monthly precipitation was determined using the Oregon 5tate climatic maps for either Colorado or Wyoming. This process caused the number of unique polygons to increase dramatically. Once done, however, like polygons (identical species, basal area, aspect, and seasonal precipitation) were then aggregated, by Forest, to reduce sample size and simplify or expedite various aspects of the modeling process. After characterization of current stand conditions was completed and all unique polygons were identified, the current stand condition (size class) was altered using inventory derived age/basal area relationships to reflect how the stand may have evolved, or developed, over time. Inventory data provided by the Forests were used to develop age/basal area relationships for each primary tree species. The age/basal area inventory data provided for the AR and MBR National Forests are the same as those used in the earlier analyses (Troendle and Nankervis 2000; Troendle et al. 2003) and included some data representative of the Route National Forest. Age/basal area data for stands on the PSI National Forest were added to the original data set, and the equations used in the earlier analyses were re-calculated. As was determined in the earlier analyses, a single equation for each species best represented the age/basal area relationships for stands on all NFS land in the South Platte River Basin. Virtually no change occurred in either the slope or the intercept of the equations derived earlier when the data for the PSI National Forest was added to the data set and the models re-fit (see Figures 3-7). Using the age/basal area relationships, the relative age and basal area of all current stands (ca year 2000, the surrogate for the reference year 1997) was then calculated. Using the same relationships, it was possible to go forward or backward in 20-year increments, and estimate a new basal area and size class appropriate to the adjusted age for each stand, or polygon. As the stands were moved forward or backward in time and new age and basal area were determined, the size class may also have changed as the stands passed through the midpoint between the average age of the differing size classes (see Figures 3-7). The estimated average ages, and therefore basal areas, for the three size classes (particularly sawtimber) tended to be lower on the PSI National Forest than for other Forests although the age/basal area data were consistent across Forests (see for example Figures 3-7). This created a slight problem in the hydrologic simulations because a lower age and basal area for sawtimber stands at "complete hydrologic utilization" (see Troendle and Leaf 1980) had to be defined for the PSI National Forest than was used for the other Forests. Because hydrologically mature stands were generally younger and less dense on the PSI National Forest, the temporal cycle from non-stocked to sawtimber 6 2/2f2007