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30 <br /> 24 <br />0 <br />°0 18 <br /> <br />X <br />N <br /> <br />`u 12 <br />Q <br /> 6 <br /> 0 <br />= <br />E 1 i i { <br />1 <br />? <br />? I f <br /> <br /> € <br /> ? f <br /> <br />? * <br />j <br /> ? <br /> <br /> ? <br />? <br />i <br />? <br />? <br />io <br />8 <br />6 d <br />{ <br />N <br />4 <br />? <br />2 <br />0 <br />1860 1880 1900 1920 1940 1960 1980 2000 2020 2040 2060 <br />0 Seed/Sap ? Pole 0 Saw D Water Yield (in) <br />Figure 15. Age class distributions and resultant water yield for spruce-fir from 1860 to <br />2060. Future streamflow declines (2020-2060) will occur at a modest rate as current <br />seedling and sapling stands continue to evolve into the pole class and poles into the <br />sawtimber class. <br />The trends in water yield and in the change in water yield over time differ slightly among <br />the three national forests (Figure 16). Both the MB and AR National Forests receive <br />greater amounts of precipitation and yield more streamflow than the PSI National Forest. <br />As noted earlier (Tablel), the forested area on the AR National Forest is predominantly <br />occupied by lodgepole pine with an important but lesser presence of ponderosa pine and <br />spruce-fir. In contrast, the PSI National Forest is dominated by ponderosa pine with <br />significant, but lesser, amounts of aspen, Douglas fir, and spruce-fir. Because inventory <br />data for the PSI National Forest indicate that the average age of the sawtimber class for <br />the four dominant species is much lower than for the same species on the other two <br />Forests, there are differences in the temporal distribution of simulated water yield and <br />water yield change. Streamflow from the PSI National Forest appears to have been the <br />greatest during the early 1900s when the presence of ponderosa pine appears to have been <br />minimal. In contrast, streamflow from the AR appears to have peaked during the late <br />1800s and, because the average age of sawtimber stands, particularly lodgepole pine, is <br />greater on the AR than on the PSI, subsequent streamflow declines were more gradual. <br />Simulated water yield from the portions of the AR and MBR National Forests within the <br />South Platte River Basin are similar to the flows simulated for that portion of the same <br />forests within in the North Platte River Basin. Simulations indicate that less water yield <br />per unit area is generated from the PSI National Forest. However, there are more forested <br />acres on NFS land in the South Platte River Basin (1.7 million acres) than in the North <br />Platte River Basin (1.1 million acres) so that the reduction in total flow from forest land <br />in the South Platte is actually greater than the simulated reduction in flow on the North <br />Platte. <br />19 2/2/2007