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however, the same trend can be expected and is already partially reflected in the <br />declining flows that were simulated to have occurred from 1920 to 1860. <br />Of the six species occupying 95 percent of the area and contributing 95 percent of the <br />total water yield from the forested portion of all NFS land in the South Platte River <br />Basin, only three species (spruce-fir, lodgepole pine, and ponderosa pine) are dominant <br />occupants and account for most simulated changes in water yield over time (Figure 9). <br />Water yield from aspen, bristlecone pine, and Douglas fir follows the same trend over <br />time, but both the total yield and the contribution to change in flow in the South Platte <br />River Basin from these species is significantly smaller (Figure 9). Simulations indicate <br />that although similar trends in water yield occurred for all six species, the combinations <br />of stand characteristics driving the simulations differ among species (Figures 10-15). <br />Douglas fir (Figure 12), lodgepole pine (Figure 13), and ponderosa pine (Figure 14), for <br />example, were nearing complete hydrologic maturity (sawtimber) during the reference <br />year 1997 and fiirther declines in streamflow due to continued succession (barring <br />intervention) are expected to be less than for the other three species. Based on current <br />stand conditions, the natural progression of aspen (Figure 10), bristlecone pine (Figure <br />11), and spruce-fir (Figure 15) have a dominant impact on future declines in water yield. <br />Table 3. Simulated water yield from the NFS land in the South Platte River Basin from <br />1860 to 2060. <br /> <br />Year Water yield' <br />(area inches) Water yieldz <br />(area inches) Water yieldz <br />(acre-feet) <br />1860 4.3 3.8 546,726 <br />1880 4.2 3.7 531,169 <br />1900 5.1 4.6 658,680 <br />1920 5.1 4.8 687,435 <br />1940 4.5 4.2 610,367 <br />1960 3.8 3.6 522,062 <br />1980 3.3 3.1 453,486 <br />2000 3.0 3.0 433,329 <br />2020 2.8 2.8 411,309 <br />2040 2.6 2.7 387,769 <br />2060 2.4 2.5 367,062 <br />'Estimate of water yield allowing spruce-fir to change with time <br />ZEstimate of water yield holding spruce-fir constant for the entire period <br />14 2/2/2007