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Table 6. Simulated water yield from all pole and sawtimber stands in the lodgepole pine <br />forest type on NFS land in the South Platte River Basin for resulting from differing <br />intensities of beetle kill. Water yield is presented in area from the impacted area. <br /> Water yield 1997 <br />(area inches) Water yield (area inches) after tree mortality (%) in <br />all pole and sawtimber stands of lodgepole pine <br /> <br />Forest <br />Baseline 50% <br />mortality <br />90% mortality 50% pole and 90% sawtimber <br />mortality (acre-feet)* <br />AR 2.9 5.6 8.3 6.5 (103,600). <br />MBR 2.8 5.7 8.6 8.0 ( 1,700) <br />PSI 2.1 4.9 6.8 5.6 ( 23,500) <br />Total 2.7 5.5 8.1 6.3 (128,800) <br />Approximate increase in water yield, in acre-feet, relative to baseline (1990 water yieia. <br />Obviously, the simulated impacts of the beetle infestation are speculative and based on an <br />assumed degree of mortality. For example, if the estimated beetle kill were reduced to 50- <br />percent mortality on the 146,431 acres of sawtimber stands alone (Table 5) and impacts <br />in the pole size stands were excluded; the increase in annual water yield would still be <br />about 35,000 acre-feet. Increasing the estimate of mortality to 90 percent in the <br />sawtimber stands alone would result in a simulated increase in water yield in the South <br />Platte River Basin of 65,000 acre-feet per year. Taking the simulation to extremes, 90- <br />percent mortality in all the pole and sawtimber stands of lodgepole pine stands on NFS <br />land in the South Platte River Basin would result in an increase in water yield of about <br />191,000 acre-feet per year. <br />The simulated changes in water yield after beetle mortality are probably reasonable for <br />the chosen scenarios. Admittedly, the extent of mortality is speculative; however flights <br />over the area and other visual observations indicate mortality will be widespread and <br />significant. A conservative assumption that 50-percent mortality will occur in sawtimber <br />stands alone results in a 35,000 acre-foot increase in water yield per year. On the other <br />hand, the assumption that mortality will approach 90 percent in all pole and sawtimber <br />stands is excessive. Alternatively, 90-percent mortality in sawtimber and 50-percent <br />mortality in pole stands over the next few years, and the resultant 129,000 acre-foot <br />increase in water yield, seems a reasonable estimate. As with the documented changes in <br />the White River watershed (Love 1955), one can expect a gradual increase in water yield <br />over the next 10 or 15 years that will persist for some time. Mortality on other <br />government or privately owned land in the South Platte River Basin can be expected to <br />follow a similar pattern. Estimates of the impact of the beetle infestation on water yield <br />from the South Platte River Basin, which are based only on estimates for NFS land, are <br />very conservative. Regarding lodgepole pine, the current beetle infestation will reset the <br />system, hydrologically, and cause notable water yield changes. Because the majority of <br />the lodgepole pine type in the South Platte River Basin occurs on the AR National Forest, <br />the majority of the water yield increase will also occur from that forest. <br />For comparison, we did a similar analysis of the potential impacts of the beetle <br />infestation on lodgepole pine in the North Platte River Basin where 530,000 acres of <br />lodgepole pine are evenly split between the pole (258,000 acres) and sawtimber (272,000 <br />24 2/2/2007