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acres) classes (Troendle and Nankervis 2000; Troendle et al. 2003). If mortality were to <br />occur to 50 percent of the trees in the pole stands and 90 percent of the trees in the <br />sawtimber stands on NFS land only, annual water yield in the North Platte River Basin <br />could also increase by about 228,000 acre-feet. The beetle infestation in the lodgepole <br />pine type will have a significant impact on water yield from the entire Platte River Basin <br />The simulated changes in water yield represent what is expected to occur a few years <br />after mortality. When the trees die, the needles are retained for some time. During that <br />time, the actual increases in water yield are somewhat less than simulated because the <br />retained needles still intercept snow and allow interception losses to occur. Trees that are <br />in the process of dying also transpire some water. After the trees are dead and the needles <br />are shed, interception losses occur but are greatly diminished while transpiration losses <br />are eliminated. Water yield increases are greatest after the needles and fine branches are <br />shed, which could take 10 to 15 years. Hydrologic recovery, or return to pre-infestation <br />hydrologic condition, probably occurs 50 to 80 years after regeneration is established. <br />Because the stand resetting process should not cause soil disturbance, it can be assumed <br />that infiltration of rainfall and snowmelt will not be impacted and overland flow is not an <br />issue after mortality. Hydrologic response to summer rainfall events may increase <br />slightly, locally because of wetter soils resulting from the reduced transpiration, but such <br />increases in peak flows should not present a significant threat. Snowmelt peaks may or <br />may not increase, depending on the proximity of the impacted lodgepole pine stands with <br />other unaffected forest types and the elevation gradient that exists for the lodgepole pine <br />type within the drainage of concern. One can expect earlier and more rapid melt to occur <br />in the impacted areas that, when combined with more snow and wetter soils can result in <br />greater and perhaps advanced contribution to peak flows. The actual impact on peak <br />flows, however, depends on how these site-specific contributions synchronize or <br />desynchronize streamflow when combined with the contributions from the balance of the <br />drainage, or landscape. More than likely, peak flows from the landscape will not increase <br />measurably and may even decrease. <br />The beetle kill simulations were made assuming the mortality was evenly distributed <br />across the polygon or area. This may or may not be the case because the mortality could <br />be uniformly distributed, or patchy in nature. Because mortality is considered a partial <br />impact, the water yield simulation is similar for either scenario. <br />25 2/2/2007