Management and Disturbance Effects on Water Yeild

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"? ra 4 would result in a simulated increase in water yield in the South Platte River Basin of 65,000 acre-feet per year. Taking the simulation to extremes, 90-percent mortality in all the pole and sawtimber stands of lodgepole pine stands on NFS land in the South Platte River Basin would result in an increase in water yield of about 191,000 acre-feet per year. In contrast to the extensive area affected by fire and insect mortality, forest management activities on the three Forests appear relatively minor since 1997. The activities implemented consist mostly of partial stand removal practices primarily intended to reduce fuels, foster tree release, or to improve stand condition. About 38,000 acres of the 1.7 millions acres of NFS land has been treated in recent years. Of that, almost 24,000 acres were treated using a natural fuels broadcast or understory burn with the objective of impacting 20 to 40 percent of the residual basal area of the stand. Data indicate that 78 percent of the activity occurred on the PSI National Forest with the balance occurring on the AR National Forest. The potential increase in water yield resulting from the net effect of recent and current management activities will increase water yield by about 5,000 acre-feet per year. If the current level of activity is projected into the future, the net increase would average 10,000 to 20,000 acre-feet per year over time. Two significant conclusions were drawn from the assessment of the historical trends in water yield from NFS land in the South Platte River Basin. First, as was the case in the North Platte study, simulations suggest that a significant decline in water yield occurred from NFS land as a result of increases in forest density. In both basins, the data also indicated that the process is cyclical. As stands mature naturally, water yield declines; as stand regenerate, water yield increases. Without human intervention, this natural process is largely propagated by fire and insects. The second conclusion is that although water yield is expected to increase after catastrophic fires, such as the Hayman Fire, the potential effect of the current insect infestation, particularly in the lodgepole pine type, is a far more extensive and may be a more significant factor in effecting water yield in the future. Mortality from the current mountain pine beetle infestation may recapture much of the decline in water yield caused by growth or re-growth of forest stands over the past 80 or 90 years. Hydrologic impacts that may result from insect infestations are also occurring to some extent in other forest types and on other public and private land that is not considered in this study so the impact of the infestations on water yield from the entire Platte River basin might be quite significant.