Management and Disturbance Effects on Water Yeild

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Simulated water yield from NFS land in the South Platte River Basin was greatest ca 1920 and has declined in proportion to the subsequent vegetation density increases that have occurred since then. The net reduction in water yield occurring from 1920 to 1997 is equivalent to about 1.8 area inches or 261,000 acre-feet from the 1.7 million acres ofNFS land in the scenario that holds spruce-fir constant. The simulated decline in water yield was slightly greater at 2.1 area inches (about 300,000 acre-feet) with spruce-fir stands altered similarly to the other species. As was found for the North Platte River Basin, simulations also indicated that water yield from the South Platte River Basin will continue to decline assuming the natural succession of the current forest vegetation is not interrupted. As current stands continue to mature, water yield from NFS land can be expected to decline another 0.5 area inches, or an additional 72,000 acre-feet by the year 2060 if the current trend in vegetation density is not interrupted by management activity or catastrophic events such as fire or mortality resulting from insect infestation. For the most part, the impacts of historic wildfires are reflected in the characterization of the current forest conditions provided by the three Forests. In 2002, however, the largest wildfire in recorded Colorado history occurred. Known as the Hayman Fire, about 110,000 acres within the fire boundary were consumed by fire. The simulated water yield from the burned area for the year 2020 is about 4.5 area inches and represents a 3.4-area inch or 30,600-acre-foot increase over the 1997 reference water yield from the same area. Simulations indicate that the water yield from the burned area will decline from 4.5 area inches in 2020 to 3.0 area inches in 2060, but still represent a 1.9-area inch (17,000 acre- foot) increase relative to the 1997 reference water yield. Currently a major infestation of insects, bark beetles and defoliators, is affecting forest stands throughout Colorado, in nearby states, and in southwestern Canada. Although a variety of tree species are affected by the mountain pine beetle (ponderosa pine, lodgepole pine, and limber pine) or by the spruce beetle (Engelmann spruce), the lodgepole pine type seems to be impacted the most severely and extensively during this current epidemic. Of the 456,306 acres of lodgepole pine on NFS land in the South Platte River Basin, almost 95 percent are classified as pole or sawtimber stands and are subject to mountain pine beetle attack. Based on input from R2 staff, we simulated three scenarios in lodgepole pine. First, we simulated the hydrologic impact of a 50-percent kill in all lodgepole pine sawtimber and pole size stands. Second, we simulated the impact of a 90-percent kill throughout both the sawtimber and the pole stands. Third, based on the consensus of the R2 staff that a 50-percent kill in the pole size stands (4.9 to 9.0 inch DBH) and a 90-percent kill in the sawtimber stands was the most likely expectation of what may actually occur during the next few years, we simulated that impact. If 90 percent of the trees in the sawtimber stands are killed along with 50 percent of the trees in the pole size stands; tlie increased annual water yield from the impacted area in the lodgepole pine type will average nearly 3.6 area inches or 129,000 acre-feet. Obviously, the simulated impacts of the beetle infestation are speculative and based on an assumed degree of mortality. For example, if the estimated beetle kill were reduced to 50- percent mortality on the 146,431 acres of sawtimber stands alone and impacts in the pole size stands were excluded; the increase in annual water yield would still be about 35,000 acre-feet. Increasing the estimate of mortality to 90 percent in the sawtimber stands alone