Estimating Additional Water Yield From Changes in Management of National Forests in the North Platte Basin

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r Ll I THE POTENTIAL IMPACT OF CATASTROPHIC EVENTS ON WATER YIELD There is concern that a spruce beetle outbreak could reach epidemic proportions as a result of the recent Routt blow down. The proximity of the potential outbreak area and the spruce -fir stands in the North Platte River Basin put those stands at risk as well. We simulated the short -term hydrologic consequence (10 year) of an outbreak in which 50 percent of the Spruce basal area in sawtimber stands and 30 percent of the spruce basal area in pole sized stands were killed by the beetle. All acres of spruce -fir , were considered subject to impact, regardless of management class, as it would be unreasonable to assume that Wilderness or lands Not Suitable for , Harvest would be spared. The baseline hydrologic simulation for year 2000, presented earlier, indicated that annual water yield averaged 16.5 area inches for all stands in the spruce -fir type. Average basal area in the sawtimber stands is currently 149 ft2 acre -' while the pole stands average 127 ft2 acre -', both of which are at nearly complete hydrologic utilization. We simulated the impact of the beetle killing 50 percent of the sawtimber and 30 percent of the pole -sized trees. Mortality was simulated to have occurred uniformly over a 10 -year period (2001— 2010). In year 1, or 2001, water yield increased from 16.5 to 16.6 inches. By year 10, water yield increased to 18.8 inches from the 306,000 acre spruce -fir type. The 2.2 inch increase is consistent with the increase of 2.0 inches Love (1955) observed following a budworm infestation on the White River in which 30 percent of the entire stand was killed. The 2.2 inch increase in year 2010 represents a 56,100 acre -foot increase in water yield per year. The increase in flow could persist, at a decaying rate, for as long as 60 -70 years. I A second hypothetical simulation addressed the effect of fire in the lodgepole pine and ponderosa pine Forest types on water yield. The simulation assessed burning 30,000 acres. In this scenario, the 30,000 acre fire consumed 26,000 acres of lodgepole pine and 4,000 acres of ponderosa pine. An assumption was made that the fire would reduce "effective" basal area to 10 percent of its original value for the stands involved. As a result of burning 26,000 of the 627,963 acres of lodgepole pine, a 0.3 inch (11.3 to , 11.6 inches) increase in flow, or 15,700 acre -foot increase in water yield was simulated. For ponderosa pine, burning 4,000 acres resulted in a 0.2 inch I 42 1