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^1 <br />}' i? <br />Final Report <br />Historical and Future Impacts of Vegetation Management <br />and Natural Disturbance on Water Yield from <br />Forest Service Lands in the South Platte River Basin <br />C. A. Troendle, J. M. Nankervis, and A. Peavy <br />IntCo Uct1on <br />Troendle and Nankervis (2000) reviewed the current knowledge on the regional effects of <br />forest disturbance or vegetation change on water yield as part of an analysis evaluating <br />changes in water yield that may have resulted from management of National Forest <br />System (NFS) lands in the North Platte River Basin. <br />Numerous studies worldwide have found that changes in forest density cause changes in <br />water yield. Hibbert (1967), Troendle and Leaf (1980), Bosch and Hewlett (1982), and <br />Stednick (1996) summarized findings from these studies. As Hibbert (1967) observed, <br />reducing forest cover increases water yield, establishing (or increasing) forest cover on <br />sparsely vegetated land decreases water yield, and response to treatment is highly <br />variable and unpredictable. The first two of these conclusions -changes in forest density <br />decreases water yield and reducing forest cover generally increases water yield--are well <br />documented and universally accepted. <br />However, the hydrologic response to changes in forest cover, although variable, is more <br />predictable than Hibbert (1967) initially concluded (Troendle and Leaf 1980; Bosch and <br />Hewlett 1982; and Stednick 1996). This change in thinking about predictability is a result <br />of increased number of observations over time as well as an improved understanding of <br />factors influencing streamflow response. Streamflow response to a change in forest cover <br />is strongly related to climate, species composition, and the percentage change in <br />vegetation density (Figure 1, Bosch and Hewlett 1982). Data from 95 watershed <br />experiments conducted throughout the United States shows that, on average, streamflow <br />increases by nearly 0.1 inch for each 1 percent of watershed area harvested (Stednick <br />1996). Because annual streamflow varies greatly, the general conclusion was that about <br />20 percent of the basal area of the vegetation, above the point of streamflow <br />measurement, must be removed before a statistically significant change in flow can be <br />detected (Hibbert 1967; Bosch and Hewlett 1982; and Stednick 1996). However, as <br />Bosch and Hewlett (1982) suggest, reducing forest cover by less than 20 percent may <br />well produce statistically non-significant responses that presumably approach zero <br />increase at zero change in forest cover. <br />A significant body of hydrologic research is specifically appropriate for assessing the <br />effects of vegetation changes on water yield. Yet, the universal relationships depicted in <br />1 2l2/2007