Summary
Forest Service scientists used a paired watershed approach to quantify changes in water yield and peak stormflow using streamflow data from an infested watershed and a nearby watershed with similar elevation, slope and disturbance history, but differing in eastern hemlock basal area. The study watershed has experienced complete mortality of eastern hemlock since infestation was first detected in 2004. Hemlock mortality resulted in a 6 percent reduction in basal area in the watershed, but this loss is primarily concentrated (26 percent) in riparian zones. The scientists expected initial increases in annual yield shortly after hemlock tree mortality, and eventual decreases in yield as the tree species replacing hemlock are higher water using tree species. They found that annual yield did not increase significantly in any year after infestation, but it did decrease significantly in 2010. Monthly yield also decreased after infestation, but changes were limited to the dormant season. The decline in yield is likely to persist as hemlock is replaced by species with higher transpiration demands. Although yield declined, peakflows, or the highest flows following storm events, increased significantly after infestation, but only during the largest storms. The distribution of peakflow values also showed greater variability. Changes in stormflow are likely to persist as a result of infestation due to the permanent alteration of the forest canopy. Thus, streams draining watersheds where eastern hemlock has been lost due to hemlock woolly adelgid infestation demonstrate both altered yield and significantly higher peakflow during the largest storms.
