LONG-TERM IMPACTS OF FOREST TREATMENTS ON WATER YIELD - A SUMMARY FOR NORTHEASTERN USA

Long-term changes in annual water yield are summarized and compared for 11 catchment studies in the northeastern USA. Substantial increases in water yield of up to 350 mm year-1 were obtained in the first year by clearing forest vegetation and controlling regrowth with herbicides. Commercial clearcutting with natural regrowth resulted in initial increases in water yield of 110-250 mm year-1. This range in response was due to differences in precipitation and configuration of cuttings. Unless regrowth was controlled with herbicides, yield increases declined quickly after cutting, seldom persisting for more than 10 years. However, yield increases were readily extended over 20 years or more with intermediate cuttings and/or repeated control of regrowth with herbicides. Nearly all increases in water yield occur during the growing season as augmentation of baseflow. Changes in species composition after forest cutting on several study catchments eventually resulted in decreased water yields compared with those from uncut, control catchments. Results are discussed in terms of implications for surface water supplies, global climate change, nutrient cycling, hydrological modeling, and long-term research.