Most of the Nation's high-quality surface water 

 comes from watersheds which support forest vegeta- 

 tion. Trees and other vegetation affect the water 

 balance of a drainage basin in two ways. First, tree 

 branches and leaves of plants may intercept up to 30 

 percent of total precipitation during light-intensity 

 storms. This part of the precipitation is evaporated 

 without adding to soil moisture storage. Second, 

 plant roots absorb large volumes of soil water which 

 are transported through the stem and removed 

 through the leaves as transpiration. Since streamflow 

 is a residual after all evaporative processes have been 

 satisfied, early foresters and watershed scientists rea- 

 soned that water yield could be increased by reducing 

 evaporative losses. Reducing vegetation density is the 

 most efficient way to reduce evaporative losses. 



Increases in streamflow after various intensities of 

 forest cutting have been demonstrated in many parts 

 of the country. Those individual studies have been 

 summarized by Hibbert^', LulP^, Sopper^^, Douglas 

 and Swank^", and Anderson et al.^' The following 

 general conclusions have been presented: 



1. Large increases in water yield following 

 forest cutting occur in areas where 

 (a) precipitation is abundant, (b) vege- 

 tation is dense, and (c) sufficient solar 

 energy is available to evaporate large 

 volumes of water. ^^ The largest increases 

 measured the first year after removal of 

 all woody vegetation were 16.8 inches in 

 western North Carolina, ^^ and 18.2 

 inches in western Oregon. ^^ 



2' Hibbert, Alden R. Forest treatment effect on water yield. In 

 Forest Hydrology. William E. Sopperand Howard W. Lull(eds.), 

 p. 527-543. Pergamon Press, New York. 1967. 



" Lull, Howard W. Management possibilities for water yield 

 increases. Proc. Joint FAO/USSR Int. Symposium on influences 

 and watershed management, p. 168-178. 1970. 



^^Sopper, W. E. Watershed management: water supply augmen- 

 tation by watershed management in wildland areas. Report to the 

 National Water Commission. Natl. Tech. Inf. Service, Springfield, 

 Va, 149 p. 1971. 



2'» Douglas, James E., and Wayne T. Swank. Streamflow 

 modification through management of eastern forests. U.S. Depart- 

 ment of Agriculture, Forest Service. Research Paper SE-94, 15 p. 

 1972. 



"Anderson, Henry W., Marvin D. Hoover, and Kenneth G. 

 Reinhart. Forests and water: effects of forest management on 

 floods, sedimentation, and water supply. U.S. Department of 

 Agriculture, Forest Service. General Tech. Rep. PSW-18, 115 p. 

 1976. 



2' Lull, op. cit. 



" Hoover, M. D. Effect of removal of forest vegetation upon 

 water yields. American Geophysical. Union Transactions, 6:969- 

 975. 1944. 



2* Rothacher, Jack. Increases in water yield following clear-cut 

 logging in the PNW. Water Resources Research, 6:653-658. 1970. 



2. Selective tree cutting has little or no 

 effect on water yield unless the cutting 

 intensity exceeds 20 percent. 2' 



3. When the cutting intensity exceeds 20 

 percent, water yield increases are pro- 

 portional to the percentage reduction in 



^ growing stock. Maximum yield in- 

 creases are achieved by clearcutting.^" 



4. In the eastern hardwood region, water 

 yield increases are maximum the first 

 year after timber harvesting; increases 

 gradually diminish as the forest is regen- 

 erated. 3' The duration of yield increases 

 after clearcutting in the eastern hard- 

 wood region is highly variable. The long- 

 est time for a clearcut experimental 

 watershed is about 40 years; the shortest 

 about 5 years. 32 



5. In the mixed conifer zones of the Rocky 

 Mountains, the most efficient way to 

 increase water yield is removing the 

 trees in small patches which together 

 occupy about half of the watershed 

 area. 33 Because of slow regrowth and 

 snow redistribution into the small open- 

 ings, yield increases are expected to per- 

 sist for up to 40 years. 34 



6. Water yield increases after forest re- 

 moval are greatest in year of abundant 

 precipitation and least in years of 

 drought, especially in regions where 

 evapotranspiration exceeds moisture 

 supplies. 35 



7. Because deciduous trees are dormant for 

 part of the year and thus use less water 

 than conifers, converting from decidu- 

 ous species to conifers reduces the an- 

 nual water yield. 3* 



8. In areas where moisture supplies are 

 limited during the growing season, con- 

 verting from deep-rooted native species 



2' Hibbert, op. cit. 



^01 hi J. 



" Kovner, J. L. Evapotranspiration and water yields following 

 forest cutting and natural regrowth. Society of American Foresters 

 Proceedings, p. 106-110. 1956. 



" Douglass and Swank, op. cit. 



" Ihid. 



M Ibid. 



" Lull, Howard W., and K. G. Reinhart. Increasing water yield 

 in the Northeast by management of forested watersheds. U.S. 

 Department of Agriculture, Forest Service, Res. Pap. NE-66, 45 p. 

 1967. 



" Swank, Wayne T., and James E. Douglass. Streamflow greatly 

 reduced by converting deciduous hardwood stands to pine. 

 Science, 185: 857-859. 1974. 



311 



