PROBLEMS FROM LAND OCCUPANCY 177 



and return of water from the soil to the atmosphere. This 

 return is calculated from determinations of evaporation from 

 soil, use of water by plants, and interception of precipitation 

 before it reaches the ground — these represent the water costs 

 for each type of cover or each condition of soil. 



The experimental areas are, generally speaking, in areas 

 where the underlying rock materials have a low permeability, 

 and clay is likely to be an important constituent in the soils. 

 The experimental forests are typical of the national forests 

 in that most of them are in mountainous areas with generally 

 a thin mantle rock upon relatively impervious bedrock. The 

 earth materials are predominantly of fine texture, and in- 

 filtration rates may vary markedly depending upon the vege- 

 tative cover and the organic material in the soil as well as the 

 soil itself. In forested areas the surficial materials, down to a 

 depth as far as roots can penetrate, commonly are far more 

 permeable than the underlying subsoil or fragmental rock or 

 bedrock. These soils have high infiltration capacities, and 

 some of the water entering them may be discharged into 

 streams within a matter of a few hours or days. This has been 

 designated "subsurface storm flow." The shallow ground- 

 water reservoir through which it moves may transmit large 

 quantities of water in a short time but is generally limited in 

 depth and therefore has only a small storage capacity. 



The effectiveness of forests in preventing overland flow and 

 consequent erosion is illustrated in the Coweeta Experimental 

 Forest, and by contrast in the Copper Basin some 50 miles to 

 the west, where the forest cover has been removed. 



Coweeta Experimental Forest, N.C. 18 The setting of the 

 Coweeta Experimental Forest provides an excellent op- 



18 References: "Coweeta Experimental Forest Watershed Management Re- 

 search," U.S. Forest Service, 1948, 40 pp. 

 Hursh, C. R., and E. F. Brater, Separating Storm Hydrographs 

 from Small Drainage into Surface and Subsurface Flow, 

 Trans. Am. Geophys. Union, vol. 22, pp. 863-870, 1941. 

 Hoover, M. D., Effect of Removal of Forest Vegetation upon 



