A NATIONAL PLAN FOR AMERICAN FORESTRY 307 



exceptional and the beneficial effects for which they are responsible 

 are so slight in comparison with the damage ordinarily wrought by 

 run-off that they may be dismissed here as entirely negligible. 



Studies of surface run-off from forested areas, and from areas in 

 other types of natural or planted vegetation have been made in Wis- 

 consin and in Mississippi. On silt loam uplands in Wisconsin l with 

 slopes averaging 36 percent, the proportion of total summer precipi- 

 tation which ran off over the surface of the ground beneath hardwood 

 forests of varying density averaged 2.8 percent. Wild pastures of 

 native grasses, in which the soil had never been cultivated, showed 

 a surface run-off about 2% times as great. Cultivation greatly in- 

 creased the percentage of surface run-off; from cultivated hay fields 

 it averaged 17.7 percent, and from small grain fields, cornfields, 

 seeded pastures, and fallow land it averaged over 25 percent. 



H. G. Meginnis of the Southern Forest Experiment Station made a 

 study of run-off and erosion from the upland loess soil of northern 

 Mississippi by means of a number of sample plots. At the time of the 

 disastrous flood in the Yazoo River in 1931-32 when 27 inches of rain 

 fell, 62 percent of the rain ran off immediately from the plots located 

 in cultivated fields, and 54 percent from those located in abandoned 

 fields. The run-off during the same period from the plots in an undis- 

 turbed oak forest was only 0.5 percent and but 2 percent in a scrub 

 oak forest. 



Total run-off can of course be measured only at the foot of 

 slopes, or wherever the precipitation which has percolated into the 

 ground is again brought to the surface by the outcropping of bed- 

 rock or impervious soil layers, and joins that which has run off over 

 the surface. The volume of streams, compared with the precipitation 

 received by the watershed above the point where stream volume is 

 measured, indicates total run-off only so far as there is no deeper 

 movement of moisture in the soil beneath the stream channel. In 

 the drier portions of the United States stream flow for an entire year 

 may be as little as 6 percent of the total precipitation on a watershed, 2 

 although averaging more, but in the more humid portions is almost 

 always higher. In the Middle West Missouri and Illinois, for 

 example the total run-off as measured by surface flow averages 20 

 to 30 percent 3 with minima of probably 15 percent. In the East the 

 average total run-off in streams is more nearly 50 per cent of the pre- 

 cipitation and rarely drops below 25 percent. King 4 gives the average 

 percentage run-off for Tennessee rivers as 45 percent with extremes 

 of 12 and 66 percent. 



The principal factors which influence the normal division of run-off 

 into useful subsurface waters and less useful or destructive surface 

 waters are the character of the precipitation, the geology and topog- 

 raphy of the surface on which it falls, and the vegetative cover on 

 that surface. The vegetative cover is the only one of these factors 

 which it is within human power to control. Hence the necessity for 

 understanding how it operates. Forest is the cover on by far the 

 greater part of the United States which is still in natural vegetation, 

 and on which important quantities of rain or snow fall. The more 



1 Bates, C. G., and Zeasman, O. R. "Soil erosion." Wise. Agr. Exp. Sta. Research Bui. No. 99, 1930. 



2 Blaney, H. F. "Discussion of 'forests and streamflow'." Proc. Amer. Soc. Civil Eng., Dec. 1932. 



3 Duley, F. F., and Miller, M. F. "Erosion and surface run-off under different soil conditions." Mo. 

 Agr. Exp. Sta. Research Bui. No. 63, 1923. 



* King, W. R. "Surface waters of Tennessee." Div. of Geol. Dept. of Educ. Bui. 40, 1931. 



