16 



BULLETIN 512, U. S. DEPARTMENT OF AGEICULTUEE. 



higher rate than the former. This is due to the fineness of the 

 particles and the compact structure of the clay soils as compared 

 with the open, porous structure and coarse particles of the sandy 

 soils. The open structure of a soil facilitates the entrance and rapid 

 circulation of both air and water, since resistance to flow varies 

 inversely as the size of the individual pore spaces. After a long 

 dry period the pores in the upper layers of a soil become filled with 

 air which, until it is expelled, tends to retard the entrance of soil 

 water. A deeply plowed soil will absorb a greater percentage of 

 rainfall than one where shallow plowing is practiced, and the greater 

 the amount of humus in a soil the greater will be its capacity to 

 absorb water. The rate of absorption after the top soil is saturated 

 with water depends upon the permeability of the subsoil. A close, 

 impervious subsoil checks the rate of percolation and thereby in- 

 creases the run-off at the surface. 



The water capacity of the top foot of farm land in good tilth has 

 been stated^ to be 4 to 5 inches; thus a soil 12 inches deep could 

 absorb this amount of rainfall provided the rain is supplied to the 

 surface at the same rate at which the soil is capable of receiving it. 

 If the former rate is greater than the latter, the excess water runs 

 off over the land surface with a velocity depending upon the slope. 

 The steeper the slope the more rapid the run-off, and correspondingly 

 less would be the time allowed for the absorption of water by the 

 soil. Hence, the steeper the slope the greater will be the percentage 

 of the rainfall flowing off. 



To assist in the determination of the percentage of rainfall flowing 

 off from any particular field, the following table was prepared : 



Probable percentages of rainfall running off, for tlie different types of soil, and 

 for a rainfall of S inches in 48 hours. 





Approxi- 

 mate 

 percent- 

 age of silt 

 and clay 

 in the 

 soil. 



Run-off expressed in percentage of rainfall. 



Kind of soil. 



Open, pervious subsoil. 



Slope of land in feet 



per hundred— 



Impervious subsoil.' 



Slope of land in feet 



per hundred— 





5 



10 



15 



20 



5 



10 



15 



20 



Sandy 



Per cent. 

 20 

 40 

 60 

 80 



Per 

 cent. 

 40 

 50 

 65 

 80 



Per 

 cent. 

 45 

 55 

 70 

 85 



Per 

 cent. 

 50 

 60 

 75 

 90 



Per 

 cent. 

 55 

 65 

 80 

 95 



Per 

 cent. 

 45 

 55 

 70 

 85 



Per 

 cent. 

 50 

 60 

 75 

 90 



Per 

 cent. 

 55 

 65 

 80 

 95 



Per 



cent. 

 60 



Sandy loam 



70 



Clay loam 



85 



Clay 



100 







1 The word impervious should be construed to mean that the subsoil admits water but much more 

 slowly than an open, pervious subsoil. 



Note. — If soil is deeply plowed and contains much humus, deduct 10 from the above values. 

 1 U. S. Geological Survey, Water Supply Paper No. 192, p. 315. 



