16 BULLETIN 512, U. S. DEPARTMENT OF AGRICULTURE. 
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 the different types of soil, and 
for a rainfall of 8 inches in 48 hours. 
Run-off expressed in percentage of rainfall. 
Approxi- 
mate 
percent- | Open, pervious subsoil. Impervious subsoil.! 
Kind of soil. age of silt Slope of land in feet Slope of land in feet 
and clay per hundred— per hundred— 
in oe 
soil. 
OO OE eee 
Per cent. | cent. | cent. | cent. | cent. | cent. | cent. | cent. | cent. 
SPT CE Day Se I A ie Ney Roe EE Sy 20 40 45 50 55 45 50 55 60 
pay, MOAT ee ee ae ee Cg 40 50°} 55 60 65 ay) 60 65 70 
COTA yMORMUG HS) tee cle one ew on See Aner 60 65 70 75 80 70 75 80 85 
VIPS 4 sce 4a aoae aoe sec ne BOP See eb adage seenel- 80 80 85 90 95 85 90 95 100 
1 The word impervious should be construed to mean that the subsoil admits water but much more 
slowly than an open, pervious subsoil. 
NoTE.—Ii soil is deeply plowed and contains much humus, deduct 10 from the above values. 
1U. S. Geological Survey, Water Supply Paper No. 192, p. 315. 
