REGION OF EVAPORATION 139 
Loss by evaporation chiefly at the surface 
Evaporation goes on from every wet substance. 
Water evaporates therefore from the wet soil grains 
under the surface as well as from those at the sur- 
face. In developing a system of practice which will 
reduce evaporation to a minimum it must be learned 
whether the water which evaporates from the soil 
particles far below the surface is carried in large 
quantities into the atmosphere and thus lost to plant 
use. Over forty years ago, Nessler subjected this 
question to experiment and found that the loss by 
evaporation occurs almost wholly at the soil surface, 
and that very little if any is lost directly by evapora- 
tion from the lower soil layers. Other experimenters 
have confirmed this conclusion, and very recently 
Buckingham, examining the same subject, found 
that while there is a very slow upward movement 
of the soil gases into the atmosphere, the total quan- 
tity of the water thus lost by direct evaporation from 
soil, a foot below the surface, amounted at most to 
one inch of rainfall in six years. This is insignificant 
even under semiarid and arid conditions. How- 
ever, the rate of loss of water by direct evaporation 
from the lower soil layers increases with the porosity 
of the soil, that is, with the space not filled with soil 
particles or water. Fine-grained soils, therefore, 
lose the least water in this manner. Nevertheless, 
if coarse-grained soils are well filled with water, 
