REGION OF EVAPORATION .139 



Loss hy 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, 



