FACTORS AFFECTING THE EVAPORATION OF 
MOISTURE FROM THE SOIL 1 
By F. S. Harris, Director and Agronomist, and J. S. Robinson, Fellow in Agronomy, 
Utah Agricultural Experiment Station 
INTRODUCTION 
The importance of soil moisture in crop production is well understood. 
No plant can grow unless moisture is present to help make food available 
and furnish the water necessary to carry on regular plant functions. In 
arid regions the growth of crops is limited more by a lack of moisture 
than by any other factor, and even in regions of high rainfall crop yields 
are often materially reduced by droughts. 
In sections where only a small amount of rain falls, practically all 
that sinks into the soil returns to the surface and is evaporated directly 
or passes through plants, from which it is evaporated. In humid re¬ 
gions also, where some of the soil moisture percolates to great depths, 
there is considerable loss by evaporation from the surface of the soil. 
Moisture evaporated from the soil is completely lost and is of no 
value to crops; hence, it is important to reduce evaporation to a mini¬ 
mum, particularly where the supply of moisture is limited. The best 
condition would be to have no evaporation of moisture except that pass¬ 
ing through the plant and assisting in its functions. Any information, 
therefore, that will lead to a better understanding of the factors involved 
in evaporation and a fuller knowledge of methods of controlling these 
factors will be of considerable practical importance as well as scientific 
interest. 
Surface losses are due to two factors: (i) Capillarity, by which the 
moisture is brought to the surface, and (2) evaporation. In many of the 
soil-moisture studies that have been made these two factors have not 
been clearly separated, but have been considered together in determining 
loss. This has led to considerable confusion, since a difference in two 
losses might, in one case, be due to a difference in the rate at which 
moisture was supplied to the surface by capillarity and, in another case, 
to the evaporation factors. 
In the experiments reported in this paper an attempt has been made 
to eliminate the factor of capillarity and to confine the studies entirely 
to evaporation in order to determine, as nearly as possible, the effect of 
a number of the factors involved in the evaporation of moisture from 
the soil. 
iThe writers wish to a ckn owledge their indebtedness to Messrs. George Stewart and N. I. Butt, of 
the Utah Experiment Station, for their assistance with certain experiments and in preparing this paper 
for publication. 
Journal of Agricultural Research, 
Dept, of Agriculture, Washington, D. C. 
gj 
Vol. VII, No. 10 
Dec. 4. 1916 
Utah—4 
(439) 
