AMOUNT OF EVAPORATION 131 
possible, direct evaporation of water from the soil 
must be prevented. 
Few farmers really realize the immense possible 
annual evaporation in the dry-farm territory. It is 
always much larger than the total annual rainfall. 
In fact, an arid region may be defined as one in which 
under natural conditions several times more water 
evaporates annually from a free water surface than 
falls as rain and snow. For that reason many stu- 
dents of aridity pay little attention to temperature, 
relative humidity, or winds, and simply measure the 
evaporation from a free water surface in 
the locality in question. In order to ob- | Br 
tain a measure of the aridity, MacDougal dE 
has constructed the following table, show- are 
ing the annual precipitation and the an- 
nual evaporation at several well-known == 
localities in the dry-farm territory. ——— 
True, the localities included in the fol- = 
lowing table are extreme, but they 
illustrate the large possible evap- = 
oration, ranging from about six to {x 
thirty-five times the precipitation. Fic. 32. Annual rain- 
fall and evaporation 
(See Fig. 32.) At the same time in arid region com- 
it must be borne in mind that pared. The high evap- 
oration rate makes 
while such rates of evaporation necessary thorough 
may occur from free water sur- farming. 
faces, the evaporation from agricultural soils under 
like conditions is very. much smaller. 
