CLIMATOLOGY IN ARID ZONE RESEARCH 69 
The latter method proves to be much more satisfactory in prac- 
tice. When it rains, any excess water drains through the soil and 
is similarly measured. Thus, evapotranspiration can be deter- 
mined as a difference since every other term in the hydrologic 
equation is measured. A number of these evapotranspirometers 
are now in operation in widely scattered areas of the world. 
Many additional installations are needed if we are to understand 
the variation of evapotranspiration from one area to another. 
The analysis of the observations of evapotranspiration from 
collaborators in various climatic regions of the earth has revealed 
that it is easy to get erroneous answers, particularly in arid areas 
where measurements give values that are likely to be too large 
(5). The errors occur when the vegetation on the area surround- 
ing the tanks is not the same as on the tanks or when the soil 
moisture inside the tanks differs from that in the soil outside. 
There are three possible sources of energy for evaporation or 
evapotranspiration: solar radiation, heat that reaches the evap- 
orating surface from the air, and heat that is stored in the 
evaporating body. However, with no external source of energy, 
the surface temperature of an evaporating body would quickly 
drop to the dew point of the air and evaporation would cease. 
Consequently, evaporation can occur as a continuing process 
only while energy is being received from some outside source. 
The sun is the original source of all energy that is involved in 
the transformation from liquid to water vapor, but not all the 
energy that is received from the sun is used in evaporating water. 
Some of the incoming solar radiation is immediately reflected 
from the surface back to the sky. For a vegetation-covered 
surface about 25% of the incoming radiation is lost in this way. 
Also a certain percentage of the incoming radiation 1s radiated 
from the surface back to the sky, the amount depending upon the 
temperature of the earth’s surface and on the sky above. This 
amount is often between 10 and 15% of the incoming radiation. 
After deducting the losses due to reflection and back radiation, 
the remainder, which is known as the net radiation, must be 
partitioned into three parts; that which heats the soil, that which 
heats the air by convection through contact with the soil surface, 
