CLIMATOLOGY IN ARID ZONE RESEARCH 73 
In this example, in a humid climate, where the lake evaporation 
is 83% as great as the evaporation from the pan, it is seen that 
size of evaporating surface is a much less important factor than 
in an arid climate. 
Thus, it is evident that measurements made with evapotrans- 
pirometers are subject to serious error. To insure that the observa- 
tions are reliable it 1s necessary to keep an area around the field 
tanks under high soil moisture. The size of this buffer area varies 
with the climate; it must be much larger in the arid climates 
then in humid. 
Conditions Affecting Evapotranspiration 
Although the various methods of determining evapotranspira- 
tion have many faults and the determinations are scattered and 
few, we get from them an idea of how much water is transpired 
and evaporated under different conditions. We find that the rate 
of evapotranspiration depends on five things: climate, soil mois- 
ture supply, plant cover, soil type and texture, and land manage- 
ment. There is considerable evidence to show that, when the root 
zone of the soil is well supplied with water, the amount used by 
the vegetation will depend more on the amount of solar energy 
received by the surface and the resultant temperature than on 
the kind of vegetation growing in the area. Soil type and texture 
and farming practices likewise have little effect on the rate of 
evapotranspiration under high moisture conditions. The water 
loss under optimum soil moisture conditions, the potential 
evapotranspiration, thus appears to be determined principally by 
climatic conditions. 
Using the most reliable measurements of evaporation and 
transpiration that are available a valid and practical relationship 
between certain climatic parameters and potential evapotrans- 
piration has been obtained. This relationship permits the com- 
putation of potential evapotranspiration for any place from 
information on air temperature and latitude alone. The relation- 
ship is given and its use described elsewhere (11). Work is pro- 
ceeding toward the development of a new formula that is based 
on sound physical principles. In the meantime, the present em- 
