70 THE FUTURE OF ARID LANDS 
and that which is utilized in evaporation. Recent measurements 
have shown that when the soil is very moist more than 80% of 
the net radiation is used in evaporation. As the soil becomes dry, 
the evaporation rate declines and more of the net radiation is 
devoted to heating the air and soil with little remaining for 
evaporation. 
The potential rate of evapotranspiration is realized only when 
the area of the evaporating surface is large enough so that all the 
energy for evaporation comes from radiation and none from 
advection. Obviously, the area of a standard evapotranspirometer 
(4 square meters) is too small and can give reliable values only 
when it is surrounded by an extensive buffer area identical in 
vegetation cover and soil moisture. If the area of the evaporat- 
ing surface is large, the influence of the air passing over it becomes 
small and solar radiation is the primary source of energy for 
evaporation. Under these circumstances the atmospheric humid- 
ity is unimportant. If the air is moist, the temperature of the 
evaporating surface will rise to a point, above the dew point of 
the air, at which the evaporation will just use the energy that is 
available. Similarly, in dry air, rapid evaporation will lower the 
temperature of the evaporating surface until the evaporation is in 
balance with the available energy. 
When a psychrometer is used to determine the humidity of the 
air, a thermometer bulb is moistened and becomes an evaporating 
surface. The water evaporating from the wet bulb thermometer 
cools the bulb. The surface area of the bulb is small, and the 
amount of water vaporized is very small. Heat flows into the 
water film on the bulb from the warmer surrounding air, and the 
evaporation process will reach equilibrium at a rate and at a wet 
bulb temperature at which the energy appropriated from the air 
is just sufficient to maintain the evaporation. Solar radiation 
contributes almost no energy to this process. The water evapo- 
rated from the wet bulb moistens the air, but the amount is so 
minute that the effect on the moisture content of the air is 
completely negligible. 
The Piche evaporimeter is also small with an "evaporating 
surface of approximately 13 square centimeters. The evapora- 
tion from the surface of a Piche evaporimeter is likewise incom- 
