Yoshida - 1981 - Fundamentals of Rice Crop Science

98 FUNDAMENTALS OF RICE CROP SCIENCE
2.19. Variations in the capacity of air at different temperatures for water
vapor.
For evaporation from a surface to continue there are several physical requirements.
First, there must be a supply of energy to provide the quite large latent heat
of vaporization, which varies with temperature; it is about 590 cal/g at around 15°C
and 580 cal/g at 30°C.
Second, the vapor pressure in the overlying air must be less than that at the
evaporating surface since evaporation (a net transfer of water vapor) is zero if there
is no gradient in vapor pressure. The gradient in vapor pressure is controlled by the
capacity of the air to take up more water vapor and the degree of turbulence in the
lower atmosphere necessary to replace the saturated air near the evaporating
surfaces. The turbulence is caused by winds or by convectional currents. While
winds are not particularly stronger in the tropics than in other regions, convection
is frequent.
The air’s capacity to retain water vapor increases rapidly with temperature (Fig.
2.19). Warm tropical air masses can, therefore, take up more water vapor than
cold ones. The actual amount also depends on the air’s relative humidity — the
lower it is, the more favorable are the conditions for further evapotranspiration.
The dry tropics have, therefore, very high rates of evapotranspiration.
Third, water must be available for evaporation, this being a limiting factor under
dry conditions.
2.5.6. The concept of potential evapotranspiration
The term potential evapotranspiration. (PE), originally proposed by Thornthwaite
(1948), was defined as the water lost by vegetation if the soil is never water
deficient. PE was more explicitly defined by Penman (1948) as the amount of