Yoshida - 1981 - Fundamentals of Rice Crop Science

CLIMATIC ENVIRONMENT AND ITS INFLUENCE 103
g = constant of the wet and dry bulb
psychrometer equation (0.49 mm Hg/°C).
The equation 2.28 is the final form of the Penman equation.
The quantity E will vary according to the value assigned to the reflection
coefficient r in the equation 2.26. Thus, if r is 0.25, E will represent the PE from a
short green crop; if r is 0.05, E willrepresent the evaporation from open water. The
net radiation ( R n ) can be measured directly with a net radiometer. If solar radiation
data are available, the net radiation of a rice crop can be estimated by equation
2.18.
The ratio, D / g, is dimensionless, and is a factor that makes allowance for the
relative significance of net radiation and evaporativity in total evaporation. For
example, at 10°, 20°, and 30°C, the D / g values are 1.3, 2.3, and 3.9, respectively.
Thus, during summer months when evaporation totals are high, the net radiation
term is given more weight than the evaporativity term. Furthermore, in humid
areas R n is usually greater than E a so R n tends to be the dominant term in the
equation.
A systematic comparison of the Penman method with seven other methods
indicated that only the Penman estimates correlate closely with pan evaporation
data throughout the year in Thailand (Juntharasri 1977).
2.5.10. The Thornthwaite method
Thornthwaite's formula for estimating potential evapotranspiration uses lysimeter
and watershed observations of water losses in the central and eastern US
(Thornthwaite 1948, Thornthwaite and Mather 1955, 1957). Physically, it is based
on the high correlation between temperature and solar radiation. His formula
reads:
(2.29)
(2.30)
(2.31)
Because it requires only monthly mean temperatures, Thornthwaite's method
probably has been most extensively used to compute potential evapotranspiration