Yoshida - 1981 - Fundamentals of Rice Crop Science

102 FUNDAMENTALS OF RICE CROP SCIENCE
2.5.9. The Penman equation
The Penman method for estimating PE combines the turbulent transfer and the
energy balance approaches (Chang 1967; Penman 1948, 1956; Ward 1975). It
requires observations on net radiation, temperature, humidity, and wind speed.
There are three basic equations, the first of which is a measure of the drying
power of the air. This increases with an increasing saturation deficit, indicating
that the air is dry and with high windspeeds:
E a = 0.35 ( e a – e d ) (0.5 + u 2 /100) (mm/day)
where E a = aerodynamic term of evaporation,
e a = saturation vapor pressure at
the mean air temperature (mm Hg),
e d = actual vapor pressure in air (mm Hg), and,
u 2 = wind speed at a height of 2 m above
the ground surface (miles/day).
(2.24)
The second equation provides an estimate of the net radiation available for
evaporation and heating at the earth’s surface:
R n = A – B (mm/day)
(2.25)
where A is the short-wave incoming radiation and B is the long-wave outgoing
radiation, as estimated in the following expressions:
(2.26)
(2.27)
where R a = theoretical radiation intensity at the ground
surface in the absence of an
atmosphere expressed in evaporation units,
r = the reflection coefficient of
the evaporation surface,
n/N = ratio of actual/possible
hours of bright sunshine,
s = Stefan-Boltzman constant,
T a = mean air temperature (°K), and
e d = as in equation 2.24.
The equations 2.24 and 2.25 can be combined when appropriate assumptions are
made:
where D = slope of the saturation vapor
pressure curve for water at
the mean air temperature mm Hg/°C, and
(2.28)