Yoshida - 1981 - Fundamentals of Rice Crop Science

100 FUNDAMENTALS OF RICE CROP SCIENCE
Under most conditions, a decrease in the water depth is recorded every morning
and the pan is refilled to the specified level. The measurement is corrected for
rainfall. The evaporation rate is for the day previous to the measurement.
The ratio of field evaporation to pan evaporation is called crop coefficient or pan
factor. The crop coefficient value of irrigated rice obtained from the use of a U.S.
Weather Bureau Class A pan is reported to be 0.86 during the wet season in
northern Australia (Chapman and Kininmonth 1972).
2.5.8. Simplified energy balance method
Application of the law of energy conservation to the streams of heat energy
arriving at, entering, and leaving the earth’s surface leads to an equation of energy
balance at the earth’s surface where the energy conversion occurs (Davies and
Robinson 1969):
R n = E + H + G + ph,
where R n = net radiation,
E = energy used for evapotranspiration,
H = sensible heat,
G = soil-water heat flux, and
ph = energy used for photosynthesis.
(2.15)
Under most conditions, the net storage of heat by soil-water and the energy
captured by photosynthesis are small, and thus the equation (2.15) can be reduced
to:
R n = E + H.
(2.16)
For freely transpiring, well-covered surfaces with no water shortage in the root
zone, evapotranspiration may use most of the net radiation, and sensible heat may
approach zero. This evapotranspiration rate gives the maximum, which may be
similar to Penman’s potential transpiration or Thornthwaite’s potential evapotranspiration.
Thus, the potential evapotranspiration (PE) is given by:
R n
PE = ,
L
(2.17)
where L is the latent heat of vaporization (590 cal/g). The net radiation can be
directly measured with a net radiometer or can be estimated from solar radiation
data.
For a paddy field, the ratio of net radiation to total incoming short-wave radiation
varies from 0.70 at the early growth stages to 0.55 at ripening, with a mean value
of 0.62 (RGE 1967a). Thus, the average net radiation of a rice crop can be
estimated by:
R n = 0.62 × S.
(2.18)
Combining equation 2.17 with 2.18 gives the complete equation for potential
evapotranspiration (Yoshida 1979).