Yoshida - 1981 - Fundamentals of Rice Crop Science

CLIMATIC ENVIRONMENT AND ITS INFLUENCE 101
R n 0.62
PE = = • S
= 0.0017 (0.62 • S )
L
L
= 0.0105 • S (mm/ T ), (2.19)
where S is expressed in calories per square centimeter per T, and T can be day,
week, or month. This equation implies that the potential evapotranspiration is
about 1 mm/100 cal. incident solar radiation.
Even under flooded conditions, however, sensible heat is not negligible (RGE
1967b). When loss of energy by sensible heat is taken into account, equation 2.19
is modified into:
0.82 . R n
E R = = 0.0086
L
. S (mm/ T ), (2.20)
where E R is a realistic estimate of evapotranspiration from a paddy field after the
crop has been established.
Although the above energy balance equation has been derived in a very simplified
manner, estimated potential evapotranspiration agrees well with direct
measurements of evapotranspiration and agronomic experiments.
Agricultural engineers at the International Rice Research Institute (IRRI) measured
evapotranspiration (ET) from a 400-m 2 field with minimized seepage loss and
obtained the following empirical formula:
ET = 0.15 + 0.01072 . S. (2.21)
The first term of equation 2.21 was the average percolation and seepage rate
during the measurement (IRRI 1965). Good agreement between equations 2.19
and 2.21 may support the common belief that the ET of flooded rice is essentially
equal to potential evapotranspiration.
The potential evapotranspiration computed from equation 2.19 correlates well
with the observed U.S. Weather Bureau Class A pan evaporation rate at IRRI, Los
Baños, Philippines:
PE = 0.93 × pan evaporation. (2.22)
Agronomic experiments on irrigation requirements in IRRI’s fields indicate that
about 7 mm water/day is necessary after the stand establishment to achieve the
maximum dry season yield (IRRI 1970). The same conclusion can be obtained
from estimates of evapotranspiration and percolation plus seepage in the following
way: Given a value of 14,964 cal/cm 2 for April 1970, the daily evapotranspiration
is obtained by equation 2.20:
0.0086 × 14,964 = 4.4 mm/day.
E R =
30
(2.23)
For IRRI fields, the best estimate of the daily percolation plus seepage is about 3
mm for the dry season. Thus, the total water consumption (evapotranspiration +
percolation plus seepage) in an IRRI field becomes 7.3 mm/day, which agrees well
with the agronomic field experiments.