Vergara - 1976 - Physiological and morphological adaptability of ri

MICROCLIINIATE or Tin: RICE CROP 125
et al.. 1974). An exponential profile of wind velocity’ observed in rice canopies
agreed well with theoretical results (e. g. Inoue. 1963).
The aerodynamical behavior of the rice crop is characterized by z“ and d.
These two quantities change with the growth of crops and plant type as tvcll
as wind velocity. Although there is somewhat large scatter in Fig. 5B, records
obtained throughout the growing period of rice plants yielded the folloiving
relations:
d = 1.04 h°*‘=
Z0 = 0.025 hm
(16)
Tani (1963) showed that the acrodvnamical properties of rice crops are also
affected by the wind blowing over them. His measurements seem to indicate
that the value of d decreases monotonically with increasing wind velocity’,
while the value of zo is an increasing function of wind velocity". The observed
behavior of d and zn may be ascribed to the bending of rice plants in wind.
In connection with the evaluation of frictional drag imposed on the atmosphere
by a rice crop, the values of drag coefficient (CD) were calculated from
w = 2r"
L”
ln 2;+Zn_d
Zn
where z, is a comtenient height. The results showed a steep decrease of CD
with wind velocity from the order of 0.1 in light winds to the order of 0.01 in
strong ‘winds (Tani, 1963). This means that the surface drag of a ricc crop is
considerably reduced by the bending of rice plants.
Turbulence in rice canopies
Using hot-wire anemometers. Nakagawa (1956) measured the wind fluctuation
in and above a rice canopy in relatively strong winds. He concluded that fundamental
relations deduced from theories for local isotropic turbulence can
applied to study statistical properties of the air flow in plant canopies. In strong
winds. the values of turbulent intensity
time mean of wind u) were found to be nearly‘ constant in the rice canopy at
about 0.3. More recently‘. Kobayashi (1973) observed a very abrupt increase in
the magnitude of (mu descending from the surface air layer into the rice
canopy. The turbulent intcnsitv in a rice canopy was three times as large as that
in the surface air layer. In light itvinds, the values of
with the movement from the top of the canopy to the inside of the canopy’ as
shown by Uchijima et al. (1974). We also reported that the magnitude of tur~
bulcnt intensity; of the vertical component of wind (fin) in thc rice canopy
was about one-third that of the horizontal wind component.
Uchijima et al. (1974) used wind data obtained with supersonic and heatedthermocouple
anemometers to study characteristics of the energy distribution
of wind at each frequency and thc effects of plant elements on the energy dis-
(l?)
be