Vergara - 1976 - Physiological and morphological adaptability of ri

1\-tlL?ROC.LI1\-I.l'(1‘}i or 'I'H.E RICE CROP 123
The net radiation available for warming of plants and underlying surfaces and
for evaporation is given as the balance of incoming and outgoing radiant fluxes.
The net radiation of the crop canopy is given by
Rn z
where R, is the net radiation, p, is the albedo. ,5, is the total shortwave radiation.
and L“ is the net loss of long-wave radiation. Long-period measurements
of the radiation balance above a rice field gave the following balance sheet for
the whole growing season of rice plants in units of MJImP ,8. = 1488, ptldS, =
260, (1 — p.) dS, = 1228. I.., = 306, and R, = 622 (RGE. 1967a). When rice
plants fully covered underlying surfaces. the following relation was also obtained
between R, and ,5, (in W/ml) above rice canopies (Franeesehini. 1959; RGE.
1967a):
R» = 11F asi
_ n:
Measurements showed the values of n, to be between 0.8 and 0.85 and the values
ofn, to range from 34.85 to 55.76 xiv/m? The above relation can be used to detennine
the net radiation of fully grown rice fields from data of solar radiation.
The attenuation of net radiation within the rice crop is also expressed by an
exponential relation as
RAL) = R10) ¢XP(~ LL) (13)
where R,,(()) and R,,(L) are the net radiation at the top of the canopy and at a
leaf area index of L. respectively‘. and k, is the extinction coefficient of net
radiation. The values of k, were found to range between 0.5 and 0.6 (Uehijirna,
1961; Iwakiri. 1964). The vertical profiles of R, calculated from Eq. (l3) was
used to evaluate the profile of leaf evaporation in a rice canopy (Uchijima.
1962a).
WIND REGIME OF THE RICE CROP
Since the wind regime is one of the most important factors controlling the
exchange of atmospheric constituents between vegetated surfaces and the air.
many workers have measured and analyzed the wind regime in and above rice
fields (eg. Nakagawa. I956; Inoue. 1963'. Tani. 1963; Seo and Yamaguchi.
1968).
Aerodynamical characteristics of rice field
Figure 5A illustrates the mean wind profile above and in a rice cantipjv. The most
obvious feature is the verv rapid change ofwind velocity near the top of the cancipv
This is because plant elements extract momentum b_v drag. The wind profile
in the air layer above the canopy can be expressed as
uz (~)= v.1 ~d+-~d _
A__ n{ZZ° tt%}zgd
2., (l4)