Vergara - 1976 - Physiological and morphological adaptability of ri

130 CLIAiL-YFE AND RICE
temperature and specific humidity between two heights above a rice field. Assuming
equality of Kb and Kw. the components of heat balance for a rice canopy
were calculated. Figure 7C represents the diurnal course in these components for
the middle part of the 1965 ricc-grovxing season (RG11. 1967b). Because the rice
canopy reached full cover and appreciable radiation was intercepted by the
canopy, the midday magnitude of (G + J) was substantially lower than that
observed in a water layer without a rice canopy (Yabuki, 1957). A large percentage
of Ru was dissipated by latent heat of evaporation from the crop. The heat
balance sheet (in percent) for the growing season (July to September) was:
Rm l00.0;l E, 78.8; C, 21.2 (RGE, 1967b).
The relation between Rn and 1 F. for the rice field nras approximately expressed
by
1 E = OBZ-Rn (23)
This relation can be used to estimate the evaporation from rice under wet climatic
conditions “there the effect of advection of heat energy from a warm and dry
air to the cooler wet rice area can be disregarded. Hoxi-‘ever. a different relation
may be observed over a wct ricc area under dry or arid climatic conditions.
Laurence and Pruitt (I971) observed the high amounts of latent heat flux which
exceeds net radiation by a factor of 1.56 under the influence ofa warm and dry
breeze.
In Japan, the vaiues of the ratio of field evaporation to pan evaporation
(diameter 20 cm) have been generally used as crop coefficient (f) in planning
field irrigation. Sato (1960) I-lanyu and Ono (1960). and lwakiri (1965) determined
the crop coefficient of rice fields using data of lysimeter measurements.
The coefticient ranged from 0.72 to 1.84 during the growing season, with an
average of 1.18. On the other hand, RGE (19678) reported, on the basis of heatbalance
analy-‘sis. that crop coefficients decrease gradually’ from 1.2 at transplanting
time to 0.59 at harvesting time‘ The discrepancy between the amounts of rice
evaporation evaluated by the tivo methods indicates the need for further studies
of the water consumption of rice fields. In the estimation of rice evaporation
from evaporation of a Class A pan. Lourence and Pruitt (1971) assumed a crop
coefficient approximately equal to unity. but Chapman and Kinininonth (1972)
used a crop coefficient of0.86 to estimate rice evaporation during the wct season.
Influence of canopy on plant environment
As already indicated, plant elements intercept much light and reduce air mixing,
thereby changing the energy partition of crop fields. From heat balance analysis,
Uchijima (1961) proposed the folloiving relations:
E, = EH11 - exp(- k|_L)}
14w
= 5.291971“ ki-Ll
(241
where ET, Ew, and E, are the transpiration from rice plants, the evaporation