Fluxes of carbon dioxide and water vapor above paddy fields

Atmospheric fluxes of carbon dioxide and water vapor were measured by the eddy correlation technique over a paddy field in 1989. The carbon dioxide was transported downward during daylight hours due to photosynthesis of the paddy crop. The downward flux of carbon dioxide increased with increasing net radiation. Maximum values of downward flux varied with the growing stage of the paddy crop: ca. 0.3 mg m−2 s−1 at early vegetative growth stage and ca. 1.3 mg m−2 s−1 at ear formation stage. The daytime totals of downward flux of carbon dioxide also showed seasonal variation reflecting the photosynthetic activity of the paddy crop: ca. 6 g m−2 at early vegetative growth stage in June and 40 g m−2 at ear formation stage in September. The seasonal variation of daily totals of carbon dioxide flux shows that carbon dioxide of about 28 t ha−1 is fixed by the paddy crop from transplanting to harvesting. Taking into account the water use efficiency, the paddy crop requires water in amounts at least 100 times that of carbon dioxide fixed by photosynthesis. It is noted that the correlation coefficients between carbon dioxide, water vapor and vertical wind velocity have constant values under near neutral and free convective regimes.