Heat-Mitigation Effects of Irrigated Rice-Paddy Fields Under Changing Atmospheric Carbon Dioxide Based on a Coupled Atmosphere and Crop Energy-Balance Model
2021
Ikawa, Hiroki | Kuwagata, Tsuneo | Haginoya, Shigenori | Ishigōoka, Yasushi | Ono, Keisuke | Maruyama, Atsushi | Sakai, Hidemitsu | Fukuoka, Minehiko | Yoshimoto, Mayumi | Ishida, Sachinobu | Chen, Charles P. | Hasegawa, Toshihiro | Watanabe, Tsutomu
Known as the heat-mitigation effect, irrigated rice-paddy fields distribute a large fraction of their received energy to the latent heat during the growing season. The present hypothesis is that increased atmospheric CO₂ concentration decreases the stomatal conductance of rice plants and increases the air temperature by means of an increased sensible heat flux. To test this hypothesis, a coupled regional atmospheric and crop energy-balance model is developed and applied to a 300 × 300 km² region in Japan. Downscaling meteorological variables from grid-mean values of mixed land use (3 × 3 km²) generates realistic typical diurnal cycles of air temperature in rice paddies and adjacent residential areas. The model simulation shows that, on a typical sunny day in summer, doubling the CO₂ concentration increases the daily maximum grid-mean air temperature, particularly where rice paddies are present, by up to 0.7 °C. This CO₂ effect on the grid-mean air temperature is approximately half the effect of the reduction in rice-paddy area that is postulated to occur on a time scale similar to that of the atmospheric CO₂ change. However, within the internal atmospheric boundary layer of the rice paddies, the CO₂ effect on the air temperature (+ 0.44 °C) still exceeds the effects of the land-use change (+ 0.11 °C). These results show a potentially important interplay of plant physiological responses regarding atmospheric CO₂ in the heat-mitigation effect of rice-paddy fields under a changing climate.
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