Water temperature and the growth of rice plants are interrelated and influence each other. To elucidate this relationship, and in particular, to quantify the effects of the growth of rice plants on water temperature, this study focused on the difference between the temperature of paddy water and that of open shallow waters devoid of rice plants. Here, we simultaneously analyzed the seasonal changes in the difference of water temperatures and the growth of rice plants. A water temperature model based on the energy balance equation and the average climate data from 1971 to 2000 were used to estimate the daily mean temperature of paddy waters in conjunction with the growth of rice plants during the growing season in Japan at a spatial resolution of approximately 1 km × 1 km. The most notable result from the simulation experiment is the fact that the temperature of paddy waters increases by approximately 0.3°C from spring to early summer and decreases by approximately 1.5-2.0°C in midsummer compared to that of open shallow waters. This is because the net radiation at the surface of paddy waters is affected not only by climate factors but also by the increase in the leaf area index of rice plants. The net radiation and the temperature of paddy waters during the growing season are significantly determined by the balance of the downward flux of long-wave radiation emitted from the plant layer and the extent to which short-wave radiation is intercepted by the foliage of rice plants. Therefore, even under identical climate conditions, a shift in the transplanting date could affect the temperature of paddy waters due to an increase in the leaf area.
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