Modeling the Water Temperature of Paddy Fields to Estimate Climate Change Impacts on Areas and Timings Suitable for Rice Cultivation in Japan.

The temperature of water in a paddy field is an important factor affecting the growth and yield of rice (Oryza sativa L.). We developed a water temperature model to estimate the impact of climate change on the thermal environment of a paddy field and on the period suitable for rice cultivation. The model was based on energy balance equations both for water surface and vegetation, and was designed to make use of daily meteorological data, that is, air temperature (daily mean, maximum, and minimum), water vapor pressure, wind speed, and solar radiation. The validity of the model was tested using observations conducted at three experimental sites in a temperate climate. The seasonal variations in the modeled water temperature showed good agreement with observed values, with root mean square errors of 0.8–1.3 °C. On the basis of this model, the impact of climate change on the water temperature of unvegetated paddy fields and the early limit date of transplanting (ELT) for rice cultivation in Japan over the past 30 years were estimated using 1 km grid square meteorological data. The ELT was defined as the earliest date in the year when the mean daily water temperature exceeded 15 °C. Water temperature showed an increasing trend as air temperature increased. In addition, the ELT generally came earlier in the year, and areas in which the ELT occurred before April expanded considerably. The proposed model can be applied to any given location and climate condition, making it possible to predict the location and extent of agricultural areas impacted by climate change, as well as the timing of rice cultivation in those areas.

Acknowledgements:
This work was supported by Cabinet Office, Government of Japan, Cross-ministerial Strategic Innovation Promotion Program (SIP), "Technologies for creating next-generation agriculture, forestry and fisheries."