Impacts of climate changes on the temperature of paddy waters and suitable land for rice cultivation in Japan

A model for the energy balance of rice fields was improved by using meteorological and geographical data to simulate the changes in the water temperature resulting from plant growth. The average climate of Japan during the period 1971–2000 was used as a baseline. The improved model was used to assess the possible effects of the future climate (2081–2100) on agricultural practices at a spatial resolution of approximately 1 km2. The most notable result from the simulations is that the water temperature during the growing season for the future climate increased by approximately 1.6–2.0 °C throughout the country. This increase can lead to a remarkable northward shift of the isochrones of safe transplanting dates for rice seedlings. This means that the rice cultivation period will be prolonged by approximately 25–30 days. Such an increase in the thermal resources allows greater flexibility of variation in the cropping season as compared with that at present; thus, resulting in a reduction in the frequency of cool summer damage in the northern districts. The area of safe cultivation expands to the northernmost region, if all the forests in the climatically suitable areas can be converted into rice fields. Conversely, climate warming will also induce high-temperature stress in rice plants in one-fifth of the current total cultivation area. The current agricultural practices and rice cultivars used in these areas will inevitably require altering to prevent the projected heat stress during summer.