Spatial characterization of recent hot summers in Japan with agro-climatic indices related to rice production

Abstract

The spatiotemporal characteristics of high summertime temperatures, which can cause high-temperature injuries to rice, during the recent period of climatic variability (1978-2010) in Japan were analyzed and quantified by using daily gridded meteorological data with high spatial resolution and GIS data. Several indices based on heat-dose (defined as the cumulative temperature exceeding a certain threshold) were introduced to assess the impact of heat stress on rice production and quality. Specifically, we examined the utility of these indices for evaluating quality declines in rice. Time series of heat-dose indices based on maximum and minimum summertime temperatures showed that heat stress intensity increased remarkably from 1994. Moreover, high values of a heat-dose index based on a daily mean temperature exceeding 26°C during the 20 days after the heading date of rice showed a clear association with a decline in observed rice quality, whereas no clear relationship was apparent between decreased rice quality due to heat stress and average temperature. We also examined the effect of heat stress on spikelet fertility and ripening in the rice cultivar Koshihikari in four representative regions of Japan by using two indices, a heat-dose index based on a daily maximum temperature exceeding 35°C during 7 days around the heading date, and one based on a daily mean temperature exceeding 26°C during the 20 days after heading. The first index showed large interannual variability in the four regions because of its high dependence on the timing of heading.