Dry-wet evolution and its influencing factors in single cropping rice growing season in Southwest China

Research on regional dry-wet evolution and its influencing factors during the crop growing season is critical for sustainable agricultural development and food security. In this study, which is based on the daily meteorological data from 316 meteorological stations in the rice-growing area of Southwest China from 1961 to 2015, and on calculation of the ratio of precipitation to reference crop evapotranspiration (ET₀) in a single cropping rice growing season in Southwest China, the characteristics of dry-wet evolution from 1961 to 2015 were analyzed. The sensitivity of ET₀ to the main climatic factors and attribution rates to ET₀ of the main climatic factors were studied, and the factors influencing dry-wet evolution were also discussed. The results showed that the semi-humid climatic regions with a single cropping rice growing season in Southwest China were mainly distributed in the south of Panxi area of Sichuan and the center and northeast of Yunnan, but that the rest of Southwest China comprised humid climatic regions. Compared with 1961-1990, the area of the semi-humid climatic region in Southwest China increased during 1991-2015, while the area of the humid climatic region decreased. During the single cropping rice growing season, 40.8% of the sites in Southwest China became wet from 1961 to 2015, and the rest of the region became dry. The climate became wet in the northeast of Sichuan basin and the northeast of Yunnan because of increasing precipitation and decreasing ET₀. The positive effect caused by the increasing precipitation on the humidity index exceeded the negative effect caused by the increasing ET₀, which resulted in the climate becoming wet in the Panxi area of Sichuan. The positive effect caused by the decreasing ET₀ on the humidity index exceeded the negative effect caused by the decreasing precipitation, which resulted in the climate becoming wet in the south of Chongqing, and in the north and west of Guizhou Province. In contrast, most of Yunnan showed a dry trend with decreasing precipitation and increasing ET₀. The climate became drier in other areas of Southwest China because the negative effect caused by the decreasing precipitation on the humidity index exceeded the positive effect of the decreasing ET₀. During the single cropping rice growing season, the ET₀ decreased with increasing average temperature and relative humidity in Southwest China, but the ET₀ increased with increasing sunshine hours and wind speed. The significant decline in sunshine hours and wind speed in Southwest China was the main reason for the decrease in ET₀. The study provided a scientific basis for reduction of the possible climate risks of the single cropping rice growing season in Southwest China under global climate change.