Temporal and spatial distribution of seasonal drought in Southwest of China based on relative moisture index

Southwest of China is an important agricultural area, and seasonal drought is the most common agro-meteorological disaster in this region. Therefore, the study of temporal and spatial distribution of seasonal drought in Southwest of China is important for drought resistance and disaster reduction. In this study, daily precipitation, temperature, sunshine hours, relative humidity, wind speed and vapor pressure of 97 meteorological stations in four provinces (city) in the recent 50 years (from 1959 to 2008) were collected and used to calculate the relative moisture index (M). The spatial distribution of drought frequency and intensity in Southwest of China was analyzed at annual and seasonal time scale, and the annual variation of drought intensity and distribution in the recent 50 years were addressed. The results showed that, drought frequency at annual time scale was higher in the west, and lower in the east, exhibiting pronounced zonation patterns in Southwest of China. The areas with high incidence of drought mainly located at the western Sichuan plateau, the mountainous area in Southwestern Sichuan, and the regions of mountains, plateau and river valley in the northwest and north center of Yunnan. In these areas, drought occurred over twice in every three years, and drought intensities were mainly moderate or severe. The variation of drought frequency was significant between seasons. Drought happened most frequently in winter, followed by spring, autumn and summer. Winter drought was the most severe among the four seasons, followed by spring drought, autumn drought and summer drought. In general, the areas with high drought frequency normally had high drought intensity. In the aspect of annual variation, Southwest of China was getting wetter in recent years. Annual drought intensity was smaller, and drought was less severe in spring and autumn, but more severe in summer and winter over the past 50 years. In the last 10 years, annual drought intensity was obviously higher, and drought was more severe in summer, autumn and winter. This was consistent with the fact of high drought occurrence in Southwest of China over the past few years.