Spatio-temporal characteristics of crop drought in southern China based on drought index of continuous days without available precipitation

This study was based on daily precipitation data from standard meteorological stations in the 15 provinces (municipalities, or autonomous regions) in southern China. We adopted continuous days without available precipitation (Dnp) as drought index, also improved the critical values of available precipitation and drought classification standard during the data process, then calculated drought index values for crop (spring sowing-summer harvesting crop, spring sowing-autumn harvesting crop, summer sowing-autumn harvesting crop, and overwintering crop) during the most recent 50 years (from 1959 to 2009) in southern China. We analyzed the spatial distribution characteristics and inter-annual variation of crop drought frequency and crop drought duration days. In addition, we introduced daily drought frequency to study dynamic change of crop drought during the growing period. The results showed: spring sowing-summer harvesting crop drought occurred sometimes in the west of Southwest China and part of Huaibei Area during spring; spring sowing-autumn harvesting crop drought often affected the middle and lower reaches of Yangtze River, as well as the northeast of South China and the east of Southwest China during summer and autumn; summer sowing-autumn harvesting crop drought often occurred in the middle and lower reaches of Yangtze River during autumn, as well as the east and north of South China; overwintering crop drought took place in the north of Yangtze River and South China during autumn and spring, especially drought occurred frequently in the west of Southwest China during autumn to next spring. Generally, the distribution of drought duration days without available precipitation was basically consistent with the distribution of drought frequency, which meant drought lasted relatively longer in drought-prone area. The characteristics of drought change trend in southern China showed that: spring sowing-summer harvesting crop drought showed a decreasing trend in covering area as well as a reducing trend in intensity; spring sowing-autumn harvesting crop drought showed a slightly increasing trend in covering area yet a slightly reducing trend in intensity; summer sowing-autumn harvesting crop drought showed a significantly increasing trend in covering area and a slightly increasing trend in intensity with an exception of the Southwest China showing a reducing trend in intensity; overwintering crop drought showed a relatively more significantly increasing trend in covering area and an reducing trend in intensity with an exception of South China showing a increasing trend in intensity. From the perspective of daily drought frequency, early spring in the south and west of South China as well as the north of Yangtze River, together with spring in the west of Southwest China, showed a relatively higher drought frequency, hence affected spring sowing crops; midsummer drought in July and August in the middle and lower reaches of Yangtze River, autumn drought in September and October in South China, together with midsummer and autumn drought in the east of Southwest China mainly affected autumn harvesting crops; overwintering crop showed a relatively higher drought frequency in the whole study area, and the main drought period was in autumn and wintering period, however, early spring in the north of Yangtze River, as well as the south of South China, and Southwest China showed a relatively higher drought frequency.