The most common pattern of drought and flood in China is "drought in the north and waterlogging in the South", which may occur once every four years, that is to say, in most years, the south is prone to waterlogging, while the north is prone to drought. The areas prone to waterlogging are mainly concentrated in the eastern region, especially in the middle and lower reaches of the Yangtze River and to the south, drought is most likely to occur in the North China Plain and the Northwest Shaanxi Gansu Ningxia region.
The most rare pattern is "waterlogging in the north and drought in the South", which occurred in the 17th and 18th centuries on average every nine years. For example, during the Qianlong period of the Qing Dynasty, there was continuous rain in the north from 1756 to 1757, and the situation of "waterlogging in the north and drought in the South" for two consecutive years was more rare in history.
Since 1950, China's climate has maintained a pattern of "drought in the north and waterlogging in the South". However, the precipitation in the arid and semi-arid areas of Northwest China has been increasing continuously in recent 30 years, showing a trend of "warm and humid".
The uneven drought and flood is closely related to China's vast territory and large interannual variability of monsoon climate. The abnormal change of summer monsoon will directly lead to the spatial and temporal differences of precipitation. When the summer monsoon is weak, the rain belt lingers in the south for a long time, resulting in long rainy season and heavy rainfall, which may cause waterlogging, and at the same time, short rainy season and less rainfall in northern China may lead to drought. At this time, drought will occur in the south. On the contrary, it will cause drought in the South and waterlogging in the north.
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