China is located in the southeastern part of the Asian continent. Its total area is 9.6 million km2, which accounts for 1/15 of the total land area of the earth. Topographically, China is divided from east to west into three areas consisting of plains, plateaus, and high mountains, which form a slope inclining toward the Pacific Ocean, with all major watercourses flowing from west to east. The highest area is the Qinghai-Tibet plateau, with an elevation higher than 4000 m and numerous mountains, valleys, and lakes. It is the source area of the Yangtze and Yellow rivers. The area to the north of the Qing-hai-Tibet plateau and the eastern part of Sichuan Province, with an elevation of 1000-2000 m, constitutes the second area of China. It is composed entirely of plateaus and mountains. The Daxing'an, Taihang, and Wushan mountains and the area to the east bordering the Yunnan-Guizhou plateau up to the coastal areas of China constitute the third area, where the hills and plains crisscross each other. Most hills are less than 1000 m, and the elevation of the coastal plain areas is less than 50 m.
The low coastal plains and hills of eastern and southern China make up 41% of the total area of the country and are densely populated, with about three-quarters of China's population. The principal plains are the northeast plain, the north China plain, and the middle and lower Yangtze River basin plain.
The vast eastern area and most of the south are affected by the eastern Asia monsoon climate. During the summer season, these areas are affected by the oceanic air current, and in winter the continental air current prevails. This results in dry winters and wet summers. The summer monsoon plays an important role in the formation of rainfall for various regions of the country. About 60-80% of the total precipitation falls during the 4 months of the rainy season.
Rainfall is unevenly distributed throughout the country, ranging from an annual average rainfall of 2000 mm at the southeast coast to 200-400 mm in the northwest. This uneven spatial and temporal distribution has caused repeated natural calamities in China. The average annual precipitation in the mainland has been estimated at 648 mm; that is equivalent to a volume of 6189 billion m3 (Department of Hydrology, Ministry of Water Resources, 1992).
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