Main Causes of Drought

The possible factors responsible for droughts in China can be categorized as natural and, to some extent, cultural, as described in the following sections.

Natural Factors

Meteorological Factors The main agricultural region in eastern China is influenced by the East Asia monsoon. In this region, the distribution of precipitation is uneven and annual variation is high. The precipitation zone jumps between the northern and the southern parts of the country according to a tropical high-pressure concentration during the April-September period, resulting in such an uneven precipitation distribution that it causes storms and floods in some regions and droughts in other regions. In addition, movement of precipitation zones also causes droughts. Lack of rains along the Yangtze River during summer can also lead to serious droughts.

The Chinese climate is mainly controlled by four semipermanent atmospheric activities: the Mongolia high pressure and the Aleutian low pressure during winter and the India low-pressure and the Pacific tropic high pressure during summer. When the Mongolia high-pressure is strong and the Pacific tropic high pressure is weak during winter, cold air drives straight into the south from the north, and the whole country experiences a cold air mass. As a result, the weather turns cold and dry (because of no rains). In summer, when the Pacific tropic high pressure and the India high pressure are strong and active, the major part of the country is influenced by warm and wet airflow from southeast and southwest. Consequently, rains occur during summer. The rainy season in China is mainly from April to September. However, the spatial distribution of precipitation and its intensity differ from year to year depending on the timing of northward movement of subtropical high pressure, the active range of the high pressure, and the large annual variation in other weather systems.

The active intensity of the western Pacific tropical high pressure is related to ENSO (El NiƱo/Southern Oscillation; chapter 3). When the ENSO

Figure 28.2 Spatial distribution of drought intensity and frequency in China, 1951-91.

Figure 28.2 Spatial distribution of drought intensity and frequency in China, 1951-91.

develops, the western Pacific tropical high pressure is strong, and its region of influence is relatively greater toward the west and the north. This results in floods in the Yangtze and the Huaihe rivers during summer due to above-average precipitation and results in droughts along the Yellow River in northern China due to below-average precipitation. Meanwhile, the south of the lower reaches of the Yangtze River experiences drought. When the sea temperatures in the East Pacific recover and their deviations are still positive, below-average precipitation occurs along the Yangtze and Huaihe rivers during summer, leading to drought. During an ENSO year, below-average precipitation occurs along the Yellow River in southern and northern China, which also causes drought.

Distribution of Water Resources Shortage of water resources is one of the reasons for drought occurrence, particularly in the north. The national average of water availability exceeds 27000 m3/ha, but only 4500 m3/ha of water is available along the Yellow and the Huaihe rivers. Water availability is even lower (about 200 m3/ha) along the Liaohe and the Hailuanhe rivers, which does not satisfy the water needs of crops at all. The national program on "water transfer from south to north" that the government is planning to undertake will alleviate the existing drought situation in northern China.

Cultural Factors

Due to the rise in industrial production and crop yields, the demand for agricultural water has increased four to five times as compared to demand in the 1950s, but the water supply has remained limited. For example, at some places two-crops-in-one-year system is transformed into three-crops-in-one-year system. This is one of the reasons that drought-damaged areas have expanded every year rather than shrunk despite the post-1949 improvement of irrigation facilities by the government. Also, sometimes farmers use water carelessly, and soil erosion occurs due to farming on steep hills. Some farmers use chemical fertilizers instead of natural fertilizers, which deteriorate soil quality and lower the water-holding capacity of soils. Such cultural activities contribute to drought occurrence, for instance, in the North China Plain.

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