Monday, March 7, 2011

Physical Geography of the Yellow River in China

The Yellow River originates from the Yueguzonglie Basin (4,500 m in elevation), which lies in the north side of the Bayankala Mountain, Qinghai-Tibet Plateau. It flows over nine provinces or regions, such as Qinghai, Sichuan, Gansu, Ningxia, Inner Mongolia, Shanxi, Shaanxi, Henan and Shandong, and finally enters into the Bohai Sea in Kenli county, Shandong province. The length of its trunk is 5,464km, and the fall head is 2,280km. Its drainage area is located within east longitude 96º-119º and north latitude 32º-42º. It runs about 1,900 km long from the east to the west and about 1,100 km from the north to the south with a drainage area of 0.795 million km² (including 42 thousand km² for the inner stream section). Together with the area affected by flood, it amounts to 0.915 million km².


          


Landform and Topography: The Yellow River's drainage area starts from the Bayankala Mountain in the west, to the Bohai Sea in the east, from the Yinshan Mountain in the north, to the Qinling Mountain in the south. It consists of 4 distinct landforms, such as Qinghai-Tibet Plateau, Inner Mongolia Plateau, Loess Plateau and North China Plain. The terrain is high in the west, low in the east, and is divided into three ladders. The first ladder is Qinghai-Tibet Plateau in the northeast of the Qinghai-Tibet Plateau which is 3000 ~5000m above the sea level and famous as ?Roof of the World? The series of mountains on it stretch from the northwest to the southeast, with snow and glacier on the tops all year around. The Bayankala Mountain on the south of the Qinghai-Tibet Plateau is the divide of the Yellow River and the Changjiang River. The Qilian Mountain on the north of the plateau serves as a boundary between the Qinghai-Tibet Plateau and the Inner Mongolia Plateau. The head-stream area of the Yellow River and its tributaries, the Heihe River, and the Baihe River, are plain district with grassland, lakes and swamps.



The second ladder ends at the Taihang Mountain in the east, and it is 1,000~2,000 m above the sea level. In this district, the north of the Baiyun Mountain including the Hetao Plain of the Yellow River and Erduos Highland is a part of the Inner Mongolia Plateau; on the south of the Baiyun Mountain lies the Loess Plateau, the Qinling Mountains and the Taihang Mountains.

Hetao Plain, 900 km long and 30~50km wide, is from Xiaheyan in Ningxia to Touketo in Inner Mongolia with an elevation of 1,200~900m. Owing to its plain terrain, rich soil and convenient irrigation conditions, this area is the chief agricultural base of Ningxia and Inner Mongolia. The Yinshan Mountain in the north and the Helan Mountain and the Langya Mountain in the west are barriers to block off the desert coming from the Tenggeli, Wulanbu and Badanjilin desert on the Arlashan Plateau from eroding the hinterland of the Yellow River drainage area.

The Erduos Plateau in the south of Hetao is surrounded by the Yellow River in the north, east , west and the Great Wall in the south, its area is about 0.13 million km2, and the elevation is 1,000~1,400m. It is a quadrate table-shaped and dry erosive highland. On the highland, wind-sand-physiognomy develops, and the north of the highland is Kubuqi desert, the south is Maowusu desert and there are many alkali lakes but few rivers. The edge zone of the Plateau is one of the main sources of the thick sand in the Yellow River.

The Loess Plateau is from the Riyue Mountain in the west to the Taihang Mountain in the east, the south border is the Qinling Mountain and the north border is Erduos Plateau, with an elevation of 1,000~2,000m. It is the largest area rich in loess in the world. Its physiognomy types include loess platform, girder, hills, ditch and so on. The Plateau, with greatly varied and rolling surface, relatively big height difference, deep layer of loess, loose layer of soil, fragmented landform, less vegetation, serious water-soil loss, is one of the main sources of the floods and sedimentation of the Yellow River. The Fenwei Basin, formed by loess pilling and river alluviation, belongs to the rift valley-structure-type basin. It is the rich area of Shanxi and Shananxi, because of the plain surface, fertile soil and long-history irrigation.

Qinling Mountain running through the south of the Loess Plateau is the divide of the sub-tropic zone and warm-temperate zone in China, and also the watershed of the Yellow River and the Changjiang River. It can block off the south warm-wet air current in summer and the north cold air current in winter. Taihang Mountain standing between the Loess Plateau and the north China Plain, is the watershed of the Yellow Rive and the Haihe River, and also an important geographical boundary in north China. The neighboring Funiu, Waifang, Taihang Mountains are barriers to keep the warm-wet air current from the south-east-sea from reaching the upper and middle reaches of the Yellow River, and they affect the climate of this area. This area is one of the important flood sources of the Yellow River because of the surface of the ground being in favor of elevating hydrosphere, rainstorm being strong and the favorable conditions of runoff generation and concentration.

The third ladder is from the Taihang Mountain to the Binhai Sea, consisting of the alluvial plain of the lower reaches of the Yellow River and the hilly area in the middle Shandong province. The alluvial plain is an important part of North China Plain, with an area of 0.25 million km² and no more than 100m above the sea level. The channel of the Yellow River is a divide, the north belonging to the Haihe River drainage area and the south to the Huaihe River drainage area. The gradient is small, water is not easy discharged and the disaster of flood, water logging, drought and alkali are very serious in the area. The hilly areas in Shandong province are composed of the Taishan Mountain, Lushan Mountain and Yimeng Mountain. Most of its altitude is 200~500m above sea level, only a small part of it is above 1000m.

        

Climate: The Yellow River basin is very huge with complex landforms. To its north is the sea, to its west is the plateau. The height difference between the west and the east is very large, and the climate is very distinct. So far as the monsoon is concerned, the area above Lanzhou in the upper reaches of the Yellow River belongs to monsoon area of the Qinghai-Tibet Plateau; the rest is monsoon area of temperate and sub-tropic zones. In terms of the climate, the southeast of the basin belongs to humid climate, the middle of it is half-drought climate, the northwest is drought climate. In winter, this basin is dominated by the high pressure of Mongolia, the north wind prevails, the climate is dry and cold and with scarce precipitation. In summer, the sub-tropic high pressure of the west Pacific Ocean becomes strong, warm-wet air mass from the sea enters this area, so the high pressure of Mongolia move northward gradually, the cold mass and the warm mass confront with each other, which results in much rainfall.

The precipitation in this area is mainly from rain, and the proportion of snowfall is small. The average annual rainfall is 370.1 billion cubic meters, taking up only 6% of the average of the whole country, namely 465mm deep (including inner stream flow area). The annual rainfall decreases from the southeast to the northwest. The district with maximum rainfall is at the north slopes of the Qinling Mountain, with an annual average of about 800mm, some area up to 900mm. The district with minimum rainfall is Hetao, with only 200~300mm annually, especially in the area from Hangjinhouqi of Inner Mongolia to Linhe, which is less than 150mm. In most of the basin, the annual rainfall is 400~600mm. The rainfall distributes very unevenly in different years. For example, its maximum in summer (June and July) takes up 54.1% of the whole year, the maximum month is July, takes up 22.1%; the minimum rainfall in winter (December and February), takes up 3.1%, the minimum month is December, takes up 0.6%. The coefficient difference of the annual rainfall is 0.15~0.40 in the whole basin.

The temperature in the Yellow River basin is higher in the southeast than in the northwest, and lower in the hills than in the plain. The average temperature of many years is 1~8ºC in the upper reaches, 8~14ºC in the middle reaches and 12~14ºC in the lower reaches. The average maximum temperature is in July, 20~29ºC in most areas and the supreme maximum temperature is 44.3ºC in Luoyang City of Henan Province. The minimum temperature is in January, under 0ºC in most areas, and the supreme minimum temperature is -48.1ºC in Maduo of Qinghai. The difference is 10~15ºC within the same day.



Water Resources: The average natural runoff of many years is 58 billion m³ in the Yellow River basin, taking up only 2.1% of the total in China, and take the 4th place among the seven large rivers of China. The annual average runoff depth is 77mm in the basin, only 28% of the average runoff depth of the whole country, which is 276mm. It is only slightly deeper than the Liaohe River among the seven large rivers of China. Each person in the drainage area takes 593 cube meters, about 23% of that of the whole country. The average water per mu of cultivated land takes is 324 m³, which equals to 18% of that of the country.

The distribution of the natural runoff of the Yellow River is uneven in different areas. The area of the upper reaches above Lanzhou takes up 29.6% of that of the country and the annual runoff is 32.3 million m³, taking up 55.6% of all the river. This area provides most coming water for the Yellow River. Though the basin area is increased by 163 thousand km² from Lanzhou to Hekou which takes up 12.5% of all the river, the runoff is decreased by 1billion cubic meters because of the dry climate, the huge loss of the evaporation and leakage of the riverbed in this area. The area from Hekou to Longmen takes up 14.8% of the whole basin area; the coming water is 7.25 billion m3, taking up 12.5% of the entire river. The area from Longmen to Sanmenxia takes up 25.4% of all the river area; the coming water is 11.33 billion m3, taking up 19.5% of the entire river. The area from Sanmenxia to Huayuankou takes up only 5.5% of all the river area, but the coming water is 6.08 billion m3, taking up 10.5% of the entire river and it is another district with more runoff generation and concentration. The area between Huayuankou and Hekou takes up 3% of the entire area, the quantity of the coming water is 2.1 billion m³, taking up 3.6% of the entire river. The natural runoff in flood season (July-October), at each station of the main stream, takes up 60% of that of the entire year, 40% is from other season. In flood season, the flood rises and falls sharply but the runoff is very small in winter. For instance, the peak runoff measured by Lanzhou Station (in upper reaches) in 1946 reached 5900 m³/s in the flood season; the minimum runoff is only 335 m³/s in other season, the difference is nearly 17 times. The peak runoff measured by Shanxian Station (in middle reaches) in 1933 reached 22000 m³/s, but the minimum is only 240 m³/s, the difference is 91 times.




With the development of national economy and the construction of water storage, diversion, pumping works, the annual consumption of the runoff of the Yellow River has reached 28~29 billion m³ in the 1980s, among which industry and rural consumption is about 1.1 billion cubic meters, the rest was consumed by irrigation. The utilization rate of the Yellow River's water recourse is about 50%, at a higher level, compared with other big rivers at home and abroad. The distribution of the Yellow River's runoff has changed very much because of the regulation of the Longyangxia Reservoir on the upper reaches of the Yellow River.

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