Water Resources in China—2004.08.02
I. PHYSICAL BACKGROUND
China is situated at the southeastern part of Eurasia on the Northern Hemisphere, closed to the Pacific Ocean on the southeastern side, stretching into the hinterlands of Asia, bordered by the subcontinent of southern Asia. The total land area is 9.60 million km2, or equivalent to about 1/5 of the world total, ranking third among all countries in the world. China borders more than ten countries with the total length of land boundaries being more than 2000 km. China has a sea area of about 3 million km2, and more than 5400 islands of various sizes, in which Taiwan is the largest. The total length of coastal lines of the mainland and islands is 32000 km, in which the coastal line of the mainland occupies 18000 km.
The topography of China is characterized by high west part and low east part, gradually descending in elevation from the west towards the east to form three steps. The highest step is the Qinghai-Tibentan Plateau with the elevation being generally more than 4000 m above sea level, hence known as "the roof of the world". To the east of the Qinghai-Tibetan Plateau is the second step with much lower elevations, ranging between 1000 m and 2000 m and alternating with huge basins. The third step stretches from the line in the west composed of Mounts Daxing'an, Taihang, Wu and the eastern rim of the Yunnan-Guizhou Plateau to the coast in the east, and embraces alternating hills and plains, and there are large areas of low mountains and hills at elevations of less than 500 m above sea level with only a few peaks being at 2000 m, and the elevation of plains is all less than 200 m. The plains are distributed in the north to south direction, including Northeast Plain, North China Plain, Middle to Lower Yangtze Plain and Pearl River Delta Plain, being important industrial and agricultural bases and economic centers of China.
Classified according to types of topography, mountains account for 33.3% of the national land area, plateaus 26.0%, basins 18.8%, hills 9.9% and plains 12.0%. Mountains, hills and rugged plateaus, which are customarily called mountainous areas, totally account for 2/3 of the national land area.
As affected by the vast area and complex topography, China's climate is endowed with a feature of complex types. China is subject to strong monsoon climate and most of the areas
are under the impacts of the southeastern and southwestern monsoons, thus making the southeastern China wet and the northwestern China dry. The precipitation varies greatly from year to year and from season to season. As affected by monsoon and topography, China becomes one of the countries with most frequent drought, flood and waterlogging hazards in the world.
The average annual precipitation in China is 648 mm, or 19% less than the world average 800 mm on land, and the volume of average annual precipitation is 6190 billion m3. The regional distribution of precipitation is very uneven, showing a general trend of gradually descending in magnitude from the southeastern coastline to the northwestern hinterlands. The annual precipitation in the southeastern coastal areas and parts of the southwestern areas is more than 2000 mm, but that in the northwestern China is usually less than 200 mm, and 50 mm in the Tarim and Turpan basins in Xinjiang and Chaidamu Basin in Qinghai, etc, even less than 25 mm at centers of those basins.
As affected by the monsoon climate, the precipitation shows not only a big inter-year variation, but also wet spans and dry spans, and inter-year variation is bigger in areas or for seasons with less precipitation. The ratio of maximum annual precipitation to minimum at observation station has the biggest value in the northwestern China, possibly exceeding 8, but only has values ranging 2-3 or less than 2 in the southern and southwestern China. Except in a few small special areas, precipitation is usually concentrated in summer and mainly occurs in rainstorms. In most of the areas, the ratio of average maximum precipitation of four consecutive months to average annual total ranges 60-80% from the south to the north. The average annual evaporation from water surface also varies greatly over regions, with the lowest value being only 400 mm, and the highest up to 2600 mm. The low values mainly occur in mountainous areas and the high ones on plains and plateaus. The average annual land evaporation also shows a trend similar to precipitation, descending in magnitude from the southeast to the northwest. It has a value of 700-800 mm in the eastern areas and usually less than 300 mm in the western areas.
3. Rivers and River Basins
China has a large number of rivers with a total length of 420000 km. There are more than 50000 rivers each with a drainage area of more than 100 km2 and more than 1500 each with a drainage area of more than 1000 km2. As affected by the topography and climate, the rivers are distributed very unevenly over regions. Most of the rivers are situated in the wet eastern monsoon climatic zone, directly flowing into the sea, with the major ones including the Yangtze, Yellow, Heilong, Pearl, Liaohe, Huaihe, Haihe, etc. The northwestern China is dry
with slight precipitation, and has only a small number of rivers, most of which are inland ones without connection to sea, and there are also large none-runoff areas.
The whole country may be divided into nine major river basins, including Yangtze, Yellow, Haihe, Huaihe, Songhua-Liaohe, Pearl, Southeast Rivers, Southwest Rivers and Inland Rivers, by combining a large river with nearby smaller rivers flowing directly to the sea or out of the region or by combining a number of neighboring rivers in a region. The Songhua-Liaohe, Haihe, Yellow, Huaihe and Inland Rivers are practically situated in the northern areas of China and the Yangtze, Pearl, Southeast Rivers, Southwest Rivers in the southern areas (see attached figure in Annex).
II. WATER RESOURCES
Water Resources are referred to the fresh waters that are renewable, closely related with uses by human societies and ecosystems, including both surface water and groundwater with precipitation being the main source of recharge.
1. Surface Water Resources
The volume of surface water resources is referred to the dynamic volume of water in rivers, lakes, glaciers, etc and generally expressed in volume of natural river runoff. The volume of annual river runoff in the country is 2711.5 billion m3, equivalent to a depth of 284 mm over the whole land area. The direct recharge by precipitation accounts for about 71% of this volume, and indirect recharge by precipitation via aquifer 27% and recharge by glacier and snow melt 2%.
The depth of natural river runoff, which depends on precipitation and conditions of watersheds, gradually descends in magnitude from the southeast towards the northwest, having values of more than 1000 mm in some southeastern areas and values of less than 10 mm in some western areas. The total volume of river runoff in the five northern river basins account for less than 20% of the national total, but the four southern river basins account for more than 80% of the national total.
The river runoff shows inter-year variation similar to precipitation, but to a bigger extent with the impacts of watershed. The ratio of maximum annual runoff to minimum usually has a value of less than 5 in the southern areas, but may exceed 10 in the northern areas. The intra-year variation of river runoff is also big as affected by the size of river basin as well as intra-year variation of precipitation. The rivers in the northern areas show a bigger intra-year runoff variation with the maximum runoff of consecutive four months accounting for more
than 80% of the annual total in some areas, and this ratio has a value of about 60% for rivers in the southern areas.
Table 1 Average annual river runoff by river basin
2. Groundwater Resources
The volume of groundwater resources is referred to the dynamic volume of water in aquifer that is directly related with precipitation and surface water via recharge and/or discharge. Most of runoff formed by precipitation flows into rivers, but there is a part of runoff that percolates into underground to form groundwater. The volume of groundwater resources is usually expressed in terms of volume of groundwater recharge. As activities of human beings affect groundwater recharge to some extent, the volume of groundwater resources is defined under specific conditions of development and utilization of water resources. The annual volume of groundwater resources in China is 828.8 billion m3, in which groundwater in mountainous areas occupies 676.2 billion m3 and that in plain areas 187.4 billion m3 with 34.8 billion m3 of overlapping volume between them. Most of groundwater in mountainous areas flows out as base flow of rivers, thus overlapping with surface water resources and being mainly exploited as surface water. The groundwater resources in plain areas are mainly in the northern areas and the five northern river basins occupy 78.8% of national groundwater resources in plain areas. Table 2 shows groundwater resources by river basin.