Water crises and hydrology in North China
The world is facing increasing problems of water crises related to floods, droughts and water pollution. This is particularly so in China. To overcome these problems, a new ethic of sustainable development is called for. This not only requires the reinforcement of established principles and technologies, but also an improved understanding of hydrological processes and their application to the realization of integrated water resources management. Hydrological processes exert a key influence on water resources management. Water supplies are threatened by climate change and by overexploitation of the resource. In most parts of the world the demand for water is rising, as a result of both population increase and increased standards of living. Competition for the resource is becoming more intense, and ways of sharing water in an equitable manner are posing great challenges to water managers, particularly in river systems shared by several nations. Preserving fragile and vulnerable ecosystems while fostering development is especially difficult.
China is the world's largest developing country, and its vast territory encompasses many zones with different climatic and geographical characteristics. China has a total area of 9Γ6 Γ° 10 6 km 2 , representing 6Γ4% of the total land area of the Earth. North China (NC) is recognized as the political, economic and cultural centre of the country, and this is where Beijing, the capital of the nation, is located. NC covers the area of the Huang (Yellow River)-Huai (Huai River)-Hai (Hai River) Plain. Its population in 2000 was about 437 million, accounting for approximately 35% of the total population of China. The GDP of NC is US$ 386 billion, representing about 32% of the total for China. The irrigated areas of NC comprise 42% of the total irrigated area of China and NC accounts for 40% of the total agricultural production of China. However, the water resource per capita in the region is only 501 m 3 , which is only 25% of the national average and one-fifteenth of the world average. As a result, water crisis issues and related eco-environmental degradation within NC represent one of the major problems currently facing China. As runoff generated from upland areas is significantly decreased, resulting in overexploitation of the regional water resources, serious water and eco-environmental problems arise. These include drying-up of river courses, groundwater depletion, lake and wetland recession and degradation, seawater intrusion along the coastal zones, and the occurrence of water pollution in most streams. In the case of the Yellow River (China's 'Mother River'), its flow has frequently ceased in its lower reaches since the 1970s. During the 27 years from 1972 to 1999, drying up has occurred 21 times, i.e. about four times in every 5 years.
Overextraction of groundwater found in the North China Plain (NCP) has now extended to cover an area of nearly 90 000 km 2 , representing 70% of the northern part of the NCP. Since the 1950s, the accumulated overextraction of groundwater has increased to 90 Γ° 10 9 m 3 . Areas subject to problems of soil loss and runoff reduction in the mountainous regions now cover 550 000 km 2 , equivalent to two-thirds of the mountainous regions. Sandstorms, resulting from desertification, are endangering Beijing and other cities. Thus, the problems of water shortage and related eco-environmental issues in NC have become the most significant issue to impact sustainable development in this very important region.
This Special Issue addresses these emergent water crisis issues by presenting case studies from NC. Advances in process understanding and the causes of these problems, linked to both natural change and human impact, are addressed. Key topics include the following: (a) studies of hydrological processes impacted by climate change and highly intensive human activity; (b) water consumption by agriculture, and associated hydrological processes, including the dynamics of evapotranspiration and subsurface water, as well as water