Many parts of the mainland are sinking because of rapid urbanization and heavy use of underground water, a study conducted by the Chinese University of Hong Kong found.

Researchers projected that coastal cities like Shanghai subsided by 5 to 7 millimeters each year and would become unsuitable for human living in 100 years.

The university's Institute of Space and Earth Information Science found that urbanization had created immense pressure on the ecology and human living environment in China.

The urbanization rate in Dongguan city has increased from 13 percent in 1979 to 51 percent in 2000. Shenzhen is housing more than 10 million people on its less than 2,000 square kilometers of land, with an annual population growth rate of 15 percent from 1990 to 2000.

The trend created surging demands for housing, infrastructure and energy, the study said.

The institute assistant professor, Zhang Yuanzhi said urbanization would result in the constructing of surfaces which were impervious to water on the ground, such as asphalt and concrete roads. Increased prevalence of impervious surfaces would lead to a higher pressure in drainage and flood prevention of the cities, especially during heavy rains.

Underground water was also pumped away to fulfill the higher demand for drinking water, Zhang said. Combined with the effect caused by impervious surfaces, land subsidence would result.

Zhang said some 50,000 square kilometers of lands in 96 cities of the nation were subsiding. About 80 percent of these subsided lands were along the eastern seaboard. The situation was more serious in the Yangtze River Delta and the Bohai-Tianjin Region.

Underground water in some cities, such as Datong in Shanxi province, was pumped away for use in power plants. Mining in Luoyang also contributed to the problem.

"It is projected that the land subsidence is ranged from 5 to 10 millimeters a year. That figure superficially appears insignificant. But serious problem will result in a longer period," Zhang said, adding that some cities would not be suitable for human activities after the land subsidence.

The impervious surfaces also replaced the vegetated lands, making it more difficult for heat to be dispersed by plants.

The heat island effect would be more serious, Zhang said.

The study found that the impervious surfaces percentage for urban areas in Kowloon and Hong Kong Island had reached 60 percent or above. The percentage for the SAR's countryside was below 40 percent.

However, the land subsidence in Hong Kong was stable, he said.

Zhang said the pace of urbanization should be slowed down, and the building intensity should be reduced.

The over-use of underground water on the mainland should be restricted, he said.

"Measures should be taken to slow down the land subsidence," he added.

(China Daily HK Edition August 7, 2007)