China's major cities rely heavily on water collected in catchment areas that are under threat from encroachment and pollution, according to a new report from The Nature Conservancy, a leading global conservation organization.

The report, which was released on Monday, said Beijing, Shanghai and 28 other major cities depend on surface water for daily consumption, but it noted that 73 percent of the water catchment areas that supply that surface water were impacted by medium to high pollution levels.

The nation's 30 largest and fastest growing cities have seen a surge in demand for water because of their rapid expansion, and they now consume around 29.6 billion cubic meters of water annually, the report said.

The report said the vast majority of water consumed by the major cities, 93 percent, came from collected surface water. The other 7 percent was drawn from underground reservoirs and other sources.

The 30 cities, which are home to one-third of the country's population, depend on 135 surface water catchment areas.

These areas have faced pressure and contamination from environmental deterioration, the loss of forests and wetlands, and pollution from agricultural and industrial production, said Andrea Erickson, director of The Nature Conservancy's water safety program.

Among the water catchment areas that supply the big 30 cities, one-third have seen more than half of the area they formerly covered lost to agriculture and urban construction.

The areas affected by medium to high pollution levels supply around 82 million people with their water, the report said.

In addition to the pollution, the nation's major cities have had to deal with water shortages, the report said. Of those, 17 percent had experienced severe shortages.

But the organization also offered suggestions to solve the problems, such as getting the cities to use market-based mechanisms and other initiatives to reverse the trend of exploiting land in the catchment areas.

The Ministry of Environmental Protection also recently released an annual assessment of water quality in China's major lakes and rivers. It highlights severe pollution levels.