When people think about environmental problems blighting Beijing, the first thing that usually comes to mind is the city's air.

This is hardly surprising: Air pollution is, after all, highly visible.

Despite the recent improvement in this area, which is now threatened by the huge numbers of new cars hitting the road here every day, Beijing is still often shrouded in acrid smog.

By contrast, we see hardly any signs of a water crisis in our day-to-day lives. Water continues to flow out of the taps and showers in our homes. The city's parks are lush and green, while their lakes and the waterways that link them together remain filled with water. One wouldn't want to drink that water, but I have seen numerous people swimming in it, especially in Houhai and Yuyuantan Park.

However, Beijing faces a worsening water crisis that is far graver than its air quality problems.

Rapid improvements in the city's air could be achieved with better public transportation, driving restrictions, cleaner vehicles, green power, and fewer factories.

The water crisis, on the other hand, is more intractable because it stems from the increasingly acute mismatch between Beijing's population and its water resources.

Between 1949 and 2010, the number of people living in Beijing rose from 4 to 17 million. During the same period, thanks to a prolonged drought, the surface water in the city's rainfall-fed reservoirs, lakes, rivers, and streams has declined significantly. This drought has been especially severe during the past decade.

The ongoing crisis has been exacerbated by the pollution of much of Beijing's dwindling supply of surface water. Indeed, pollution was one factor in the 1997 closure of the Guanting Reservoir, which was formerly one of Beijing's most important sources of drinking water.

The reservoir was recently reopened, but with a much smaller capacity, making it incapable of serving as even an emergency backup water supplier for the capital.

Faced with shrinking water supplies, Beijing has turned to neighboring Hebei province to slake its thirst. But Hebei also faces a severe water shortage.

Beijing also now pumps about 2.1 billion cubic meters of groundwater each year. The long-term pumping has led to the decline of the groundwater level.

Finally, even if Beijing receives significant amounts of water from the mammoth Yangtze River diversion project, its long-term water situation may well continue to deteriorate.

That's because the decades-long drought shows no sign of abating and could be made worse by global-warming induced extreme weather.

In addition, the Yangtze water will have to be shared with parched Hebei province and Tianjin and the latter is set for further residential and industrial growth.

Beijing is also likely to demand more water for golf courses, car washes, spas and private pools as its population becomes more and more affluent.

Currently, Beijing residents have, on average, just 248 cubic meters of water to use each year. That's significantly less than the Chinese average and well below the international guideline of 1,000 cubic meters.

Thus, Hu Kanping the deputy editor of the magazine, Environmental Protection, and co-author of the 2010 Chinese Environmental Greenbook stated: "People in other countries have a kettle of water, people in China have a cup of water, people in Beijing have sip of water."

Unless drastic action is soon taken to address the capital's water crisis, it won't be long before we're down a drop of water in Beijing.