Asia's cities are ballooning, and the accompanying upsurge in the consumption of water and production of waste in urban areas is placing new pressures on the environment.

Home to 53% of the world's urban population, Asia has the highest concentration of megacities, including Shanghai, Tokyo, Karachi and Beijing. Not only are Asia's cities big and numerous, they are among the most polluted. The urban explosion has made providing safe water and sanitation a massive challenge for the region.

Historically, the availability of local water resources has determined not only where major cities have been established but how well they have fared. But in Asia, rapid -- and often unplanned -- urban growth in recent decades has overwhelmed water systems.

Asia's per capita water availability is already the lowest of any continent. Fast economic growth, coupled with breakneck urbanization and changing lifestyles, has made a difficult situation worse. In 2012, slightly over half of the world's population lived in urban areas. By 2050, that ratio is projected to jump to more than two-thirds, with much of that growth taking placing in Asia.

The region's urbanization is fueling demand for water not just for municipal use but also for manufacturing and agriculture. And changing diets, especially an increased preference for meat -- the production of which is notoriously water-intensive -- are compounding water challenges. Asia needs to make substantial water savings in agriculture to quench the thirst of its expanding cities. Some of the largest urban centers -- from Beijing and Manila to Jakarta and Dhaka -- are already at risk of running out of water.

The challenge of providing safe drinking water is compounded by the growing incidence of floods and droughts in Asia. According to the Asian Development Bank, people living in the Asia-Pacific region are "four times more likely to be affected by natural disasters than those living in Africa, and 25 times more likely than those living in Europe or North America." Most Asian megacities are in coastal areas, making them vulnerable to global warming-induced rises in ocean levels.

As cities across the region struggle to access adequate water supplies, many of their residents are beginning to rely on bottled water. This practice, however, has fueled a serious waste-management problem. Due to very low recycling rates, billions of plastic bottles end up as garbage every year, taking up increasing space in landfills or even littering the landscape. Some cities are running out of places to put those bottles.

The environmental problems do not end there: The retreat of megadeltas due to China's upstream damming of rivers originating on the Tibetan Plateau has become a serious issue. According to several scientific studies, heavy upstream damming, which can obstruct the flow of silt to plains and estuaries, is contributing to the retreat and subsidence of Asia's big deltas, which are home to such megacities as Tianjin, Shanghai, Guangzhou, Bangkok, Kolkata and Dhaka. This development, in turn, is causing seawater to flow into coastal freshwater aquifers, affecting municipal supplies.

UNCOVENTIONAL SOLUTIONS Yet despite this deepening crisis, a water-stressed Asia continues to live beyond its means environmentally, overexploiting water resources while hoping to postpone the day of reckoning. Some countries have responded to these challenges by implementing grand but environmentally questionable projects, from China's South-North Water Transfer Project (the world's biggest hydraulic initiative) to India's now-stalled proposal to link up its most important rivers.

With the first two of its three legs already operational, the $62 billion Chinese undertaking is aimed at moving water from the south to the parched north, all the way to Beijing and Tianjin. But the environmental costs are mounting: Energy-hogging treatment plants along the transfer routes seek to tackle water degradation and pollution, even as water quality deteriorates in the source river, the Yangtze. Given the project's energy intensity, swelling costs and environmental impact, a better alternative for China would have been desalination, wastewater treatment and recycling, and reduced irrigated farming in its arid north.

Asian cities have little choice but to tap unconventional sources for their water supply. One such option is recycled -- or "reclaimed" -- water. Singapore has embraced, on a commercial scale, the use of chemical processes to turn wastewater into clean water. The water-scarce city-state has found this option to be less expensive than desalinating seawater.

The toilet-to-tap concept has long been in use in manned spacecraft. Still, the public is far less keen on recycled water than on desalinated water. To help ease the "yuck factor" among reluctant citizens, Singapore -- like London and San Diego -- mixes treated wastewater with conventional water in the city's supply system.

Even if the reclaimed water is channeled strictly for nonportable uses, such as gardening, flushing toilets and doing laundry, it can help alleviate a city's water crisis. Reclaimed water can also be used to artificially replenish aquifers, rivers and reservoirs and for ecological purposes, such as restoring or enhancing wetlands and riparian habitats. With many Asian cities increasingly desperate for additional water resources, more metropolises will likely be forced to recycle wastewater to augment their supplies.

Another option for Asian cities is rainwater harvesting, a relatively low-cost technique invented in Asia in the 9th or 10th centuries. Some cities are already trying it. For example, new apartment complexes and commercial buildings in the southern Indian metropolises of Bangalore and Chennai are required to have rainwater-harvesting systems. In much of Asia, heavy rains in the monsoon season make it easier to trap and store rainwater for dry-season use.

Most Asian cities also need greater public and private investment to upgrade and maintain water-distribution networks so as to plug leakages and prevent contamination. In Asia, losses of treated water from leaky distribution were conservatively estimated at $9 billion in 2011, according to the Asian Development bank.

Water scarcity is set to become Asia's defining crisis, creating an obstacle in the continent's path toward continued economic growth. Competition between cities, industries and farms over limited water resources is already intensifying. Addressing these challenges demands new skills, technologies, management practices and approaches, including building demand-side efficiency and tapping nontraditional water sources.

Brahma Chellaney is a geostrategist and the author of "Water, Peace, and War," and the award-winning "Water: Asia's New Battleground," among other books.