Large cities currently take up one percent of the world's land area, but draw water from 41 percent of the globe, thereby impacting environmental and socio-economic conditions in far flung areas repercussions of which reverberate globally, says a study.
"Our world is increasingly connected in such complex ways that the decisions we make must be well informed across long distances if we want to protect both the environment and the well being of people," said one of the researchers Jianguo Liu from Michigan State University (MSU) in the US.
Drawing water to quench the water demand of megacities means transporting water long ways, and flooding the world with change, the researchers said.
The study focused on Beijing to apply a novel approach called "telecoupling" to gain a full appreciation of moving tonnes of water.
"The telecoupling approach allows us to consider positive and negative impacts on source regions alongside those to the city. We need this full picture of all the moving parts to fully assess the sustainability of modern cities' water supplies," said the study's lead author Jill Deines from MSU.
The researchers noted that in China, there are programmes that compensate people to change their businesses or habits to improve water quantity or quality.
An example is Beijing's Paddy Land-to-Dry Land programme with neighbouring farming regions, which pays farmers for lost revenue if they convert their fields from water-intensive rice paddy to corn.
The study indicated these services were cost effective, and improved the quality of available water, but did not add much to the quantity.
Beijing's distant water sources provide much needed water volume for the city, but these water connections have complicated impacts on far-flung regions, the study said.
The findings appeared in the journal Water International.