As Mexican President Vicente Fox prepares to open the 4th World Water Forum, the BBC's Claire Marshall looks at the severe water problems facing Mexico City.
With a population of more than 20 million people, and dwindling water supplies, the Mexican capital is a stark example of the severe water supply issues facing many of the world's rapidly developing mega-cities.
Many in the city have just one hour of running water per week
The parched ground crunches beneath your feet as you walk through the Texcoco area on the outskirts of the city. The bleached, cracked terrain stretches out in all directions. Nothing can grow here.
It is very difficult to imagine that, just 70 years ago, this area was filled with water. This was one of five lakes that used to enrich the Mexico City valley.
Today, in a prime example of what more than a century of water mismanagement can do, they have all but disappeared.
Population growth, the over-exploitation of subterranean aquifers, and a failure to recycle limited water supplies have turned a once-fertile region into a barren desert.
Many of Mexico City's inhabitants get by on just one hour of running water per week.
And, most people consider the city's tap water to be undrinkable - though water officials say it is now safe to drink - so Mexico has become the second-highest consumer of bottled drinking water in the world.
In her office, Marta Delgado, the president of the Mexico City Water Commission, points at a wall map of the capital.
Her arm makes a wide arc as she points out where there used to be water. Then she points at the few isolated blue patches where it remains.
"I think that if we do nothing, then we are heading for a crisis," she says.
"People think that a water war starts when nothing comes out of the tap. But it doesn't. It happens when the water becomes more expensive, or is of a lower quality. That is already happening here in Mexico City."
At one of the city's few sewage treatment plants, a pungent smell fills the air.
The manager of the site says that they manage to process half a cubic metre of waste per second.
However, he points out that the metropolis produces 50 cubic metres per second.
Less than 10% of Mexico City's waste water is recycled, compared with London where that figure is more than 90%. Most rain water is also lost.
Looking for solutions, the World Water Forum delegates will be hearing about the progress being made by community projects from around the world.
Ron Sawyer heads one called TepozEco, not far from Mexico City. With state money, and investment from their NGO, he has helped to install 30 ecological toilets in one village where there is no piped water.
Less than 10% of Mexico City's waste water is recycled
"I think it is a major hurdle, and extremely important that there be more dialogue between the actual users of water and the government, and the private sector in terms of certain niches where it can help more effectively," he says.
Marcela is one villager who is now saving and recycling her precious water supplies.
Running water from a tap connected to a large storage tank, she says: "During the dry season, we used to have to go and get buckets of water on our donkey. It was a long journey. Now we have stored enough to last us for many months."
The planet's population is swelling, and fears about climate change are growing. This Water Forum desperately needs to come up with answers for a global problem.