Many of the 20 million people of this metropolis get by on as little as one hour of running water per week, while almost all the copious rainfall is flushed unused down the sewers, creating a gargantuan flow of wastewater the city's few treatment plants can't handle.

As with New Orleans, Mexico City is on life support, but on a much larger scale.

Huge pumps work day and night to suck sewage-laced water out of the rapidly sinking, mountain-ringed bowl in which the city lies. Some areas suffer floods of sewage. Around seven in every eight toilet flushes goes untreated.

Mexico City has paved over its rivers and made them into underground sewers or expressways - often both at the same time - while pumping so much water from underground aquifers that some neighborhoods sink by up to a foot a year.

The city would probably flunk in all the five topics to be discussed at the fourth World Water Forum starting Thursday: how water can be harnessed for growth, be provided more efficiently, better benefit the poor, be used environmentally, and be prevented from causing natural disasters.

Mexico City's system serves no one well. Almost everyone buys bottled water or expensive home water systems. But it serves the poor worst. For many, bad water or none at all is just another fact of life.

''We don't want to ask for the impossible, but it would be nice if the water could come twice a week,'' said Juan Maria Bautista Ortiz, 42, whose family of four gets as little as one hour of running water per week at their tarpaper shack.

Plastic drums in their dirt yard catch the precious liquid when it comes and store about 400 gallons for an entire week of toilet use, sponge baths, and washing clothes and dishes.

The city water system isn't bad because it's cheap. Because it's bad, it's terribly expensive.

City water pipes are leaky, low-pressure and often dry, so every home must have an underground storage tank, as well as a system to pump the water up to a rooftop storage tank from which to flow down.

''It costs more than it would to build a well-made water system,'' said Jesus Campos, the assistant director of Mexico's National Water Commission.

Officials occasionally launch halfhearted campaigns to get people to drink tap water, but while they swear it's safe when it leaves treatment plants, they say it's often contaminated in aging, ill-maintained home tanks and plumbing.