Water tight.

• Author: Postel, Sandra
• Position: Water conservation

Four cities' innovative conservation efforts prove that saving water makes economic and environmental sense.

Mexico City's historic plaza offers a strange sight. The imposing Metropolitan Cathedral, built soon after the 16th-century Spanish conquest, droops rather dramatically on its right side, and slightly less so on the left. Inside, an array of tension wires and green metallic beams support the weakening edifice. The capital's revered cathedral is sinking, and it has little to do with the engineering skills of the early Spaniards. Large parcels of land are slumping as the city siphons its underground water supply, and the resulting structural damage is just the most visible of many consequences.

Mexico City is an extreme case, but there are many cities around the world that have overstepped the limits of their water supply. Homes, apartments, offices, stores, restaurants, and government buildings account for less than a tenth of all the water used in the world today - the remainder goes to farmers and industries - but their demands are concentrated in relatively small geographic areas. And, in many cases, those demands are escalating. As cities expand, they strain the capacity of local water supplies and force engineers to reach out to ever more distant sources.

Beyond the challenge of finding enough water, it costs a lot to build and maintain the reservoirs, canals, pumping stations, pipes, sewers, and treatment plants that make up modern water and wastewater systems. Collecting and treating water and wastewater also takes a great deal of energy and chemicals, adding to environmental pollution and the operating costs of a community's water system. Under this financial strain, many cities are having difficulty meeting the water needs of their residents, and many low-income residents in developing countries get no service at all.

Conservation, once viewed as just an emergency response to drought, has been transformed in recent years into a sophisticated package of measures that offers one of the most cost-effective and environmentally sound ways of balancing urban water budgets. Just as energy planners have discovered that it is often cheaper to save energy up front - for instance, by investing in home insulation and compact fluorescent lights - than to build more power plants, water planners are realizing that water efficiency measures can yield permanent water savings and thereby delay or avert the need for expensive new dams and reservoirs, groundwater wells, and treatment plants.

Slowly the idea is catching on that managing demand rather than continuously striving to meet it is a surer path to a secure water supply - and will save money and protect the environment at the same time.

Water Balance

Many urban areas simply have no feasible way to balance supply and demand without conservation and more efficient water use. Mexico City is a prime example. This sprawling metropolis of 18 million people relies on groundwater for more than 80 percent of its supplies. Pumping exceeds natural replenishment by 50 to 80 percent, which has caused water tables to drop and aquifers - water-holding geologic formations - to compress. This, in turn, has led to sinking of the land and damage to surface structures, including the cathedral.

Mexico City, which sits in a cramped, high mountain bowl, has outstripped its supply of groundwater and is forced to search out water on the other side of the mountains that hem it in. The city now meets 17 percent of its demand by bringing water from the Cutzamala River system 80 miles away and lifting it 3,900 vertical feet - all at enormous cost. With the metropolitan area expanding by more than half a million people each year, officials are racing against time to achieve some degree of water stability.

Faced with such an intractable problem, the Mexican government and city officials are orchestrating an aggressive water conservation effort. In 1989, the federal government took a bold step in adopting a strict set of nationwide efficiency standards for household plumbing fixtures and appliances. These require toilets - the biggest water guzzlers in the home - to use no more than 1.6 gallons of water per flush. Maximum limits have been set for showers, faucets, dishwashers, and washing machines as well.

Mexico City has launched an ambitious program to replace conventional toilets using about 4.2 gallons with the 1.6-gallon models in public places, commercial buildings, and private residences. By late 1991...