Watering Mexico City

In 1950, Mexico City was home to just 2.8 million people. In the following 50 years, the population of the city soared to 18 million — and it is expected to exceed 19 million by 2015.

The water requirement figures are daunting: 19 million people each using over 300 liters of water per day works out to the equivalent of a lake one meter deep, one kilometer wide and 5.7 kilometers long — everyday.

Finding a source of that much water — and getting it to the consumers — is a challenge of dimensions that no city has ever faced.

No wonder hydrologists and water supply engineers from around the world look at Mexico City as a test case in the provision and management of urban water supplies.

Mexico City is built over the site of Tenochtitlan, capital of the Aztec empire, whose rulers controlled a vast territory and the destiny of millions during the 15th and 16th centuries.

The heartland of the Aztec empire is a high mountain valley known as the Basin of Mexico. Lying some 2,250 meters above sea level, the Basin is the highest valley in the region. It is surrounded by a succession of magnificent volcanic mountain ranges on three sides and a series of hills and low ranges on the fourth. This makes it a closed watershed and ecological unit of about 7,500 squared kilometers in the area.

Water cascaded from the mountains into the Basin, but no one drained from it in Aztec times. A chain of five shallow lakes filled about 1,500 squared kilometers of the Basin floor — lakes with many islands and irregular shorelines of bays and promontories and marshland.

Numerous villages and towns were located close to the lakes and Tenochtitlan itself occupied a large island in the southwest corner of the largest lake, which was connected to the shore by causeways up to eight kilometers in length.

Archaeologists have estimated that by 1519, there were about 1.5 million people living in the Basin of Mexico (where Mexico City is now located) distributed among more than 100 towns. That made it one of the world's most densely settled urbanized regions at that time — comparable with parts of China, perhaps.

At Tenochtitlan, the area of continuous urban habitation was between 12 and 15 squared kilometers, housing a population in the vicinity of 150,000 to 200,000 people.

Like most cities, Tenochtitlan was dependent on the supplies and goods it received from outlying communities. But unlike other cities, its relationship with its suppliers was aggressively parasitic and not at all symbiotic.

The fact is that, although the basin was environmentally diverse, its productivity was limited. And as population growth in Tenochtitlan and its urban satellites had begun to outstrip available resources, the Aztecs resorted to warfare with their neighbors as a means of meeting their needs.

But they were not interested in occupying or settling in the conquered territories. They simply harnessed the available resources to their own ends.

The vanquished were forced to pay tribute and the appropriation of food and other goods in this way became more and more essential as the Aztec ruling system evolved and its population expanded. In effect, warfare and tribute became fundamental to the economy of the Aztec Empire.

When the Spanish arrived in 1519, they found that a total of 37 towns were paying tribute to Tenochtitlan. In a single year, the city imported 7,000 tons of maize, 5,000 tons of beans and 8,000 tons of sundry other produce — which adds up to 20,000 tons of produce in all, an average of 55 tons per day.

Large quantities of dried fish, chilies, cacao seeds, cotton, henequen fibers, vanilla, honey and fruits were among the many other products regularly brought into the city.

Not surprisingly, the Aztecs’ dependence on a warfare and tribute economy caused deep resentment among the subjugated groups. Cortés, the leader of the Spanish army in the Americas, was not slow to realize that this discontent could be used to his advantage.

He made alliances with those hardest hit by the Aztecs’ demands for tribute, and with their help, he was able to take Tenochtitlan with only a few dozen Spanish soldiers. By August 1521, he controlled the entire Aztec Empire.

With the Spanish conquest, Tenochtitlan was renamed Mexico City. Both transportation and agricultural practices in the Mexico Basin changed too — with serious effect.

Many of the water supply and irrigation canals built by the Aztecs were filled in to make roads for the horses, mules, wheeled carts and wagons that the Spanish introduced to Mexico.

A 17th century source states that more than 3,000 mules arrived in Mexico City everyday with supplies of wheat, corn, sugar and other goods for the city.

Apart from mules, donkeys and horses, the Spanish also brought in cattle, sheep, goats and pigs. These of course were needed as a source of meat, but their introduction severely affected the environment, with ultimately disastrous consequences for the water balance of the region.

Since Aztec times, wells have supplemented the springs and aqueducts, which brought water to the city. By the late 20th century, they were its principal source of supply, with the city authorities pumping a massive volume of water each day from 347 wells — some up to 200 meters deep.

In addition, over 3,000 officially registered private wells and some 5,000 to 10,000 illegal wells were also pumping water from the aquifer.

And still, it was not enough — either in terms of quantity or quality. Even though over 90% of the Basin's inhabitants had direct access to water — either through a connection to the house or from standpipes in the neighborhood — few believed the city's water supply clean enough to drink straight from the tap. They preferred to buy their drinking water from public or private tankers. And it was not cheap.

In theory, Mexico City should not have had a problem with its supply. The aquifer is huge — with reserves large enough to sustain current rates of extraction for another 212 to 344 years — even though natural recharge is little more than half the current offtake.

But there is a price for taking out more water than natural recharge replaces: subsidence. With wells drawing out more than double the amount of water flowing in, the water table under the city is dropping at a rate of 15 to 20 centimeters per year in some parts of the city.

Subsidence in the central areas has stabilized at about six centimeters per year since 1954 — when new wells were banned there and some old ones closed off. Even so, parts of downtown Mexico City have sunk up to nine meters during the past 100 years.

The change that subsidence has brought to the city is dramatic. In 1900, the original lakebed was three meters below the average level of the city center — and still covered in water. By 1974, it was two meters above — and bone dry, if not covered in tarmac.

With Mexico City's water supply problems becoming ever more serious, the authorities resorted to a solution in the Aztec tradition (though without the warfare). In the 1970s, they drew up plans to harness supplies from outside the Basin. By the late 1990s, Mexico City was receiving almost a third of its water supply from the Lerma and Cutzamala watersheds.

But since some points of extraction were over 100 kilometers away and 1,300 meters below the altitude of Mexico City, transferring the water from source to city has required a lot of investment. This included the construction of eight reservoirs, a 127 kilometer aqueduct, 21 kilometers of tunnels, a 7.5 kilometer canal and six pumping stations to raise the water from source to city.

At the beginning of the 21st century, the situation is critical. Mexico City’s rate of growth is not slowing down enough to alleviate its problems to any significant degree in the short term. But at least this growth is offering some hope that it can be contained.

Cities © 2004 by John Reader, and reprinted with the permission of the publishers, Grove/Atlantic, Inc. and William Heinemann.