If you approach Mexico City by airplane, a visitor will see a vast city stretched over a plateau, punctuated by two beautiful snow-capped volcanoes - Popocat�ptl and Iztacc�huatl - nicknamed "Popo" and "Itza" by Mexico City's inhabitants. Mexico City's challenges in supplying water for a huge urban population are intensified by this unique biophysical environment. The city is located in the southwestern region of the Basin of Mexico in the altiplano, or high plateau, region of Mexico. The city has the largest urban population in the world living in an enclosed basin with no natural outflow to the sea. Unlike most other megacities, there is little wind to cleanse the air and no ocean or river to exchange water and sewage (Weiner, 2000).
The basin is the highest valley in the region, at an altitude of 2,200 metres above sea level, and encircled by a ring of mostly extinct volcanoes that reach elevations of over 5,000 metres. These mountains, or sierras, serve as the "lungs of the city" and also capture precipitation to feed the area's aquifers and springs. The city receives an annual average rainfall of 700 millimetres, most of which is concentrated in a few severe storms from June to September. In pre-hispanic times, the basin contained a series of interconnected shallow lakes - Tzompanco (Zumbango), Xaltoca, Texcoco, Xochimilco and Chalco - covering approximately 1,500 km2 of the basin floor. The entire lake system drained toward Lake Texcoco, the largest and lowest elevated of the lakes (Aguilar et al, 1995).
The Basin of Mexico has been one of the world's most densely populated areas for almost two millennia (Aguilar et al, 1995). At the time of Spanish conquest (AD 1519), the estimated population was over one million people (Ezcurra and Mazari Hiriart, 1993). Tenochtitl�n, the centre of Aztec civilisation in the Basin, was one of the most sophisticated and technologically advanced cities of its time. The Aztec people constructed Tenochtitl�n on land ingeniously reclaimed from the shallow Lake Texcoco. Through a sophisticated system of dikes, canals and sluices, the Aztec modified and controlled the lake system, which was vital not only as a resource for military defence of their capital city, but also formed the basis of an extremely productive agricultural system. Lakes Chalco, Xochimilco and part of Texcoco were used for chinampa cultivation. The chinampa system was a highly creative water management solution to cultivation in the shallow lakes, based on the construction of canals on the wetland and lake floors and platforms or raised beds of earth in between (Wirth, 1997). The lake beds provided rich soils, and Aztec farmers could raise the level of water in the surrounding canals, which prevented seasonal flooding and enabled careful regulation of soil moisture on a year round basis (Rojas Rabiela, 1991). Much of the surrounding land was terraced and irrigated, and an elaborate network of aqueducts carried water from the abundant springs in the higher elevations of the basin down to the city.
After Conquest and the subsequent destruction of Tenochtitl�n, the Spanish initiated a series of land-use changes. The colonisers began draining the central lakes in an attempt to control flooding, as well as to satisfy their demand for pasture and rangeland for livestock. The use of the plough and draft animals meant that the new inhabitants of the basin were less dependent on centralised food production. Consequently, the Spanish drained many of the canals to construct roads and chinampa cultivation declined. In the mid-nineteenth century, the process of lake drainage intensified due to the demands for more land by cattle ranchers, hacienda owners and developers. The old Aztec aqueduct system was still in use to deliver plentiful spring water until the mid-1850s, when the discovery of potable groundwater motivated the large-scale drilling of wells (NRC, 1995). Over the next century, the process of water extraction and drainage continued. Today, the original lakes have almost entirely disappeared and much of the basin is paved. All that remains of the lake system is dry beds and some of the old chinampa canals in Xochimilco.
Modern Mexico City is the cosmopolitan and vibrant cultural, economic, political and industrial centre of Mexico. It is characterised by impressive avenues and colonial buildings, fine museums and galleries, and the huge Z�calo, or city square, which is the energetic focal point of Mexico City. Alternatively, it is also known for its congestion and air pollution, and as one of the largest urban areas in the world, with an estimated population of close to 20 million people. The city's population has doubled about every 15 years since the early 20th century, when its population was less than one million. Since 1940, the population has increased constantly at rates of 4-5 per cent per year (Aguilar et al, 1995). To a large extent, this extraordinary rate of growth is due to the centralising policies of the federal government, which favoured urban over rural development. The federal government concentrated industrial production, wealth and services in Mexico City (NRC, 1995). The city, therefore, had access to electricity, oil and other power sources, the provision of water and drainage facilities, and was the focus of major infrastructural investments such as roads and public transportation. In turn, this centralisation led to an intense concentration of population. People were drawn to the capital for employment opportunities and to improve their quality of life, as it provided services such as education, health, potable water, and sewerage that were not available in rural areas. Today, Mexico City accounts for 45 per cent of Mexico's industrial activity, 38 per cent of GNP, and 25 per cent of the population within the country (Tapia et al, 2000).
Since the 1950s, the city's population has spilled over the original political and administrative boundaries of the Federal District (or D.F.), into the surrounding jurisdictions, especially the State of Mexico (NRC, 1995). The current urban area - called the Mexico City Metropolitan Zone or MCMZ - is comprised of 16 subunits, or delegaciones, of the Federal District, as well as 21 municipalities, or municipios, of the State of Mexico. The rate of population growth within the north and central portions of the City has slowed and even declined since the 1980s, but the MCMZ is still growing. The fastest rate of growth has occurred in the form of illegal or "informal" settlements in the outskirts of the city, often in the steep, ecologically-sensitive upland areas of the valley. Many of these settlements, known as "lost cities" (ciudades perdidas) or "popular colonies" (colonias populares), become more or less established over time (NRC, 1995). Because of this pattern of growth, the political jurisdictions of Mexico City do not necessarily correspond with its geography. This complicates an already complex administrative situation, and makes it extremely difficult for officials and planners to provide services for Mexico City's enormous population. Urban settlement has expanded from 90 km2 in 1940 to 1,160km2 in 1990, and now, with an extended corridor of industry and people to the cities of Toluca and Cuernavaca, the urban "conglomeration" extends outside of the Basin of Mexico as well (Tapia et al, 2000).
Mexico City's intense process of economic development and population growth has taxed the ecosystem that sustains it to near exhaustion. Now, with most of its agricultural lands paved and watersheds taken over by settlement, the basin generates only a small fraction of its resource needs. The city is dependent on other ecosystems to subsidise the vast amounts of food, energy and water that it requires, a subsidisation that is only possible because Mexico City is the hub of an immense concentration of economic and political power that permits it to concentrate resources as well (Aguilar et al, 1995).
The Mexican Constitution of 1917 entrenched water resources as public property under the control of the federal government. Water management in Mexico, as in many other countries, is highly centralised. In 1989, in an effort to make federal management more efficient, the government created the National Water Commission (Comisi�n Nacional de Agua, or CNA) as the sole federal authority to deal with water management. The CNA operates as an autonomous agency within the Ministry of Environment and Natural Resources (SEMARNAT). The CNA is responsible for implementing Mexico's Law on National Waters (1992), as well as the operation of an extensive hydraulic infrastructure for the sourcing and delivery of bulk water supplies. Management of water and wastewater within the MCMZ is shared by the Federal District and the State of Mexico, who are responsible for providing potable water, and wastewater collection and disposal within their jurisdictional boundaries.
Mexico City receives 70 per cent of its water as groundwater from the aquifer system that has supplied the population of the basin for hundreds of years. Natural springs and runoff of summer rains from the sierras and mountains surrounding the city are the main source of water to the aquifer (Tapia et al, 2000). This source is so bountiful that water was not a scarce resource in Mexico City until about 35 years ago, when its population reached over six million people (Morgan, 1996). Today, however, Mexico City faces a serious water deficit. As a result of increased demand from consumers and industry, and the rapid deforestation in the surrounding hills that have served as aquifer recharge areas, more water is now leaving the system than entering it. It is estimated that 63m3/second of water is needed to support the potable and agricultural irrigation needs of Mexico City's population (Tapia et al, 2000). The main aquifer is being pumped at a rate of 55.5m3/s, but is only being replaced at 28m3/s, or about half of the extraction rate, leaving a shortfall of 27.5m3/s. (Tapia et al, 2000). The Rio Magdalena, one of the last small surface water sources in Mexico City, supplements 2 per cent of this shortfall, but this river is increasingly contaminated by urban pollution (Ezcurra and Mazari, 1996).
While engineers attempt to find a solution to this water shortage, the damaging consequence of aquifer overexploitation is alarmingly visible to the inhabitants of the capital, whose city is literally sinking beneath them. This sinking, or subsidence, is caused by the depletion of water volume and pressure from the lowering of the aquifer, which causes the clay soils below the city to consolidate and the land that rests on top to collapse. Subsidence has been a problem since the early 1900s as an effect of the diversion and draining of lake water from the basin floor. Since this time, some areas in downtown Mexico have sunk 9 metres (Tapia et al, 2000). Today, Mexico City is sinking between 5 and 40 centimetres a year (Tapia et al, 2000). The Angel Statue, "El �ngel" or El Monumento de la Independencia, in downtown Mexico is one of the most dramatic examples of this phenomenon. Built in 1910 to commemorate the centenary of Mexico's independence from Spain, it was anchored by a foundation deep beneath the surface of the street. However, due to the sinking of the land around the statue, 23 steps had to be added to its base (Morgan, 1996). In some areas of the city, this sinking is so perceptible that children reportedly mark their height on a pole to see if they grow faster than the ground sinks (NRC, 1995).
Subsidence not only threatens the foundations and structure of Mexico City's many important colonial buildings, it also causes serious damage to the city's water supply and sewer infrastructure. Water pipes crack or break as the city sinks, which causes leaks and enables potential contaminants into Mexico City's distribution system. The dense clays that overlie much of the aquifer were previously considered an impervious barrier to the downward migration of water and contaminants, but when the soil sinks the dry clay becomes fissured and allows waste from the city above to seep into the water table below. Due to this infiltration, the water quality of the aquifer has become as large a concern as the quantity of water in the aquifer (Mazari and Mackay, 1993). Subsidence has also exacerbated Mexico City's enduring flooding problem. Flooding has always been a seasonal concern in Mexico City, but continued subsidence has necessitated the construction of dikes and a deep drainage canal, as the city has sunk below the natural lake floor. The situation is now so serious that it takes numerous pumping stations which run 24 hours a day all year round to keep the summer rains from washing sewage and runoff back to the city (Morgan, 1996).
To help make up the city's water shortfall and to slow or stop the sinking landscape, Mexico City's water authority and engineers sought water from other sources outside the basin. As early as 1941, a 15 kilometre long aqueduct was constructed to divert water from the adjacent Lerma river catchment area to Mexico City. In 1982, work began on the large-scale Cutzamala River project - a system to transfer surface water to Mexico City from a distance of 120 kilometres and over elevations of 1,200 metres. These two supplementary sources contribute 6 m3/s of groundwater and 13 m3/s of surface waters respectively, amounting to 30 per cent of the total supply to Mexico City (Morgan, 1996). The combined Lerma-Cutzamala projects represent a massive capital investment. According to Mexico City's hydrological planners, it requires 443 km of pipelines, which supply 202 storage tanks, with a joint capacity of 1.5 million m3. There are 102 plants to pump the water to the upper zones of the west and south of the Federal District. For its distribution, there are 560km of primary pipeline network and 12,044 km of secondary network (DDF, 1997). It also requires a tremendous amount of energy -equivalent to an 800 MW reactor running permanently - because water must be conducted uphill over one kilometre (Morgan, 1996). Before it reaches its final destination, chlorination plants treat the water to make it suitable for human consumption.
Despite these remarkable examples of engineering, testimony to the ingenuity of the city's modern engineers, Mexico City still cannot meet the water demands of its population. Water projects on this grand scale are highly capital intensive. In past decades, the government placed its emphasis on water supply at the expense of its aging domestic distribution system. Thus, although the bulk supply of water has increased steadily in an effort to keep up to the demand, water is consumed or lost through leaks as fast as it is supplied. This is a source of frustration for the CNA and hydrological engineers, because after water leaves their distribution system, an estimated 40 per cent of the total water supplied to the MCMZ is lost through leaks in the aging municipal systems, and through ineffective coordination policies between the various levels of government.Next: Sharing common water resources