|Water for Urban Areas (UNU, 2000, 243 p.)|
|5. Water supply and distribution in the metropolitan area of Mexico City|
Mexico is a country with an area of 2 million km2 and a population of more than 92 million. The mean annual rainfall is 780 mm (representing a volume of 1,522 km3), the average annual runoff is 410 km3/year, and the renewable annual groundwater is estimated at 55 km3. The average annual per capita water availability is about 5,000 m3, which is twice the world average. However, water is scarce in the north of the country and abundant in the south. In fact, 79 per cent of the natural groundwater recharge occurs in the south-eastern region of the country (SEMARNAP/CNA, 1996). Mexico's per capita water availability thus varies regionally, depending on both the mean annual rainfall and population concentrations: in regions with less water but high population, availability ranges between 211 and 1,478 m3/year; in regions with more water and less population, availability varies between 14,445 and 33,285 m3/year (CNA, 1994).
In 1995, the total water abstracted in Mexico for all purposes was estimated at about 300 km3. Of this, 26.5 per cent was allocated to produce hydroelectric energy and 73.5 per cent for other purposes: 61.2 per cent for agriculture, 8.5 per cent for domestic use, 2.5 per cent for industry, and 1.3 per cent for aquaculture (fig. 5.1). It is estimated that, by the year 2000, 142 km3/year will be required to produce hydroelectric energy and 2.89 km3/year for thermal power (mainly cooled by seawater) (SEMARNAP/CNA, 1996).
Current annual average water abstraction represents about 43 per cent of total annual renewable water. Viewed nationally, this may give a false sense of water abundance since it does not indicate the problems of scarcity and contamination that can now be observed in most of the basins and aquifers. Both water scarcity and water contamination have already contributed to the development of a series of conflicts between the various uses and different users in many parts of the country (Naranjo and Biswas, 1997; CNA, 1994).
The total annual wastewater generated in Mexico is estimated at 7.3 km3 (or 231 rn3/sec). Even though infrastructures exist to treat 1.4 km3/year, only 0.53 km3/year are actually treated. Thus, nearly 93 per cent of the wastewater is discharged without any treatment whatsoever (SEMARNAP/CNA, 1996). The 1994 studies by Mexico's National Water Commission (Comisiacional del Agua, CNA) on water quality in 218 basins in the country (representing 77 per cent of the territory, 93 per cent of the population, 72 per cent of the industrial sector, and 98 per cent of irrigated areas) indicated that most of the basins are at present contaminated with organic, industrial, and/or agrochemical wastes (CNA, 1994).
The number of people being supplied with water has increased in recent years (fig. 5.2), but not all of this water is treated (fig. 5.3).
According to the CNA (1997a), out of a total population of 91.6 million, 15.1 million people did not have access to clean drinking water and 30.6 million did not have access to sanitation facilities in 1995. The problem is most serious in the rural areas, where 48 per cent of the population do not receive safe drinking water and 79 per cent do not have access to sanitation services (SEMARNAP/CNA, 1996).
Current estimates indicate that, by 2000, total demand for drinking water in Mexico will increase to 9.4 km3/year (299 m3/sec), and 7.7 km3/year (244 m3/sec) of wastewater will be generated (SEMARNAP/CNA, 1996). Already, however, Mexico faces serious problems of water availability because of the mismatch between the centres of water demand and of available water. The 75 per cent of the territory in which most of the large cities, the industrial sector, and irrigated land are concentrated accounts for only about one-third of the water available in the country.
The problems related to water management have become very complex. The increasing population, the very serious problems related to lack of sanitation and clean water, as well as permanently high investments in infrastructure without achieving definite solutions are forcing the government to search for different approaches other than purely engineering and supply-oriented ones. The institutions concerned are realizing that successful water resources management requires a long-term planning process from the technical, economic, political, social, and environmental points of view.
A good example of unsustainability from all the different points of view is Mexico City, where both water supply and wastewater treatment have become serious problems. So far, no long-term sustainable strategies have been proposed by the government, either federal or state, and, unless programmes on demand management for the city are seriously considered and implemented, it will be only a matter of time before more acute conflicts arise between the different users of clean water.