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La Sierra Madre occidentale

 | 
Luc Descroix
, 
Juan Estrada
, 
José Luis Gonzalez Barrios
, 
et al.

Summary

Traduction de Simon Barnard

Texte intégral

1The western Sierra Madre is a relatively empty part of México, forming both a 'frontier zone' in the sense of a possible area for colonisation in a new country and, paradoxically, a reserve of labour whose inhabitants have been leaving in recent years. It is also close to the other frontier in the geopolitical and administrative sense - between north and south - formed by the boundary between México and the United States of America.

2This long range of mountains of volcanic origin is also the longest rhy-olitic range in the world; it consists of a vast set of staggered plateaux cut by rifts lying parallel to the range and to the coast (northwest/south-east). These rifts are sometimes as much as 1,000 metres deep, but the edges usually slope fairly gently for a mountain region, with fault steps breaking the overall difference in level. The main watercourses flow at the bottom of these rifts and leave through gorges to flow to the Pacific or to the Mexican central northern altiplano; in the latter case, some such as the Río Conchos flow into the Rio Bravo del Norte (the frontier river that the Americans call the Río Grande) while others like the Río Nazas provide temporary supplies for the evaporation lagoons in the large closed depressions in the north of the country. The two sides of the western Sierra Madre are extremely asymmetrical, with a gentle, regular eastern slope from the high plateaux (at an elevation of 1,000 to 2,000 m) of the altiplano, while the slope is much steeper on the western slope, and the main watercourses have cut deep gorges to descend more rapidly. This is the region of the renowned Barrancas del Cobre that the inhabitants of the state of Chihuahua hold are the deepest in the world (they are 1,850 m deep in two places).

3Water is scarce in northern México: 54 percent of the arable land is in the northern half of the country, which receives only 7 percent of precipitation. This results in problems in the use of the resource, with many users often competing for small quantities available. The western Sierra Madre clearly seems to be a ‘water tower’ in the centre of arid and semi-arid northern México. Precipitation is locally over 1,000 mm per year and is over 500 mm almost everywhere. However, the range is bracketed by deserts—in the west by the Sonora desert, the driest of North America (precipitation less than 50 mm per year at Mexicali) and in the east by the Chihuahua desert, the larger of the two and where precipitation is generally less than 200 mm per year in the endorheic depressions.

4However, the rainy season lasts for only three or four months everywhere, making it necessary to store water for numerous purposes. Many dams have therefore been built on the watercourses flowing either to the Pacific (Río Yaqui, Río Humaya, Rio Sonora, Rio San Lorenzo, Río Matapé, etc.) or eastward to the large interior depressions (mainly the Río Conchos and the Rio Nazas that have already been mentioned).

5The comparative abundance of water means that the Sierra Madre has better grazing land than that in the neighbouring arid or semi-arid areas. Above 2,400 m, these pastures are dominated by pine forests that are natural cover in the greater part of the high plateaux. These conditions are favourable for agropastoral exploitation and account for the very ancient human presence in the area.

6Over-exploitation of the environment and its resources is observed in both the pasture and forest storeys. This appears to have been the case for several decades. The sharing out of the hacienda land in application of the 1936 agrarian reform was performed from the 1940s to the 1970s in the Sierra Madre, which was then little occupied. This resulted in a considerable increase in population, and a resulting increase in livestock and grazing pressure and the disorganisation of the grazing management system. The latter feature then caused the rapid deterioration of the quality of pastures and damage to the soil—under increasing pressure as livestock numbers continue to increase. Indeed, cattle farming (calf production) is devoted mainly to supplying 9-month calves to the extremely buoyant and profitable US market.

7At the same time, the western Sierra Madre was opened to forestry exploitation at the beginning of the 1970s. Durango very soon became México's leading state for timber production (it is reminded that México is a federal state, whose official name is Estados Unidos Mexicanos— United Mexican States). The largest towns, that had often grown with mining operations (Durango, Parral, Santiago Papasquiaro, etc.) opened to wood and paper industries, also causing very strong demand for wood and over-exploitation of resources, with a proportion extracted on a clandestine basis. At the end of the 1990s, production exceeded 5 million m3 per year while authorised production was 1.7 million m3. A ‘mining’ approach to exploitation with hardly any attention paid to the renewal of resources combined with the corruptive power of private companies that cared little about the state of the environment on México led to very rapid deforestation. No reforestation was performed, very little maintenance was carried out and the villagers were ready to let all their wood go for cash. The situation has improved in the last few years.

8These two environmental deterioration processes have already affected water resources and have led to the launching of the research programme on 'Water management in a basin in northern México' in 1991, whose results and conclusions were delivered in the form of technical reports from 1998 to 2001 and then as scientific publications since 2001. Numerous studies have been conducted within the framework of this programme managed by IRD continental water specialists in association with the CENID RASPA (Centro Nacional de Investigación Disciplinaria en la Relación Agua Suelo Planta Atmósfera) at the INIFAP research station (Instituto Nacional de Investigaciôn Forestal y Agro-Pecuaria) in Gómez Palacio, Durango state, in northern México.

9This research showed that the over-exploitation of the environment in the western Sierra Madre resulting from over-grazing and deforestation, as has been seen above, caused land degradation in a large proportion of the area. This resulted in marked changes in the hydro-dynamic characteristics of the land. Trampling by cattle, the uprooting of Gramineae, and the stripping of increasing large areas led to compaction and the stripping of the finest, richest material in the surface horizons.

10In turn, deterioration of land and vegetation has reduced soil moisture absorption capacity and hence the total natural water retention of the drainage basin, increased runoff, more serious flood peaks and, in parallel, more marked low water levels. As a result, the capacity of the western Sierra Madre to supply water to downstream sectors has been strongly reduced. The 1990s was a somewhat dry period in northern México and the reservoirs of the large dams built on the two slopes of the sierra were very often practically empty even at the end of the rainy season.

11Analysis of hydrological records and the data collected during the 8-year period of measurements revealed the following phenomena at the scale of the experimental plot, the small experimental drainage area and the first order drainage basin (several thousand square kilometres in this case):

  • the runoff coefficient is stable: no decrease in flows has yet been observed;
  • a decrease in lag time in the basins after precipitations, meaning that runoff is faster than before;
  • an increase in flood discharges in comparison with the base flows; this means that water flows more rapidly during floods, the land and groundwater are recharged to a smaller extent and less water is stored in the drainage basin for sustainable flow, which is very useful during the long eight-month dry season;
  • a decrease in the moisture content and the infiltration and water storage capacities of soils;
  • shorter, more marked floods and longer, more marked low water periods.

12All these features make the Sierra Madre's ‘water tower’ function more fragile and will make water management even more delicate in the years to come. Indeed, the ‘consumer’ regions downstream have sometimes not managed water very economically in the still recent past. Irrigation was developed excessively from the 1940s onwards with the construction of large dams and the establishment of large irrigated perimeters. These first used all the renewable water flowing from the mountains. They then tapped underground water in order to continue growth without improving water efficiency. The cheap energy policy led to seeking water at increasing depths instead of optimising efficiency and investing in irrigation systems that are more expensive but more economical with regard to water consumption; likewise, leaks and conveyance losses were neglected for a long time. The 1990s saw the combination of a long drought and the setting up of NAFTA, resulting in the end of subsidies and the beginning of real farm prices (on the Mexican side of the border of course, as on the other side farmers received substantially increased subsidies, enabling them to flood the Mexican market with maize and frijoles!).

13At the same time, México was affected—fortunately—by the fashion for 'integrated water resource management' and the new Water Code, adopted with due ceremony in April 2004, gives a fine position to users' associations for the setting up and management of a user-payer system (or polluter-payer if the user modifies the water instead of or in addition to using it) based on drainage basins and not just on watercourses. The idea is that any user of an area in a drainage basin who might modify water resources must be considered as a water user. This is the case of livestock farmers whose herds change the behaviour of soils and of timber traders who upset the water cycle.

14The new code should help to solve numerous latent problems concerning water use and conflicts that have emerged in recent decades:

  • at the US frontier for the sharing of transfrontier water (Río Tijuana, Río Colorado and Rio Bravo del Notre/ Río Grande);
  • between the Mexican states for the same sharing operation (water conveyance projects have had to be cancelled for lack of agreement between the states);
  • above all between users as there is now payment for water and only high value-added activities will be able to subsist in the arid and semi-arid zones where water will necessarily be expensive as it is in short supply.

15With a few rules for the good management of land and of land-vegetation-water resources (and not just water), the western Sierra Madre will continue to play the role of water tower for northern México. The return of satisfactory rainfall (in 2004) and the application of the new water law should make it possible to conserve the irrigated perimeters in the foothills while allowing the development of mountain areas thanks to specialised agriculture (orchards, horticulture, etc.) and ecotourism enhanced by the richness and variety of the natural areas in this range of mountains.

© IRD Éditions, 2005

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