Modifications physiques du milieu et conséquences sur le comportement hydrologique des cours d'eau de la Sierra Madre Occidentale (Mexique) Physical modifications of the environment and consequences on the hydrological behaviour of rivers in the western Sierra Madre (Mexico)

Résumé

Des recherches ont été menées dans la Sierra Madre Occidentale du Mexique afin de déterminer l'impact des modifications physiques du milieu (surpâturage et déboisement) sur le fonctionnement hydrodynamique des bassins. L'étude des régimes hydrologiques des deux principaux cours d'eau (le río Ramos avec un bassin de 7.100 km² et le Sextin de 4.700 km²) composant le haut Nazas montre une variation possible du régime hydrologique qui pourrait être liée aux dégradations de l'espace. Cette modification du régime n'est pas visible dans les tendances annuelles des pluies et des écoulements. De ce fait, d'autres indicateurs sont utilisés dans la recherche de tendances. En utilisant des algorithmes ordinaires et un modèle global simple (le modèle Nazas) nous testons et proposons ici trois indicateurs du régime hydrologique:

1. la comparaison entre les coefficients d'écoulement de base et de crue montre une possible diminution de l'écoulement de base;

2. Les temps de réponse des bassins montrent une probable diminution. Cela signifie que la transformation de la pluie en débit pourrait devenir de plus en plus rapide

3. le degré de participation de l'humidité préalable du milieu montre une possible diminution des temps de ressuyage du sol.

Si les résultats sont peu significatifs statistiquement dans cette zone, tous les indicateurs montrent une possible modification du régime hydrologique. L'enjeu est de taille ; la sierra approvisionne en eau plus du quart de la superficie du Mexique. Les modifications du régime hydrologique pourraient avoir des conséquences graves sur le développement.

Mots-clés:

• Bassin versant,
• écoulement de base,
• temps de réponse,
• rétention en eau,
• surpâturage,
• déboisement,
• Nord-Mexique

Abstract

Environmental impacts can alter the runoff regime of river basins. However, if the transformations are progressive, the hydrologic repercussions may not be always clearly visible. The influence that the physical parameters of the environment (soil surface features, vegetation, soil occupation, etc.) have on the hydrodynamic functioning of river basins is a current research concern in hydrological sciences. Generally these studies conclude that a decrease in vegetative cover leads to an increase in runoff, irrespective of the climate of the region.

For decades, the Western Sierra Madre has suffered progressive degradation by overgrazing and deforestation. This subtropical climate has a long dry season during the year (8 to 9 months), and the soils are generally thin (mostly Phaeozems and Lithosol). The vegetative cover of this zone is significantly degraded. Forests were reduced by more than 50 % from 1972 to 1992 as a result of deforestation. Overgrazing also has impacted the landscape, leaving stony slopes and crusted surfaces observable all over the mountain.

Due to both overgrazing and deforestation, increased runoff coefficients and erosion rates have been observed at the plot and micro-plot scale in this region. The response is different at the basin scale, due to the spatial variability of the hydrodynamic behaviour of the slopes. The aim of this paper is to determine if over-exploitation of the terrestrial system could have hydrological consequences. New methods and indexes are proposed, which are able to highlight the hydrological responses of the basins. Examples are given for two watersheds of the Upper Nazas basin.

An examination of the two main rivers of the High Nazas (the Rio Ramos with a basin of 7100 km2 and the Sextin of 4700 km2) shows a likely variation of the hydrologic regime, which could be linked to spatial degradation. This modification of the regime is not visible in the annual trends of rain and runoff. As a result, ordinary algorithms and a simple global model (the Nazas model) were used to describe the annual trends. Three indicators of hydrologic regime are proposed here :

1. A comparison between the base-flow and flood-flow based on the algorithm of Gustard et al. (1989), which allows the separation of flood flow from base flow;

2. The lag time (or response time) of watersheds, the time necessary for the rain to cause an increase in river flow;

3. Changes in the water content of soil. Infiltration and evapo-transpiration rates are considered to be the main elements of water balance and it is generally assumed that they increase with increasing vegetation cover. This allows a greater volume of water to stay in the soil. When there is no soil water content assessment, the Antecedent Precipitation Index (API) can be used to compensate. This takes into account the volume of the previous rainfall events, the time passed since this event, and a parameter Alpha of time water depletion.

Changes in land use were evaluated by remote sensing. The data collected were used in a calibration field campaign, to verify the hypothesis that there is a strong increase in overexploitation of the environment.

The results of the hydrological analysis were not always statistically significant in this zone. However, the changes are concomitant and all of the chosen indicators show a modification of the hydrologic regime. The analysis of 30 years of data shows that there is an evolution of the hydrologic behaviour of the watersheds. The following observations were made :

- A reduction of the base-flow was noticeable during the years 1970-1979 to 1990-1998. It appears that the floods have more and more importance in the hydrographs of the watersheds.

- A reduction in response time, which indicates that the transformation of the rainfall into runoff has become more and more rapid.

- The contribution of previous humidity in the environment in the formation of runoff also supports the earlier observations. It would seem that the drying time of the soil decreased. It appears to be a consequence of a reduction in soil water content, due to erosion and the degradation of vegetation.

The Western Sierra Madre supplies water to more than a quarter of the total area of Mexico. Changes in the hydrologic regime could have serious consequences on development in this region. The southern part of the country is mountainous, and 54% of the fields suitable for cultivation are located in the north of Mexico, but this region receives only 7% of the total water available in the whole country. The purpose of this research was to illustrate the importance of forests in the maintenance of the sustainability in the great "Laguna" irrigation district.

Keywords:

• Watershed,
• base flow,
• lag time,
• soil water retention,
• overgrazing,
• deforestation,
• Northern Mexico