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 :
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 :
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.
Watershed, base flow, lag time, soil water retention, overgrazing, deforestation, Northern Mexico.
David Viramontes, Instituto
Tecnología del Agua (IMTA), Paseo Cuauhnáhuac 8532, CP 62550
Jiutepec, Morelos, MEXIQUE