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International audienceSoil surface features strongly determine whether rain water will infiltrate or runoff. This results in a segregation between several kinds of hydrological functioning of hillslopes and catchments. Using a deterministic model, it is attempted to define the spatial and temporal boundaries of Hortonian (infiltration excess runoff) and Hewlettian (saturation excess overland flow) hydrological behaviour. The model allows to calculate the role of the antecedent precipitation index and the soil water holding capacity in the runoff yield. These factors depend on soil hydrodynamic properties. In Northern Mexico, data collected in four experimental networks are used: one in the sub-humid Western Sierra Madre, one in its semi-arid foothill, one in the centre of the endoreic Bolson de Mapimi (the southern part of Chihuahuan desert) and the last one on the southern edge of the latter, in a limestone range. There is a regional distribution of these parameters because of rainfall distribution and overall because of the whole ecological context. The value of α parameter (which determines the depletion time of soil water content) and the proportion of bare soils are the most important explaining factors of geographical segregation between Hortonian and Hewlettian contexts. This study determines that the Western Sierra Madre, with its temperate climate, is mostly characterized by a Hewlettian hydrology, despite an increase in Hortonian behaviour due particularly to land degradation. Inversely, as it is well known, Hortonian runoff dominates completely the semi-arid and arid areas; however in certain circumstances, saturation excess overland flow can appear due to landscape or local roughness, and local or temporal possibility to infiltrate a great proportion of rainwater, i.e. during low intensity–large duration events (hurricanes crossing the mountains, winter rainy events linked to El Niño Southern Oscillation configuration)

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Abstract:The western Sierra Madre is the main water-providing area of northern Mexico. However, most of this mountain range has suffered a progressive degradation of soils and vegetation due to overgrazing and deforestation, for four or five decades. The objective of this study is to determine the impacts of these changes on water balance and hydrodynamic basin behaviour.The hydrological data of two basins (the Sextin basin, 4660 km 2 and the Ramos basin, 7130 km 2 ) of this area were analysed. Annual runoff coefficients have not changed. Therefore, other indices were used to determine changes in the streamflow regime:ž an index of irregularity of daily discharge; ž the separation of flood flow and base flow; ž the lag time of the watersheds; ž the baseflow recession index; ž the two-day recession index. Some changes were noticeable at the basin scale in the water balance of the catchments: ž the ratio of the flood runoff coefficient to the base runoff coefficient increased from the 1970s to the 1990s; ž the basin lag time decreased 2Ð1% in the Ramos basin and 6Ð1% in the Sextin basin; ž the two-day recession index and the baseflow recession index increased, reflecting a more rapid decrease of streamflow after the peak flow; ž the mean annual runoff coefficient and the irregularity as estimated by the index used here showed no significant evolution.The convergence of such observations confirms that dramatic changes in the evolution of water resources in the near future in this area are to be expected, if the current land use is not strongly modified. Water management and dam operation would also be seriously affected.

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Origine de l'étude L'évolution du milieu peut se refléter sur l'évolution du régime des cours d'eau. Les impacts des changements du milieu à une échelle de temps rapide et sur une grande superficie (construction de barrages, déviation des cours d'eau pour l'irrigation, nouvelles zones mises en culture, modifications du modelé dues aux séismes, etc.) peuvent rapidement se traduire par des modifications du régime des écoulements. Cependant, les modifications progressives dans le temps et diffuses dans l'espace (surpâturage, deforestation, urbanisation, etc.), n'ont pas toujours de répercussions visibles sur les régimes des cours d'eau sauf si ces changements sont déjà très marqués dans l'espace. Des travaux effectués sur des bassins versants expérimentaux comparables et sur des tendances statistiques sont souvent nécessaires pour détecter des modifications des régimes des écoulements. Néanmoins, la plupart de ces études ont mené à la recherche d' effets quantitatifs sur le bilan hydrique. C'est-à-dire que l'intérêt principal est de savoir si un site modifié physiquement produit plus ou moins d'écoulement. Par ailleurs, le rôle hydrologique d'une formation végétale ou d'un paysage déterminé dépend de la combinaison de conditions climatiques et pédologiques. Ainsi, plus les sols ont une réserve en eau importante, et plus le climat présente des périodes de surplus et de déficit en eau dans l'année, plus les changements de cette formation ont des répercussions sur le bilan d'écoulement annuel (Cosandey, 1995). Ainsi, une forêt médi terranéenne joue un rôle plus important qu'une prairie continentale, et encore plus qu'une zone aride.

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