Soil 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 alpha 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 and 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 Nino Southern Oscillation configuration). (c) 2007 Elsevier B.V. All rights reserved.
