Génèse des débits dans les petits bassins versants ruraux en milieu tempéré: 1 - Processus et facteurs Streamflow generation within small rural catchments in a temperate environment: 1 - Processes and factors

Résumé

La première des deux parties de cette synthèse bibliographique sur la genèse des débits montre que la complexité et la diversité des organisations et fonctionnements hydrologiques constatées dans les petits bassins versants ruraux peuvent s'analyser et s'interpréter à l'aide de "clés de lecture" simples, issues d'une approche systémique et dynamique, et utiles aussi pour les modéliser (cf. Partie 2). Elle présente les différents processus tant superficiels que souterrains pouvant contribuer à cette genèse, ainsi que les facteurs du milieu qui les contrôlent: forçages atmosphériques aux limites, conditions hydriques et hydrologiques initiales, propriétés hydrodynamiques des milieux et interfaces traversés, topographie et morphométrie en 3-D du bassin. Elle rappelle ou introduit plusieurs concepts utiles pour caractériser dans chaque cas les combinaisons de processus et facteurs en jeu et leurs effets hydrologiques: seuils fonctionnels et grandeurs caractéristiques contrôlant la forme et la non-linéarité de la réponse du bassin, concepts de "zone ou période active variable" pour un processus donné et de "zone ou période contributive variable" pour un flux aux limites donné décrivant son organisation interne. Elle discute les avantages et limites des différentes méthodes (graphiques, isotopiques, géochimiques) de décomposition des hydrogrammes de crue ainsi que leur complémentarité dans l'étude du système bassin versant.

Mots-clés:

• Petit bassin versant,
• débits,
• processus,
• facteur,
• seuil fonctionnel,
• zone/période active variable,
• zone/période contributive variable,
• décomposition des hydrogrammes,
• non-linéarité

Abstract

This 2-part review on streamflow generation presents the state of the art in both field studies and modelling of the hydrologic behavior of rural catchments. It focuses mainly on temperate environments and water flows within small catchments, but many points have a more general significance.

The first part presents the main results of hillslope hydrology since the 1960s, mainly obtained on small research catchments. It appears that floods can be generated by a large range of both surface and subsurface processes, and not only by infiltration-excess surface runoff, as is still assumed by some hydrologists and modellers. In each case, the processes involved and their combinations are very variable in time and space, depending on the variable combinations of several environmental factors: precipitation and energy inputs imposed by atmosphere forcings at the upper boundary, variations in initial hydric (soil) and hydrologic (catchment) conditions which cause nonlinearities in catchment responses, water storage and resistance-to-transfer properties of the various compartments (vegetation, surface, soil, subsoil) and their interfaces, catchment 3-D topography and morphometry controlling compartment geometry and gravity forces.

The non uniform and non random distributions of these processes and factors determine the catchment functional, spatial and temporal organization: (1) at each point, process activation or deactivation results from a balance between water supply from above and local water storage or transfer capacities depending on functional thresholds related to these water properties; (2) spatio-temporal variations of factors lead to some recurrence of conditions favorable or unfavorable to each process in some areas of variable extent and some periods of variable duration: this leads to the concepts of "variable active area and/or period" (for a given process); (3) these active areas and periods contribute to outfluxes only if they are hydraulically connected to the catchment boundaries: this leads to the complementary concepts of "variable contributing area and/or period" (for a given global outflux). Several hydrograph separation methods are used to estimate various contributions to streamflowwhich are difficult to measure in situ. They all have severe limitations: graphical methods are rather arbitrary, tracer methods are based on simplifying assumptions (end-member homogeneity, conservative tracer behaviour,...) that are not very realistic. Moreover, considering the same streamflow from different points of view, they give results that are not comparable but rather complementary: velocity criterion (rapid, delayed, slow flows) for graphical methods, time origin criterion ("pre-event"/"event" water) for water-related isotope tracers, space origin criterion ("source" reservoirs) for other physico-chemical tracers. Lastly, none of them identifies directly the processes involved. Nevertheless, they are very useful in showing that streamflow is a complex mixing of various water types, with high proportions of subsurface and pre-event water in many cases - contrary to classical hydrologic interpretations.

Thus, the complexity and diversity of hydrologic patterns and behaviors observed in small rural catchments, and especially the continuum of streamflow generation situations (from pure surface to pure subsurface contributions), can be analysed and characterized using these simple concepts and methods provided by a dynamic systems approach. They are therefore useful for catchment modelling also (see Part 2).

Keywords:

• Small catchment,
• streamflow,
• process,
• factor,
• functional threshold,
• variable active area/period,
• variable contributing area/period,
• hydrograph separation,
• non-linearity