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International audience; In this study, we have attempted to determine the effective hydraulic properties of a highly heterogeneous soil horizon composed of two elementary pedological volumes (EPVs). Our upscaling approach was guided by the “scaleway” approach, in which the properties of a complex system can be estimated by multiple discrete upscaling steps. This approach was tested on a data set from laboratory measurements of hydraulic conductivity at the EPV scale, while an explicit three-dimensional soil structure was considered at the horizon scale. We then formulated a decision tree to guide the choice of the appropriate upscaling method to determine the effective hydraulic conductivity at the horizon scale. In the case of low contrast between hydraulic conductivities at the EPV scale, the effective hydraulic conductivity at the horizon scale can be achieved by calculating the Wiener bounds, which requires only the proportion of the different EPVs. In the case of high contrast between hydraulic conductivities at the EPV scale, we recommend either calculating the Cardwell and Parson bounds, or performing a direct three-dimensional numerical simulation to solve Richards' equation, which requires an explicit representation of the three-dimensional structure of the soil horizon. The Cardwell and Parsons bounds remains a good and easily available approximation. Otherwise, more accurate estimation can be obtained by numerical simulation, although this is time consuming. A decision map is proposed to help choose the best method to estimate the effective hydraulic conductivity.

Ce projet de recherche a consisté à déterminer les propriétés hydriques effectives de l’horizon E&Bt d’un Albeluvisol, horizon fortement hétérogène. La démarche employée a été i) de mesurer les propriétés hydriques des deux sous-unités homogènes de l’horizon (les Volumes Elémentaires Représentatifs, VPE), ii) de caractériser l’organisation 3-D de celui-ci et iii) de modéliser ses propriétés hydriques effectives selon l’approche du Scaleway. Les propriétés hydriques ont été obtenues par des mesures de laboratoire. En particulier, la détermination de la conductivité hydraulique en régime insaturé sur des échantillons pluri-centimétriques a nécessité le développement d’une nouvelle méthode de mesure. Les propriétés hydriques des VPE ocre argileux et des VPE clairs limoneux sont significativement différentes. La géométrie 3-D de l’horizon a été caractérisée par la méthode géophysique de résistivité électrique. La structure électrique obtenue est discrétisée en deux phases dont l’organisation s’apparente à la structure de l’horizon. Sur des coupes verticales de cette structure, l’estimation des propriétés hydriques effectives de l’horizon a été réalisée par des calculs analytiques et par des simulations numériques. Les résultats montrent que la conductivité hydraulique est très dépendante de l’orientation de la structure du milieu. In fine, nous avons proposé une démarche opérationnelle permettant de déterminer rapidement la conductivité hydraulique effective d’horizons hétérogènes., Our research aims at determining the effective hydraulic properties of a highly heterogeneous horizon in an Albeluvisol. We first measured the hydraulic properties of the two homogeneous sub-units (Elementary Pedological Volume, EPV) of the horizon by developing a new laboratory protocol dedicated to the measurement of the unsaturated hydraulic conductivity on small pluri-centimetric samples. We demonstrated that the hydraulic properties of the clayey ochre EPV and the silty white EPV were significantly different. Secondly, we characterized the 3-D organization of the heterogeneous horizon by the geophysical method of electrical resistivity. The signal was sensitive to the difference of texture between the two EPV and, as a consequence, the electrical structure derived from the field measurements showed an organization close to the structure of the horizon. Thirdly, we estimated the effective hydraulic properties of the horizon using the Scaleway approach. The effective hydraulic properties were determined from analytical calculations or numerical modelling on 2-D slices of the 3-D structure. The results showed a strong relationship between the hydraulic conductivity and the structure of the soil horizon. Thank to our results of modelling, we finally proposed a simple practical approach to obtain the effective hydraulic conductivity of heterogeneous horizons.