Three-dimensional structure characterisation and transient flow modelling of a variably saturated heterogeneous monolith

In this study, we investigate the possibility of modelling the transient hydraulic behaviour of an undisturbed monolithic subsoil sample of 0.5 m3, based on a detailed characterisation of the soil structure. At the larger scale, the subsoil core structure is defined by the presence of discontinuous clay and a stone layer within a large sandy matrix. A second structural level is characterised by the spatial distribution of hydraulic properties in the sandy matrix, and described using the Vogel-similarity concept [Vogel, T., Cislerova, M., Hopmans, J.W. 1991. Porous media with linearly variable hydraulic properties. Water Resources Research 27, 2735–2741]. This exhaustive structural information is subsequently implemented into a 3-D transient and variably saturated flow model. Simulation results are compared with experimental observations and it is showed that the presence of a discontinuous clay and stone layer is sensitive in predicting effective outflow. Given the fact that no calibration is performed, we consider that the outflow predictions are good. The results are however disappointing for the prediction of time series of pressure head. Moreover, the local scale Ksat had to be scaled based on structural and hydraulic monolith-scale observations. This study points out that: (i) it would be necessary to characterise the continuity of flow, and related parameters such as the hydraulic conductivity at the appropriate scale, to adequately simulate variable saturated transient flow at that scale and (ii) a smaller spacing in the sub-sampling would be needed to properly predict local scale time series.