Speaker
Description
Soil macropores left by the soil fauna or decayed roots act as preferential pathways where gravity-driven flow bypasses most of the soil matrix. These fast, out-of-equilibrium, water transfers co-exist with slower capillary-driven flow in the soil matrix. Some water and the contaminants it contains can transfer from the macropores to the matrix.
These lateral exchanges are considered in dual-permeability models coupling preferential and matrix flow that have been used for over 50 years. Water transfer in the matrix is usually modeled by the Richards’ equation while a kinematic wave is often used in the macropores. Macropore-matrix lateral exchanges are modeled by simplified physics-based equations, generally first-order terms that are calibrated to match the horizontal Richards’ equation. The lateral exchange term also involves a parameter characterizing the mean half-distance between macropores, d.
Surprisingly d values estimated from soil structure observations, when used to model experimental hydrographs recorded at the column scale or in the field, induce an overestimation of water exchange from the macropore to the matrix (Saxena et al., 1994; Larsson and Jarvis, 1999; Lissy et al., 2020). For this reason, in practice, values of d are calibrated to fit the hydrographs, resulting in values 3 to 10 times higher than observed.
In this talk, we will explore the reasons of this higher-than-expected values of d and, in particular, the fact that the first-order term, by essence, cannot consider the lateral spatial variations of water content that occur in the soil matrix compartment, leading eventually to an inappropriate water exchange dynamic.
We will evaluate a new water exchange term defined as the product of a wetted macropore-matrix specific interfacial area and the water flux density from macropores. The former will be estimated harnessing time-series of X-ray Computed Tomography images recorded during simulated rainfall events on undisturbed soil cores. The same images will also be used to determine a priori five of the seven model-parameters. The water flux density from macropores was estimated by solving the Richards’ equation in a second —horizontal — representation of the soil matrix.
Compared to a model describing the macropore-matrix exchange with an average pressure head, the new pseudo-2D exchange term improved the modeled temporal evolution of drained and stored water in the soil, and predicted a macropore-matrix water exchange dynamics in line with that expected from physics. It opens up the possibility to model water and contaminant retention transfer at the macropore-matrix interface and of using values of the transfer-term parameters determined experimentally or calculated by another model.
-Larsson, Jarvis 1999.Evaluation of a dual-porosity model to predict field-scale solute transport in a macroporous soilJ. Hydrol. 215 (1–4), 153–171.
-Lissy,Sammartino, Ruy, 2020. Can structure data obtained from CT images substitute for parameters of a preferential flow model?][4] Geoderma 380, 114643.
Saxena, Jarvis, Bergström, 1994. Interpreting non-steady state tracer breakthrough experiments in sand and clay soils using a dual-porosity model. J. Hydrol. 162 (3–4), 279–298.
| Country | France |
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