The exchange flow between preferential flow paths and matrix in soils: from pore to continuum scale with tensors

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Preferential flow still poses a great challenge when trying to quantitatively describe water and solute movement in structured soils. Empirical models are based on phenomenological approaches that consider flow domains for the macropore network and the porous matrix and as a key component the exchange between domains. However, many aspects of preferential flow cannot be explained without a physically adequate pore-scale understanding of both, the flow within macropores and the flow exchange between preferential flow paths and the soil matrix. Problems include the use of the continuum-assumption for the macropore system and flow in highly heterogeneous pores with spatially variable local properties. The objective is to create a foundation for novel class of digital soil physics models with explicit preferential flow where exchange fluxes with soil matrix are informed by pore-scale physics with accounting for spatially heterogeneous wettability and tensorial flow properties of soils for non-scalar and off-diagonal terms. Flow exchange coefficients will be explicitly calculated based on the pore-scale simulations. Conditions for the occurrence of non-creeping flow effects in larger macropores will be studied by using Navier-Stokes equations. Finally, more physically-based soil hydrological continuum models will be developed, which are based on results of pore-scale simulations used to derive "functions" for upscaling pore-scale flow in macropores and flow-interaction with porous matrix. For comparison, Richards-based models and individual fracture models will be used. This project will analyze available preferential flow observations and breakthrough data and intends to improve fluorescence dye tracing techniques for quantification of the flow exchange for testing the pore scale modelling. The collaborative project combines pore and continuum scale modeling with experimental validation. The two groups from RAS (Moscow) and ZALF (Müncheberg) collaborate with respect to fluorescence tracer experiments, dual-continuum model evaluation, and comparison of pore-scale with multi-domain approaches. Available soil data, preferential flow observations, and staining imaging will be used; additional Uranine tracer techniques developed. The group of Dr. Kirill Gerke will carry out pore-scale analyses for a stochastic reconstruction to compute tensorial properties, pore-scale flow simulations, and to derived functions for upscaling. The group of Dr. Horst H. Gerke (ZALF) focusses on experimental and tracing and conventional dual-continuum modeling. Both German and Russian groups will finally re-interpret previous studies to obtain improved knowledge on preferential and exchange flow based on pore-scale information. The improved modelling may help explaining preferential flow in unsaturated soils as a pore-scale effect and upscaled by transferring and comparison into larger scale models.