EGU General Assembly 2007
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  Information - SSS12 Transport in preferential flow domains of the soil porous system: Measuring, interpretation, models, upscaling (co-listed in HS)

Event Information
Soils often exhibit a variety of small-scale heterogeneities such as cracks, inter-aggregate macropores, decayed root channels and other types of macropores, which partition flow into separate regions. Due to these local-scale heterogeneities, a non-ideality of transport (preferential flow) generally occurs thereby creating local-scale non-equilibrium conditions in pressure head and solute concentrations between faster and slower flow pore regions.
Accounting for the mechanisms involved, we distinguish the following types of preferential flow at the ‘pedon scale’
1. Preferential flow in real macropores (or non-capillary pores). At least in principle, Richards’ equation is not applicable. The kinematic wave, or the simple Hagen-Poiseuille equation, are preferentially applied. Relevant fluxes are usually notably accelerated;
2. Preferential flow in interpedal (interaggregate) pores. Richards’ equation is applicable since the pores are in the category of capillary pores. The fluxes are accelerated compared to those in the matrix (intrapedal pores), when infiltration and redistribution are considered, but less accelerated compared to the fluxes in item 1;
3. Fingering due to the instability on the wetting front. This occurs most frequently at the interface of a less permeable layer above a more permeable one. Solution by Richards’ equation exists only if the geometry of fingering is defined. Rate of fluxes is comparable to item 2;
4. Preferential flow due to spatial irregularities or temporal dynamics in soil wettability (or waer repellency). This type of preferential flow is often assumed to occur when initial soil water content is below a critical value. Rate of fluxes is comparable to type 2.

Local-scale heterogeneities and non-equilibrium type of preferential flow are obviously expected to be highly related, but quantification of this relationship remains a great challenge.
The goal of this session is to analyze /discuss approaches for quantitative characterization of the soil structure and related water flow and solute transport processes in heterogeneous soils, in the presence of structural inter-aggregate pores or even shrinkage cracks.
In such soils the use of morphometric data in combination with soil physical characteristics is particularly relevant. The complex geometry of the pore space may now be quantified by means of tomography, magnetic resonance and image analysis. Additionally, as hydrological processes integrate the effects of soil pore arrangement, insight into the physical influence of local-scale heterogeneities on soil processes should be gained by appropriately investigating and modeling the water and solute transport behavior in natural soils with different degrees or forms (geometries) of structure.
Fractal theory also seems to be an effective tool for characterizing soil heterogeneities, since size and shape of different combinations of functional inter-aggregate pores can be quantified and coupled to physical characteristics and behavior. Both test descriptive and predictive property of fractals are of concern.
Finally, an objective of the session is to inspect the influences of physical heterogeneities and preferential pathways on flow and transport processes across spatial scales and related scale-dependent parameterizations in lab and field-scale transport experiments.
Keywords: Preferential flow; soil porous systems; soil micromorphology; fractals; tomography; magnetic resonance; upscaling

Preliminary List of Solicited Speakers
Dani Or
Ecole Polytechnique Federale de Lausanne (EPF
CH-1015 Lausanne, Switzerland

Hans-Joerg Vogel
Environmental Research Center
Leipzig/Halle, Germany


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