Workshop on Interfaces and Interfacial Displacement in Unsaturated Porous Media
A workshop on Interfaces and Interfacial Displacement in Unsaturated Porous Media will take place from the 10th-12th of March 2015 in Potsdam, Germany.
The link to the workshop is
Forschergruppe MUSIS (FOR 1083)
Subsurface fluxes of water and energy near the soil surface play an important role for the distribution of water into surface and subsurface water, for agriculture and for the transport of contaminant into deeper zones and of nutrients to plants. Estimations of the water fluxes on a decimeter scale and above are usually based on continuum modeling concepts where the pore scale is averaged out. Water movement is usually modeled with the Richards equation, which is a mass conservation equation for water coupled to the Buckingham Darcy equation for the water fluxes. The movement of water in the pores could be modeled using first order principles with a given pore space geometry, however, to resolve this scale is for practical questions not feasible. The Richards equation is the established physically based model to predict flow and it is used on intermediate scales up to kilometers.
The hypothesis of MUSIS is that it is the role of different types of interfaces that is not appropriately captured in models, which makes flow models close to the soil surface so problematic. First, the movement of fluid during infiltration or evaporation invokes the movement of a sharp water-air interface. The interaction between moving interfaces and the complex morphology of the material yields non-trivial flow patterns. The Richards model describes the water content as an averaged quantity and does not capture the influence of different fluid interface patterns. This concerns in particular infiltration fronts, which can form instabilities at the pore scale and beyond. The second interface is the soil surface itself. It is usually incorporated in a model as a boundary condition, where either a pressure head or a water flux is prescribed. However, this interface has specific hydraulic and morphological properties, which determine flow patterns and resulting fluxes across the soil surface. Water flux that is driven up-wards by evaporation is not prescribed by a flux, but by atmospheric demand and by the complex transport processes of water vapor through the rough soil surface and through the flowing air in the layer just above the soil surface. The third type of interfaces is material interfaces. On the pore scale this would be the interface between pore and grain or the interface between aggregates that touch in different ways, while on larger scales these are interfaces between soil of different texture or chemical properties. Such interfaces can act as barriers for evaporation flow or lead to non-equilibrium of water pressure on all relevant length scales. These interfaces and their combination can cause flow patterns that cannot be reproduced by the Richards equation with the usual types of boundary conditions and constitutive relations of state variables. This is also true if averaged properties are used at larger scales. The effect of interfaces on flow is important over all scales, starting from the pore scale on.
The goal of MUSIS is to explore the transition from Richards to non-Richards flow behavior which is supposed to be determined by structural properties of the soil and the external forcing through atmospheric boundary conditions. Hence, criteria for such a transition should be developed based on quantifications of structures and flow regimes as defined by dimensionless numbers. The group works with experimental and theoretical methods, on different length scales ranging from the pore scale to the plot scale. We consider water fluxes due to evaporation and due to infiltration and redistribution.