Evaporation from porous media

Drying of porous media is ubiquitous in various hydrologic and engineering processes ranging from evaporation from terrestrial surfaces to drying of food, building materials, pharmaceutical products, and preservation of historical buildings. Water evaporation from soil, arguably the porous medium ‘par excellence’, plays an important role in water management, land-atmosphere interactions, soil salinization, vegetation and crop production, biodiversity and ecosystem functioning. Therefore, it is important to have a clear understanding of the processes controlling water evaporation from porous media.

Typically at the early stages, the evaporation rate from saturated porous media is relatively high and rather constant, the so-called stage-1 evaporation. During this stage, the evaporative flux is supplied by the capillary-induced liquid flow hydraulically connecting a receding drying front (defined as the interface between saturated and unsaturated zone) to the surface of porous media. During this period, the evaporation rate is limited primarily by the atmospheric conditions and the surface remains partially wet (see Figure 1 for a conceptual picture). At a certain cumulative water losses or the drying front depth (controlled by the transport properties of porous media), the downward gravity and viscous forces overcome the upward capillary forces disrupting the hydraulic connection with the surface. This marks the end of stage-1 evaporation.  The depths of the drying front at the end of stage-1 evaporation under different boundary conditions can be predicted using the hydraulic characteristics of the porous media. 

At the end of stage-1, the liquid meniscus detach from the surface and pin to a level below during a transition period from stage-1 to stage-2 evaporation. Disconnection of all liquid pathways from the surface leads to the formation of a dry layer below the surface which marks the onset of stage-2 evaporation (see Figure 1). During this period, the evaporation rate is supplied by the capillary induced liquid flow from the wet zone to the bottom of the dry layer, liquid vaporization at that level and vapor diffusion through the overlying dry layer. Stage-2 evaporation is limited primarily by the transport properties of the soil. 

Figure 1. The conceptual sketch illustrating the mechanisms controlling evaporation from porous media. (a) Detachment of the liquid meniscus from the surface and pinning to a level below during the transition from stage-1 to stage-2 evaporation. (b) Formation of a dry layer above the surface at the onset of stage-2 evaporation. During this period, the evaporation is proceeded by liquid from the wet zone to the bottom of the dry layer supplied by the capillary flow, liquid vaporization at that level and vapor diffusion through the overlying dry layer (after Shokri and Or, 2011).

Accurate description of evaporative fluxes from porous media without invoking adjusting parameters is still a grand challenge due to the complexity of the processes controlling the evaporation dynamics including but not limited to the role of heterogeneity, the coupling between the atmospheric conditions and transport processes occurring inside porous media, properties of the evaporating fluid, and the mass exchange across turbulent boundary layers formed above the surface.