Measurement of Preferential Flow During Infiltration and Evaporation in Porous Media

Abstract

Infiltration and evaporation are governing processes for water exchange between soil and atmosphere. In addition to atmospheric supply or demand, infiltration and evaporation rates are controlled by the material properties of the subsurface and the interplay between capillary, viscous and gravitational forces. This is commonly modeled with semi-empirical approaches using continuum models, such as the Richards equation for unsaturated flow. However, preferential flow phenomena often occur, limiting or even entirely suspending the applicability of continuum-based models. During infiltration, unstable fingers may form in homogeneous or heterogeneous porous media. On the other hand, the evaporation process may be driven by the hydraulic coupling of materials with different hydraulic functions found in heterogeneous systems. To analyze such preferential flow processes, water distribution was monitored in infiltration and evaporation lab experiments using neutron transmission techniques. Measurements were performed in 2D and 3D, using homogeneous and heterogeneous setups. The experimental findings demonstrate the fingering effect in infiltration and how it is influenced by the presence of fine inclusions in coarse background material. During evaporation processes, the hydraulic coupling effect is found to control the evaporation rate, limiting the modeling of water balances between soil and surface based on surface information alone.

Particle Deposition in Drying Porous Media

“…As water evaporates from the free surface, it is partially replaced by water flowing from the samples [ 13 , 36 , 37 ]. Indeed, the general process of drying is driven by capillary forces [ 8 , 9 , 10 , 11 ]. In most permeable porous media and without gravity effects, the sample remains uniformly wet owing to the capillary effect [ 12 , 13 ].…”

“…Many studies described the evaporation of pure water from porous media with micrometric pores. The general process of drying/drainage is driven by capillary forces [ 8 , 9 , 10 , 11 ]. Drying does not occur in a dry area extending progressively from the free surface.…”

Partitioning of subsurface evaporation in water limited invironments

“…Fourth, Papafotiou et al (2010) studied the effect of preferential evaporation patterns in porous material with both numerical modelling and laboratory experiments. Preferential evaporation in soils originates from heterogeneities in the soil texture, structure and compaction.…”