Evaporation from Porous Media in the Presence of a Water Table

Abstract

Evaporation from porous media in the presence of a water table is influenced by the water table depth and the hydraulic characteristics of the medium, among other factors. For shallow water tables, the upward capillary liquid flow maintains continuous liquid pathways connecting the water table to the surface. For deeper water tables, however, the hydraulic continuity may be disrupted due to the opposing gravity and viscous forces. The main objective of this study was to demonstrate the limitation of the common assumption of liquid continuity through the unsaturated zone across large distances above a water table. The concept of the evaporation characteristic length was used to predict the maximum depth of the water table hydraulically connected to the surface. When the water table depth exceeded this characteristic length, the evaporative water loss was significantly suppressed due to the loss of liquid continuity. For model comparison, experimental data from the literature and data from laboratory evaporation experiments were used. Experimental results from cylindrical columns packed with sand with different particle size distributions connected to water tables fixed at various depths below the surface confirmed the existence of a finite characteristic length indicating the maximum depth of the water table hydraulically connected to the surface. The experimentally determined characteristic lengths were in a good agreement with the theoretical predictions. The obtained results also provided new insights regarding the solute transport and deposition patterns during evaporation in the presence of a water table.

Column-scale unsaturated hydraulic conductivity estimates in coarse-textured homogeneous and layered soils derived under steady-state evaporation from a water table

“…(7) for D max , can be analytically derived only for simple algebraic K l (h) relationships such as the BC power model (Warrick, 1988;Sadeghi et al, 2012a). Lehmann et al (2008) and Shokri and Salvucci (2011) incorporated the van Genuchten (1980) retention model and derived an approximate analytical solution for D max of coarse-textured soils by balancing capillary and gravitational forces. But in general, there are numerous mathematically more complex expressions for K l (h) for which an analytical solution for Eq.…”

“…This approach seems to be an attractive alternative, as e and D max are easier to measure in a steady-state evaporation experiment than K l and h. The evaporation rate may be accurately measured with a balance as the temporal change of mass of a constant head device (e.g., Mariotte bottle) used to establish a constant water level within the evaporation column. As D max is a macroscopic length between water table and drying front, it may be measured by means of dye tracers (Shokri et al, 2009;Shokri and Salvucci, 2011) or image analysis (as demonstrated in Lehmann et al (2008)). Beyond these simple methods, the drying front position can also be determined with advanced measurement technologies such as neutron radiography (Shokri et al, 2008) or heat pulse techniques (Sakai et al, 2011;Deol et al, 2012).…”

“…One-dimensional steady-state evaporation from a fixed shallow water table (WT) is dependent on the WT depth and on the soil hydraulic properties Shokri and Salvucci, 2011;Assouline et al, 2013;Or et al, 2013). Two distinct scenarios are often considered; the first when the WT is shallow enough for the soil to sustain hydraulic continuity between the WT and the surface due to capillary forces, and the second when the WT reaches a critical depth (denoted as D max ) below which the hydraulic continuity between the WT and soil surface is no longer maintained.…”

“…Steady-state water flow from the WT to the soil surface was extensively studied theoretically and experimentally for homogeneous soils (Moore, 1939;Anat, 1965;Warrick, 1988;Gowing et al, 2006;Salvucci, 1993;Shokri and Salvucci, 2011;Sadeghi et al, 2012a) as well as for stratified soil profiles (Willis, 1960;Ripple et al, 1970;Warrick and Yeh, 1990;Lu and Zhang, 2004) and other heterogeneous media Zhu and Mohanty, 2002;Bechtold et al, 2012). The majority of developed models are based on the Buckingham-Darcy law (Buckingham, 1907) and its analytical or numerical solutions within the liquid flow domain.…”

Calculation of Steady-State Evaporation for an Arbitrary Matric Potential at Bare Ground Surface

“…The second scenario deals with an evaporative front that is below the ground surface. For such a scenario, the upward capillary flow may not be strong enough to overcome the downward gravity, resulting in discontinuity of the vertical liquid profile at a certain level below the soil surface [11]. In other words, a water-depletion dry layer will be formed between the ground surface and the evaporative front, which marks the discontinuity of the liquid profile.…”