Analysis of the impact of surface layer properties on evaporation from porous systems using column experiments and modified definition of characteristic length

Abstract

The hydraulic properties of the layer at the vicinity of the soil surface have significant impact on evaporation and could be harnessed to reduce water losses. The effect of the properties of the upper layer on the evolution of phase distribution during the evaporation process is first illustrated from threedimensional pore network simulations. This effect is then studied from experiments carried out on soil columns under laboratory conditions. Comparisons between homogeneous columns packed with coarse (sand) and fine (sandy loam) materials and heterogeneous columns packed with layers of fine overlying coarse material and coarse overlying fine material of different thicknesses are performed to assess the impact of upper layer properties on evaporation. Experiments are analyzed using the classical approach based on the numerical solution of Richards equation and semianalytical theoretical predictions. The theoretical analysis is based on the clear distinction between two drying regimes, namely, the capillary regime and the gravity-capillary regime, which are the prevailing regimes in our experiments. Simple relationships enabling to estimate the duration of stage 1 evaporation (S1) for both regimes are proposed. In particular, this led to defining the characteristic length for the gravity-capillary regime from the consideration of viscous effects at low water content differently from available expressions. The duration of S1, during which most of the water losses occur, for both the homogeneous and two-layer columns is presented and discussed. Finally, the impact of liquid films and its consequences on the soil hydraulic conductivity function are briefly discussed.

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

“…However, as Fig. 4 shows and similarly reported by Assouline et al (2014), the Lehmann et al (2008) approximation (h max % a À1 ((n À 1)/n) (1 À 2n)/n ) obtained by linearization of the soil-water retention curve underestimates h max especially for finer soils.…”

“…sand or gravel layer) at the soil surface can drastically reduce soil evaporation (Willis, 1960;Yuan et al, 2009;Assouline et al, 2014). In order to study mulch effects with this new approach, a 40-cm soil profile of k = 0.8 with a 3-cm coarse layer (k = 1.2) at the surface and a WT at the bottom was considered in Fig.…”

“…Below h max , liquid water flow dominates transport and above h max vapor flow is dominant. This definition is similar to that of Assouline et al (2014) (their Eq. (25)) who described h max as the pressure head corresponding to the water content marking the onset of stage II (vapor diffusion limiting stage) evaporation.…”

Linking evaporative fluxes from bare soil across surface viscous sublayer with the Monin–Obukhov atmospheric flux-profile estimates

“…The quantification of vertical fluxes from measured profiles of wind speed, air temperature, and water vapor density has been a central focus of micrometeorological research (Monin and Obukhov, 1954;Dyer and Hicks, 1970;Businger et al, 1971;Pruitt et al, 1973;Dyer, 1974). Nevertheless, even the simplest scenario of bare soil evaporation may exhibit complex dynamics due to internal transport mechanisms unrelated to atmospheric BCs (Fisher, 1923;Yiotis et al, 2004;Prat, 2007;Lehmann et al, 2008;Shokri and Or, 2011;Or et al, 2013;Assouline et al, 2014). Hence reliance on atmospheric-based methods may lead to overestimation of evaporation from drying soil surfaces (Camillo and Gurney, 1986;Kondo et al, 1990;Bittelli et al, 2008).…”

Natural length scales define the range of applicability of the Richards equation for capillary flows

“…The combination of reduced hydraulic conductivity and high flow rates (e.g., such as imposed by high evaporation rates) may give rise to significant viscous effects [ Assouline et al ., ]. Viscous effects reduce the characteristic length, and are quantified by introducing an effective hydraulic conductivity for the capillary flow transmission zone (e.g., the region connecting a drying front with an evaporating surface).…”