Evaporation from a shallow water table: Diurnal dynamics of water and heat at the surface of drying sand

Abstract

[1] Accurate estimates of water losses by evaporation from shallow water tables are important for hydrological, agricultural, and climatic purposes. An experiment was conducted in a weighing lysimeter to characterize the diurnal dynamics of evaporation under natural conditions. Sampling revealed a completely dry surface sand layer after 5 days of evaporation. Its thickness was <1 cm early in the morning, increasing to reach 4-5 cm in the evening. This evidence points out fundamental limitations of the approaches that assume hydraulic connectivity from the water table up to the surface, as well as those that suppose monotonic drying when unsteady conditions prevail. The computed vapor phase diffusion rates from the apparent drying front based on Fick's law failed to reproduce the measured cumulative evaporation during the sampling day. We propose that two processes rule natural evaporation resulting from daily fluctuations of climatic variables: (i) evaporation of water, stored during nighttime due to redistribution and vapor condensation, directly into the atmosphere from the soil surface during the early morning hours, that could be simulated using a mass transfer approach and (ii) subsurface evaporation limited by Fickian diffusion, afterward. For the conditions prevailing during the sampling day, the amount of water stored at the vicinity of the soil surface was 0.3 mm and was depleted before 11:00. Combining evaporation from the surface before 11:00 and subsurface evaporation limited by Fickian diffusion after that time, the agreement between the estimated and measured cumulative evaporation was significantly improved.

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

“…While Anat (1965) contained all required data, D max was not provided for all other considered datasets. As both theory and measurements (Deol et al, 2012;Assouline et al, 2013) suggest that the DF depth can be commonly much smaller than the WT depth, we assumed D max % D (WT depth) for the experiments of Willis (1960), Kumar (1999), Gardner and Fireman (1958), and Hassan and Ghaibeh (1977).…”

“…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.…”

Continuum-scale investigation of evaporation from bare soil under different boundary and initial conditions: An evaluation of nonequilibrium phase change

“…Subsurface airflow drives conditions away from equilibrium through a combination of vapor pressure lowering and enhanced rates of vapor transport [ Armstrong et al ., ]. Elevated or fluctuating temperatures in the context of diurnal cycles also play an important role in defining nonequilibrium conditions near the land‐atmosphere interface [ Assouline et al ., ]. These experimental findings have been further substantiated numerically by Smits et al .…”

Modelling evaporation processes in soils from the Huasco salt flat basin, Chile

“…When cyclic thermal conditions are present, two processes can drive soil evaporation: evaporation from the soil surface into the atmosphere during early morning and subsurface evaporation limited by Fickian Diffusion until late afternoon. As explained by Assouline et al (), evaporation from the soil surface depletes the water condensed and redistributed during nighttime. After the depletion of this water, Fickian diffusion becomes the governing process and increases the thickness of the dry layer of soil observed at the soil surface.…”