TY - JOUR

T1 - Capillary, Film, and Vapor Flow in Transient Bare Soil Evaporation (2)

T2 - Experimental Identification of Hydraulic Conductivity in the Medium to Dry Moisture Range

AU - Iden, Sascha C.

AU - Diamantopoulos, Efstathios

AU - Durner, Wolfgang

PY - 2021

Y1 - 2021

N2 - Bare-soil evaporation involves coupled flow of liquid water, water vapor, and heat. As evaporation results in non-isothermal conditions in the soil, the temperature dependence of transport properties and thermal fluxes of water and vapor must be accounted for. In a companion paper, we showed that the Richards equation, that is, a single-phase flow model assuming isothermal conditions, is applicable to accurately determine soil hydraulic properties including the medium to dry range from evaporation experiments by inverse modeling. This is warranted if pressure head data across a wide moisture range, that is, from almost saturated to almost air-dry, are used in the objective function and a suitable parameterization of the hydraulic conductivity function including vapor and non-capillary flow is used. In this article, we confirm the theoretical results by examining real evaporation experiments, in which we measured the temporal dynamics of evaporation rate, soil temperature, and pressure head in laboratory soil columns. Pressure head was measured with mini-tensiometers and relative humidity sensors. The measurements were evaluated by inverse modeling with the Richards equation assuming isothermal conditions and ambient temperature in the soil. Our results for a sandy and a loamy soil show that the observed transient water and vapor dynamics in the drying soil could be accurately matched, provided the hydraulic conductivity curve considered isothermal vapor diffusion and film flow. These components dominate hydraulic conductivity in the medium to dry soil moisture range and were uniquely identified in agreement with the theoretical analysis in the companion article.

AB - Bare-soil evaporation involves coupled flow of liquid water, water vapor, and heat. As evaporation results in non-isothermal conditions in the soil, the temperature dependence of transport properties and thermal fluxes of water and vapor must be accounted for. In a companion paper, we showed that the Richards equation, that is, a single-phase flow model assuming isothermal conditions, is applicable to accurately determine soil hydraulic properties including the medium to dry range from evaporation experiments by inverse modeling. This is warranted if pressure head data across a wide moisture range, that is, from almost saturated to almost air-dry, are used in the objective function and a suitable parameterization of the hydraulic conductivity function including vapor and non-capillary flow is used. In this article, we confirm the theoretical results by examining real evaporation experiments, in which we measured the temporal dynamics of evaporation rate, soil temperature, and pressure head in laboratory soil columns. Pressure head was measured with mini-tensiometers and relative humidity sensors. The measurements were evaluated by inverse modeling with the Richards equation assuming isothermal conditions and ambient temperature in the soil. Our results for a sandy and a loamy soil show that the observed transient water and vapor dynamics in the drying soil could be accurately matched, provided the hydraulic conductivity curve considered isothermal vapor diffusion and film flow. These components dominate hydraulic conductivity in the medium to dry soil moisture range and were uniquely identified in agreement with the theoretical analysis in the companion article.

KW - Evaporation

KW - hydraulic conductivity

KW - inverse modeling

KW - soil hydraulic properties

KW - vadose zone

KW - water retention curve

U2 - 10.1029/2020WR028514

DO - 10.1029/2020WR028514

M3 - Journal article

AN - SCOPUS:85106712690

VL - 57

JO - Water Resources Research

JF - Water Resources Research

SN - 0043-1397

IS - 5

M1 - e2020WR028514

ER -