Capillary, Film, and Vapor Flow in Transient Bare Soil Evaporation (2): Experimental Identification of Hydraulic Conductivity in the Medium to Dry Moisture Range

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.

Key Points: Identification of soil hydraulic properties across the full moisture range by inverse modeling of evaporation experiments. Advanced instrumentation with tensiometers and relative humidity sensors allows to identify hydraulic conductivity in medium to dry soil. Evaporation experiments can be modeled correctly with Richards’ equation, provided hydraulic properties account for vapor and film flow.

Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659