Revisiting the simplified evaporation method: Identification of hydraulic functions considering vapor, film and corner flow

Abstract

The simplified evaporation method (SEM) is a technique to determine soil hydraulic properties, i.e. the water retention curve and the hydraulic conductivity curve, from evaporation experiments in the laboratory. The method relies on simplifying assumptions like spatial linearity in pressure head and water fluxes, which are violated in the evaporation process. Therefore, its validity and accuracy has been checked in the past by numerical simulation using the Richards equation and subsequent evaluation of the computer-generated data. These numerical tests were conducted using models of the soil hydraulic properties like the van Genuchten–Mualem model, which account only for water retention and conductivity in completely filled capillaries and thus give biased descriptions of the hydraulic properties at low moisture contents. We reinvestigate the accuracy of the SEM by a numerical simulation study for a broad spectrum of soils with textures ranging from clay to pure sand, using more realistic functions of the soil hydraulic properties, which account for water adsorption, flow in incompletely filled capillaries and isothermal vapor flow. Our results show that the previous numerical studies underestimated the non-linearities of suction and water content profiles which occur during bare-soil evaporation, in particular for sandy soils. Nevertheless, the SEM approximates the true hydraulic functions of most soils very well, with mean deviations below 0.1% volumetric water content and below 0.1for log10 of hydraulic conductivity. Larger columns or higher evaporation rates increase this error only slightly. Only for the extremes in the texture spectrum, we found bias in the estimated soil hydraulic properties. We furthermore investigate different strategies for calculating a mean suction and show that a weighted combination of the arithmetic and geometric mean leads to the best results.