Validation and Evaluation of Vapor Pressure and Evaporation Methods for Estimating of Soil Water Characteristic Curves.

Hydrological models based on physical properties of soils depend on soil hydraulic properties for proper model-runs. Commonly, the soil water characteristic curve (SWCC) is required to run different model outputs, including agricultural, environmental and hydrological processes such as soil moisture content over time and time lag of contaminants. Pressure plate apparatus at different suctions can determine this SWCC, but this procedure is time-consuming and laborious. Likewise, 1) the pressurization system may not be available in many laboratories and 2) it is developed at only predefined suction values. An option for this approach is the combination of two techniques: evaporation for the wettest and vapor pressure for the driest part of the SWCC. This could be implemented using the HYPROP and WP4C equipment. Together, they offer continuous measurement of suction at different water contents, allowing to obtain a SWCC with a high number of data points.

In this study, the first two horizons were collected from three pedons. First, an interlaboratory run was performed to validate field capacity and wilting point measurements, which were determined by a) pressure plate apparatus b) estimation after saturation until free drainage stops and c) from different fit models of SWCC described by vapor pressure and evaporation methods.

Secondly, the Van Genuchten model (VGM) was applied in order to fit the curve and estimate the VGM parameters θ_r, the residual water content, θ_s is the saturated water content, α, n and m (m = 1 − 1/n). These water retention parameters obtained by the vapor pressure and evaporation methods were compared with those obtained with the Rosetta model (V2.0-alpha). This work reviews the advantages, disadvantages and applicability of each of these fitting approaches for SWCC estimation.