Laboratory Method for Estimating Water Retention Properties of Unsaturated Soil

Soil hydraulic properties are necessary for modeling water flow and solute transport in the vadose zone. However, direct measurement of these characteristics in field conditions is tedious, time-consuming and expensive. In this study, a laboratory method was used to characterize soil water retention curves of three soil samples in the region of Bouhajla (Central Tunisia). For experimental purposes, volumetric water content and pressure head values were measured using the gravimetric method and Watermark sensor, of a small disturbed soil core, respectively, during a drying cycle under the effect of evaporation. The van Genuchten model was fitted to the measured retention curves with the RETC software to determine residual water content (θr), saturated water content (θs) and the two shape parameters; α and n. Strong correlations were found between the fitted and measured retention curves. The van Genuchten model was also fitted to the retention curves measured by pressure chamber (as the reference method). The results were evaluated by calculating the root mean square error (RMSE) and the geometric mean error ratio (GMER). Statistical analysis proved the success of the proposed method for estimating van Genuchten soil retention parameters of the studied soils. A Mann-Whitney test performed at the significance level of 0.05 showed no significant difference between the two methods.