Comparison Between Predicted and Measured Hydraulic Conductivity of an Unsaturated Soil

The hydraulic conductivity — volumetric water content relationship (k-function) of an unsaturated soil is often estimated using predictive models based on the soil-water retention curve (SWRC). The accuracy of such models in predicting the actual soil k-function is seldom verified by direct measurements of hydraulic conductivities, mainly due to the considerable large timeframe necessary to evaluate precisely several magnitudes of hydraulic conductivities at different volumetric water contents. In order to contribute to the understanding of the accuracy of soil k-function estimates using predictive models based on SWRC, the manuscript presents a comparison between the predicted k-function using a SWRC based model and direct measurements of unsaturated hydraulic conductivity of a sandy soil. The soil k-function was inferred using the Mualen-Van Genuchten model, with SWRC data obtained from pressure extractor and free drainage column tests. The soil unsaturated hydraulic conductivities were obtained experimentally using steady-state unsaturated centrifugal flow at different increased gravities. Results comparison indicates that the predicted k-function using a SWRC based model can differ considerable from the actual unsaturated hydraulic conductivities measured experimentally. The difference between the predicted and measured magnitudes of hydraulic conductivity increases as the volumetric water content decreases. The analysis indicates that the accuracy in predicting the soil k-function relies on defining properly the soil residual volumetric water content. Inferring the soil k-function using the residual volumetric water content magnitude obtained from pressure extractor tests, leads to an underestimation of the actual soil k-function, while inferring the soil k-function using the residual volumetric water content obtained from evaporation tests leads to an overestimation of the actual soil k-function.