A Combined and Extended Procedure for Measuring the Soil Water Retention and Hydraulic Conductivity Curves

Soil-specific properties like water retention and hydraulic conductivity are largely used in soil and environmental modelling and are typically obtained after laboratory analyses. So far, no single method is available to measure the entire suction range for water retention or hydraulic conductivity. Common methods for describing the soil water retention curve (SWRC) include simplified evaporation, pressure plates, neutron spectroscopy, and dewpoint. Regarding hydraulic conductivity, the techniques vary for the saturated or unsaturated condition, using tension disks and transient evaporation methods. In the search for a procedure to describe the entire water retention and hydraulic conductivity curves, the objective of this study was to illustrate the combination and use of a series of laboratory methods in eight different semi-hierarchical combinations to cover the whole suction range (0 $\le$ pF $\le$ 7). The data obtained from each combination was used to fit the van Genuchten-Mualem equation and compared using the RMSE and Akaike statistics. The main results show that using a combination of many methods for the water retention and hydraulic conductivity curves did not necessarily improve the curve fitting. However, adding data points at near saturation (pF close to 0) or from the driest part of the curve (pF $\ge$ 4) improved the estimates on both curves. Specifically, for the clay soil, the RMSE for the hydraulic conductivity curve decreased from 0.0372 to 0.0369 cm/d when measurements from near saturation were added. For the sandy loam 2 soil, the RMSE for the water retention curve decreased from 0.039 to 0038 when including data from the driest part of the curve. Among all the soil-water-related parameters tested in this study, the estimates for the water retention content at the permanent wilting point (θ_{1500 kPa}) showed the largest difference among all the combinations of methods, up to 52%. In contrast, the difference in the water content at field capacity (θ_{33 kPa}) estimates was only 3%. This study provides an evaluation and insights to identify the best combination of methods when measuring or parametrizing the soil water retention and hydraulic conductivity curves.