New transient method for determining soil hydraulic conductivity function

The instantaneous profile method (IPM) is a transient method for measuring a soil’s hydraulic conductivity function (SHCF), which relates soil hydraulic conductivity with suction. In the existing interpretation method of the IPM, boundary flux during testing must be known to integrate instantaneous profiles of water content for obtaining the water flow rate. However, it is usually difficult and expensive to measure a boundary flux and if not known, assumptions that may not be easily justified (especially in the field condition) have to be made. In this study, a new method is proposed so that boundary flux does not need to be measured, controlled or assumed during a test. The new method is evaluated through (i) hypothetical column tests using transient seepage analyses and (ii) five case studies. The new method is capable of determining an SHCF with good accuracy. Normalized root-mean-square deviation (NRMSD) for the old and new methods is less than 5% and 10%, respectively. The accuracy of the new method can be increased substantially (i.e., NRMSD <5%) when the spacing of sensors installed along a soil column is reduced. Closer sensor spacing reduces error propagation due to numerical differentiation of instantaneous profiles of hydraulic head for determining hydraulic gradient.