A Transient Water Release and Imbibitions Method for Rapidly Measuring Wetting and Drying Soil Water Retention and Hydraulic Conductivity Functions

A transient water release and imbibitions (TWRI) method for measuring the soil water retention curve (SWRC) and hydraulic conductivity function (HCF) under drying and wetting states is presented. The intellectual merit of the TWRI method is its integration of physical and numerical experiments: it employs the simple and reliable measurement of transient water content by electronic balance to record the signature of transient unsaturated flow in soil sample, and it takes advantage of the robust inverse modeling capability to simulate the physical process. The TWRI method; therefore, has two integrated components: testing and modeling. The testing consists of water release upon two-steps increase in matric suction and water imbibitions upon one-step decrease in matric suction. Each process lasts sufficient time in order to obtain transient water content. The data is then used as an objective function in an inverse numerical modeling process to obtain the unsaturated hydrologic parameters that fully define the SWRC and HCF of the soil. A novel feature of the TWRI method is its capability to measure SWRC and HCF under wetting state. The apparatus can accommodate both undisturbed and remolded samples. The testing time required for completing a full drying and wetting loop is approximately one week for most soils. Validation of the technique has been performed both experimentally and numerically. Comparisons between data obtained with the TWRI method and the independent Tempe cell method verify the reliability, applicability, and accuracy of the TWRI method. Results from three different soils are presented to illustrate the procedure and performance of the TWRI method for different soils from fine sand to clayey silt. The TWRI method provides a fast, accurate, and simple testing tool for obtaining SWRC and HCF of various types of soils under both wetting and drying states with high range of matric suction several orders of magnitude above the air-entry pressure of the ceramic stone used in the experimental setup.