Improved evaporation method for the measurement of the hydraulic conductivity of unsaturated soil in the wet range

The evaporation method (EM) is an effective technique for measuring soil hydraulic properties. However, the EM generally fails to measure hydraulic conductivity curves (K-h; K and h are the hydraulic conductivity and the pressure head, respectively) in the wet (low suction) range when the hydraulic gradient within soil samples exceeds the minimum accuracy of the pressure transducer. In this study, we developed a new experimental device that uses the EM to perform conductivity measurements in the wet range. By making two major improvements, we successfully expanded the measurable range of K-h to cover the wet range, enabling accurate and effective measurements. Firstly, the accuracy of the hydraulic gradient measurement was remarkably improved by the use of a differential pressure transducer. Assuming the minimum evaporation speed (0.18 cm/day at the center of soil sample), the estimated measurable range of K-h is up to 27 cm/day for any type of soil. That is, our device has expanded the range of K-h measurable by the EM by more than two orders of magnitude on the wet end. Secondly, our device can execute the saturated/unsaturated steady-state infiltration method (SSIM/USIM) and the EM on the same sample. This diminishes not only time and cost but also errors between different methods, ensuring good accuracy and representativeness. The range of K-h measurable by the USIM was determined to be more than 21 cm/day, which overlaps with the range measurable by the EM. As a result, our device can obtain consecutive K-h curves from saturation to the medium pressure range (-500 < h <= 0 cm) for any type of soil.