Effect of effective saturation and ceramic cup properties on the response time of tensiometers

In recent years, more studies have been focusing on the transient behaviors of the movement of soil water and changes in matric potential, which require the measurement of the rapid changes in matric potential or capillary pressure. Because of the delayed response, the value of the matric potential measured using a tensiometer may not precisely reflect the actual matric potential during a transient condition. We performed a series of experiments to analyze the effect of grain size, effective saturation, and the hydraulic conductivities of ceramic cup and porous media on the response time of a tensiometer. The outcomes suggested that a decrease of hydraulic conductivity of the ceramic cup significantly increases the response time. Moreover, the response time measured under unsaturated conditions did not follow an ideal exponential decay. This finding was consistent with the existing theories, based on which, we proposed three models to describe the time-varied response function. These models were applied to correct the measured matric potential for dynamic water retention curve (WRC), and the findings were compared with those of other existing methods used for measuring the matric potential. We found that the incorporation of the theory of response time significantly reduced the discrepancy between the matric potential measured using two ceramic cups with different response times during dynamic drainage. We concluded that the delayed response of a tensiometer can have a significant effect on the measurement of the transient matric potential or capillary pressure and can be corrected using response time equations.