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A Transient Method for Measuring Soil Water Diffusivity and Unsaturated Hydraulic Conductivity¹

ABSTRACT

A method for measuring the unsaturated diffusivity of soil is described. It involves measurement of the matric potential transient at one end of a soil sample following a step change in the potential at the other end. Small potential steps can be used in contrast with the analogous outflow methods, because the difficult task of accurately measuring small flow rates is obviated. The analysis used to obtain the diffusivity from the potential data relies mainly on the delay in the transient and does not depend critically on its shape, thus avoiding the other major problem associated with outflow methods. With the use of the graphs provided, data analysis is simple. Measurement of the total outflow induced by each potential step allows the hydraulic conductivity to be calculated from the diffusivity. Measurements made on repacked soil during desorption and sorption showed no hysteresis in the hydraulic conductivity vs. water content relationship and agreed with independent measurements using a steady-state method. Data for field soil cores are also presented. Differences in shape between the predicted and observed matric potential transients are discussed in terms of entrapped air.

¹ Contribution from Soil Science Dep., Massey Univ. and Plant Physiology Div., DSIR, Palmerston North, New Zealand.

² Senior Lecturer, Massey University, and Scientist, DSIR, respectively.

Received for publication February 7, 1983. Accepted for publication June 16, 1983.