Abstract

Soil hydraulic properties are important in many vadose zone processes, but the measurement of these properties is usually tedious and often difficult. We describe a continuous flow method that allows very rapid and accurate measurement of hydraulic conductivity, K(h), and moisture retention, θ(h), functions including hysteresis. The experimental design described here employs simultaneous tensiometry and water flow measurements that are easily automated. The design provides a highly versatile approach to controlling draining and/or wetting rates. The analysis uses a combination of direct Darcian analysis and numerical inversion of Richards' equation for estimation of the hydraulic properties. This combination allows for estimation of wetting and/or draining K(h) and θ(h) over the entire tensiometer range of measurement, while retaining the physical significance of the hydraulic parameter estimates. Estimation of K(h) and θ(h) to lower soil water pressures can be accomplished by supplying the inverse analysis with an independently measured water content such as θ(1.5 MPa). The key to applying the inverse analysis to variably saturated conditions unambiguously is identification of the air-entry pressure of the draining soil. We show how the air-entry pressure can be identified from an inflection in the soil water pressure and changes in drainage flux. The air entry identification is verified by measurement of the soil air pressure. The continuous flow approach is applied successfully to a variety of soil textures and structures. The effects of drainage rate and range of measurement are also demonstrated and discussed.

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