Simplified Multistep Outflow Method to Estimate Unsaturated Hydraulic Functions for Coarse‐Textured Soils

Abstract

Although the multistep outflow (MSO) method is well suited for the estimation of soil hydraulic properties by inverse solution techniques, this method has not been widely adopted because it requires advanced instrumentation and is time consuming. The objective of this study was to develop a modified version of the multistep outflow technique that largely simplifies laboratory procedures and reduces costs and time. The numerical inversion procedures require applying user‐friendly HYDRUS software to estimate fitting parameters for soil water retention and unsaturated hydraulic conductivity curves. Whereas values of saturated water content and saturated hydraulic conductivity must be measured independently, the remaining functional parameters are estimated using an inverse solution of a transient drainage experiment using multiple suction steps and a hanging water column, with drainage outflows measured during drainage. A comparison test showed that the simplified experiment without tensiometric measurements provided sufficient information in the parameter identification compared with a traditional pressure outflow experiment with tensiometric measurements for an Oso Flaco sand and a loamy sand field soil in the suction range of 0 to 17 kPa.