Conjunctive Use of Tension Infiltrometry and Time‐Domain Reflectometry for Inverse Estimation of Soil Hydraulic Properties

Abstract

Infiltration from a tension disc infiltrometer can be applied conjointly with time‐domain reflectometry (TDR) measurements of soil water content to improve estimates of field hydraulic parameters. However, interpretation of TDR‐measured water contents for use in inverse optimizations may be problematic when rods are partially within the wetted zone. The objective of this study was to assess if TDR‐measured soil water contents in addition to cumulative infiltration could improve parameter estimability for the inverse optimization problem. Infiltration experiments were conducted with a 0.58‐m‐diam. cylinder packed with a loamy sand. Three trifilar TDR probes were inserted diagonally into the soil to measure transient water contents during infiltration. Inverse optimizations utilized cumulative infiltration, water contents from diagonally placed TDR probes, and a branch of the wetting water characteristic θ(h) from extracted soil cores. Measured θ(h) at one or more pressure heads was required in optimizations to provide a satisfactory description of the water characteristic in the dry region. Optimizations for three infiltration experiments yielded similar parameter estimates with overlapping 95% confidence intervals. The use of diagonal TDR‐measured water contents improved the predicted redistribution of soil water and decreased covariances between parameter pairs that led to better parameter estimability. Optimized simulations predicted water contents in a three‐dimensional region within 0.03 m3 m−3 of values measured by buried horizontal TDR probes. Parameter estimates were relatively insensitive to changes in the assumed averaging depth transverse to TDR rods. For the diagonally placed probes, the dominant gradients in water content were in directions that minimized errors associated with assuming a uniform weighting of water content within the TDR sampling volume.