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

Moreover, the presence of a wetting front can influence the spatial sensitivity of TDR probes and bias the esti- Infiltration from a tension disc infiltrometer can be applied conjointly mates of optimized parameters (Ferreet al., 2002). with time-domain reflectometry (TDR) measurements of soil water Several studies have measured transient soil water content to improve estimates of field hydraulic parameters. However, interpretation of TDR-measured water contents for use in inverse contents using TDR during three-dimensional axial- optimizations may be problematic when rods are partially within the symmetric flow. Kachanoski et al. (1990) used both wetted zone. The objective of this study was to assess if TDR-mea- curved- and straight-wave guides to monitor the wetting sured soil water contents in addition to cumulative infiltration could front progression and estimate cumulative infiltration improve parameter estimability for the inverse optimization problem. from a point source. Wang et al. (1998) used horizontally Infiltration experiments were conducted with a 0.58-m-diam. cylinder buried probes to measure soil water contents during packed with a loamy sand. Three trifilar TDR probes were inserted infiltration and estimate soil hydraulic parameters based diagonally into the soil to measure transient water contents during on quasianalytical solutions. Because probes were bur- infiltration. Inverse optimizations utilized cumulative infiltration, wa- ied, this precludes the use of these methodologies in ter contents from diagonally placed TDR probes, and a branch of the field applications where an undisturbed soil condition wetting water characteristic (h ) from extracted soil cores. Measured is desired. Schwartz and Evett (2002) used TDR in con- (h ) at one or more pressure heads was required in optimizations to junction with cumulative infiltration to inversely opti- provide a satisfactory description of the water characteristic in the mize hydraulic parameters for a fine-textured soil in the dry region. Optimizations for three infiltration experiments yielded similar parameter estimates with overlapping 95% confidence inter- field. Probes were inserted diagonally at 45 from the vals. The use of diagonal TDR-measured water contents improved soil surface to minimize soil disturbance. They obtained the predicted redistribution of soil water and decreased covariances good agreement between measured and predicted water between parameter pairs that led to better parameter estimability. contents at late times when the wetting front had ex- Optimized simulations predicted water contents in a three-dimen- tended deeper into the profile. However, measured TDR sional region within 0.03 m 3 m 3 of values measured by buried hori- water contents were significantly underestimated by the zontal TDR probes. Parameter estimates were relatively insensitive optimized solution at early times. They attributed the to changes in the assumed averaging depth transverse to TDR rods. poor estimation of TDR water contents at early times For the diagonally placed probes, the dominant gradients in water to physical nonequilibrium processes during transient content were in directions that minimized errors associated with flow near the margins of the wetting front. The assump- assuming a uniform weighting of water content within the TDR sam- tion of uniform weighting of water content (and the pling volume. dielectric constant) across the three-dimensional sam- pling volume of the trifilar probes in the presence of the wetting front (Knight et al., 1994; Ferreet al., 1998)