AN IMPROVED ANALYSIS OF GRAVITY DRAINAGE EXPERIMENTS FOR ESTIMATING THE UNSATURATED SOIL HYDRAULIC FUNCTIONS

The unsaturated hydraulic properties are important parameters in any quantitative description of water and solute transport in partially saturated soils. Currently, most in situ methods for estimating the unsaturated hydraulic conductivity (K) are based on analyses that require estimates of the soil water flux and the pressure head gradient. These analyses typically involve differencing of field-measured pressure head (h) and volumetric water content (theta) data, a process that can significantly amplify instrumental and measurement errors. More reliable methods result when differencing of field data can be avoided. One such method is based on estimates of the gravity drainage curve K'(theta) = dK/d-theta which may be computed from observations of theta and/or h during the drainage phase of infiltration drainage experiments assuming unit gradient hydraulic conditions. The purpose of this study was to compare estimates of the unsaturated soil hydraulic functions on the basis of different combinations of field data theta, h, K, and K'. Five different data sets were used for the analysis: (1) theta-h, (2) K-theta, (3) K'-theta; (4) K-theta-h, and (5) K'-theta-h. The analysis was applied to previously published data for the Norfolk, Troup, and Bethany soils. The K-theta-h and K'-theta-h data sets consistently produced nearly identical estimates of the hydraulic functions. The k-theta and K'-theta data also resulted in similar curves, although results in this case were less consistent than those produced by the K-theta-h and K'-theta-h data sets. We conclude from this study that differencing of field data can be avoided and hence that there is no need to calculate soil water fluxes and pressure head gradients from inherently noisy field-measured theta and h data. The gravity drainage analysis also provides results over a much broader range of hydraulic conductivity values than is possible with the more standard instantaneous profile analysis, especially when augmented with independently measured soil water retention data.