TEMPORAL AND SPATIAL (DOWN PROFILE) VARIABILITY OF UNSATURATED SOIL HYDRAULIC PROPERTIES DETERMINED FROM A COMBINATION OF REPEATED FIELD EXPERIMENTS AND INVERSE MODELING

Abstract

An in situ method for estimating parameters necessary for characterizing unsaturated flow in porous media has been applied to 37 individual data sets from the same study site (a sand dune on the edge of a wetland). The measured field data include beginning and ending soil moisture profiles, and continuous records of net surface flux (infiltration/evaporation) and pressure head at the base of the profile. For each study period, the initial profile was used along with the upper and lower boundary conditions to simulate flow and storage changes in the profile from a form of the Richard's equation. Following each model run, the parameters in van Genuchten's characteristic equations were adjusted using the Levenberg-Marquardt procedure. Parameter adjustment and forward modeling continued until a minimum was reached in the difference between measured and simulated moisture profiles. An analysis of the resulting (optimized) parameter estimates indicated that repeated experiments, using the same method, resulted in a very consistent set of parameter values, particularly when parameters were estimated for individual soil layers rather than for the profile as a whole. Statistical analyses of the estimated parameter values indicated that most of the parameters have small coefficients of variability and the mean values are consistent with those generally considered to be valid for uniform sand. The optimization procedure also produced reasonable (and unique) values of saturated moisture content for a buried clayey layer within the otherwise homogeneous deposit of dune sand.