Uncertainty Assessment of a Parameterization Method for Vadose Zone Modeling

A method for parameterizing heterogeneous vadose zone models is being evaluated to determine the contribution of various sources of uncertainty to flow and transport prediction uncertainty. The method uses similar media scaling, geostatistical analysis, and conditional simulation methods to estimate soil hydraulic parameters at unsampled locations from field-measured water content data and scale-mean hydraulic parameters determined from available site characterization data. Data from a large-scale field experiment is being used in the evaluation. Sources of uncertainty identified include error in the measurement of site data, limited sampling of spatially variable soil properties, and errors related to modeling scale-mean parameters and site geostatistics. In practical applications, upscaling introduces additional uncertainty. Results indicate that the uncertainty in scale-mean parameters is significant and influenced equally by variability in soil properties and by the water retention modeling. Estimation of water retention using soil texture and bulk density measurements provides a reasonable assessment of uncertainty, but introduces a significant bias that may be difficult to evaluate. The relative contributions to uncertainty of spatial variability and the method of water content measurements are also evaluated.