Estimating Vadose Zone Hydraulic Properties of Tile-Drained Fields Using Inverse Modeling

The prediction of water flow and solute transport through the vadose zone requires knowledge of the spatial distribution of soil hydraulic properties, such as the conductivity and retention parameters. Several point-scale techniques are available for measuring soil hydraulic properties. However, in applications at the field to regional scale, a prohibitively large number of sampling sites are needed to characterize the vadose zone. An alternative approach is to estimate effective values for the hydraulic parameters by inverse modeling. In this study, the inverse approach is applied to a 9600-acres irrigation district located in the San Joaquin Valley, California. The area consists of a number of tile-drained fields for which weekly drain flows are available. A numerical model is used to simulate three-dimensional variably-saturated subsurface water flow as well as field-scale and district-scale drain flows. Input data include spatially distributed crop types and weekly evapotranspiration and irrigation amounts for each field in the district. An inverse algorithm is then applied for optimization to observed drain flows. Optimization parameters are the soil hydraulic parameters, described here by a single spatially distributed scaling factor, and the drain conductances.