Modeling Flow and Nitrate Transport in a Heterogeneous Vadose Zone: Comparison of Three Approaches

We developed three different representations of two-dimensional and three-dimensional water flow and nitrate transport in a 16 m thick alluvial unsaturated zone in an irrigated orchard in semi-arid Fresno County, California,. The first model representation is a homogeneous lithofacies representation which explicitly identifies major textural facies within the unsaturated zone. The second is a heterogeneous lithofacies model which includes heterogeneity within each facies using scaling factors technique. The third is also a heterogeneous model where the heterogeneity in each lithofacies is characterized by using different random fields of each of the van Genuchten parameters. Scaling factors and van Genuchten parameters are generated using a geostatistical model of the field site. The geostastical model is based on extensive borehole data and undisturbed core analyses. We compared the effect of the different modeling representation on water flow and nitrate transport during seven years of two different fertilization treatments. Estimated nitrate levels in the vadose zone and nitrate leaching to groundwater differ significantly between the fertilizer treatments, yet are nearly identical between the three model representations and model dimensionality. In all cases, the deep vadose zone nitrate mass is at least four times larger than the measured nitrate mass at the site. The high discrepancies between the measured and model estimated nitrate at the site underscore the need for a rigorous examination of alternative modeling strategies to simulating deep vadose zone flow and transport processes.