Hysteretic, variably saturated, transient flow and transport models with numerical inversion techniques to characterize a field soil in central Arizona

Abstract

Field-measured data collected from infiltration experiments conducted at the Maricopa Agricultural Center, Arizona, and one-dimensional numerical models were used in an iterative approach to develop conceptual models of unsaturated flow and transport. Conceptual models were developed using field data, estimates of hydraulic and transport properties, and assumptions regarding the types of processes occurring in the field plot. Using the field data and numerical models in an iterative approach revealed that adjustments to the parameterization of the conceptual models were necessary to allow for acceptable predictions of flow and transport. In addition, the numerical models delineated where the conceptual models needed adjustment and indicated where further analysis of the field data was necessary. Specifically, in regards to the unsaturated water flow, it was found that: (1) without increasing the laboratory determined value of Ks tenfold, it was impossible to make acceptable predictions for the infiltration experiments; (2) the laboratory-derived hydraulic properties provided a superior first approximation to texturally based hydraulic properties taken from an internal database; (3) incorporation of hysteresis into the soil hydraulic functions resulted in predictions of the water contents through time and soil water content profiles that were marginally better than not incorporating hysteresis; and (4) based on the mean square errors of the predictions and a post-audit based on the adjusted hydraulic properties, the use of a homogeneous soil profile was supported. In regard to the unsaturated solute transport, it was found that: (1) inversion techniques to estimate the unsaturated transport properties at each location (local approach) yielded the best results with respect to the fit of the data compared to the techniques which fit data from all depths simultaneously (global approach); (2) predictions using unsaturated transport properties averaged from the global approach yielded the best results compared to the average local approach properties; (3) incorporating hysteresis had little effect on the predicted bromide breakthrough curves and concentration profiles, yielding only marginally better results than predictions from models with no hysteresis; (4) it was impossible to accurately predict the breakthrough of bromide at depths deeper than 200 cm without using a retardation factor less then one.