Uncertainty analysis of groundwater flow and solute transport in unsaturated-saturated porous medium: Maricopa case

Abstract

Various methods are used to postulate, compare and rank alternative conceptual-mathematical models of unsaturated flow and transport during infiltration and tracer experiments at the Maricopa Agricultural Center (MAC) near Phoenix, Arizona. The models include one- and two-dimensional flow and transport in a uniform soil, a soil consisting of uniform layers, and a stratified soil having laterally varying properties. Characterization of the soil as a uniform medium is based on information obtained from public sources. Characterization of the soil as a layered medium is based on site data. Soil hydraulic properties are estimated from pedologic data. Uniform soil hydraulic properties are ascribed to each layer on the basis of soil type using mean values of three generic databases. Variable soil hydraulic properties are ascribed to individual soil samples using pedotransfer models based on site soil data. By treating these variable hydraulic property estimates as measurements, Bayesian updates of their mean values together with the variance of each estimate are obtained. Geostatistical analysis of soil pedologic and hydraulic properties provides support for a layered conceptual model with relatively large-scale lateral variability in each layer. The above conceptual-mathematical models and hydraulic parameter estimates are applied to analyze flow and transport under uncertainty and to compare simulated and observed water contents during one of the infiltration experiments at the MAC. It is shown that in order to reproduce observed behavior, it is necessary to further modify the hydraulic parameter estimates through inverse modeling. Various conceptual-mathematical models and parameter estimates are compared and ranked using likelihood-based model discrimination criteria. The inverse results are confirmed by using those to simulate the flow of an earlier experiment. Finally, the inverse estimates of soil hydraulic parameters based on one infiltration experiment are used for solute transport modeling. An advection-dispersion model with linear mass transfer to and from a zone of immobile water, or anion exclusion, is employed in forward and inverse modes. The study illustrates how a combination of methods and data sets can be used sequentially and in tandem to improve one's understanding of unsaturated flow and transport conditions at a site.