Modeling Nonequilibrium Flow and Transport Processes Using HYDRUS

Accurate process-based modeling of nonequilibrium water fl ow and solute transport remains a major challenge in vadose zone hydrology. Our objecƟ ve here was to describe a wide range of nonequilibrium fl ow and transport modeling approaches available within the latest version of the HYDRUS-1D soŌ ware package. The formulaƟ ons range from classical models simulaƟ ng uniform fl ow and transport, to relaƟ vely tradiƟ onal mobile-immobile water physical and two-site chemical nonequilibrium models, to more complex dual-permeability models that consider both physical and chemical nonequilibrium. The models are divided into three groups: (i) physical nonequilibrium transport models, (ii) chemical nonequilibrium transport models, and (iii) physical and chemical nonequilibrium transport models. Physical nonequilibrium models include the Mobile-Immobile Water Model, Dual-Porosity Model, Dual-Permeability Model, and Dual-Permeability Model with Immobile Water. Chemical nonequilibrium models include the One KineƟ c Site Model, the Two-Site Model, and the Two KineƟ c Sites Model. Finally, physical and chemical nonequilibrium transport models include the Dual-Porosity Model with One KineƟ c Site and the Dual-Permeability Model with Two-Site SorpƟ on. Example calculaƟ ons using the diff erent types of nonequilibrium models are presented. ImplicaƟ ons for the formulaƟ on of the inverse problem are also discussed. The many diff erent models that have been developed over the years for nonequilibrium fl ow and transport refl ect the mulƟ tude of oŌ en simultaneous processes that can govern nonequilibrium and preferenƟ al fl ow at the fi eld scale.