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United States Department of Agriculture

Agricultural Research Service

Title: Modeling Nonequilibrium Flow and Transport Processes Using HYDRUS)

item Simunek, Jirka
item Van Genuchten, Martinus

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2007
Publication Date: 5/1/2008
Publication URL:
Citation: Simunek, J., Van Genuchten, M.T. 2008. Modeling Nonequilibrium Flow and Transport Processes Using HYDRUS. Vadose Zone Journal. Vol 7:782-797

Interpretive Summary: The problem of preferential flow in the subsurface has received much attention in the soil and agricultural sciences because of its implications in accelerating the movement of agricultural contaminants (fertilizers, pesticides, pathogenic microorganisms, toxic trace elements) through the vadose zone between the soil surface and the groundwater table. The potentially rapid migration of radionuclides from low- and high-level nuclear waste disposal facilities, and the preferential movement of non-aqueous liquids or other pollutants from underground storage tanks, waste disposal sites and mine tailings, has also become a concern for hydrologists, geophysicists, and environmental scientists. Preferential flow is probably the most frustrating process in terms of hampering accurate predictions of contaminant transport in soils and fractured rocks. Preferential flow, as opposed to uniform flow, results in irregular wetting of the soil profile as a direct consequence of water moving faster in certain parts of the soil profile than in others. In this paper, we review various approaches for modeling preferential flow in the vadose zone between the soil surface and the groundwater table. Existing approaches range from relatively simplistic models to more complex physically based dual-porosity, dual-permeability, and multi-region type models. Several models invoke terms accounting for the exchange of water and solutes between the soil or rock matrix (micropores) and the soil macropores or rock fractures. All of the various nonequilibrium flow and transport modeling approaches are incoporated in the latest version of the HYDRUS-1D software packag, which can be downloaded freely from or Several examples and comparisons of equilibrium and various nonequilibrium flow and transport models are also discussed in this paper. Results are important to better understand and predict the fate and transport of agricultural contaminants move in the subsurface.

Technical Abstract: Accurate process-based modeling of nonequilibrium water flow and solute transport remains a major challenge in vadose zone hydrology. The objective of this paper is to describe a wide range of nonequilibrium flow and transport modeling approaches available within the latest version of the HYDRUS-1D software package. The formulations range from classical models simulating uniform flow and transport, to relatively traditional 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: a) physical nonequilibrium transport models, b) chemical nonequilibrium transport models, and c) physical and chemical nonequilibrium transport models. Physical nonequilibrium models include the Mobile-Immobile Water Model, a Dual-Porosity Model, a Dual-Permeability Model, and a Dual-Permeability Model with Immobile Water. Chemical nonequilibrium models include the One Kinetic Site Model, the Two-Site Model, and the Two-Kinetic Sites Model. Finally, physical and chemical nonequilibrium transport models include the Dual-Porosity Model with one Kinetic Site, and the Dual-Permeability Model with Two-Site Sorption. Example calculations using the different types of nonequilibrium models are presented. Implications for the formulation of the inverse problem are also discussed. The many different models that have been developed over the years for nonequilibrium flow and transport reflect the multitude of often simultaneous processes that can govern nonequilibrium and preferential flow at the field scale.

Last Modified: 05/23/2017
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