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Title: Two-dimensional modeling of preferential water flow and pesticide transport from a tile-drained field

Author
item GARGENAS, ANNEMIEKE - SWEDISH UNIVERSITY
item SIMUNEK, JIRKA - UC RIVERSIDE, CA
item JARVIS, NICHOLAS - SWEDISH UNIVERSITY
item Van Genuchten, Martinus

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2006
Publication Date: 3/21/2006
Citation: Gargenas, A.I., Simunek, J., Jarvis, N., Van Genuchten, M.T. 2006. Two-dimensional modeling of preferential water flow and pesticide transport from a tile-drained field. Journal of Hydrology. Vol 329: 647-660

Interpretive Summary: Rapid non-equilibrium flow of water and dissolved chemicals through soil macropores and fractures can profoundly affect solute transport into and through the unsaturated zone between the soil surface and the groundwater table. The resulting preferential flow process dramatically increases the risk of groundwater pollution by surface-applied agricultural chemicals. Although the importance of preferential flow has been demonstrated repeatedly by means of experiments on relatively small undisturbed soil columns in the laboratory, quantitative information about its significance for solute transport at the field-scale is still very much lacking. The objective of this study was to compare various preferential flow and transport models in terms of their ability to simulate the measured dynamics of water drainage and a pesticide (MCPA) leaching from an tile-drained agricultural field in Southern Sweden. Four different conceptual approaches of increasing complexity were implemented in HYDRUS-2D, a popularly used software package for simulating two-dimensional water flow and solute transport in soils and groundwater. The approaches ranged from a traditional model assuming equilibrium transport, to a relatively sophisticated dual-permeability approach that accounts for water and pesticide transport through both the soil macropores and the soil matrix (the latter made up of smaller micropores). The dual-permeability approach most accurately simulated the measured dynamics of pesticide leaching from the tile-drained field. This model gave good predicitons of both peak flow resulting from macropore flow, and base flow reflecting the soil matrix (mictopore) characteristics. The equilibrium and other approaches failed to simulate the preferential nature of the flow process. The study shows that two-dimensional models can be very helpful tools for studying pesticide leaching from large sloping fields with much spatial variability in soil properties. Results should be of interest to scientists and engineers trying to better understand and predict water flow and solute transport processes in tile-drained field systems.

Technical Abstract: Preferential flow through soil macropores in tile-drained soils can significantly increase the risk of pollution of surface water bodies by agricultural chemicals such as pesticides. While many field studies have shown the importance of preferential flow in tile-drained fields, few have included detailed numerical modelling of the processes involved. The objective of this study was to compare four conceptually different preferential flow and/or transport approaches for their ability to simulate drainage and pesticide leaching to tile drains. The different approaches included an equilibrium approach using modified hydraulic properties near saturation, and three non-equilibrium approaches: a mobile–immobile solute transport model, a dual-porosity approach, and a dual-permeability formulation. They were implemented into the HYDRUS-2D software package. The model predictions were compared against measurements of drainage and pesticide concentrations made at an undulating, tile-drained field in southern Sweden (Na¨sbyga°rd) during a period of 6 weeks following spray application of the herbicide MCPA. he dual-permeability approach most accurately simulated preferential drainage flow, even though this approach somewhat overestimated the drainage rates. The equilibrium and mobile–immobile approaches largely failed to capture the preferential flow process. The dual-porosity approach predicted much more distinct and higher drainage flow events as compared to the dual-permeability approach. Differences between measured and simulated tile drainage rates using the dual-permeability approach could be partly explained by water by-passing the tile drains and recharging the deeper aquifer. The dual-permeability and dual-porosity approaches closely captured the dynamics in measured pesticide concentrations. Both the equilibrium and mobile–immobile approaches completely failed to match measured MCPA leaching by underestimating the peak concentrations by more than two orders of magnitude. We conclude that two-dimensional models are suitable tools for studying pesticide leaching from undulating fields with large spatial variability in soil properties.