|Van Genuchten, Martinus|
Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/7/2004
Publication Date: 7/18/2005
Citation: Coquet, Y., Coutadeur, C., Labat, C., Vachier, P., Van Genuchten, M.T., Roger-Estrade, J., Simunek, J. 2005. Water and solute transport in a cultivated silt loam soil: 1. Field Observations. Vadose Zone Journal. 4:573-586. Interpretive Summary: Water and solute transport processes at the field scale are known to be very heterogeneous. Previous studies have shown the importance of soil structure and texture in regulating field-scale solute transport. The presence of different horizons or layers within a soil can be a major factor causing flow heterogeneity. In addition to soil structural properties associated with soil type, flow and transport processes are also affected by such external factors as tillage practices, and traffic by agricultural or other machinery. The purpose of this study was to understand how the presence of different layers, compartments or compacted zones resulting from plowing, surface tillage and trafficking, affect water and solute transport in an agricultural soil profile. The study involved a detailed field experiment (discussed in this paper), as well as the use of a numerical model for optimal analysis of the data (described in a companion paper). The experiment was performed on a small (4m by 2m) field plot perpendicular to the path of a tractor that had pulled a harrow for seedbed preparation. The plot was irrigated with a rainfall simulator, while a solute tracer (bromide) bromide was applied during part of the infiltration experiments. The water and bromide fronts in the subsurface were found to be highly heterogeneous. The heterogeneity could be explained by the particular soil structural features created by the agricultural practices, in particular by the location of compacted soil zones. Very little water and bromide had penetrated the large compacted zones under the wheel tracks. Our measurements indicated the presence of substantial preferential flow of water and bromide along paths immediately bordering the wheel tracks. Compacted clods from previous years, located in the plow layer between the wheel tracks, furthermore acted as low-permeability barriers that diverted the infiltrating water and bromide flow around them. Detailed studies like this one are helpful to explaining the very heterogeneous nature of field-scale leaching patterns. Results are important for better understanding the effects of alternative cropping and tillage systems on water and solute transport processes in the field.
Technical Abstract: Vadose zone flow and transport processes are known to be strongly affected by both soil structure and soil texture. We conducted a field experiment to explore the impact of heterogeneity in soil structure created by agricultural operations such as wheel traffic, plowing and surface tillage on water and solute transport. The experiment was performed on a 4 by 2 m2 field plot perpendicular to the path of a tractor that had pulled a harrow for seedbed preparation. The plot was irrigated with a rainfall simulator at a rate of 21 mm/h for 2 hand 20 min. An 850 mgIL bromide solution was subsequently applied at a rate of 26 mm/h for 2 h. Soil water contents and pressure heads during the experiment were monitored with time domain reflectometry (TDR) probes and tensiometers. The soil was sampled for resident bromide (Br ) concentrations at the end of the experiment. Water and bromide fronts were found to be highly heterogeneous. The heterogeneity could be explained by the particular soil structural features created by the agricultural practices, in particular by the locations and sizes of compacted soil zones. Very little water and bromide had penetrated the large compacted zones under the wheel tracks. Bromide, TDR, and tensiometer measurements all indicated the presence of preferential flow of water and bromide along paths immediately bordering the wheel tracks. The compacted clods in the plow layer furthermore acted as low-permeability barriers that diverted water and bromide flow around them. The highly heterogeneous plow layer between the wheel tracks produced a much higher solute dispersivity as compared to the compacted soil below the wheel tracks.