Water and solute transport in a cultivated silt loam soil. 1. Field observations

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-1 for 2 h and 20 min. An 850 mg L-1 bromide solution was subsequently applied at a rate of 26 mm h-1 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.