Water and herbicide transient flow transport in field dried topsoils during controlled infiltration: I. Water capillary and gravity-driven transient flows: A preliminary examination

A disc infiltrometer with a disc diameter of 80 mm has been operated in the field at − 0.2 kPa few days after the Spring 2003 spraying of a mixture of bentazone and isoproturon on three dried tilled topsoils differing in texture but under the same climatic and vegetation conditions. We have derived both infiltration parameters from the disc infiltrometer records and capillary invasion parameters from the combined analyses of soil structure and soil water carried out on horizontal soil core slices of approximately 7 mm thickness. We assumed that the recorded transient flow can be subdivided into the capillary flow, estimated from sorptivity (differentiated linearization method), and the gravity-driven transient flow, calculated as the difference between the recorded total flow and the estimated capillary flow. Although gravity-driven flow occurred in this study, we assumed that capillary invasion theory is applicable because the topsoils were dry, leading to predominance of capillary flow, and the experiment was ended shortly after reaching the steady-state flow. Capillary invasion was the predominant transient flow process occurring into the silt loam soil whereas a combination of capillary flow and gravity-driven transient flow occurred into the sandy loam soil and the silty clay soil. Finally, capillary invasion theory was partly validated but more experimental and conceptual work is required.