Preferential flow in sandy loam soils as affected by irrigation intensity

Dye-tracer studies in the field using Brilliant Blue FCF as tracer were performed to investigate the effect of irrigation intensity and soil heterogeneity on preferential flow. In two fields, both level and newly tilled in terms of seed bed preparation, to plots of 1.6 × 1.6 m were applied 50 mm of dye solution at rates of 10 and 50 mm h⁻¹. In the second year level, plots of grass of similar size were applied with 25 mm dye solution at a rate of 3.1, 6.2, 12.5, and 25 mm h⁻¹. For all plots the stained patterns were examined one or two days after application of dye solution by the excavation of 11 vertical cross sections of 100 × 100 cm and 10 cm apart from each other. Flow patterns were digitized and depth functions for the degree of dye coverage and the number of activated flow channels were calculated. Furthermore, the structural features of each cross section were examined visually. The results show that deep penetration of water into the soil profile took place as preferential flow through macropores, mainly earthworm channels, with much of the water thus bypassing the soil matrix. In the top 0–25 cm layer, the degree of dye coverage tended to be larger for the lower irrigation intensities indicating that water flow in the top soil took place through a relatively great proportion of the pores in the soil matrix. In the 35–100 cm subsoil layer the number of stained macropores tended to be larger for the higher irrigation intensities. Thus, at higher irrigation intensity a positive pressure potential apparently developed more extensively in the topsoil initiating preferential flow through a greater number of macropores in the subsoil. In the newly tilled soil, water flow took place through a relatively great part of the topsoil matrix. Deeply penetrating stained earthworm channels originated, predominantly, in the well defined transition zone between topsoil and subsoil. In the soil left untilled and grass covered for about one year the continuity of macropores was more pronounced, and stained channels could frequently be traced from the subsoil all the way to the soil surface, in particular at low irrigation intensity.