Field study of preferential flow pathways in and between drain trenches

This study examined the number, distribution, and connectivity of biopores (>1 mm) in a sandy loam till with tile drains located at 1.2-m depth. Two areas (6.5 by 1 m, 10 m apart) were irrigated within 6 to 8 h with 50 mm of water containing the dye Brilliant Blue (2.2 g L⁻¹) using a field sprayer. Groundwater was initially below drain depth. The distribution of stained and unstained biopores was examined in 15- and 30-cm-wide horizontal terraces in 0.5-m-long sections along the 6.5-m irrigated transect at up to eight depths (15–175 cm), for a total investigated area of 14 m² This extensive data set showed that the number of biopores were of similar magnitude at both study sites in and outside the drain trench, ranging from 0 to 1114 m⁻² in the 14-m² examined section. The number of stained biopores (0–833 m⁻²) was unevenly distributed in the horizontal direction and seemed unaffected by distance to the drain trench (0–5.5 m), while in vertical sections with numerous dyed biopores at 150-cm depth, staining continued farther down in fractures. Staining in the drain trench was associated with biopores, voids, fractures, and the soil along the pipe, which may indicate that the tile drain took over the role of fractures in the till. Consequently, the connectivity of biopores with fractures or drains may have an important impact on staining patterns and on preferential flow phenomena.