In structured agricultural soils, preferential flow and transport processes can often dominate water and solute movement. To evaluate and simulate the effects of preferential flow on both groundwater recharge and the potential for tile drains to rapidly capture infiltrating water and solutes, knowledge of macropore spatial distribution is required; however, very little of this type of detailed information currently exists.

In this work, macroporosity spatial distribution relative to tile drains was investigated at three sites within a single field that had different A – B horizon soil type combinations. A total of one hundred and forty nine 0.25 m² horizontal soil surfaces were excavated at depths ranging from 0.02 to 1 m below ground surface, and at distances from 0 to 3.5 m from tile drains; and on each surface, macropores were counted and classified into 0.5-5 mm, 5-8 mm, 8-10 mm, and 10-12 mm equivalent circular diameter categories. In addition, hydraulically effective macroporosity was quantified on the field surface overlying each excavated subsurface location using tension infiltrometry methodology.

Results from macropore counting show that macropore area fraction (MAF) (defined as macropore area / total soil area) does not vary significantly (P < 0.1) relative to lateral distance from the tile drain. Visible macropores in the 0.5 to 5 mm, and 5 to 12 mm, size ranges were most abundant near surface, and the shallow B horizon, respectively; and when present, macropores at tile depth were mostly 0.5 to 5mm in size. Maximum depth averaged total visible MAF was significantly different between the three plots, and ranged from 0.0036 to 0.0087; whereas minimum MAF was less than 0.001 and located at tile depth. Hydraulically effective macroporosity was approximately 100 times less than visible MAF observed 0.02 m below the field surface and was also not influenced by drain position.