Atypical early-time infiltration into a structured soil near field capacity: The dynamic interplay between sorptivity, hydrophobicity, and air encapsulation

Accurate measurement of infiltration attributes for the soilʹs surface layer is critical for understanding the dynamics of soil water in response to rainfall or irrigation. This study used four large lysimeters to measure the early-time infiltration behaviour of a structured, well-drained, silt loam Dystric Cambisol. For each lysimeter there were four separate infiltration experiments, with a 480-mm-diameter tension infiltrometer used to supply infiltrating water under suctions of 0, 0.5, 1, and 1.5 kPa. We consistently observed a lack of clear sorptivity-driven infiltration behaviour for all lysimeters under all surface-imposed suctions. Having ruled out artefacts of the tension infiltrometer–lysimeter system, surface seal development, air confinement, and a limited ‘infiltration capacity’ of the soilʹs top 50 mm layer, we conclude that direct and indirect effects of weak hydrophobicity and air encapsulation are the most important controls. Hydrophobicity, which appears to restrict the first 5–10 mm of infiltration, occurs despite the soils being close to field capacity. During unsaturated infiltration some macropores are non-fillable to infiltrating water, and we suggest that these pores may also act to isolate parts of the pore network that could otherwise exhibit sorptivity, and thus restrict early-time infiltration. The interaction between hydrophobicity and the “non-fillable” pore network may be an important mechanism causing non-uniform infiltration through preferential flowpaths. This may enhance air encapsulation by effectively blocking the escape path for the displaced air from parts of the pore network, further restricting the expression of sorptivity. In this sense the early-time infiltration behaviour of this soil is seen to be governed by the dynamic interaction between sorptivity, hydrophobicity, the network of air-filled pores, preferential flow and air encapsulation.