Preferential Flow through Intact Soil Cores Effects of Matric Head

Abstract

Continuous soil pores may act as pathways for preferential flow depending on their size and water status (filled or drained), the latter being largely controlled by the soil matric head (h). The literature contains a wide range of proposed minimal pore sizes that may contribute to preferential flow. The objective of this study was to examine the relationship between h (and corresponding pore sizes) and preferential solute transport in a naturally structured soil. Tracer (3H2O and pentafluorobenzoic acid, [PFBA]) miscible displacement experiments were performed at several h values in intact soil cores (15‐cm diameter, 30‐cm length) using an apparatus especially suited to maintain constant h while collecting large effluent volumes. To test for the occurrence of preferential flow, observed breakthrough curves (BTCs) were evaluated for physical nonequilibrium (PNE) using a comparison between fitted local equilibrium (LE) and PNE models. Fitting results of the observed BTCs indicated absence of PNE in all solute transport experiments at h ≤ −10 cm. Experiments at h ≥ −5 cm consistently exhibited PNE conditions, indicating the presence of preferential flow. These results suggest that soil pores with effective radii of 150 μm and smaller (water‐filled at h = −10 cm) do not contribute to preferential flow. Observed pore water velocities were not indicative of the presence or absence of preferential flow conditions. Continuous measurements of soil water content (θ) using time domain reflectometry (TDR) revealed that at h = −10 cm, <2% of the soil volume (1–5% of θ at saturation) had drained.