Multiscale Characterization of Vadose Zone Macroporosity in Relation to Hydraulic Conductivity and Subsurface Drainage

The purpose of this study was to quantify macroporosity distribution in relation to tile drains, and to determine any correlation between macroporosity and hydraulic conductivity (K). Macropore-tile drain relations were investigated at three plots within a single field. Macropores on 1490.25-m(2) horizontal surfaces, located at depths of 0.02 to 1 m, and distances of 0 to 3.5 m from the tile drains, were counted and classified into 0.5 to 5, 5 to 8, 8 to 10, and 10 to 12 mm equivalent circular diameter categories. Results show no significant (P < 0.1) difference in macropore area fraction (MAF) (defined as macropore area/total soil area) distribution relative to tile drain position; however, MAF did vary significantly between plots. The coefficient of variation for plot-specific, depth-averaged, total MAF ranged from 10 to 48%. Macropores in the 0.5- to 5-mm and 5- to 12-mm size ranges were most abundant near the surface and the shallow B horizon, respectively. When present, macropores at tile depth were mostly 0.5 to 5mm in size. Maximum depth-averaged total MAF ranged from 0.0036 to 0.0087 within the plots, whereas the minimum depth averaged MAF was less than 0.001 and located at tile depth. Strong correlation was observed between field surface K and MAF below 0.45 m depth (r = 0.79), and B horizon K and MAF below 0.02 m depth (r >= 0.83). Dye staining patterns revealed that > 20-yr-old tile installation scars were channeling infiltrate to tile drains at two plots.