Spatial and Temporal Influences on Hydraulic Properties in Macroporous Tile‐Drained Soil

This study investigated post‐harvest temporal changes in macroporosity and hydraulic properties, relative to subsurface drain position, at three different locations within a single field. Tension and double‐ring infiltrometer tests were conducted on the field surface at distances between 0 and 4 m from drains to determine hydraulic conductivity and a soil structural parameter as a function of pressure head [ K (ψ) and α′(ψ), respectively], and field‐saturated hydraulic conductivity ( K fs ). At two of the locations, duplicate sets of infiltration tests were conducted on significantly drier soils about 1 mo after the initial tests. Macroporosity values at each location were determined visually and with capillary theory (i.e., hydraulically effective macroporosity,ε ¯ m ), and results from the two methods were compared. At the two field locations with relatively low B horizon permeability, surface soil K fs was greatest above the drains; however, at the location with the greatest B horizon permeability, surface soil K fs increased away from the drain. The K fs was also significantly ( P < 0.1) greater under drier soil conditions. Although α′(ψ) and K (ψ) relationships were not influenced by drain position, they did exhibit temporal variability. For ψ < −2 cm, both α′(ψ) and K (ψ) were less in dry soil, whereas for ψ > −1 cm, both α′(ψ) and K (ψ) tended to be greater in dry soil. Theε ¯ mwas not influenced by drain position but tended to be greater in wetter soil, althoughε ¯ mattributable to pores with equivalent diameter >0.3 cm tended to be greater in dry soil. Theε ¯ mwas approximately 100 times less than visible macroporosity.