Air Flow Measurements to Describe Field Variation in Porosity and Permeability of Soil Macropores

Abstract

Limited data are available on spatial variation in hydrologic behavior of soil macropores. This study examined field variatiou of soil water release and permeability in the pressure head range that influences macropores. One hundred twenty‐seven undisturbed soil cores were collected from a no‐tillage field containing a Lobdell silt loam (fine‐loamy, mixed, mesic Fluvaquentic Eutrochrept) soil. An air flow device that eliminates the gravitational gradient was used to measure water release, θ( h ), and air permeability, k a ( h ), in the pressure head, h , range of 0 to ‐240 mm. Mean air‐filled porosity, ε a , of 0.044 m 3 m ‐3 and geometric mean k a equal to 1.3 µm 2 at h = 0 indicated that macroporosity in a majority of these cores failed to exhibit negative air‐entry pressure heads. Generalized scaling of water release curves and air permeabilities using water contents expressed as degree of capillary saturation ([θ( h ) + ε a (0)]/θ s ) yielded considerable scatter reduction, implying a high degree of pore geometry and flow similarity for this soil. The spherical semivariogram model of water release scaling factors, α( h ) r , had a range of 14.1 m, whereas the linear semivariogram of air permeability scaling factors, α( k a ) r , had a range of 21.1 m. Further, sill values for α( k a ) r were 7.6 times greater than for α( h ) r . The poor correlation, dissimilar statistical distributions, and dissimilar spatial dependence of scaling factors suggested that characteristic lengths related to total macroporosity and fluid‐conducting macroporosity were substantially different.