AIR-FLOW MEASUREMENTS TO DESCRIBE FIELD VARIATION IN POROSITY AND PERMEABILITY OF SOIL MACROPORES

Limited data are available on spatial variation in hydrologic behavior of soil macropore. This study examined field variation of soil water release and permeability in the pressure head range that influences ma cropores. One hundred twenty-seven undisturbed soil cores were collect ed 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 releas e, theta(h), and air permeability, k(a)(h), in the pressure head, h, r ange of 0 to -240 mm. Mean air-filled porosity, epsilon(a), of 0.044 m (3) m(-3) and geometric mean k(a) equal to 1.3 mu m(2) at h = 0 indica ted that macroporosity in a majority of these cores failed to exhibit negative air-entry pressure heads. Generalized scaling of water releas e curves and air permeabilities using mater contents expressed as degr ee of capillary saturation ([theta(h) + epsilon(a)(theta)]/theta(s)) y ielded considerable scatter reduction, implying a high degree of pore geometry and now similarity for this soil. The spherical semivariogram model of water release scaling factors, alpha(h)(r), had a range of 1 4.1 m, whereas the linear semivariogram of air permeability scaling fa ctors, alpha(k(a))(r), had a range of 21.1 m. Further, sill values for alpha(k(a))(r) were 7.6 times greater than for alpha(h)(r). The poor correlation, dissimilar statistical distributions, and dissimilar spat ial dependence of scaling factors suggested that characteristic length s related to total macroporosity and fluid-conducting macroporosity we re substantially different.