MACROPORE CHARACTERIZATION BY INDIRECT METHODS

Macropores are important in preferential flow; therefore, function and continuity of macropores are often more important than visual observa tion. The objective of this research was to compare several indirect m ethods of determining macropore volume. Ponded and tension infiltratio n measurements were made in situ at the soil surface and 0.25 m deep i n a Waukegan silt loam (fine-silty over sandy, mixed, mesic Typic Hapl udoll), and air-filled porosity at each pressure head was calculated f rom conductivity determinations. Undisturbed, unconfined samples were taken from the 0.25- to 0.35-m depth. Air-filled porosity as a functio n of pressure head was determined on these samples using wetting and d raining equilibrium curves in rotated cores (to even out the gravitati onal gradient), measured/calculated after steady-state lab conductivit y, and calculated from ped size distribution (assuming a ped shape fac tor and ped size/crack size relation, with known bulk density and ped density). Water adsorption/desorption in rotated cores in only four ou t of 20 samples displayed an ''air-entry'' pressure head, and no sampl es displayed a ''water-entry'' pressure head (that is the head at whic h no more water enters pores during equilibrium soil water adsorption) . All methods tested estimated air-filled porosity in the same range. Air-filled porosity determined from ped-size distribution and from pon ded and tension infiltration measurements was much less tedious than f rom rotated core measurements.