CHARACTERIZATION OF WATER ENTRY INTO A SOIL WITH A CONTRASTING TEXTURAL CLASS

Parameters characterizing water entry into a texture contrast soil (loamy sand over clay) were obtained from measurements along a transect. Spatial variability of selected infiltration parameters was studied using geostatistics and frequency distribution statistics. The occurrence of transmission macropores, stained with methylene blue, was recorded and their influence on measured parameters assessed. Sorptivity measured at a small suction (-100 mm potential), which is not influenced by macropores >0.3 mm equivalent diameter, exhibited slight spatial dependence, whereas in situ steady-state ponded infiltration rates had no spatial dependence. Cylindrical transmission macropores with measured diameters greater than 2 mm strongly affected the magnitude and frequency distribution of “detached” Ksat values, but did not affect in situ ponded sorptivity, presumably because the surface area available for adsorption was not increased significantly by pores of this size. Excluding all pores greater than 0.3 mm equivalent diameter, however, reduced sorptivity by 40%. The results indicate that in modeling runoff processes there is a need to discriminate between macropore and matrix contributions to the variability of infiltration parameters in the field.