PHYSICAL AND MORPHOLOGICAL CHARACTERIZATION OF BYPASS FLOW IN A WELL-STRUCTURED CLAY SOIL

Bypass flow governs solute movement in well-structured clay soils. Combined use of physical and morphological methods was made in this study to better characterize the process. Flow processes in five 200-mm-long undisturbed cores of 200-mm diameter were monitored at 11-min intervals by using 102 small transducer tensiometers, divided among five samples and installed at three depths. Flow patterns along macropores were stained with methylene blue. Twenty-five tensiometers in or within a distance of a few millimeters of stained macropores reacted very quickly when 10 mm of simulated rain was applied at an intensity of 13 mm h-1, and showed a drying pattern after the end of the simulated rainfall. Forty-three tensiometers inside peds reacted more slowly and showed continued wetting after the end of the simulated rainfall indicating internal catchment of water in the bottom of discontinuous macropores followed by redistribution of water. Internal catchment increased with depth in the samples, as indicated by both physical and morphological data. Using tensiometer measurements as a point count, it is estimated that 33% of the soil volume was in close contact with continuous macropores, while 42% was influenced by the effects of internal catchment. An average of 5.2 +/- 0.96 mm of the applied 10 mm of water left the cores through bypass flow, while an estimated average of 3.3 +/- 0.96 mm contributed to internal catchment. The observed patterns of water movement illustrate the inadequacy of the concept of mobile/immobile water.