Film and Pulse Flow in Artificial Macropores

It is generally recognized that macropore flow may occur in unsaturated soil and that this should be taken into account when modelling flow of water and solutes in soil. The objective of the present study was to examine the water pressure potential conditions at which macropore flow may occur and to elucidate the nature of macropore flow. A laboratory setup consisting of a sand column 20 cm in diameter and 20 cm high with outlet into a 50 cm long glass tube was used. The outlet characteristics of the glass tube was either free to the atmosphere or the glass tube terminated in gravel. The tube diameters used were 3, 4, and 6 mm while the water application rates used were 1.2, 2.2, 12, and 18.5 mm h ⁻¹. Experiments covering 26 combinations of tube diameter, water application rate and outlet characteristics were conducted. Water entered the macropores at a water pressure potential in the overlying sand more or less equivalent to the water entry pressure potential of the macropore according to capillary theory. The nature of flow in the macropore was for the tubes of 3 mm and 4 mm in diameter predominantly pulse flow while film flow was more likely to occur in the tube of 6 mm in diameter. During pulse flow events a pressure potential gradient was consistently created in the sand column, the pressure potential decreasing to -20 cm to -30 cm corresponding approximately to the air entry pressure potential of the sand. The pulse flow events occurred repeatedly during the infiltration as long as water application was continued. When film flow occurred after a pulse flow event, the film flow continued at a water pressure potential less than -20 cm in the sand close to the opening of the artificial macropore.