Mechanism of Water Entry into Simulated Macropores

Abstract It is commonly believed that, when the soil is unsaturated, water cannot enter soil macropores that begin below the soil surface. The objective of this research was to show that water under negative pressure can enter simulated macropores, after first establishing a continuous water film on the full length of the macropore walls. The simulated macropores were hollow, cylindrical, straight‐walled glass tubes 4.88, 2.60, and 0.85 mm in diameter. Water flux through the simulated macropores was obtained down to a negative water pressure of −1.02 kPa. This was shown by placing simulated macropores against a porous fritted‐glass plate 60 mm in diameter with pore diameters between 40 and 60 µm after establishing a water film on the walls of the simulated macropores. The flux of water down the simulated macropores varied from 0 to 67 mm 3 s −1 depending on the contact of the simulated macropore with the porous plate, which influenced the continuity of water in the porous plate with water films on the walls of the simulated macropores. A water manometer was connected to the bubbler of the burette in order to measure the total negative water pressure operating in the simulated macropores. The total negative water pressure operating in the simulated macropores was equal to the sum of (i) the negative water pressure due to the difference in elevation of water menisci of the water manometer and (ii) the matrix potential in the pores of the fritted‐glass plate.