The use of tension infiltrometers to assess routes and rates of infiltration in a clay soil

This paper describes the design and operation of a simple tension infiltrometer which imposes pressure potentials at the soil surface (h_b) of –0.5, – 2 and –9 cm water, corresponding to equivalent diameters ( e _d) of the largest conducting pore of 6, 1.5 and 0.3 mm respectively. Infiltration measurements obtained in Evesham series clay soil were fitted to Philip's (1957) two-term infiltration equation. Results were interpreted using information on the number, type and size of conducting macropores obtained in a dye tracing experiment. Significantly larger values of effective hydraulic conductivity at h _b=–0.5 cm were attributed to the flow of water in large shrinkage cracks which constituted nearly 90% of the total conducting macroporosity. Measured fluxes at h _b=–9 cm were related to sorption and swelling in the clay matrix, since within the range of infiltration times considered ( t <0.5 h), the gravity or steady state component of infiltration was negligible. Rainfall intensity/duration data for a large number of storms at Silsoe were used to demonstrate that in dry soil nearly 70% of rainfall infiltrates in the clay matrix, and that the infiltration capacity of macropores of e _d≤1.5 mm is only rarely exceeded.