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Experiments to determine the unsaturated flow properties of soils are based either on the steady-state Darcy equation or on the transient diffusion equation. An apparatus has been designed to compare the steady-state and transient methods on the same sample. Results indicate that the transient outflow pattern at high moisture contents (using a coarse sand) differs considerably from that predicted by diffusion theory. The extended theory which takes into account the air as well as the water phase also does not agree with the observed transient behaviour at the high moisture contents. At low moisture contents the agreement between the two methods is considerably better. The transient outflow method was found to be inapplicable at the high moisture contents. Much of the difficulty is attributed to the inaccessibility of air to drainable pores under these conditions.
DE Elrick (Sat,) studied this question.