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Later work has revealed wider implications and applications of the similarity solution for redistribution of soil water with capillary hysteresis [Philip, 1991]. The analysis involves continuity of both water potential and flow velocity across the interface between the draining and wetting regions. Two recent papers, however, claimed that in special circumstances, water potential is discontinuous across the interface and an “extended pressure condition” is needed. The apparent difficulty may be removed by recognizing that in the special case, hydraulics is not enough: volume conservation is maintained by diffusion of dissolved air. The extended pressure condition gives a useful approximation to the distribution of saturation but gets the potential distribution wrong and fails to recognize the air pressure differential between the two regions. Analogous conclusions apply to more general systems such as water/oil. |
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