ABSTRACT
The common assumption when modeling transient liquid flow in an unsaturated porous medium is that the capillary pressure-saturation degree relationship is independent of the macroscopic liquid flux. Thus, the usual practice in solving transient flow problems is to incorporate the capillary head (ψ ) - saturation degree (,S'e) relationships, measured under hydrostatic or steady-state flow conditions, into the continuity equation [1], The measurement of the dynamic Ý(Se) relationship under transient conditions is a complex task, mainly because of the finite response time of the devices used for capillary pressure determination. Nevertheless, several experimental studies have demonstrated differences between water retention curves determined under transient conditions and those determined under steady-state or hydrostatic conditions [2-4] as well as differences among determinations at different capillary pressure increments, i.e. at different water fluxes [5-7]. Several possible phenomena have been proposed to explain the liquid flux dependence of the Ý(Se) relationship: (i) a reduction in the amount of entrapped air when smaller pressure increments are applied (smaller water fluxes) in a wetting process [5]; (ii) ‘salt sieving’, i.e. greater free salt concentration within the sample and less soil water retained when larger pressure gradients are applied in a drainage process [5]; (iii) a complementary effect of contaminants being accumulated near the water-air interface, causing a reduction in the interfacial tension and, therefore, in the amount of water retained [2]; (iv) the formation of isolated (from the continuous liquid phase) pendular rings, whose size and redistribution rates depend on the rate of drainage [2, 7-9]; (v) limited, local, flux-dependent availability of air to replace draining soil water [4]; and (vi) the disruption of the quasi-crystalline structure of the water molecules near the solid surfaces, caused by flow, which displaces them from their minimum-energy positions [10].
