In the introduction to the special Vadose Zone Journal section, Preferential and Unstable Flow: From the Pore to the Catchment Scale, Gerke et al. (2010) considered research stimulation that emanates from the unbiased discussion of the interplay among the three major flow-driving forces: capillarity, gravity, and viscosity. On one side, Richards' (1931) diffusivity equation evolves when stressing capillarity and back-setting gravity and viscosity. On the other side, bringing gravity and viscosity to the forefront leads to Stokes flow, and wetting proceeds in films. Burdine (1953) demonstrated the applicability of Stokes principles while linking pore geometry with flow. However, only recently...

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