HESS Opinions: Unsaturated infiltration: from unproven capillary flow to viscous flow

Abstract. Briggs (1897) deduced capillary flow from deviation of the equilibrium between capillarity and gravity. Richards (1931) raised capillary flow to an unproven soil hydrological dogma. Apparent corrections of the dogma led to non-equilibrium flow, macropore flow, and preferential flow during infiltration. Viscous flow is proposed as alternative to capillary flow during unsaturated infiltration.