Data from an infiltration and drainage experiment into and from a sand tank model of 5 by 3 m and 2 m deep were reevaluated. The uniform sand overlaid a 0.2-m-thick gravel layer from which five perforated pipes collected drainage flow. The data include time series of capillary potentials, ψ, volumetric water contents, θ, both from nine levels, and drainage flow, q, that resulted from sprinkler infiltration with a constant rate of 15.6 mm h−1 that lasted >16 h. The wetting and the pressure fronts (i.e., the increases of water content and of capillary potential) moved with the same constant velocity of 3.25 × 10−5 m s−1. The capillary potentials about 0.2 to 0.4 m behind the wetting front remained constant at about −2.5 kPa. The maximum degree of saturation during infiltration was <0.6 and the mobile water content involved in flow was about 0.15 m3 m−3. The constant velocity indicates a continuous and dynamic balance of forces acting on the mobile water. Thus, viscosity is considered to have balanced gravity as the driving force, and a momentum dissipation approach to infiltration evolved. Capillary and Bond numbers indicate that flow was subjected to considerable capillary potential, yet its gradient was only active throughout a depth range of 0.2 to 0.4 m behind the wetting front. The constant velocity prevailed despite local variations in the mobile water content. Constant velocity of wetting is proposed as the global key parameter during infiltration on which local features, like volume flux density and variation of water content, may depend.

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