Summary Preferential flow needs self‐dependent fluid mechanical characterization and quantification. A theoretical and experimental approach is presented to describe preferential flow with the three variables volume of flowing water and its kinetic energy and momentum. Two soils were irrigated at rates of 50, 75 and 100 mm hour−1 at one site and three times at 100 mm hour−1 at a second site. The variations of soil water content caused by irrigation were measured at five depths with horizontally installed TDR wave guides at 300‐s intervals. The volume flux densities were calculated from the water balances. The computation of momentum and kinetic energy of preferential flow was based on the velocities of the wetting fronts and the amplitudes of the variations of soil water. Kinetic energy per unit volume of flow in the soil exceeds the one of irrigation by factors between 100 and 1000, which indicates concentration of flow. However, kinetic energy per unit volume of flow in the soil is less by factors of 106 to 108 than the capillary potential of soil water. Kinetic energy per unit volume of flow seems a reasonable variable for discriminating between preferential and ordinary flow at a threshold of 2 × 10−6 Pa.

The basic principles are derived of steady momentum dissipation during laminar boundary-layer flow in macroporous soils. The principles lead to a flux law and to geometrical measures of the flow paths. The theory of kinematic waves evolves, and the shock fronts are routed with the method of characteristics. The approach is applied to TDR soil moisture readings at depths 0.15, 0.35 and 0.55 m in a soil under meadow during an infiltration experiment (rate and duration of sprinkling were 2.78 x 10" 5 m s" 1 and 4500 s, respectively). Momentum dissipation during flow within the soil profile is independent of momentum added to the soil surface by sprinkling. Specific momentum dissipation per unit depth in the soil profile was about 0.02 kg m 2 s' 1 , with a tendency to increase with depth.