Rate, kinetic energy and momentum of preferential flow estimated from in situ water content measurements

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.