Poiseuille Flow Geometry Inferred from Velocities of Wetting Fronts in Soils

Soil structure can significantly enhance or modify infiltration rates and flow pathways in undisturbed field soils. Relations between features of soil structure and features of infiltration were elucidated from the velocities of wetting fronts. To study flow geometries during wetting, 100 sprinkler infiltration experiments were performed in situ at 25 different sites. Applied sprinkler irrigation rates ranged from 20 to 100 mm h-1 and lasted 1 h. Time domain reflectometry (TDR)-based observations of soil moisture time series at various depths yielded 215 velocities of wetting fronts between 0.2 and 5.5 mm s-1. Application of Poiseuille's Law to the velocities resulted in radii and densities of equivalent Poiseuille pores in the ranges of 5 to 30 micrometer and of 2 x 10(6) to 2 x 10(8) m-2, respectively. From the equivalent Poiseuille pores capillary heads were estimated to be in the range of -2.0 to-0.2 m. They served as initial conditions for modeling infiltration with the HYDRUS-2D code using hydraulic properties representative of the sand, silt, loam, and clay soil textural classes. The observed in situ wetting velocities indicated preferential flow; however, the calculated equivalent pore radii suggested that no macropores were required to initiate preferential flow. HYDRUS-2D reproduced the bulk of observed wetting velocities when hydraulic properties typical of sandy soils were used.