Infiltration in homogeneous sands and a mechanistic model of unstable flow

One-dimensional infiltration experiments were conducted to study the mechanics of unstable flow in homogeneous soils under non-ponding infiltration. A mechanistic model is presented which explains soil water pressure gradients that are characteristic of stable and unstable flow in homogeneous soils and is based on a dynamic soil water entry pressure, the Darcy-Buckingham flux equation, and hysteretic moisture retention functions. Infiltration experiments were conducted with five sand samples under applied fluxes of 2, 5, 20, and 50% of their saturated hydraulic: conductivity. Soil water pressures were measured at fixed depths following passage of the wetting front. A trend of decreasing soil water pressure over time following passage of the wetting front is not predicted by Richards' equation and produces unstable flow. Under air-dry initial soil water conditions, soil water pressures were unstable for all fluxes in the three coarser sands. In the two finer sands, unstable flow occurred at infiltration rates of 20 and 50% of their respective saturated hydraulic conductivities but stable flow occurred at lower fluxes. Soil water pressure measured just behind the wetting front was found to be an increasing function of applied flux and average grain size of the media for infiltration with air-dry initial soil water content. Additional tests showed that systems that produced unstable now under air-dry initial soil water contents exhibited stable now during infiltration with initial soil water contents that were greater than air dry.