SUSCEPTIBILITY AND PREDICTABILITY OF CONDITIONS FOR PREFERENTIAL FLOW

Preferential flow in the field might be caused by various factors and is difficult to observe in situ. This experimental study was designed to identify the combined effects of air entrapment, surface desaturati on (suction head), soil layering, and water repellency (hydrophobicity ) of the porous media on unstable preferential flow (or fingering) in the vadose zone. The predictability of unstable flow was studied on th e basis of two existing criteria for gravity fingering: (1) a velocity criterion proposed by Hill and Parlange [1972] and (2) a pressure hea d criterion by Raats [1973] and Philip [1975]. Two-dimensional transpa rent chambers (60 cm high, 41.5 cm wide, and 2.8 cm thick and 90 cm de ep, 74.5 cm wide, and 1.8 cm thick) were used to visualize water infil tration into a water-wettable sand, a water-wettable loam, differently layered sand and loam, and a water-repellent sand. The results sugges ted that infiltration into the homogeneous sand and a sand-over-loam s ystem, without the effects of air entrapment and surface desaturation, was unconditionally stable. Infiltration in the loam vias also stable as observed in the limited chambers. The flow was unconditionally uns table in a fine- over-coarse stratified sublayer and conditionally uns table in the homogeneous sand under the effects of air entrapment and surface desaturation. In multiple-layered systems, infiltration flow w as semiunstable; fingers developed in the sand layer and were stabiliz ed in the loam. In the repellent sand the wetting front was unstable u nder low pending conditions; however, it was stabilized when the pendi ng depth exceeded the water-bubbling (entry) value of the hydrophobic medium. Both the velocity and pressure head criteria predicted fingeri ng in the sand (layers) with the effects of gravity. However, the crit eria failed to predict stable flow in the loam, indicating that the ca pillary (stabilizing) effects on the flow need to be included in theor etical developments. Finally, the observed width and speed of the fing ers and the system flux were found to be always higher under air-drain ing fingering conditions than with fingering under air-confined condit ions.