Effects of soil water repellency on infiltration rate and flow instability

Laboratory infiltration experiments were carried out to quantify the effect s of soil water-repellency on infiltration rate and the wetting front insta bility. A two-dimensional transparent chamber (41.5 cm wide, 50 cm high and 2.8 cm thick) was constructed for infiltration experiments using three wat er-repellent Ouddorp sands (The Netherlands) and a wettable silicon sand. T he results showed that if the water-pending depth (h(0)) at the soil surfac e was lower than the water-entry value (h(we)) of repellent sands, infiltra tion would not start until the water drop penetration time (WDPT) is exceed ed; and contrary to infiltration in wettable soils, the infiltration rate i ncreased with time. However, infiltration could immediately start at any ti me when h(0) > h(we). The wetting front was unconditionally unstable for h( 0) < h(we), resulting in fingered flow. However, the flow was conditionally stable for h(0) > h(we) if the soil was not layered in a fine-over-coarse or wettable-over-repellent configuration, and if soil air was not compresse d during infiltration. The occurrence of stable and unstable dow in repelle nt soils was consistent with the prediction based on a linear instability a nalysis. The findings can be used to improve irrigation efficiencies in wat er repellent soils, e.g, using high-ponding irrigation methods. (C) 2000 El sevier Science B.V. All rights reserved.