Modeling fingered flow of water in soils owing to wetting front instability: a review

Fingered flow in the unsaturated zone caused by wetting front instability enhances solute leaching to the groundwater. This paper reviews recent progress in fingered flow research, focusing on theoretical results and model development. A variety of stability criteria have been derived to predict wetting front instability, mainly through linear, and sometimes non-linear, stability analysis, but also by theoretically analyzing infiltration into dry soils, and by stochastic methods that take into account random variations of the soil properties, fluid pressure, and front location. These stability criteria are discussed and compared. Eight expressions fur finger size are presented. They fall into three categories based on the dependence on the ratio of the infiltration rate to the soil hydraulic conductivity in the finger. Next, the modeling of finger growth is discussed. Because of its relevance to solute transport, recent advances in the measurement and modeling of finger behavior in moist soils are thoroughly reviewed. Various models (ranging from easily applied closed-form equations for risk assessment to sophisticated transient numerical codes) have been developed over the past 15 years. These are discussed and selected results are shown. Finally, some remaining gaps in our understanding are given. Among these are the role of the initial water content, the nature of the flow in the distribution zone above the fingers, the effect of soil heterogeneity on wetting front instability, and the combined effect of soil heterogeneity and wetting front instability on field-scale solute transport. (C) 2000 Elsevier Science B.V. All rights reserved.