Flow of water in porous media with saturation overshoot: A review

Different from classic wetting patterns, saturation overshoot (also called saturation-bump or non-monotonic saturation profile) is widely observed in unsaturated porous media. Saturation overshoot is the case of a high and uniform water content zone directly behind the wetting front followed by another region with a low and uniform saturation. This paper reviews recent research progress on this anomalous phenomenon, focusing on experimental observations and theoretical model development. Experimental conditions for the occurrence of overshoot generally coincide with those of gravity-driven fingered flow. Saturation overshoot is associated with the corresponding capillary pressure overshoot. The Richards equation in its basic form cannot describe the non-monotonic saturation field due to its parabolic nature and incompleteness in physics. Several attempts to modify the Richards equation and other approaches proposed recently have been reviewed and assessed. The knowledge gaps and research opportunities on saturation overshoot are summarized at the end.