Unsaturated-Zone Water Dynamics From Combined Darcian and Free-Surface Film Flow.

Infiltration and redistribution lead to dynamic soil-water conditions and fluxes that frequently are not adequately treated using the traditional Darcy-Buckingham-Richards approach alone. Typical cases of difficulty include water tables that fluctuate rapidly during rainstorms, spatial distributions of transported solute that are inconsistent with diffuse flow, saturation overshoot at wetting fronts, and rapid lateral unsaturated-zone transport to streams and ditches. Increasing evidence suggests that free-surface films lining the walls of certain pores, having an unconstrained air-water interface that causes pressure, volume, and flux relations to differ from those of capillary flow, can account for some of these behaviors.

A new dual-domain model provides improved representation of many dynamic moisture conditions. The domain of preferential flow, called source-responsive because it responds sensitively to changing conditions at the source of water input, can be physically conceptualized as laminar flow in free-surface films. The average film thickness is taken to be constant, in correspondence to evidence that preferential flow moves at an approximately uniform rate when generated by a steady and ample water supply. For a given medium, the source-responsive domain is characterized by an effective areal density (area per unit bulk volume) of internal faces of pores larger than a certain size that are capable of accommodating free-surface films.

Where both preferential and diffuse flow are significant, the interactive combination of source-responsive and Darcian flow can improve prediction of unsaturated-zone fluxes in response to hydraulic inputs and the evolving distribution of soil moisture. Examples for which this approach is efficient and physically plausible include (1) rainstorm-generated rapid fluctuations of a deep water table in a fractured-rock unsaturated zone and (2) space- and time-dependent soil water content response to infiltration in a soil of highly developed structure.