Source-Responsive Representation of Nonequilibrium Preferential Flow Processes.

A continuous progression of equilibrium states, as represented by the Darcy-Buckingham law and Richards’ equation, does not encompass all possible flow modes of soil water. For example: (1) subvolumes or pores may wet up out of sequence or noncontiguously, (2) different processes may occur simultaneously on vastly different time scales, (3) a state variable such as pressure may propagate independently of water transport. The consequent discrepancies with traditional-model predictions have motivated a new approach, called source-responsive because it allows for water at depth to sensitively respond to changing conditions at the source of water input.

To predict unsaturated flow this model requires two characterizations: internal macropore facial area as a function of depth M(z), representing a capacity for preferential flow; and an active-area fraction f(z,t), indicating how much of that capacity is active at given depth and time. The source-responsive flow itself can be conceptualized as laminar flow in free-surface films. The M and f functions do not in general depend on moisture state but rather on profile-scale properties of the medium and water-input conditions (e.g. rainfall rate). Independence from local moisture state allows these functions to quantify processes that proceed without immediate interdomain equilibration.

Case studies from field and lab experiments on infiltration, aquifer recharge, and surface water-groundwater interaction show this model reasonably captures features of subsurface dynamics that are underrepresented by most models of soil-water flow. Though imperfectly understood, source-responsive processes may represent a substantial portion of unsaturated flow with minimal mathematical and measurement difficulties.