Abstract Traditional macroscopic models for water flow in the vadose zone often assume local equilibrium within a representative element volume. However, under fingering flow this assumption breaks down on a macroscale involving sub‐grid figuring because local equilibrium does not hold between the fingering and non‐fingering domains. To address this important gap, the optimality‐based active region model (ARM) provides a macroscopic framework for describing fingering flow in the vadose zone. Grounded in nonlinear thermodynamics, ARM is based on the optimality principle that unsaturated flow can self‐organize into pathways of least resistance, thereby maximizing overall flow efficiency. This leads to a relationship between water flux and hydraulic gradient that departs from the Darcy–Buckingham law by allowing hydraulic conductivity to depend not only on water saturation (or capillary pressure) but also on the magnitude of water flux. This review presents the theoretical basis of ARM, summarizes its validation with laboratory and field observations, and discusses its extension to account for the low‐velocity flow in a non‑fingering domain. Overall, the results indicate that ARM provides a robust and practical framework for understanding and predicting water movement in unsaturated soils.
Liu et al. (Fri,) studied this question.