The concept of field capacity revisited: Defining intrinsic static and dynamic criteria for soil internal drainage dynamics

Across many soil types and conditions, post wetting soil internal drainage exhibits predictable dynamics that lead to a stable and repeatable hydration state termed “field capacity” (FC). Soil regulation of internal drainage toward FC has long been recognized as producing a useful hydrologic benchmark for modeling and for estimation of plant available soil water. To overcome ambiguities and inconsistencies in various ad hoc definitions of FC, we propose using a soil intrinsic characteristic length (a matric potential value derived from drainable soil pore size distribution) to characterize the loss of hydraulic continuity associated with the attainment of FC. The resulting static criterion for FC was extended to formulate a self‐consistent dynamic criterion based on soil internal drainage dynamics. A systematic evaluation of the proposed definitions of FC using numerical simulations and experimental data reveals remarkable consistency and predictability across a wide range of soil types. The new metrics add definitiveness and robustness of this widely used concept with potential expansion to additional agronomic, hydrologic, ecological, and climatic applications. Key Points A soil intrinsic characteristic length defines attainment of field capacity This static criterion was extended to define a self‐consistent dynamic one Field capacity is characterized by a constant relative flux