Path Asymmetry in Soil Water Retention: The State Resilience Index (SRI) as a Transferable Hysteresis Descriptor

Soil hydraulic hysteresis—the divergence between drying and wetting trajectories of the water retention curve—encodes quantitative information on soil structural organization that traditional modelling approaches do not explicitly capture. This study introduces the State Resilience Index (SRI), a dimensionless descriptor defined as the normalized area enclosed between the drying and wetting branches of a hysteresis loop and evaluates its interpretive value across five contrasting datasets from the published literature. The datasets encompass compaction gradients, repeated wetting–drying cycles, depth-resolved soil profiles, and water vapor sorption isotherms from soils differing in clay content and mineralogy. SRI values ranged from 0.023 to 0.333, responding consistently to the dominant structural drivers of hysteresis in each system. High values were associated with heterogeneous, structurally complex pore networks characteristic of productive soils; declining values reflected progressive loss of structural memory under compaction, repeated cycling, or mineralogical rigidity. A provisional interpretive framework is proposed, linking SRI ranges to soil structural quality and agronomic potential. The index is dimensionless, model-free, and applicable across measurement domains and forcing regimes, positioning it as a broadly transferable tool for the comparative assessment of soil hydraulic path asymmetry.