Novel Perspective on Field Capacity: Characterizing Soil Water Content at the Inflection Point of the Soil Water Characteristic Curve

The objectives of this study are: ① to validate whether the soil water content (θ) at the inflection point (θI) of the soil water characteristic curve (SWCC) is equal to the field capacity (θFC), and ② to establish θFC and soil water suction (h) corresponding to θFC (hFC) as pivotal parameters in SWCC models, enabling the characterization of SWCC using hFC and θFC. Based on 50 soil samples, three SWCC models, namely the van Genuchten (vG), complementary error function (CEF), and modified Brooks-Corey (MBC) models, were employed to calculate θI. Results show that θI calculated from the CEF model (θI(CEF)) was statistically equivalent to the measured θFC, while θI from the vG and MBC models exhibit significant differences. The CEF model was transformed using hFC as one of parameters, and its performance was evaluated. The transformed CEF model showed a high degree of fitting performance, with adjusted R-squared values ranging from 0.953 to 0.999 and root mean square error values varying between 0.001 and 0.018 cm3 cm–3 across the 50 soil samples. This study demonstrates that hFC, as the matric potential at the inflection point of the modified CEF model, can effectively characterize the SWCC. It offers a novel perspective and physical interpretation for field capacity.