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Soil matric potential and osmotic potential are widely accepted as two independent components of total soil water potential. However, laboratory observations repeatedly demonstrated that matric potential can vary with salt concentration, implying a potential coupling between matric potential and osmotic potential. To date, it remains elusive whether matric potential and osmotic potential are independent or not and why so, and a theoretical theory for quantifying the coupling between them is still missing. Herein, a theoretical model is developed to quantitatively explain this problem via a lens provided by a recent concept of soil sorptive potential (SSP). The proposed model substantiates that matric potential and osmotic potential are not independent. The increasing salt concentration can notably depress two variables underpinning SSP, namely relative permittivity and electrical double layer thickness, leading to non‐negligible decreasing (more negative) of matric potential in the high suction range, and increasing (less negative) of it in the low suction range. In turn, the soil‐water interactions redistribute ions in soil water, raising osmotic potential especially for clay with high cation exchange capacity. The proposed model shows excellent performance in capturing experimental data, validating its accuracy. A parametric study implies that the neglection of coupling effects can lead to a significant underestimation of soil hydraulic conductivity in the film flow regime.

Water potential (Pa) refers to the Gibbs free energy of water per unit volume relative to free water. In soil, water is not inert to soils but instead unfailingly interacts with them, giving rise to enormous complexities in understanding soil behavior. Such interaction between soil and water is conventionally quantified via the concept of soil water potential, which is defined as the free energy change per unit volume of water when moving from the free water state to the soil water state (Noy-Meir & Ginzburg, 1967). In the past decades, soil water potential is treated as a cornerstone concept in soil physics, unsaturated soil mechanics, poromechanics, and vadose zone hydrology, and is adopted as the state variable in modeling multiphase and multiphysical processes occurring in soil and many other porous media, for example, water retention behavior (e.g.

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