Developing a Structural Framework to Estimate Forest Floor Evapotranspiration With the Relative Yield Index (Ry)

Forest floor evapotranspiration ( Ef ) is a critical hydrological process in forested landscapes, accounting for up to 50% of total evapotranspiration. However, existing Ef estimation models primarily rely on meteorological inputs and overlook forest structural influences, limiting their applicability across diverse forest types. This study developed a modelling framework that explicitly incorporates the relative yield index ( Ry ) as a three‐dimensional structural indicator to estimate Ef using a modified Penman–Monteith equation. Ry represents the ratio of current to maximum stand volume for given tree height and stand density, providing a comprehensive measure of forest structure. The framework estimates under‐canopy meteorological conditions including net radiation, temperature, vapour pressure deficit and wind speed based on Ry and above‐canopy meteorological data. We tested the framework using lysimeter measurements in Japanese cypress plantations with contrasting structural characteristics ( Ry = 0.57 and 0.82). Significant relationships were established between Ry , leaf area index ( R 2 = 0.34) and canopy openness ( R 2 = 0.71) using data from field inventory and literature review. The model successfully reproduced observed daily Ef with totals of 112.2–165.9 mm, representing 6.9%–13.2% of annual precipitation. Mean daily Ef ranged from 0.1 to 2.5 mm/day, comparable to previous studies. Residuals were smallest during low vapour pressure deficit conditions (< 0.5 kPa), with larger deviations during high atmospheric demand periods when stomatal regulation and soil moisture limitations become important. This Ry ‐driven framework provides a transferable, inventory‐based tool for estimating forest floor evapotranspiration across managed plantations.