Effect of magnification on determination of water retention curve using image‐based analysis

Abstract The development of an image‐based method for determining soil water retention curves (WRCs) is effective for predicting infiltration processes into soil. We investigated the effects of microscopy magnification on image resolution for estimating WRCs. Images of the soil surface were acquired at magnification of 50×, 100×, and 200×. Large‐sized pores (>30 µm in radius) captured at lower magnifications of 50× and 100× were closer to values obtained from the conventional method, pressure plate method, because images at low magnification could capture wide areas and measure representative values. The image‐based method at high magnification (200×), showed spatial distributions and captured local tendencies. All parameters of hydraulic property of lognormal model ( θ e , ψ m , and σ ) had the same areas with large values caused by structural development. This means that WRCs had variabilities on the surface of the core samplers with 5‐cm inner diameter. Relationships between magnification and WRC parameters were investigated. The θ e and σ could be captured exactly at a magnification of 50×. In this study, ψ m was underestimated by the image‐based method at all magnifications. In conclusion, for estimating WRCs, image obtained at magnification of 50× showed the highest accuracy. However, images at 50× magnification could not capture the microscopic spatial distribution of soil pore structure.