TITLE:
Modelling of In-Situ Soil Water Characteristics Based on Multiple-Years Monitoring
AUTHORS:
Ippei Iiyama
KEYWORDS:
Drying and Wetting Cycles, Main Curves, Moisture Characteristics, Scanning Curves, Soil Water Hysteresis, Water Content, Water Potential, Water Retention
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.14 No.6,
June
24,
2026
ABSTRACT: Soil water characteristics are one of the essential factors for understanding storage and flow of water in soils, recognized as a series of relations between soil water potential ψ [J?kg?1] and soil water content θ [m3?m?3]. Because a series of ψ-θ relations shows hysteretic behaviors, it is often difficult to correctly express it by using a single-valued function θ (ψ). Aiming at describing in-situ soil water hysteresis, this study formulated a model of soil water hysteresis based on ψ-θ relations observed in a field. The model assumption derived from the observed features was that any scanning curve is a similitude of a corresponding main curve on the ψ-θ plane. The model can work along randomly-ordered drying-wetting cycles by citing only a pair of the main curves. The two main curves should share the driest and wettest extremities, while there is almost no restriction on the types of mathematical functions to express any of the main curves. The model algorithm allows for calculating an unknown water content θ1 [m3?m?3] for a given water potential ψ1 [J?kg?1] by citing a latest known moisture condition (ψ0, θ0). The model reproduced well the general trend of the measured ψ-θ relations throughout the study period, except for the cases in which the chances of reversals in drying-wetting directions were wrongly input into the model in the series of measurements of ψ [J?kg?1]. It was also difficult for the model to reproduce a long continual wetting process found in the domain of ψ ≥ ?1 [J?kg?1], implying the possible improvement of the model performance by modifying the model so that the model can deal with multi-modal pore systems like volcanic ash soils or other types of structured soils rich in soil aggregates.