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【Objective】 Heavy-textured soils tend to swell or shrink following rewetting or drying, while most soil water characteristic curves measured from such soils do not consider such effects. The purpose of this paper is to investigate these and proposes an improved model to account for soil swelling and shrink. 【Method】 Changes in soil moisture and matric potential during three wetting-drying cycles in three soils were measured using thermo-TDR probe and soil water potential probe, respectively, and the results obtained from each cycle for each soil were fitted to the van Genuchten formula. We introduced a shrinking percentage parameter to improve the fitting of the van Genuchten formula. 【Result】 The heat pulse probe improved soil volume measurement, with the relative error reduced from 10%~40% to less than 10% and the associated root mean square errors reduced from 0.3~0.4 g/cm3 to less than 0.1 g/cm3. Increasing dry-wet cycles reduced the values of the parameters α and n in the van Genuchten formula. The influence of soil water on soil volume waned as soil water content decreased. The values of the parameters α and n in the van Genuchten formula both decreased as clay content increased. Accounting for the shrinkage in the modified model improved the fitting compared to the original van Genuchten formula that does not consider soil deformation. 【Conclusion】 The heat pulse probe can accurately measure change in soil volume induced by wetting and drying, and the proposed model considering volumetric change of soils following wetting or drying improves the fitting of the van Genuchten formula to the measured data.

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