The water vapour distribution profile in a dry soil layer during evaporation with high spatial resolution monitored by distributed fibre-optic sensing technology

The dry soil layer (DSL) typically forms on the surface of sandy soils during the evaporation process, with water in the DSL moving solely as vapour. Understanding the DSL thickness and its water vapour distribution is crucial for accurately estimating soil water flux, improving evaporation mechanisms and enhancing ecological conservation efforts. In this study, distributed fibre-optic sensing technology was employed to achieve high spatial resolution measurements (up to 1 mm) of the water vapour distribution within the DSL. Soil water vapour profiles in a sand column subjected to evaporation over 24 days were measured using a 60 cm long relative humidity sensing probe and an optical frequency domain reflection interrogator. The downward migration of the evaporating surface from the sand surface was effectively captured and the inhibitory effect of DSL thickness on evaporation was quantified using a logarithmic function. In addition, a linear distribution of water vapour with depth in the DSL was identified. The findings offer novel insights into the role of DSL in the hydrological behaviour of unsaturated soils and demonstrate the potential of the approach used by the authors for investigating dynamic changes in in situ DSLs, particularly in arid and semi-arid regions.