Water Vapor Transport in Dry Sand During Evaporation Monitored by Quasi-Distributed Fiber-Optic Sensing Technology

Water vapor transport in the dry soil layer (DSL) plays a critical role in water and energy exchange between soil and atmosphere in semi-arid and arid regions. However, monitoring water vapor transport in extremely dry soils remains challenging. This study directly measured changes in water vapor content and temperature within sand pores during evaporation using fiber Bragg grating relative humidity sensing technology. Results indicated that soil temperature reached a minimum when relative humidity dropped from 100%, confirming that the sensors successfully captured the evaporating front and its dynamic migration within the soil. Water vapor fluxes exhibited a mono-convex temporal pattern, peaking at the evaporating front. Additionally, deeper evaporating fronts migrated more slowly in sands with varying initial water content. Furthermore, the relative humidity distribution within the DSL was found to be depth-dependent and could be described by a nonlinear function of depth. These findings suggest that our method offers a novel approach for investigating the mechanisms of water vapor transport in dry soils in semi-arid and arid regions.