Journal Article

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

Guo, Junyi, Sun, Mengya, Zhang, Chengcheng, Lienhart, Werner, Shi, Bin, Jiang, Hongtao

2025

Overview

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. Plain Language Summary In semi‐arid and arid regions, shallow soil typically has very low water content, with the evaporating front remaining within the soil most of the year. At this subsurface evaporating front, liquid water converts to vapor, which moves through the dry soil layer (DSL) into the atmosphere. Direct measurement of water vapor transport within DSLs has been challenging with current technology. In this study, optical fiber sensing technology directly measured changes in relative humidity, temperature, water vapor density, and water vapor flux during evaporation in four sand samples with very low water content. As relative humidity decreased from 100%, the soil temperature reached its minimum, confirming the sensor accurately captured the evaporating front. The water vapor flux exhibited a characteristic pattern, rising from zero, peaking at the evaporating front, and then declining back to zero. Moreover, in sand with varying initial moisture content, a deeper evaporating front was associated with a slower downward migration rate. The relative humidity distribution within the DSL was depth‐dependent and could be characterized by a nonlinear function of depth. This study introduces a novel approach to investigating the mechanisms of water vapor transport in dry soils in semi‐arid and arid regions. Key Points The drop in soil relative humidity from 100% during evaporation precisely aligns with the moment of minimum soil temperature Sensors effectively captured the evaporating front and tracked its dynamic migration within the soil Water vapor fluxes exhibited a mono‐convex temporal pattern, peaking at the evaporating front

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Arid regions / Arid zones / Atmosphere / Bragg gratings / Depth / dry sand / Energy exchange / Energy transfer / Evaporation / fiber optics / Fronts / Humidity / Moisture content / optical fiber / Optical fibers / Optical fibres / Relative humidity / Sand / Soil / Soil layers / Soil temperature / soil-atmosphere interactions / Soils / Temperature / Vapor density / Water / Water content / Water monitoring / Water vapor / Water vapor content / Water vapor flux / Water vapor monitoring / Water vapor transport / Water vapour