Soil–atmosphere water vapor exchange in semi-arid Northwest China: New insights from fiber-optic relative humidity sensing

Abstract. Soil–atmosphere water vapor exchange in arid and semi-arid regions is a key process in near-surface hydrology, reflecting the dynamic coupling of surface energy and moisture. In this study, a novel fiber-optic sensing technique was employed to measure vertical water vapor fluxes across the soil–atmosphere interface in a semi-arid region of the Loess Plateau, Yanan, China. The observations captured vapor flux dynamics across a 7-mm dry soil layer beneath the interface (hereafter referred to as Flux Layer _soil) and a 10-mm molecular diffusion layer in the air above it (Flux Layer _air), revealing how meteorological factors modulate near-surface vapor transport. Solar radiation enhanced vapor fluxes primarily by increasing the vapor pressure deficit (VPD), with Flux Layer _soil exhibiting a slower response than Flux Layer _air. This lag was most pronounced in winter, reaching up to 120 minutes. During rainfall, fluxes in both layers declined sharply as VPD dropped to near zero. Following precipitation, Flux Layer _air recovered rapidly, driven by surface evaporation, while Flux Layer _soil increased more gradually due to the progressive drying of subsurface moisture. Structural equation modeling based on 5657 observations revealed distinct influence pathways: Flux Layer _air was more sensitive to solar radiation, air temperature, and VPD, while Flux Layer _soil was predominantly governed by VPD. These findings advance the quantitative understanding of near-surface vapor transport mechanisms and improve insight into the coupled feedbacks governing soil–atmosphere interactions under variable climatic conditions.