Seasonal variations of soil water vapor distribution transport and phase transition in the loess unsaturated zone from fixed volume monitoring

Abstract

Abstract Water vapor is a crucial yet often overlooked water resource in the soil’s unsaturated zone of arid and semi-arid regions. Understanding water vapor behavior in the soil’s unsaturated zone is crucial for research on soil and energy balance, vegetation water stress, and water utilization in arid regions. We conducted field experiments on the Loess Plateau and hilly areas to study short-term seasonal changes of water vapor within fixed-volume spaces, quantify vapor flux and condensation frequency, and assess the influences of micrometeorological factors. The results show that water vapor concentration, expressed as absolute humidity (AH), has a strongly coupled linear relationship with temperature (T) and varies seasonally. In summer, higher temperatures and AH in shallow soil layers cause downward vapor transport; in winter, the opposite occurs. The temporal oscillations of temperature and AH are significant in regions shallower than 0.7 m, with the timing of peak and trough periods varying by season. Overall, the temperature gradient exerts a stronger driving force on water vapor transport than the water vapor pressure gradient. The water vapor flux driven by the water vapor pressure gradient is most pronounced at depths less than 0.2 m, accounting for 56% to 80.4% of the total gradient flux across the profile, with a peak value of 0.78 mm/d. Bayesian estimation path analysis indicate that air temperature is the main micrometeorological factor influencing the distribution of water vapor in shallow soil layers. Free condensed water exhibits distinct depth stratification and seasonal variation with higher occurrence in cold season. This study provides scientific guidance for in-depth research on non-rainfall water sources in the unsaturated zone of loess regions and offer methods to validate the relevant water vapor models.