Assessing the Spatiotemporal Dynamics of Soil Water Vapor Adsorption Using a Global Observation Network

The movement of water vapor between the soil and the atmosphere plays a crucial role in soil-atmosphere interactions, especially under dry conditions. A previously little-noticed process known as water vapor adsorption in soil occurs when water vapor from the atmosphere is adsorbed in the soil during the night, caused by cooling at the soil surface. This process is based on the fundamental principle that equilibrium vapor pressure decreases in the vicinity of dry soil material, creating conditions under which evaporation turns into condensation. While this phenomenon is well understood at small scales under controlled conditions, its effects on ecosystems at larger scales remain poorly understood due to the lack of continuous, direct measurements.

In this study, we investigate across a worldwide network of eddy covariance measurements under which conditions negative latent heat fluxes (vapor movement towards the soil) are consistent with an established theoretical understanding of soil water vapor adsorption. We find an emerging functional relationship between latent heat flux direction, soil water content, and near-surface relative humidity which facilitates the investigation of adsorption events across the eddy covariance network. Our results confirm that soil water vapor adsorption occurs most frequently in arid areas with sparse vegetation, such as savannahs or dry shrublands. The average duration of soil water vapor adsorption is 4 hours per day in all ecosystems and up to 9 hours per day in some sites. The number of days per year where soil water vapor adsorption was measurable for three hours or longer varied by ecosystem, reaching up to 150 days per year. Our results further suggest that soil texture has a relatively minor influence on the occurrence under field conditions compared to the results of laboratory experiments.

Our analysis confirms recent findings that soil water adsorption can be isolated from eddy covariance measurements. It not only expands our knowledge of the spatial distribution of soil water vapor adsorption in different ecosystems but also facilitates future research to investigate interannual dynamics, management, and extremes. Thus, the study contributes to the understanding of a long-overlooked aspect of soil-atmosphere interaction.