Mapping soil moisture uptake by dry soils across Eddy covariance measurement sites

Soils take up water vapor from the atmosphere through processes that involve vapor diffusion and water retention. This can theoretically occur in any ecosystem under the preconditions of a humid atmosphere and dry soil pores. It can play a critical role in dry ecosystems because it can provide a substantial proportion of the total water inputs at the daily timescale. However, it remains insufficiently investigated in many regions, partly due to the absence of continuous, dedicated measurements.

In this study, we use a recently developed algorithm to detect and filter Eddy Covariance (EC) derived negative latent heat flux data collected at semi-arid and arid sites to identify soil water vapor adsorption. In a previous study, we successfully used EC data to detect soil water vapor adsorption for a Mediterranean ecosystem. 

Our findings indicate that these negative latent heat fluxes exhibit a correlation with soil water content and relative humidity at various sites suggesting that a part of the negative latent heat flux is related to soil water vapor adsorption. Building on these findings, we demonstrate that soil water vapor adsorption occurs during the dry season in various ecosystems, including woody savannas, grasslands, shrublands, and even some forests. The flux magnitude reaches values comparable to daily evaporation, which is in line with existing literature on the few previously measured ecosystems.

Furthermore, we analyze the drivers of the occurrence and dynamics of soil water vapor across sites. Thereby we study the influence of e.g. soil texture or vegetation height. This way, our study expands our knowledge of the spatial extent and inter-annual dynamics of soil water vapor adsorption in natural ecosystems and, more generally, sheds light on a mostly overlooked aspect of land-atmosphere interaction.