The potential contribution of soil moisture to fog formation in the Namib Desert

Fog is considered as an important source of water in many drylands, and the knowledge of possible sources of its formation is essential to make future predictions. Prior studies have suggested the presence of locally generated fog in drylands; however, its formation mechanism remains unclear. There have been studies on the effects of fog on soil moisture dynamics. On the contrary, no research has yet been conducted to understand the potential contribution of soil moisture to fog formation. This study, therefore, for the first time intends to examine such possibility in a fog-dominated dryland ecosystem, the Namib Desert. The study was conducted at two sites representing two different land forms (sand dunes and gravel plains) in the Namib Desert. We first examined evidence of fog formation through water vapor movement using field observations, and then simulated water vapor transport using HYDRUS-1D model. In the first part of the study, soil moisture, soil temperature and air temperature data were analyzed, and the relationships between these variables were taken as one of the key indicators for the linkage between soil water and fog formation. The analysis showed that increase in soil moisture generally corresponded to similar increase in air or soil temperature near the soil surface, which implied that variation in soil moisture might be the result of water vapor movement (evaporated soil moisture) from lower depths to the soil surface. In the second part of the study, surface fluxes of water vapor were simulated using the HYDRUS-1D model to explore whether the available surface flux was sufficient to support fog formation. The surface flux and cumulative evaporation obtained from the model showed positive surface fluxes of water vapor. Based on the field observations and the HYDRUS-1D model results, it can be concluded that water vapor from soil layers is transported through the vadose zone to the surface and this water vapor likely contributes to the fog formation in fog-dominated drylands, like the Namib Desert.