Abstract

Climate change and human pressures upon traditional water sources provoke decreases in life conditions, especially in areas where the desert is expanding. Despite this, ecosystems supplied by other more subtle water sources, specifically marine fog and dew, are widely recognized. Comprehending the finer hydrological cycle components is fundamental to preserving natural life forms and facing droughts in water stress areas. In this context, oases ecosystems show exceptional adaptive capacities and high efficiency using coastal fog and dew water, giving signals of its variability and distribution. These atmospheric water inputs are an underestimated phenomena due to the difficulty of their detection and quantification, but also the lack of spatially-distributed and long-term observations that allow us to understand their spatio-temporal dynamics. In this research, we developed a methodology with radar and optical satellite systems to provide high spatial and temporal resolution data about the influence of fog and dew on ecosystems. Sentinel-1 C-band data was exploited to describe the dynamics between dry and wet events with a co-polarized radar backscatter (vertical-vertical-VV) change detection approach. In addition, we used the reflectance responses of the multispectral instrument (MSI) over optical and infrared bands provided by the Sentinel-2 satellite to analyze vegetation structure types and dynamics concerning surface moisture. Both data were analyzed to give an approximation of the spatial variability of soil moisture and vegetation responses as proxies for these atmospheric water inputs. We have assembled a satellite-based Fog and Dew Water Index (FDWI) and applied it to the Las Lomitas fog oasis in Pan de Azucar National Park (25ºS-70ºW) from 2022-2023. This location is at 730 m.a.s.l., about 2 km from the sea, and has a regular presence of advective fog throughout the year. To verify the accuracy and sensitivity of outputs obtained for the FDWI, it has correlated with data from a standard fog collector, standard flat dew condenser, leaf wetness, and soil moisture, all provided by a main ground control station. We describe temporal variations of fog and dew in the Oasis and their association with FDWI. The analysis shows that fog is the main driver of the water supply to the Oasis. Furthermore, from a spatial perspective, the FDWI demonstrates the influences of these water inputs have a limited spatial extent, providing a local-scale map of the oasis ecosystem boundaries and gradients. Map of fog and dew water contributions represents an approach to understanding their spatial and temporal dynamics, giving estimations of the potential water use of such ecosystems