Ecohydrological controls on soil moisture and temperature along an aridity gradient in the Atacama Desert: the role of fog, dew, and rainfall in the maintenance of fog-dependent ecosystems

The northern and central coast of Chile exhibits a pronounced aridity gradient extending over 1,000 km, from the hyperarid Atacama Desert (~18°S) to the Mediterranean shrublands of central Chile (~34°S). This gradient is controlled by the interaction between the South Pacific Anticyclone, the Andean rain shadow, and the Humboldt Current. Despite these extreme arid conditions, the coastal zone is characterized by the recurrent presence of fog associated with marine stratocumulus clouds, which, upon interacting with the coastal range, generate persistent fog banks over land. Marine fog constitutes the main water source for highly specialized ecosystems such as fog oases and fog-dependent forests distributed along this gradient. In addition to fog and rainfall, dew has also been recognized as an important water source for biological communities. While the meteorology of these water inputs is well documented, their combined influence on surface soil moisture and temperature dynamics remains poorly understood.

This study evaluated the ecohydrological response of surface soil (2 cm depth) to atmospheric water regimes at four fog-dependent ecosystems (20°S-32°S): Alto Patache Research Station (hyperarid), Pan de Azúcar National Park (arid), Bosque Fray Jorge National Park (semiarid), and El Boldo Private Park (Mediterranean), during winter and spring 2025.

During the winter-spring study period, results reveal a marked north-south increase in rainfall, from 0.2 mm at Alto Patache to 155 mm at El Boldo, whereas fog exhibited an inverse pattern, peaking at the hyperarid site (>1,100 L m⁻²) and reaching minimum values in the Mediterranean zone (41 L m⁻²). Dew emerged as a relevant water source in arid and hyperarid sites (≈31 L m⁻²), exceeding values in semiarid and Mediterranean environments (10 and 26 L m⁻², respectively). Soil moisture dynamics indicate that Mediterranean and semiarid sites exhibit high temporal variability driven by rainfall pulses (mean ≈0.14 m³ m⁻³, SD = 0.055, and 0.06 m³ m⁻³, SD = 0.018, respectively), whereas hyperarid and arid sites maintain relatively stable moisture regimes (SD ≈ 0.0075) closely associated with fog and dew at weekly timescales. At daily scales, soil temperature showed significant negative correlations with non-rainfall water inputs across all sites, highlighting fog and dew as dominant thermal regulators that buffer soil heating.

We conclude that soil moisture and temperature regimes along this aridity gradient are governed by distinct hydrological drivers. A hydrological compensation mechanism emerges, whereby fog and dew sustain soil moisture and regulate soil temperature during rainless periods, particularly under hyperarid conditions. These findings underscore the critical role of non-rainfall water inputs in maintaining the ecohydrological resilience of drylands soils under future climate change.