Contributions of Non-rainfall water inputs to soil surface moisture in arid coastal ecosystems: A case study in Pan de Azúcar National Park (25°59' S and 70°36' O), Atacama Desert, Chile.

In the coastal areas of the Atacama Desert, non-rainfall water inputs (NRWI), including fog, dew, and direct vapor adsorption, sustain life in an environment where rainfall is virtually absent. These water sources provide critical moisture to the soil surface, fostering conditions for ecosystems to survive and adapt to extreme water scarcity. However, while the physical processes behind NRWI are better understood, the dynamics of each vector's events and their contributions to soil moisture remain poorly quantified.

Specifically, this research aims to characterize the dynamics of fog, dew, and direct vapor adsorption events and their contributions to soil surface moisture. A high-resolution experimental setup monitored the conditions facilitating NRWI formation—air temperature, relative humidity, surface temperature, and water vapor gradients between the air and soil pores—alongside soil surface moisture fluctuations. Our experimental design integrated a meteorological station, an infrared surface thermometer, a standard fog collector, a flat dew condenser, sensors for soil temperature, relative humidity (RH), volumetric water content (VWC), and a ground-based fog observation camera. These instruments enabled the analysis of individual NRWI vectors to investigate the timing, magnitude, and duration of events in relation to soil moisture changes.

A case study was conducted in the Las Lomitas oasis of Pan de Azúcar National Park (25°59' S, 70°36' W), situated at 731 m a.s.l. and 1.7 km from the coastline. Directly influenced by the marine boundary layer, this site provided an ideal setting to observe NRWI dynamics. Preliminary data collected between October 21 and December 5, 2024, revealed that direct vapor adsorption was the most frequently observed NRWI contributor, driving daily variations of 1–3% in VWC. Dew formation conditions were observed during 28% of the study period, primarily driven by high relative humidity (averaging 96.4%). However, dew events were short and intermittent, averaging 15 minutes, limiting their contribution to soil moisture when analyzed at the scale of individual events.

In contrast, 217 fog events were recorded, with an average duration of 3 hours and a water yield of 1.88 L/m² per event. Interestingly, only 5% of these fog events were associated with significant increases in soil moisture, with VWC rising by more than 1%. The largest fog event lasted 66.83 hours, yielding 83.58 L/m² and resulting in a 14.5% increase in VWC, the highest recorded moisture increase during the study.

The analysis shows the need to characterize and classify fog and dew events to identify strong correlations and demonstrate their impact on soil moisture. In the case of direct water vapor adsorption, it provided the most frequent moisture inputs, although in small magnitudes, that rarely contributed significantly on a daily basis. This research improves our understanding of the dynamics of NRWI events and their influence on soil surface moisture in arid environments. The findings reveal subtle mechanisms supporting coastal ecosystems and highlight their relevance for conservation and adaptation strategies in the face of climate change.