Satellite-Based Fraction of Available Water Reveals Soil Moisture Deficits Preceding Major Wildfires

Wildfires, exacerbated by climate change, land-use alterations, and extreme weather conditions, can have catastrophic impacts on both people and ecosystems. Recent research highlights the role of Soil Moisture (SM) as a predisposing factor to large fires, yet critical thresholds remain poorly characterized across different data sources. Volumetric SM measurements differ in magnitude and dynamic range across large spatial extents and satellite products, making direct comparisons challenging. To address this, we calculated the Fraction of Available Water (FAW), which ranges from 0 to 1 as SM varies from wilting point to field capacity. Using satellite observations from the Soil Moisture and Ocean Salinity (SMOS), Advanced Microwave Scanning Radiometer 2 (AMSR2), Soil Moisture Active Passive (SMAP), and the Climate Change Initiative (CCI) programs, we explored antecedent conditions in south-central Chile that favored an extreme fire spread in early February 2023, when over 240 000 ha burned in just four days. Our analysis showed that FAW was low—not only in the days immediately before the fires, but throughout the preceding month. Critical thresholds emerged across multiple satellite products, revealing plant stress (FAW < 0.50) and extreme drought (FAW < 0.20). Even drier conditions (FAW < 0.10) were widespread, affecting both burned and nonburned areas and reducing moisture constraints across the region. Our findings demonstrate that FAW thresholds derived from multiple satellite products, including SMOS, SMAP, and CCI, provide a robust framework for identifying SM levels that may predispose areas to wildfire danger.