Heat capacity, cooling efficiency and drought stress of vegetated surfaces

Drought stress has profound impacts on ecosystems and societies, particularly in the context of climate change. Traditional drought indicators, which rely on integrated surface water budget anomalies at various time scales and thresholds derived from past climate variability, provide valuable insights but often fail to deliver clear and direct real-time assessments of drought stress on vegetation.

This study introduces the Cooling Efficiency Factor (CEF), a novel metric derived from geostationary satellite observations, to detect drought stress by analyzing daytime surface warming anomalies. The CEF is based on the principle that dry surfaces warm more rapidly than wet ones under identical radiative forcing due to reduced evapotranspiration caused by soil moisture limitation and by stomatal closure, altering the effective heat capacity of the land surface.

By leveraging high-frequency, high-resolution retrievals of land surface temperature (LST) and radiation data from geostationary satellites, this study demonstrates the CEF's ability to assess drought stress conditions. The CEF correlates strongly with evapotranspiration anomalies from established datasets, including GLEAM, ERA5-Land, and TerraClimate. Results underscore the CEF's sensitivity to vegetation type, soil moisture variability, and environmental conditions, illustrating its effectiveness in identifying drought stress compared to traditional indicators.

The CEF represents a promising tool for real-time drought monitoring and integration into early warning systems, particularly for arid and semi-arid regions. By complementing existing drought assessment methods, the CEF paves the way for advancements in land-surface process studies and improved drought risk management.