Exploring the Potential of Sub-daily Microwave Remote Sensing Observations for Estimating Evaporation in Forests

Terrestrial evaporation (<inline-formula><tex-math notation="LaTeX">$E$</tex-math></inline-formula>) plays a crucial role in the water, energy, and carbon cycles and modulates climate change through multiple feedback mechanisms. While process-based models estimate <inline-formula><tex-math notation="LaTeX">$E$</tex-math></inline-formula> using satellite-derived drivers, they typically operate at daily or lower temporal resolutions. Key components of <inline-formula><tex-math notation="LaTeX">$E$</tex-math></inline-formula>, such as transpiration and interception loss, exhibit strong diurnal variability, especially under water stress and during or shortly after precipitation events. Therefore, capturing the sub-daily variability of these variables is essential for improved process understanding and <inline-formula><tex-math notation="LaTeX">$E$</tex-math></inline-formula> monitoring at fine temporal resolutions. Sub-Daily microwave observations offer the potential to resolve these short-term processes while providing all-sky retrievals. The Sub-daily Land Atmosphere INTEractions (SLAINTE) mission, proposed as part of European Space Agency&#x2019;s New Earth Observation Mission Ideas, aims to provide sub-daily Synthetic Aperture Radar (SAR) observations of surface soil moisture (SSM), vegetation optical depth (VOD), and wet/dry canopy state (WDCS). These observations are expected to enhance the estimation of <inline-formula><tex-math notation="LaTeX">$E$</tex-math></inline-formula> beyond current capabilities. This study explores the added value of such observations through observing system simulation experiments conducted at four European eddy-covariance forest sites, constraining a sub-daily version of the Global Land Evaporation Amsterdam Model (GLEAM) with synthetic sub-daily microwave observations. Three experiments assess the impact of: 1) sub-daily SSM on bare soil evaporation and transpiration; 2) sub-daily VOD on transpiration; and 3) sub-daily WDCS on interception loss. Results demonstrate that prospective sub-daily microwave data can substantially improve <inline-formula><tex-math notation="LaTeX">$E$</tex-math></inline-formula> estimates and its components, showing average relative improvements in terms of <inline-formula><tex-math notation="LaTeX">$\Delta \text{RMSE}$</tex-math></inline-formula> of up to 25&#x0025; for interception loss when assimilating sub-daily WDCS, and up to 33&#x0025; for transpiration when using sub-daily VOD. Our results highlight the need for satellite missions that provide sub-daily microwave data to better understand forest responses to environmental stress.