Sub-daily microwave observations to constrain evaporation modelling over forest ecosystems

Microwave remote sensing observations provide unique information on soil–plant water status as they are directly sensitive to changes in soil moisture, internal vegetation water content, and the water content present on vegetated surfaces due to precipitation, dew, or irrigation. These variables are key drivers of terrestrial evaporation (E) and important indicators of ecosystem functioning and health. Current E models that exploit microwave observations are largely constrained to daily time scales, due to the lack of sub-daily satellite microwave data. However, water transport within the soil–plant–atmosphere continuum exhibits strong diurnal dynamics, and E shows a pronounced diurnal hysteresis, with higher rates typically occurring during the morning compared to the afternoon. This highlights the need to monitor vegetation responses to environmental stress at sub-daily scales and to model transpiration (E_t) at high temporal resolution. Sub-daily resolutions are also required to accurately represent rainfall interception loss (E_i), which exhibits strong intra-day variability, particularly during and shortly after precipitation events. Sub-daily microwave observations offer the potential to resolve these fast processes, thereby advancing the understanding of E, stomatal regulation, and the coupling between water, energy, and carbon cycles. In particular, sub-daily observations of vegetation optical depth (VOD) and canopy surface wetness are expected to improve the estimation of E_t and E_i, respectively.

This study is motivated by the continued development of a sub-daily SAR mission concept to enable global monitoring of vegetation water dynamics, health, and stress. One of the challenges in the early development of the SLAINTE mission, as an ESA New Earth Observation Mission Idea (NEOMI) and as a concept in response to ESA’s 12^th call for Earth Explorers, was the scarcity of sub-daily observations (Steele-Dunne et al., 2024; Matar et al., 2024). Here, the potential to constrain E estimates using sub-daily VOD data is revisited using observations from a new network of in-situ GNSS-based sensors.  

We assess the potential value of sub-daily microwave observations by constraining a sub-daily version of the Global Land Evaporation Amsterdam Model (GLEAM; Miralles et al., 2011), using sub-daily observations of VOD and binary wet/dry canopy state (WDCS). These observations are derived from in-situ GNSS-based sensors deployed across several European forest ecosystems. By analyzing the diurnal cycle of VOD, potential descriptors of vegetation water stress can be identified and incorporated as constraints in this sub-daily version of GLEAM. This study presents a methodology to exploit sub-daily VOD and provides a pathway to consolidate observation requirements for estimating E at sub-daily scales and for quantifying the impact of environmental stress. 

Matar, J., et al. “A Concept for an Interferometric SAR Mission with Sub-daily Revisit”, EUSAR 2024; 15th European Conference on Synthetic Aperture Radar, Munich, Germany, 2024, pp. 18-22.

Miralles, D. G., et al. “Global land-surface evaporation estimated from satellite-based observations”, Hydrology and Earth System Sciences, vol. 15, no. 2, pp. 453–469, 2011.

Steele-Dunne, S. C., et al. “SLAINTE: A SAR mission concept for sub-daily microwave remote sensing of vegetation”, EUSAR 2024; 15th European Conference on Synthetic Aperture Radar, Munich, Germany, 2024, pp. 870-872.