Towards a global perspective on plant hydraulics: Challenges and opportunities extending microwave remote sensing to sub-daily scales

The aim of this presentation is to highlight the confluence of developments in plant physiology, biogeosciences and microwave remote sensing and a potential route to a global perspective on plant hydraulics. The context is the continued development of a satellite mission concept based on a Low Earth Orbit (LEO) constellation of Synthetic Aperture Radars (SAR) that would provide sub-daily observations including vegetation water content and vegetation wet/dry state (Steele-Dunne et al., 2023, Matar et al., 2023). A recurring challenge in mission concept development has been the scarcity sub-daily microwave data, specifically radar data. These are critical to consolidate measurement and observation requirements, and to demonstrate the science case.
Here, we will highlight research activities centred on our installation of a network of GNSS transmissivity (GNSS-T) sensors at existing forest monitoring sites across Europe. GNSS-T is an emerging measurement technique that provide crucial insight into sub-daily changes in the vegetation as a dielectric medium. Because GNSS-T is relatively inexpensive, it enables data collection across a wide range of biomes, complementing sparser tower-based sensors and providing critical observations to support mission development. 
We will outline how we are using GNSS-T observations with radiative transfer modeling to consolidate observation and measurement requirements. We will illustrate how we using GNSS-T observations to investigate the link between microwave observations and biogeophysical variables at the heart of plant water relations and the surface water and energy balances. We will also discuss how the exploitation of GNSS-T for these purposes is not trivial, highlighting some of the theoretical considerations we have encountered and our attempts to handle them. 
Finally, we will put our activities in the wider context of developments in plant physiology and biogeosciences to discuss opportunities to bring these fields closer together. This is essential to reach the global perspective needed to address urgent scientific and societal challenges.