1,2,3,5,5,6,7,8,1,1,- 1Max Planck Institute for Biogeochemistry, Biogeochemical Processes, Jena, Germany (konstantin.schellenberg@uni-jena.de)
- 2Department of Earth Observation, Institute for Geography, Friedrich Schiller University Jena, Jena, Germany
- 3Microwaves and Radar Institute, German Aerospace Center, Oberpfaffenhofen, Germany
- 4CREAF, Cerdanyola del Vallès, 08193 Barcelona, Spain
- 5GFZ Helmholtz Centre for Geosciences, Potsdam, Germany
- 6Biospheric Sciences Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA
- 7Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD, USA
- 8Department of Earth System Science, Stanford University, Stanford, CA, USA
- 9Julius Kühn-Institute for Forest Protection, Federal Research Institute for Cultivated Plants, Quedlinburg, Germany
- 10Faculty of Forest sciences and Forest Ecology, Georg-August-University Göttingen, Göttingen, Germany
- 11Institute of Geography, University of Augsburg, Augsburg, Germany
Monitoring vegetation water status is key to understanding forest canopy hydraulics, stomatal regulation, and ultimately the biosphere's drought response under a changing climate. Yet direct, in situ measurements of hydraulic state are labor-intensive and rarely sustained long enough to produce the multi-year time series needed for model development and drought-impact forecasting. Continuous proxies such as sap flow or stem water potential provide vital information about fluxes, but their representativeness for entire trees and stand-scale canopy water status remains very limited.
Here, we highlight the potential of Global Navigation Satellite Systems Transmissometry (GNSS-T) to bridge this observation gap. GNSS-T retrieves vegetation optical depth (VOD), the effective canopy opacity at L-band (1-2 GHz), by measuring one-way attenuation of GNSS microwave signals along their path from the transmitting satellite to a receiver located below the canopy. GNSS-T VOD integrates information on canopy biomass and water content of the canopy (plant and interception storage) and has demonstrated sensitivity to stand-scale vegetation water dynamics. However, its sensitivity to changes in vegetation water dynamics is expected to vary with stand biomass and canopy cover, species hydraulic strategies, and climatic conditions. These dependencies remain poorly quantified. To date, progress has been limited due to the novelty of this emerging technique as existing GNSS-T records are rather short in time and largely confined to individual sites.
In this contribution, we present the first data from VODnet, a community-driven network that builds, maintains, and advances GNSS-T for ecological research. The emerging dataset spans 10 forest stations across diverse biomes, including temperate, Mediterranean, savanna, and tropical ecosystems in South America, and Southern and Central Europe, enabling cross-site analyses of GNSS-T VOD sensitivity under contrasting climate conditions and vegetation properties.
The goal of this study is to understand the sensitivity of GNSS-T VOD to changes in vegetation water status across climate gradients, plant traits, and forest structural conditions. We do this by calculating partial correlations of VOD with hydrological drivers such as soil moisture deficit, sap flow and water potential anomalies while accounting for structural properties such as LAI, total biomass and canopy cover, and measure the degree to which site factors drive this correlation. Beyond in situ applications, VODnet provides a unique opportunity to study uncertainty in widely used spaceborne VOD data sets (e.g., SMAP, AMSR-2) through validation across forest ecosystems. Based on our results, we can now provide a first assessment of whether GNSS-T can serve as a validation reference for satellite-derived VOD.
How to cite: Schellenberg, K., Chaparro, D., Brede, B., Stanley, V., Feldman, A., Konings, A. G., Duveiller, G., Paulus, S. J., Stassin, T., Hartmann, H., Schmullius, C., and Jagdhuber, T.: Tracking Water Status and Drought Response with GNSS-T VOD Across Tropical to Temperate Forest Ecosystems, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14924, https://doi.org/10.5194/egusphere-egu26-14924, 2026.
