Tracking Canopy Water Content with GNSS-Derived VOD in a Highly Instrumented Multi-Sensor Forest Field Infrastructure

Global Navigation Satellite System (GNSS) signal attenuation offers a novel approach to estimate Vegetation Optical Depth (VOD) and thereby monitor vegetation structure and vegetation water status at high temporal resolution. At the FAIR site in Mieming (Austria), a GNSS receiver system has recently been installed, opening new opportunities to explore the applicability and added value of GNSS-based VOD in a well-instrumented forest ecosystem. The exceptional strength of FAIR lies in its dense and diverse sensor infrastructure, including eddy covariance measurements above and below the canopy, dendrometer observations, stem water potential measurements, sapflow systems, cosmic-ray neutron sensing (CRNS), soil water content and soil water potential profiles, detailed observations of precipitation and throughfall, as well as periodic, manual measurements of leaf water content.

The co-location of these measurements enables a unique framework to investigate how GNSS-derived VOD relates to plant water status, biomass dynamics, and ecosystem-scale fluxes. Key research questions include the sensitivity of GNSS-VOD to short-term vegetation water dynamics, its coupling with transpiration and carbon exchange at ecosystem levels, and its response to soil moisture variability and atmospheric demand. The FAIR site thus provides an ideal testbed to assess the potential of GNSS-based VOD as an integrative indicator of vegetation–soil–atmosphere interactions and to evaluate its role in multi-sensor ecohydrological monitoring.

In addition, we present first GNSS-VOD time series from the newly installed system and present a first draft of a data processing routine, providing a basis for future analyses and for the integration of GNSS-derived VOD into the existing multi-sensor framework at FAIR.