Monitoring Vegetation Optical Depth and Canopy Water Content with GPS Signals: First Results

Canopy water content is a direct indicator of vegetation water use and hydraulic stress, reflecting how ecosystems respond and adapt to droughts and heatwaves. However, in-situ estimates of vegetation water content often have to rely on infrequent and time-consuming destructive samplings, which are not necessarily representative of the canopy scale. On the other hand, satellite microwave remote sensing has demonstrated a promising potential for monitoring vegetation optical depth (VOD), biomass, and water content, but these large-scale measurements are still difficult to reference or compare against field observations. Here, we present a simple technique based on Global Navigation Satellite Systems (GNSS) to bridge this persisting scale gap. Because GNSS microwave L-band signals are obstructed and scattered by vegetation and liquid water, placing a GNSS receiver under a forest canopy and measuring changes in signal quality over time provides continuous information on VOD, biomass, canopy water content, and forest structure. With over 100 available GNSS satellites moving around the sky, a full hemispherical scan of the canopy can be obtained each day. We show how variations in GNSS signal attenuation reflect changes in the distribution of biomass and liquid water in the canopy. Such changes can be monitored continuously over the course of a season, providing a useful in-situ reference for satellite-based analyses. At high temporal scales, the presented technique can resolve diurnal variations in canopy water content at hourly time steps, giving continuous information on plant water stress and rehydration. Our results also show that canopy rainfall interception can be monitored at 1-minute intervals, while dew formation can be monitored at hourly scale. We discuss future strategies and requirements for deploying such cheap and easy to use GNSS-based radar systems at existing ecohydrological networks such as FluxNet or SapfluxNet, and provide further examples from newly equipped eddy-covariance towers located in Missouri and Switzerland.