31 Mar 2022
31 Mar 2022
Status: this preprint is currently under review for the journal BG.

Continuous ground monitoring of vegetation optical depth and water content with GPS signals

Vincent Humphrey1,2 and Christian Frankenberg1,3 Vincent Humphrey and Christian Frankenberg
  • 1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena
  • 2Department of Geography, University of Zürich, Zürich, Switzerland
  • 3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States

Abstract. Satellite microwave remote sensing techniques can be used to monitor vegetation optical depth (VOD), a metric which is directly linked to vegetation biomass and water content. However, these large-scale measurements are still difficult to reference against either rare or not directly comparable field observations. So far, in-situ estimates of biomass or water status often rely on infrequent and time-consuming samplings, which are not necessarily representative of the canopy scale. Here, we present a simple technique based on Global Navigation Satellite Systems (GNSS) with the potential to bridge this persisting scale gap. Because GNSS microwave signals are attenuated and scattered by vegetation and liquid water, placing a GNSS sensor under a vegetated canopy and measuring changes in signal quality over time can provide continuous information on VOD, and thus on vegetation biomass and water content. We test this technique at forested site in Southern California for a period of 8 months. We show that variations in GNSS signal to noise ratios reflect the overall distribution of biomass density in the canopy and can be monitored continuously. For the first time, we show that this technique can resolve diurnal variations in VOD and canopy water content at hourly to sub-hourly time steps. Using a model of canopy transmissivity to assess these diurnal signals, we find that temperature effects on the vegetation dielectric constant, and thus on VOD, may be non-negligible at the diurnal scale or during extreme events like heatwaves. The rainfall and dew deposition events also suggest that canopy water interception can be monitored with this approach. The technique presented here has the potential to resolve two important knowledge gaps, namely the lack of ground truth observations for satellite-based VOD, as well as the need of a reliable proxy to extrapolate isolated and labour-intensive in-situ measurements of biomass, canopy water content, or leaf water potential. We provide recommendations for deploying such off-the-shelf and easy-to-use radar systems at existing ecohydrological monitoring networks such as FluxNet or SapfluxNet.

Vincent Humphrey and Christian Frankenberg

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-84', Anonymous Referee #1, 18 Apr 2022
    • AC1: 'Reply on RC1', Vincent Humphrey, 19 Jun 2022
  • RC2: 'Comment on bg-2022-84', Anonymous Referee #2, 20 Apr 2022
    • AC2: 'Reply on RC2', Vincent Humphrey, 19 Jun 2022
  • RC3: 'Comment on bg-2022-84', Anonymous Referee #3, 12 May 2022
    • AC3: 'Reply on RC3', Vincent Humphrey, 19 Jun 2022

Vincent Humphrey and Christian Frankenberg

Vincent Humphrey and Christian Frankenberg


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Short summary
Microwave satellites can be used to monitor how vegetation biomass changes over time, or how droughts affect the world's forests. However, such satellite data is still difficult to validate and interpret because of a lack of comparable field observations. Here, we present a remote sensing technique which uses the Global Navigation Satellite Systems (GNSS) as a makeshift radar, making it possible to observe canopy transmissivity at any existing environmental research site in a cost-efficient way.