Understanding Plant Water Dynamics Using Vegetation Optical Depth Inferred From GNSS Signals

February 20, 2025

Yitong Yao

Hosted by Dien Wu

Abstract

Forest carbon sinks play a pivotal role in achieving carbon neutrality, yet water availability has become a key limiting factor in regulating their capacity. Understanding plant water dynamics is therefore essential for accurately estimating carbon sequestration potential and assessing associated risks under climate change. While existing methods from field observations to airborne and spaceborne microwave satellite measurements provide insights into daily to monthly variations, they fail to capture diurnal cycles. To address this gap, I introduced a novel technique that leverages Global Navigation Satellite System (GNSS) signals to monitor plant water dynamics, enabling the investigation of both diurnal and seasonal variations in plant water content—measured as vegetation optical depth (VOD). This new VOD dataset revealed that diurnal cycles are influenced by soil water content, plant transpiration, and leaf surface water, while seasonal variations are co-regulated by water availability and biomass changes. Additionally, this microwave observable is affected by temperature-induced dielectric changes in canopy water, a confounding factor that has been largely overlooked until recently. To quantify how well VOD measurements can constrain the effects of water content and temperature, I employed an Observing System Simulation Experiment (OSSE) combined with a Bayesian inversion approach to assess retrieval efficiency. This retrieval framework was then applied to GNSS VOD measurements to disentangle the temperature-confounding effects on water content estimation. My work highlights the potential of GNSS-based technique as a cost-effective and robust approach for monitoring plant water dynamics. It also underscores the need to reevaluate temperature-related confounding effects, particularly during compound extreme events such as heatwaves and droughts.