Seminar: Konstantin Schellenberg

Vegetation optical depth (VOD) is a measure of “thickness” of the canopy and contains information about water content and dry biomass/structure. VOD has become a key remote sensing variable to measure vegetation water status at global scales, as model constraint, to infer hydraulic traits, and to characterize drought episodes in canopies. However, satellite VOD is still poorly validated due to a substantial scale mismatch of kilometers down to plot/stand scale where in-situ measurements are carried out.In this talk, I will introduce the novel experimental setup called Global Navigation Satellite Systems Transmissometry (GNSS-T) that can generate local-scale VOD time series which could bridge this observation and validation gap. Beyond validation, GNSS‑T VOD has the potential to quantify canopy water dynamics at stand scale (including both vegetation water content and intercepted water), which could provide an additional constraint for increasingly complex vegetation and hydraulic models. Also, physiological measurements from tissue to stand level can be integrated with VOD and improve our understanding of vegetation drought responses, locally with GNSS‑T and, once validated, globally with space‑borne VOD.To gain trust in the system, we carried out a range of experiments. This includes quantifying the sensitivity of GNSS-T VOD to vegetation water dynamics, rainfall interception and dew in the research stations in Tharandt (ENF). We found VOD strongly correlating with interception water, making it a promising proxy for independent interception measurements. Sensitivity to vegetation water dynamics is lower, and was limited by wet climate during the research period, and conservative hydraulic traits in spruce trees. In Hainich (DBF), we found correlations between VOD and modeled water status (here: leaf/stem water potential) when using prediction from a QUNICY-coupled hydraulic model; the interpretation of these results is not final and opinions warmly welcome. First results from the comparison of GNSS-T with a tower-based radiometer (at MOFLUX, Missouri, USA) reveal a good potential as a validation instrument, while acknowledging differences in footprint geometry, wave interaction, and algorithms. Finally, I will highlight the potential of our community-based network “VODnet” to scale those types of experiments, which could inform satellite validation efforts by space agencies, i.e., ESA.