Paul Vermunt, PhD Candidate at Delft University of Technology, The Netherlands, in between a drought-stressed (left) and non-stressed (right) corn plant. Photo by Saeed Khabbazan.
Hi! I am a Ph.D. candidate at Delft University of Technology in The Netherlands. With an academic background in environmental sciences and hydrology, I am intrigued by understanding and measuring processes in the soil-plant-atmosphere continuum. Currently, I am researching the potential to monitor vegetation water dynamics with radar remote sensing. In my time as a Ph.D. candidate, I get the opportunity to get to the root of the matter by conducting experimental fieldwork and electromagnetic modelling. My project is supervised by Prof. Dr. ir. Susan Steele-Dunne and Prof. Dr. ir. Nick van de Giesen.
Background of my research
Experimental research in the 1960s and 70s laid the foundation for what we know now: radar signals are sensitive to water content in the topsoil and vegetation. Up until today, this discovery led to the integration of satellite-based radar products into hydrological and atmospheric research.
Until a decade ago, however, it was mainly the sensitivity to soil moisture which intrigued hydrologists and atmospheric scientists. The effect of vegetation on the radar observation was often viewed as obstructive for soil moisture retrieval. But in recent years, it is increasingly acknowledged that this part of the signal contains valuable information, for example about the hydraulic state of the vegetation. In my Ph.D. project, I try to understand which hidden information about water dynamics in a canopy is captured in a radar signal!
An important development for my research is the development of subdaily spaceborne radar observations, either via satellite constellations or by geostationary satellites. So far, users of radar satellite observations could rely on about one observation in every 4-12 days, depending on the satellite and place on Earth. But will we be able to track diurnal cycles of plant water content on large scales from space, once subdaily observations become available? This would allow for much more detailed studies on the hydraulic state of vegetation.
In other words, I try to understand what multiple radar observations per day could actually tell us about a vegetated area.
Experiments in corn fields
Together with two other students, I got the opportunity to conduct extensive fieldwork in Florida in 2018, in collaboration with University of Florida’s Center for Remote Sensing. To mimic subdaily spaceborne observations, a prototype radar instrument was mounted to a cherry picker, placed next to a corn field, and programmed to take multiple measurements from that field per day.
Experimental cornfield at the Plant Science Research and Education Unit (PSREU) of the University of Florida and the Institute of Food and Agricultural Sciences (UF|IFAS), with radar instruments mounted to cherry pickers.
We installed a bunch of different hydrometeorological sensors in the field to monitor water dynamics in the soil and plants. Meanwhile, we also sampled corn plants to capture their water content throughout the season and throughout the day.
In a similar field campaign in The Netherlands in 2019, we intensified the sampling, to better capture the diurnal cycle of vegetation water content. Dutch summer days are long, and to understand the dynamics between sunrise and sunset meant that 16 hours in the field was no exception.
Taking corn plant samples from the field to measure the wet and dry biomass, at the terrain of Van den Borne Aardappelen, The Netherlands, in 2019. Photo by Saeed Khabbazan.
But the effort was worth it. Our results showed that our subdaily radar observations were highly sensitive to daily cycles of vegetation water content, particularly on dry days. Also nocturnal dew formation and dissipation was clearly visible in our time series of radar observations.
This provides unique observational evidence that there is a lot more information in the ‘noisy signal’ than assumed so far. More importantly, it means that subdaily radar can reveal rapid vegetation water dynamics!
A single-day example of our data is shown in the Figure below. For detailed figures and more context, see ‘Related papers’ below.
Example of the relation between (a) a diurnally changing radar signal (VV-polarized L-band backscatter), and (b) diurnally changing stores of moisture in the topsoil (θ), vegetation water content (VWC), and dew, plotted on top of the VWC. The date shown here is June 9, 2018.
Our results give experimental evidence for the potential to monitor vegetation water dynamics to scale with subdaily spaceborne radar observations.
These observations would aid a more detailed assessment of the hydraulic state of the vegetation, improve soil moisture estimates, and provide insight into the ‘change of storage’ component of the water balance, which would improve our understanding of the diurnal water cycle.
But there is still a lot to discover. Do we see the same patterns with other types of vegetation? What is the effect of the moisture distribution inside the plants on the signal? Which configurations of the radar instrument are most suitable for monitoring vegetation water dynamics? And, most importantly, how can we translate this radar information into high-quality products for users? We know what we have to do.
Installing a sap flow system (The Netherlands, 2019). Photo by Saeed Khabbazan.
Vermunt, P. C., Khabbazan, S., Steele-Dunne, S. C., Judge, J., Monsivais-Huertero, A., Guerriero, L., & Liu, P. W. (2021). Response of Subdaily L-Band Backscatter to Internal and Surface Canopy Water Dynamics. IEEE Transactions on Geoscience and Remote Sensing, 59(9), 7322-7337.
Vermunt, P. C., Steele-Dunne, S. C., Khabbazan, S., Judge, J., & van de Giesen, N. C. (2021). Reconstructing Continuous Vegetation Water Content To Understand Sub-daily Backscatter Variations. Hydrology and Earth System Sciences Discussions, 1-26.
Measuring at sunset (Florida, 2018).
By: Paul Vermunt, PhD Candidate, Delft University of Technology, The Netherlands.