With the grant, Paul Vermunt will research how to use satellites from ESA and others to measure forest water content. To do so, he will regularly go to the Speulderbos on the Veluwe. On and around a 46-metre-high tower in that forest, he will test various sensors that measure the moisture levels and growth of trees.
In a warming climate, droughts are an increasing threat to the health of forests. Not only are droughts becoming more frequent, but when they occur they are on average also more severe. To make various forests resilient to droughts, it is important to improve the understanding of how these forests respond to dry periods. For this, it is important to be able to measure tree water content properly and preferably continuously. Satellites are increasingly being used for this purpose. Using microwaves, these satellites collect all kinds of data on what is happening on Earth.
INFLUENCE WATER CONTENT ON SATELLITE MEASUREMENTS
The effect of tree water content on the satellite measurements is not yet entirely clear. "To eventually measure droughts in forests with satellites, we need to understand how those measurements are affected by water," Vermunt says. To find out, Vermunt uses a lot of sensors that measure water content in different ways. Some sensors measure on the cubic centimetre, while others map almost the entire forest. "This way, we get a complete picture of the forest," Vermunt says.
As an example, Vermunt mentions the GNSS satellites which include GPS and Galileo. In a forest, your GPS reception is lower than in an open field. This is partly due to disturbance by water in the forest. If you know exactly how the signal is affected, you can use that data to measure tree water content. "Actually, we want to use GNSS signals for measurements they were not developed for. That’s called opportunistic use."
VARIOUS SENSORS
This is the first time that so many different sensors have been combined and such a detailed image of tree water content has been created. Using this image, we can validate and calibrate the satellite measurements, and thus ultimately improve them. Making the satellite data suitable for measuring moisture levels ensures that in the near future much more data will be available for large-scale research on the effect of droughts on forests.
MORE INFORMATION
Dr Paul Vermunt is a postdoctoral researcher in the Department of Water Resources (WRS; Faculty of ITC). His research and teaching focus on remote sensing of vegetation and soil moisture using microwaves. The Living Planet Fellowship is an initiative of the European Space Agency (ESA) to support young researchers with a PhD who want to start a scientific career in the context of Earth observation and Earth system science.
