SENSITIVITY OF TOMOGRAPHIC RADAR BACKSCATTER TO TREE WATER CONTENT

Paper in proceeding, 2025

It is commonly accepted that radar backscatter is sensitive to vegetation water content. For forest canopies this connection has not yet been validated using reliable in situ tree water content observations at time scales over which tree water content fluctuations occur. Using tomographic radar time series observations from the BorealScat-2 radar tower in northern Sweden, we show that radar backscatter co-varies closely with tree stem water content measured using soil moisture probes inserted in tree stems. A site-specific calibration was applied to convert the soil moisture probe measurements at 1.3 m and 8 m height to volumetric vegetation (stem) water content. The correlation between vegetation water content and radar backscatter was best at 8 m height and a frequency of 600 MHz due to sufficient vertical resolution of the tomographic radar and negligible effects of canopy attenuation. Intercepted rainfall caused large deviations between radar backscatter and vegetation water content time series. These results show that radar backscatter is directly sensitive to tree stem water content, given sufficient ground backscatter suppression, negligible canopy attenuation and no precipitation.