C-band microwave sensors reflect the spring water uptake of temperate deciduous broadleaf trees

Observations from the C-band scatterometers ERS ESCAT and Metop ASCAT have been used to monitor vegetation dynamics predominantly in agricultural areas and grasslands (Schroeder et al., 2016, Vreugdenhil et al., 2016, Vreugdenhil et al., 2017). In particular, the slope  between the measured radar backscatter and the incidence angle of the observations has been found to reflect structural changes in the vegetation (e.g., size and orientation of stems and leaves) and vegetation water content, as well as deficits therein, as for example during an extensive drought period in North American grasslands (Steele-Dunne 2019).

Often, a peak in the slope time series is observed during spring. This peak occurs predominantly in regions covered by deciduous broadleaf forests (DBF), and recurs in most years around the beginning of April. We carried out a detailed study of the causes of such spring peaks over Austria by comparing the timing of the peaks to phenology observations of leaf emergence, leaf area index and temperature conditions. The comparison showed a good agreement between the timing of the ASCAT spring peaks and the reference datasets, even in regions with low coverage of DBF, with a median absolute difference between the peak in ASCAT and the reference datasets of less than 14 days for grid cells with at least 10% DBF (Pfeil et al., in prep.).

In this presentation, we assess if similar spring peaks occur in passive microwave satellite observations. Therefore we investigate the spring behavior of vegetation optical depth (VOD) time series from the radiometers AMSR-E and AMSR2 over DBF and find similar peaks, which are less pronounced but occur very close in time to the ASCAT peaks. It can thus be said that the spring peak is not a sensor-dependent phenomenon, but reflects the sensitivity of C-band microwave sensors to leaf development in deciduous trees. In summary, the results of the study suggest that spring water uptake in deciduous trees manifests in active and passive C-band microwave observations, as it causes increased scattering from the bare twigs and branches, followed by an attenuation of the twigs- and branches scattering by the emerging leaves.

 

References

• Schroeder, R., McDonald, K. C., Azarderakhsh, M., & Zimmermann, R. (2016). ASCAT MetOp-A diurnal backscatter observations of recent vegetation drought patterns over the contiguous US: An assessment of spatial extent and relationship with precipitation and crop yield. Remote sensing of environment, 177, 153-159.
• Steele-Dunne, S. C., Hahn, S., Wagner, W., & Vreugdenhil, M. (2019). Investigating vegetation water dynamics and drought using Metop ASCAT over the North American Grasslands. Remote Sensing of Environment, 224, 219-235.
• Vreugdenhil, M., Dorigo, W. A., Wagner, W., De Jeu, R. A., Hahn, S., & Van Marle, M. J. (2016). Analyzing the vegetation parameterization in the TU-Wien ASCAT soil moisture retrieval. IEEE Transactions on Geoscience and Remote Sensing, 54(6), 3513-3531.
• Vreugdenhil, M., Hahn, S., Melzer, T., Bauer-Marschallinger, B., Reimer, C., Dorigo, W. A., & Wagner, W. (2017). Assessing vegetation dynamics over mainland Australia with metop ASCAT. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 10(5), 2240-2248.