Preprints
https://doi.org/10.5194/hess-2021-406
https://doi.org/10.5194/hess-2021-406

  10 Aug 2021

10 Aug 2021

Review status: this preprint is currently under review for the journal HESS.

The influence of vegetation water dynamics on the ASCAT backscatter-incidence angle relationship in the Amazon

Ashwini Petchiappan1, Susan C. Steele-Dunne2, Mariette Vreugdenhil3, Sebastian Hahn3, Wolfgang Wagner3, and Rafael Oliveira4 Ashwini Petchiappan et al.
  • 1Department of Water Management, Delft University of Technology, Stevinweg 1, Delft 2600 GA, The Netherlands
  • 2Department of Geoscience and Remote Sensing, Delft University of Technology, Stevinweg 1, Delft 2600 GA, The Netherlands
  • 3Department of Geodesy and Geo-Information, TU Wien, Vienna 1040, Austria
  • 4Department of Plant Biology, Institute of Biology P.O.Box: 6109, University of Campinas – UNICAMP 13083-970, Campinas, SP, Brazil

Abstract. Microwave observations are sensitive to plant water content and could therefore provide essential information on biomass and plant water status in ecological and agricultural applications. The combined data record of the C-band scatterometers on ERS 1/2, the Metop series and the planned Metop Second Generation satellites will span over 40 years, which would provide a long-term perspective on the role of vegetation in the climate system. Recent research has indicated that the unique viewing geometry of ASCAT could be exploited to observe vegetation water dynamics. The incidence angle dependence of backscatter can be described with a second order polynomial, the slope and curvature of which are related to vegetation. In a study limited to grasslands, seasonal cycles, spatial patterns and interannual variability in the slope and curvature were found to vary among grassland types and were attributed to differences in moisture availability, growing season length and phenological changes. To exploit ASCAT slope and curvature for global vegetation monitoring, their dynamics over a wider range of vegetation types needs to be quantified and explained in terms of vegetation water dynamics. Here, we compare ASCAT data with meteorological data and GRACE Equivalent Water Thickness (EWT) to explain the dynamics of ASCAT backscatter, slope and curvature in terms of moisture availability and demand. We consider differences in the seasonal cycle, diurnal differences, and the response to the 2010 and 2015 droughts across ecoregions in the Amazon basin and surroundings. Results show that spatial and temporal patterns in backscatter reflect moisture availability indicated by GRACE EWT. Slope and curvature dynamics vary considerably among the ecoregions. The evergreen forests, often used as a calibration target, exhibit very stable behaviour even under drought conditions. The limited seasonal variation follows changes in the radiation cycle, and may indicate phenological changes such as litterfall. In contrast, the diversity of land cover types within the Cerrado region results in considerable heterogeneity in terms of the seasonal cycle and the influence of drought on both slope and curvature. Seasonal flooding in forest and savanna areas also produced a distinctive signature in terms of the backscatter as a function of incidence angle. This improved understanding of the incidence angle behaviour of backscatter increases our ability to interpret and make optimal use of the ASCAT data record and VOD products for vegetation monitoring.

Ashwini Petchiappan et al.

Status: open (until 05 Oct 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Ashwini Petchiappan et al.

Ashwini Petchiappan et al.

Viewed

Total article views: 226 (including HTML, PDF, and XML)
HTML PDF XML Total BibTeX EndNote
182 41 3 226 1 2
  • HTML: 182
  • PDF: 41
  • XML: 3
  • Total: 226
  • BibTeX: 1
  • EndNote: 2
Views and downloads (calculated since 10 Aug 2021)
Cumulative views and downloads (calculated since 10 Aug 2021)

Viewed (geographical distribution)

Total article views: 196 (including HTML, PDF, and XML) Thereof 196 with geography defined and 0 with unknown origin.
Country # Views %
  • 1
1
 
 
 
 
Latest update: 23 Sep 2021
Download
Short summary
This study investigates spatial and temporal patterns in the incidence angle dependence of backscatter from the ASCAT C-band scatterometer, and relates those to precipitation, humidity and radiation data and GRACE equivalent water thickness in ecoregions in the Amazon. The results show that the ASCAT data record offers a unique perspective on vegetation water dynamics exhibiting sensitivity to moisture availability and demand and phenological change at interannual, seasonal and diurnal scales.