Abstract
Vegetation products based on microwave remote sensing observations, such as Vegetation Optical Depth (VOD), are increasingly used in a variety of applications. One disadvantage is the often coarse spatial resolution of tens of kilometers of products retrieved from microwave observations from spaceborne radiometers and scatterometers. This can potentially be overcome by using new high-resolution Synthetic Aperture Radar (SAR) observations from Sentinel-1. However, the sensitivity of Sentinel-1 backscatter to vegetation dynamics, or its use in radiative transfer models, such as the water cloud model, has only been tested at field to regional scale. In this study, we compared the cross-polarization ratio (CR) to vegetation dynamics as observed in microwave-based Vegetation Optical Depth from coarse-scale satellites over Europe. CR was obtained from Sentinel-1 VH and VV backscatter observations at 500 m sampling and resampled to the spatial resolution of VOD from the Advanced SCATterometer (ASCAT) on-board the Metop satellite series. Spatial patterns between median CR and ASCAT VOD correspond to each other and to vegetation patterns over Europe. Analysis of temporal correlation between CR and ASCAT VOD shows that high Pearson correlation coefficients (Rp) are found over croplands and grasslands (median Rp > 0.75). Over deciduous broadleaf forests, negative correlations are found. This is attributed to the effect of structural changes in the vegetation canopy which affect CR and ASCAT VOD in different ways. Additional analysis comparing CR to passive microwave-based VOD shows similar effects in deciduous broadleaf forests and high correlations over crop- and grasslands. Though the relationship between CR and VOD over deciduous forests is unclear, results suggest that CR is useful for monitoring vegetation dynamics over crop- and grassland and a potential path to high-resolution VOD.
Highlights
High-resolution monitoring of vegetation is of increasing importance for many research fields and applications
This study presents the first large scale quantitative comparison of Sentinel-1 cross-polarization ratio (CR) to Advanced SCATterometer (ASCAT) Vegetation Optical Depth (VOD) over Europe in the years 2016–2018
The spatial patterns between CR and ASCAT VOD are similar and follow vegetation patterns corresponding to land cover
Summary
High-resolution monitoring of vegetation is of increasing importance for many research fields and applications. One recent VOD dataset from passive microwave observations is the long-term Vegetation Optical Depth Climate Archive (VODCA) dataset [7], which combines VOD retrievals using the Land Parameter Retrieval Model from different sensors. This dataset was validated with MODIS Leaf Area Index (LAI) and Vegetation Continuous Fields, showing similar spatial distributions, temporal dynamics and trends. The temporal variability in the ASCAT VOD stems from the variability in the slope and curvature of the backscatter—incidence angle relationship, which is sensitive to vegetation dynamics: an increase in vegetation increases the slope. The ASCAT VOD corresponded to VOD from passive microwave observations, both in spatial distribution as in temporal dynamics. Over Australia ASCAT VOD corresponded to MODIS LAI [10]
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