Abstract
A synergic integration of Synthetic Aperture Radar (SAR) and optical time series offers an unprecedented opportunity in vegetation phenology monitoring for mountain agriculture management. In this paper, we performed a correlation analysis of radar signal to vegetation and soil conditions by using a time series of Sentinel-1 C-band dual-polarized (VV and VH) SAR images acquired in the South Tyrol region (Italy) from October 2014 to September 2016. Together with Sentinel-1 images, we exploited corresponding Sentinel-2 images and ground measurements. Results show that Sentinel-1 cross-polarized VH backscattering coefficients have a strong vegetation contribution and are well correlated with the Normalized Difference Vegetation Index (NDVI) values retrieved from optical sensors, thus allowing the extraction of meadow phenological phases. Particularly for the Start Of Season (SOS) at low altitudes, the mean difference in days between Sentinel-1 and ground sensors is compatible with the acquisition time of the SAR sensor. However, the results show a decrease in accuracy with increasing altitude. The same trend is observed for senescence. The main outcomes of our investigations in terms of inter-satellite comparison show that Sentinel-1 is less effective than Sentinel-2 in detecting the SOS. At the same time, Sentinel-1 is as robust as Sentinel-2 in defining mowing events. Our study shows that SAR-Optical data integration is a promising approach for phenology detection in mountain regions.
Highlights
Agricultural management in European mountain regions is a key strategy for preserving ecosystem stability and regional economies [1,2]
Even though the current study focuses on meadows phenology, as a first step, S-1 backscattering coefficients in VV and VH polarization were extracted over different crops and land-cover types to understand the radar signal dynamics to different vegetation types
The trends belonging to vineyards, orchards, and deciduous forest show a higher level of the signal, ranging from −11.5 dB to −7.5 dB and from −18.5 dB to −13.5 dB for VV and VH polarization, respectively
Summary
Agricultural management in European mountain regions is a key strategy for preserving ecosystem stability and regional economies [1,2]. Phenological stage monitoring is crucial in the decision-making process of the agricultural management [4]. Agricultural areas are generally of small size and the vegetation is characterized by a heterogeneous distribution. The Normalized Difference Vegetation Index (NDVI) [12] has been widely used to detect phenological phases [13,14,15,16,17,18]. In this case, cloud contamination and topographic effects in mountain regions compromise data significantly in the optical domain [19]
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