Vegetation phenology from Sentinel-2 and field cameras for a Dutch barrier island

Abstract

Remote sensing studies of vegetation phenology increasingly benefit from freely available satellite imagery\nacquired with high temporal frequency at fine spatial resolution. Particularly for heterogeneous landscapes this\nis good news, given the drawback of medium-resolution sensors commonly used for phenology retrieval (e.g.,\nMODIS) to properly represent the fine-scale spatial variability of vegetation types. The Sentinel-2 mission acquires\nspectral data globally at 10 to 60m resolution every five days. To illustrate the mission's potential for\nstudying vegetation phenology, we retrieved phenological parameters for the Dutch barrier island\nSchiermonnikoog for a full season of Sentinel-2A observations in 2016. Overlapping orbits resulted in two acquisitions\nper 10 days, similar to what is achieved globally since the launch of Sentinel-2B. For eight locations on\nthe island's salt marsh we compared greenness chromatic coordinate (GCC) series derived from digital repeat\nRGB-cameras with vegetation index series derived from Sentinel-2 (NDVI and GCC). For each series, a double\nhyperbolic tangent model was fitted and thresholds were applied to the modelled data to estimate start-, peak-,\nand end-of-season (SOS/PS/EOS). Variability in Sentinel-2 derived SOS, when taken as the midpoint between\nminimum and peak NDVI, was well-explained by camera GCC-based SOS (R2=0.74, MSD=8.0 days,\nRMSD=13.0 days). However, EOS estimates from camera GCC series were on average almost two months\nbefore NDVI-based estimates. This could partially be explained by the observed exponential relationship between\nGCC and NDVI, as well as by the combined effect of viewing angle differences and the presence of nonphotosynthetic\nelements in the vegetation canopy. A two-layer canopy radiative transfer model incorporating\nreduced chlorophyll levels in the upper layer provided a physically-based explanation of the viewing angle\neffect. Finally, we applied the phenology retrieval approach to NDVI series for all pixels of the island in order to\nmap spatial patterns of phenology at fine resolution. Our results demonstrate the potential of the Sentinel-2\nmission for providing spatially-detailed retrievals of phenology.