Potential of C-band Synthetic Aperture Radar Sentinel-1 time-series for the monitoring of phenological cycles in a deciduous forest

International Journal of Applied Earth Observations and Geoinformation

Nicolas Delpierre¹  Gabriel Hmimina²  Alexandre Morfin³  Kamel Soudani⁴  Éric Dufrêne⁴  Daniel Berveiller⁴  Gaëlle Vincent⁴ 

[1] Corresponding author.;Institut Universitaire de France (IUF), France;Laboratoire de Météorologie Dynamique, IPSL, CNRS/UPMC, Paris, France;Université Paris-Saclay, CNRS, AgroParisTech, Ecologie Systématique et Evolution, 91405 Orsay, France;

关键词: Phenology;    Forest;    Sentinel 1;    VV/VH;    NDVI;    LAI;   

DOI  :  

来源: DOAJ

【 摘 要 】

Annual time-series of the two satellites C-band SAR (Synthetic Aperture Radar) Sentinel-1A and 1B data over five years were used to characterize the phenological cycle of a temperate deciduous forest. Six phenological metrics of the start (SOS), middle (MOS) and end (EOS) of budburst and leaf expansion stage in spring, and the start (SOF), middle (MOF) and end (EOF) of leaf senescence in autumn were extracted using an asymmetric double sigmoid function (ADS) fitted to the time-series of the ratio (VV/VH) of backscattering at co-polarization VV (vertical–vertical) and at cross polarization VH (vertical-horizontal). Phenological metrics were also derived from other four vegetation proxies (Normalized Difference Vegetation Index NDVI time-series from Sentinel-2A and 2B images, and in situ measurements of NDVI measurements, Leaf Area Index LAI and litterfall temporal dynamics). These estimated phenological metrics were compared to phenological observations obtained by visual observations from the ground, achieved using binoculars by three inter-calibrated observers, on a bi-weekly basis during the budburst and weekly during the senescence. We observe a decrease in the backscattering coefficient (σ0) at VH cross polarization during the leaf development and the expansion phase in spring and an increase during the senescence phase, contrary to what is usually observed on various types of crops. In vertical polarization, σ0VV shows very little variation throughout the year. S-1 time-series of VV/VH ratio provide a good description of the seasonal vegetation cycle allowing the estimation of spring and autumn phenological metrics. Estimates provided by VV/VH of budburst dates using MOS criterion differ by approximately 8 days on average (mean average deviation) from phenological observations. During senescence phase, estimates using MOF criterion are later and deviate by about 20 days from phenological observations of leaf senescence while the differences are of the order of 2 to 4 days between the phenological observations and estimates based on in situ NDVI and LAI time-series, respectively. A deviation of about 7 days, comparable to that observed during budburst, is obtained between the estimates of senescence (MOF) from S-1 and those determined from the in situ monitoring of litterfall. While in spring, leaf emergence and expansion described by LAI or NDVI explain the increase of VV/VH (or the decrease of σ0VH), during senescence, S-1 VV/VH is decorrelated from LAI or NDVI and is better explained by litterfall temporal dynamics. This behavior resulted in a hysteresis phenomenon observed on the relationships between VV/VH and NDVI or LAI. For the same LAI or NDVI, the response of VV/VH is different depending on the phenological phase considered. This study shows the high potential offered by Sentinel-1 SAR C-band time-series for the detection of forest phenology, thus overcoming the limitations caused by cloud cover in optical remote sensing of vegetation phenology.

【 授权许可】

Unknown