New covariate: Phenology

The mean growing season period of vegetation is defined by use the satellite-derived phenology computed on the long-term average of 10-day MODIS NDVI at 1 km resolution. Phenology was extracted using the SPIRITS software applied to the historical average of the smoothed NDVI over the period 2013-2016.
The following key parameters are retrieved for each pixel: number of growing season per year (i.e. one or two); start of season (SOS, occurring at the time at which NDVI grows above the 25% the ascending amplitude of the seasonal profile); time of maximum NDVI (TOM); start of senescence period (SEN, when NDVI drops below 75% of the descending amplitude); and end of the season (EOS, when NDVI drops below 35%). Phenology timings: the value V in the image is expressed over a 3 years period, from 1 to 108 dekads (i.e. ten day periods). This done to be able to treat the circular variable dekad as an ordinal variable (the mean of SOS cam be computed as the usual average).

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