The phenology is the study of the apparition of periodic events determined by seasonal variations due to the climate. The vegetation phenology is the most studied, but also animals (notably birds and insects) and even glaciers are studied. The periodic events are for example the blooming, the emergence of leaves, the fructifying or the vegetation leaf coloring. The appearing of these different events each year is determined by different factors, notably the temperature, the day duration as well as the water content of the soils. Evidently, the altitude has also an important role (cf Wikipedia).
With a two-day repetitivity, Venµs data will enable to obtain a minimum of 2 or 3 cloud free observations each month, and usuallly more. Venµµs data will thus provide accurate observations of the phenology over a hundred of sites, while waiting for an operational system arrival enabling high repetitivity and high resolution observations at the scale of continents.
Example 1: Senescence of forests
The example below shows the appearing of autumn colors in Alpine forests (Bardonecchia region, in Italy), from Formosat-2 data. The altitude effect on the leaf senescence date can clearly be seen.
NB This image series could also be used to monitor the snow cover (left side of the images).
Formosat-2 image series in true colors (Blue in blue, Green in green, Red in red)
Example 2: identifications of rape plots
Image from Wikipedia
Some phenological features enable to identify vegetation cover, but only if image at the right date is available. It is the case of rape, for which start and senescence dates are very near the winter wheat ones. This could render discrimination of these two crops rather difficult. Fortunately, when an image of the colza spring blooming is available, the identification of this crop is straightforward, as it may be seen on the riddle below.
Riddle: where are the rape plots?
Formosat-2 image of May 30th, 2007, in true colors
Even if it is possible to program an acquisition at the right date with a satellite like SPOT, it should be known that dispite its long blooming (3 weeks), on a small area, the blooming period varies greatly depending on the sowing date, the rape variety or the sun exposure of the plot. The blooming date can also change from one year to the next depending on the meteorology. Consequently, only a frequent and regular acquisition of images can ensure (except in case of exceptional cloud cover) the availability of images during the blooming.
Example 3: deciduous or conifer forests
One could mention numerous other examples: to discriminate a deciduous forest from a conifer forest is a child game if an image taken in winter is available, after the leaves have fallen.
Riddle: where are the conifer forests ?
Formosat-2 image on September 8th, 2007, in false colors. The forests appear in dark red.