Geoscience Reference
In-Depth Information
During the summer of 2003, the high temperatures and lack of water limited
vegetation growth. The NDVI NOAA AVHRR temporal series shows a strong
decrease in the NDVI and a decrease in the total amount of vegetation cover that
was present in agricultural areas from June 2003 onwards. This decrease, which
only occurred in September in previous years, had never been as pronounced before
(see Figure 3.9).
Figure 3.9.
A comparison of the annual NDVI NOAA-AVHRR
values on surfaces mainly used to provide fodder for animals
To carry out a more detailed study of droughts on a regional scale, it is necessary
to associate the NDVI with the lack of water. The maps in Figure 3.10 show the
water deficit (calculated by the Turc potential evapotranspiration formula and with a
reserve of 125 mm) and monthly vegetation index (provided by NOAA-AVHRR
data) for each of the 4 months in the summer of 2003. On the images it is possible to
compare Western Brittany (with its low water deficit index and its high vegetation
indexes) with Eastern Brittany, where the drought was more intense (especially in
August) and where the vegetation indexes were much lower. The water deficit
experienced in August (and which continued into the month of September in the
East) was reflected in the NDVI values for September. This time delay of 1 month is
due to the time taken for the vegetation to react to the stressful climate conditions
and to the way in which the data were collected by monthly MVC.
The region of Brittany has the reputation of being a wet region. However, it can
experience prolonged and pronounced dry periods, and in certain years, these dry
periods can have a quite serious effect on the region's agricultural production. The
detailed spatialization of a phenomenon as complex and diffuse as drought requires
the use of data that has been produced by the process of remote sensing. The
compatibility of the results produced by water analysis methods and by monitoring
the NDVI leads us to believe that an inverse correlation exists between the NDVI
value and the value corresponding to the water deficit. In theory, a high water deficit
value corresponds to a low NDVI value, which would lead to the creation of
drought. However, if the value of the water deficit is quite high then it becomes
more difficult to analyze the NDVI value. It is necessary to take into consideration
different the farming methods practiced, as well as the different types of vegetation,
before carrying out any analysis on the variations of a specific satellite signal.
