A procedure to derive intra- and inter-annual changes on vegetation from NDVI time series. A case study in Spain.

Abstract

The NDVI time series are characterized by patterns like seasonality, trends and localized abrupt changes or discontinuities resulting from disturbance events. Therefore, they allow us to characterize vegetation dynamics on the basis of different temporal scales. In particular, the subtle, long-term changes convey relevant information on land degradation. The objective of this work is to study the spatial patterns of vegetation activity over Spain and its temporal variability throughout the period 1989-2002. A multi-resolution analysis (MRA) based on the wavelet transform has been implemented on NDVI time series from the MEDOKADS database. The MRA decomposes the original signal as a sum of series associated with different temporal scales. Specifically, the intra-annual series is processed to define several key features in relation with the vegetation phenology. In contrast, the interannual component of the signal is used to detect trends by means of a Mann-Kendall test and map the magnitude of the land-cover change. Finally, a comprehensive identification of the areas presenting a negative value of the magnitude of change is carried out to select those linked to land degradation processes. Results show a major presence of these areas the Southeast of Spain.