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This paper analyses the role of climatic aridity on the spatial differences of vegetation activity and its inter-annual variability in a semi-arid region of the North East of the Iberian Peninsula. The vegetation activity was quantified by means of a monthly Normalized Difference Vegetation Index (NDVI) database from NOAA-AVHRR satellite images (1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000) at 1 km 2 of spatial resolution. Coefficients of variation (CoV) from temporal NDVI series were also calculated. The greater temporal variability of NDVI was recorded in areas with low vegetation cover (steppe and dry farming lands), whereas the lowest temporal variability of NDVI was recorded in the irrigated lands and forests located in the most humid areas. There is a strong and negative relationship between the CoV and the NDVI distribution, but we recorded important differences among land uses. Using maps of annual precipitation and temperature obtained by means of a regression-based method, we studied the influence of aridity on spatial patterns of NDVI and CoV values. The results show the high influence of aridity on the spatial distribution of vegetation activity in the Middle Ebro Valley. Aridity causes a general decrease of NDVI and an increase of CoV. Nevertheless, non-linear ARTICLE IN PRESS www.elsevier.com/locate/jnlabr/yjare 0140-1963/$ -see front matter r (S.M. Vicente-Serrano).relationships between aridity and NDVI and CoV were recorded for the whole of the study area and for the different land uses. The relationships between aridity and vegetation patterns in the semi-arid region studied are discussed in depth. r

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