Remotely sensed and field surveyed early-warning signals of desertification in the Ebro basin, Spain

Abstract

The Ebro Basin near Zaragoza, Spain is the northern most semi-arid region in Europe with shrubland vegetation, a large water deficit and high grazing pressure in the last century. Early warning signals of desertification in the form of spatial and temporal variation in vegetation cover derived using remote sensing and field measurements were studied. They may provide enough time to act before a critical shift to a barren state induced by overgrazing in semi-arid zones takes place. Vegetation cover derived using a spectral mixture analysis from 18 Landsat images (1984-2008) was used to calculate spatial variation using the coefficient of variation (C_v) with a 5x5 moving window. Temporal variation was calculated using a moving window of 5 images. Vegetation cover showed a significant moderate correlation with spatial variation (-0.49) and temporal variation (-0.35). 80% of the study area showed this negative relationship. Differences in the mean of the environmental factors slope, aspect and vegetation cover show differences between these areas and itself could influence the relationship between early warning signal and vegetation cover. Early warning signals were also measured in the field using the line intercept method in 24 500m transects. Spatial variation again showed a moderate correlation (-0.46) with vegetation cover. Remotely sensed spatial variation correlated with spatial variation measured in the field (0.47). This shows that the early warning signals spatial and temporal variation can possibly contribute to remotely mapping of desertification.