This led UNEP (United Nations Environment Programme) in 1992 to propose a classification based on a more simple approach of calculating the potential evapotranspiration, proposed by Thornthwaite. The resulting Aridity Index, is also used by FAO (Food and Agriculture Organization). Besides, Drought indices can be derived from satellite data of the MODIS sensor onboard the polar orbiting NASA-EOS Terra and Aqua spacecrafts with equator crossing times of 10:30 and 13:30 Local Solar Time, respectively (Levy et al., 2007). These indices concern radiometrically corrected values for vegetation, derived from the red and near infrared spectral regions of the electromagnetic radiation (Huete et al., 1997 and Huete et al., 2002). These areas are sensitive to the study of vegetation (vegetation type, stage of development, etc. (Clevers and Verhoef, 1993 and Thenkabail, 2003)). NDVI (Normalized Difference Vegetation Index) was proposed firstly by Tucker (1979) as an index that reflects the health and density of vegetation: