where λred and λNIR are the reflectance values within the areas of red and near infrared part of the electromagnetic spectrum, respectively. NDVI values range from − 1 (strong dryness) to + 1 (healthy vegetation). The index NDVI, by itself, does not depict drought or not drought conditions, but severity of drought can be defined as the deviation from the mean NDVI value of a long period (DEVNDVI).