where and are the reflectance in the near infrared (NIR) and red bands respectively. NDVI reflects vegetation vigor, density and biomass and is effectively determined by both – landscape and meteorology. The NDVI is the most commonly used vegetation index. It varies in a range of -1 to + 1.

NDVI itself does not reflect drought or non-drought conditions. But the severity of a drought (or the extent of wetness – on the other end of the spectrum) may be defined as NDVI deviation from its long-term mean (DEV_NDVI). This deviation is calculated as the difference between the NDVI for the current time step (e.g., January 1995) and a long-term mean NDVI for that month (e.g., a 20-year long mean NDVI of all Januaries from 1982 to 2001) for each pixel:

where NDVI_i is the NDVI value for month i and NDVI_mean,m is the long-term mean NDVI for the same month m, (there are twelve long-term NDVI means for each pixel: one mean for each calendar month). When DEV_NDVI is negative, it indicates the below-normal vegetation condition/health and therefore suggests a prevailing drought situation. The greater the negative departure, the greater is the magnitude of a drought.

where NDVI_maxand NDVI_minare calculated from the long-term record for that month (or week) and j is the index of the current month (week). The condition/health of the ground vegetation presented by VCI is measured in percent. VCI values around 50% reflect fair vegetation conditions. The VCI values between 50 to 100 % indicate optimal or above-normal conditions. At the VCI value of 100% the NDVI value for this month (or week) is equal to NDVI_max. Different degrees of a drought severity are indicated by VCI values below 50%. Kogan (1995) illustrated that the VCI threshold of 35% may be used to identify extreme drought conditions. The VCI value close to zero % reflects an extremely dry month, when NDVI value is close to its long-term minimum. Low VCI values over several consecutive time intervals point to drought development.