Vegetation Health and vegetative drought conditions: Case Study of AVHRR/VIIRS Data in Tigray Region, Ethiopia
In this study, weekly mean of vegetation health index (VHI) for Tigray region from 1982 to 2016 was compared with mean VHI during 2017. The Vegetation Health index (VHI) is based on a combination of products extracted from vegetation signals, namely the Normalized Difference Vegetation Index (NDVI) and from the brightness temperatures, both derived from the NOAA Advanced Very High-Resolution Radiometer (AVHRR) sensor. The data and images have 4 km spatial and 7-day composite temporal resolution. VH is calculated based on strong inverse correlation between NDVI and land surface temperature, since increasing land temperatures are assumed to act negatively on vegetation and consequently to cause stress. The VHI values range from 0 to 100, the low values representing stressed vegetation health conditions, middle values representing fair health conditions, and high values representing optimal or above-normal vegetation health conditions. Vegetative drought conditions were calculated using vegetation health conditions as proposed by Kogan. Extreme Drought<10 VHI, Severe Drought<20 VHI, Moderate Drought<30VHI, Mild Drought <40 VHI, No Drought>40 VHI. The results showed during 1985, 1990 – 91, 2009 and 2015-16 there was large area covered under drought conditions in Tigray Region. Drought conditions were found in Tigray region during 32nd week to 42nd week of 2017. Results shows eastern and south eastern part of Tigray has not been as much affected as western and central part of Tigray during 2017.