Drought monitoring with NDVI-based Standardized Vegetation Index

Drought is onee of the major natural hazards affecting the environment and economy of countries worldwide. Reliance on weather data alone is not sufficient to monitor areas of drought, particularly when these data can be untimely, sparse, and incomplete. Augmenting weather data with satellite images to identify the location and severity of droughts is a must for complete, up-to-date, and comprehensive coverage of current drought conditions. The objective of this research was to standardize, by time of year, the Normalized Difference Vegetation Index (NDVI) to augment drought-monitoring techniques. The Standardized Vegetation Index (SVI) describes the probability of vegetation condition deviation from normal, based on calculations from weekly NDVI values. The study was conducted with 12 years (1989-2000) of Advanced Very High-Resolution Radiometer (AVHRR) satellite images. Z-scores of the NDVI distribution are used to estimate the probability of occurrence of the present vegetation condition at a given location relative to the possible range of vegetative vigor, historically. The SVI can be interpreted as vegetation condition based on the fact that vegetation is an efficient integrator of climatic and anthropogenic impacts in the boundary layer of the atmosphere. It thereby provides a spatially and temporally continuous short-term indicator of climatic conditions. Findings indicate that the SVI, along with other drought monitoring tools, is useful for assessing the extent and severity of drought at a spatial resolution of 1 km. The SVI is capable of providing a near-real-time indicator of vegetation condition within drought regions, and more specifically areas of varying drought conditions.