Evaluation of the Effect of Drought on the Vegetation of Iran Using Satellite Images and Meteorological Data

Introduction: Drought conditions can vary from moderate to severe and have different durations, necessitating continuous and operational monitoring. The longer the drought persists, the more pronounced its impact on vegetation and water resources becomes, and the more severe the drought, the greater the limitation of services for humans and the alteration of natural systems. Habitat destruction for wildlife, reduced water quality, and reduced access to water resources could be consider as most effects of drought. Drought monitoring is essential for researchers, managers, and decision-makers to identify vulnerable areas, which can be used to reduce the consequences of drought.
Material and Methods: In this study, an attempt has been made to investigate the vegetation drought situation in Iran by using Suomi NPP infrared sensor images obtained from the Earth Data website (earthdata.nasa.gov) and using (NDVI), (VCI), (TCI), and (VHI) indices. The study period, spanning from April 1st to July (the 13th to 26th week), was selected as it encompasses the typical drought duration in Iran. The Standard Precipitation Index (SPI) was calculated for Iran using daily precipitation data from 143 synoptic stations. Subsequently, the correlation coefficient was calculated between SPI and each of the indices (NDVI), (VCI), (TCI), and (VHI). In infrared images, M bands have a resolution of 750 meters, while I bands have a resolution of 375 meters.
Results and Discussion: Based on the rainfall data recorded in synoptic meteorological stations, there is minimal rainfall during the summer months (July, August, and September). Conversely, the majority of rainfall occurs during the autumn, winter, and spring seasons. Consequently, the water year in most regions of Iran commences approximately in the third decade of September and continues until the second and third decade of June annually. In this study area, the optimal temporal base for monitoring and estimating drought on the vegetation is from April 1st to June 30th. In this article, the effect of precipitation on vegetation conditions was investigated using the standardized precipitation index (SPI), derived from monthly precipitation data from synoptic meteorological stations. Iran experiences a dry season in summer, with August being the driest month of the year. The temporal and spatial changes in drought for each vegetation indicator are markedly different.
Conclusion: Based on the majority of years experiencing drought, the vegetation cover is expected to face mild or severe drought. This is demonstrated by a decrease in the values of each indicator. In years  that the vegetation was affected by drought, the values of the indices show a decrease in April, followed by an increase in June and July. This suggests the beginning of a severe drought. Based on the calculated SPI, it was determined that the area experiences low precipitation during the hot months, indicating a lower rate compared to other months.