Linkage of agricultural drought with meteorological drought in different climates of Iran

This study investigates the linkage between agricultural drought and meteorological drought, using normalization different vegetation index (NDVI) and land surface temperature (LST) for the whole of Iran in four periods of 16 days of late March, April, May, and June between 2000 and 2017. LST and NDVI were obtained from the production of MODIS MOD13A2 and MOD11A2. In the next step, the basic time synchronization of LST and NDVI was performed by the mean of two 8-day LST images and their conversion to 16-day LST for each year. The vegetation health index (VHI) was calculated from the combination of temperature condition index (TCI) and vegetation condition index (VCI). Then, Iran's map of standardized precipitation-evapotranspiration index (SPEI) was calculated on a 12-month timescale for 68 meteorological stations with the inverse distance weighted (IDW) method. By using ArcGIS 10.5, the Pearson correlation coefficient and the slope of linear regression were calculated between 12-month SPEI and VHI for all four periods in different climates: hyper dry, dry, semi-dry, semi-humid, and humid. Results showed that the correlation increased when the temperature increased. This increase occurred in dry and hyper-dry climates. As the temperature rose, the slope of the linear regression for 12-month standardized precipitation-evapotranspiration index (SPEI) on vegetation health index (VHI) increased. The highest and lowest average effects of the slope were observed in dry and climatic class, respectively. With increasing temperature, the need for water in plants will increase. Hence, there is a direct relation or a positive correlation between temperature and correlation strength and slope effect. Therefore, plants more seriously will face drought stress during the drought period, and this drought stress has a positive correlation with drought intensity. From this study, it was concluded that the highest correlation and the highest slope effect between the 12-month SPEI and VHI happened in the dry climatic class in June.