Analysis of the effect of drought on the phenology parameters of vegetation indexes from the time series of MODIS sensor images (case study: Hamadan province)

Drought is one of the consequences of climate change that slowly and over a relatively long period of time affects climate, environment, agriculture, vegetation, water resources and even economic and social sectors. The serious outcome of drought is the reduction of vegetation cover. In this research, using MODIS sensor satellite images of 2001-2020 (20-year period) and CHIRPS monthly rainfall data in Hamedan province, to analyze the effect of drought on phenology parameters (maximum NDVI value, range, base values) NDVI vegetation index has been discussed. For this purpose, the phenology parameters of the NDVI vegetation index were first extracted through the TIMESAT software, next the changes of these parameters were analyzed in relation to the elevation data and the land cover map of the region, and the correlation and RMSE error between the phenology parameters and the elevation data of the region were estimated. Moreover, through CHIRPS raster rainfall data, the annual SPI index was calculated. The results obtained from the analysis of these parameters in different elevations and uses have shown that in 2008, the maximum value of NDVI and the range of NDVI vegetation index have decreased 2008 compared to other years. On the other hand, in 2019, these parameters had higher values compared to other years. In addition, among other phenological parameters, the parameter of base values with a correlation of 0.925 has the highest correlation with the elevation data of the region, and its RMSE is 0.021. Furthermore, through monthly rainfall data for the years 2001 to 2020, it has been shown that in 2008, the average annual rainfall was lower than in other years, and the value of the annual SPI index was -1.79. Therefore, in 2008, a moderate drought occurred in the region, and on the other hand, according to the average rainfall in the years 2007, 2018 to 2020 and the value of the SPI index, during these years, a very severe drought occurred in the region.