An investigation on impact of drought on rangeland and forest vegetation changes in Chaharmahal & Bakhtiari province using MODIS satellite dat

The aim of this study was to investigate the relationships between vegetation changes and climate variables including rainfall and drought using MODIS satellite images during the years 2000-2014 in Chaharmahal and Bakhtiari Province. In order to evaluate the impact of rainfall and drought on vegetation changes in 32 synoptic, climatology and rain gauge stations in and around the province, SPI index was calculated in short time series (3, 6, 9 months) and long time series (12, 24 months). All the time scales were leading to early June in which the maximum growth of vegetation of the province is occurred. To this end, SPI drought maps and NDVI images were prepared at different time scales by kriging method and linear regression analysis was performed to define their relationships. Comparisons of NDVI and SPI over the study period showed that the vulnerability of vegetation to drought mostly depends on the life forms and plant species. In this study, depending on the type of vegetation, different correlation coefficients were observed between vegetation and SPI indices. Among different rangeland and forest vegetation types, higher correlations were found between NDVI and SPI in rangeland vegetation types compared to the forest ones and the highest correlations belonged to the annual grass and annual forb vegetation cover. This confirms the higher impact of reducing rainfall on the decline of rangeland cover compared to the forest vegetation. Overall, the findings of this study indicate that the MODIS NDVI can be used to study SPI in the assessment and monitoring of drought effects on vegetation cover. By studying and recognition of vegetation cover and applying NDVI index to assess changes in vegetation cover, vulnerable types of vegetation to drought could be identified.