A comparative study of satellite and meteorological drought indices of the desertification process in the spring vegetation of Semnan province

In this study, meteorological data and remote sensing were used to investigate the effect of drought on vegetation in Semnan province. At first, the Standard Precipitation Index (SPI) for the years 2006-2015 was calculated at Semnan station. In the next step, based on 30 Aqua MODIS images from the platform in April, May and June, the normalized difference vegetation index (NDVI) was calculated. Considering this index, the vegetation was classified into four groups including lack of vegetation, thin vegetation, semi-dense vegetation, and dense vegetation. Then, the area of each class was calculated. Finally, the NDVI and SPI indices were compared. The results of the study showed a reverse relationship between NDVI and SPI on one hand and the rainfall in the thin and semi-dense vegetation on the other hand. During all the three spring months, with an increase in the amount of rainfall and the SPI value, the vegetation on the thin floor grew to be semi-dense. Conversely, as SPI and precipitation reduced, the area of semi-dense vegetation reduced and turned into thin vegetation. This is what usually occurs in the case of droughts. With regard to the considerable size of these two vegetation areas, which account for 93 percent of the whole province area, droughts can cause irreparable damages. As it was found, the desertification process invariably turns semi-dense vegetation into poor vegetation. If this process is not stopped, desertification will go on in the region.