Evaluation of drought and its effects on vegetation in southern regions of Iran

Introduction
Climate is one of the most important factors affecting vegetation conditions. The spatial distribution of vegetation is closely related to climatic conditions. One of the main features of the Inner Plateau of Iran is its categorization in the dry belt of continental Europe and Asia (Eurasia). This feature and the occurrence of drought due to the unevenness and climate has always had a significant impact on the occurrence of desert characteristics. Drought is the amount of rainfall deviation of one year in a place, compared to the long-term average rainfall in the same place. In areas with low and non-uniform rainfall, the effects of drought are intensive, especially on water resources, agriculture and vegetation. Drought indices are mathematical equations that report drought as a numerical value of one or more variables such as rainfall and evapotranspiration. The purpose of this study was to investigate the changes in vegetation cover in southern Iran (Khuzestan, Fars, Kohgiluyeh and Boyer-Ahmad, Chaharmahal and Bakhtiari, Bushehr, Hormozgan and Sistan and Baluchestan provinces) during the period 2008 to 2017 and its relationship with the drought index in the years studied.
Materials and methods
In this research, MODIS satellite image has been used as an image with low terrestrial resolution and high temporal resolution.
Normalized Differential Plant Index NDVI, is one of the most famous and simple plant indicators used, which is based on two red bands. Near infrared is defined as follows:
NDVI= (NER – RED)/ (NER + RED) 
Standard Precipitation Index (SPI), in this case is the standardized rainfall index used to calculate the probability of rainfall occurrence for all time scales, but is mostly used in the time scales of 1, 3, 6, 12, 24 and 48 months and is one of the most important global indicators for drought.
Discussion and conclusion
The amount of poor vegetation in 2008 was 8302.66 hectares, but in 2017, it decreased to 3436.10 hectares. In general, the average rainfall in the southern regions of the country is 216.03 mm. Studies show that the highest rainfall belongs to Hormozgan province with 326.02 and then Khuzestan and Fars with 305.51 and 286.5 mm respectively and the lowest rainfall, belongs to Kerman and Sistan and Baluchestan provinces with 136.51 and 103.45 mm, respectively. In 2008, southern Iran experienced moderate drought conditions. Drought has been more paramount in Khuzestan, Chaharmahal and Bakhtiari, Kohgiluyeh and Boyer-Ahmad provinces. In some parts of Hormozgan province, drought conditions have been more normal and wet. In 2009, the situation in the whole study area has become more normal and according to the SPI index is close to normal. In order to investigate the amount of vegetation lost, land use changes in the period (2017-2001) were reviewed and the results were presented. According to the results, the vegetation has decreased 138418.4 km from 2008 to 2017 and the amount of agricultural lands has decreased 8155.63 km. Arid land has also increased significantly from 70.9797341 km in 2008 to 2896724.66 km in 2017.
Results
The aim of this study was to investigate the relationship between satellite indices (vegetation parameter) and moving averages of SPI index (climate parameter). According to the results, it can be acknowledged that vegetation parameters have always been affected by climate and parameters affecting it. Therefore, it is certain that in the future, by quantitatively detailed examining of the parameters of vegetation (such as canopy, density, etc.) and climate (such as temperature, humidity, etc.), in the long run, drought warning systems can be established in order to reduce the damage of drought.