Drought Effects on Vegetation Changes in Kashkan Basin of Lorestan Using ETM + and OLI Landsat Data

Background and objectives: Vegetation is one of the most important elements of an ecosystem, which is directly related to the moisture conditions of the land. Estimation of plant drought indices such as VCI, TCI, VHI is an effective tool to assess the effects of drought by calculating NDVI. In this regard, several studies have been conducted that explain how to calculate and use these indicators. The purpose of this study was to investigate the effect of drought on vegetation in Kashkan basin of Lorestan using Landsat satellite images and rainfall data. Methodology: Rainfall data of Kashkan basin stations during the statistical period of 1999 to 2019 were collected from the Meteorological Organization. Landsat Satellite images for the years of 1999 and 2019 from ETM and OLI sensors (L Needs 7 and 8) were obtained from the USGS database. Finally, using the SPI index, dry and wet years were distinguished during the statistical period and were selected as representatives of dry and wet years in 1999 and 2019. Vegetation indices were calculated (NDVI, TCI, VCI and VHI) using the ENVI 5/3 software. Because the area is located at different satellite passes 165 and 166, the Seamless Mosaic operator in ENVI 5.3 software was used to mosaic the images. Data with L1T level corrections (including systematic radiometric correction, geometric correction using ground control points, and topographic displacement error correction) were obtained from the USGS database. A 1: 25000 topographic map of the study area was used to control the geometric correction of satellite images. Atmospheric corrections of the images were performed by applying flash algorithm in the software and the images were prepared to extract the research indicators. Results:  Ready data was used to classify the map of NDVI, VCI, TCI and VHI indices in 1999 and 2019. Based on the obtained results, in 1999, the amount of poor vegetation was the highest with an area of ​​4869.7 square kilometers, equivalent to 52.49 percent of the area.The highest vegetation belonged to the good vegetation class with an area of ​​3311.44 square kilometers, equivalent to 69 Percentage of the area. The results of the VCI index show that the vegetation situation was good in 1999 and 4749.1 square kilometers of the area have experienced drought-free conditions. In addition, 4034.9, 111.3 and 83.5 square kilometers of the region had mild drought, severe drought and very severe drought, respectively. In 2019, according to the VCI index, 7531.8 square kilometers of the basin, i.e, 81.2% of the area, have vegetation-free conditions in terms of vegetation. According to the TCI results in 1999, limited parts of the western and southwestern parts of the basin with an area of ​​742.1 and 278.3 square kilometers had severe and very severe drought conditions, respectively. In 2019, in terms of heat, in 3005 square kilometers of the area in the northern, central and southeastern parts of the basin, no drought conditions were observed. According to the VHI results, in 1999, about 102 and 1159 square kilometers of the area had severe and moderate drought conditions, respectively, and no severe drought occurred in the area. In 2019, there was no severe drought. In general, the situation has been very good for vegetation Conclusion: The results showed that drought significantly reduced the density of vegetation in basin, and the existing vegetation was affected by drought stress. According to the VHI index, which indicates the health status of plant coverings affected by drought, two VCI and TCI index. In 1999, the eastern and western regions and the southwest had moderate to severe droughts, and most of regions had a mild drought. In 2019, due to the health of vegetation, in most areas, vegetation was under drought-free conditions and did not suffer from drought stress, but some parts also suffered from mild drought.