Drought Analysis of Alvand Boundary River Using Remote Sensing Data

Extended abstract 1- Introduction       The study of the behavior of rivers in the arid and dry areas is one of the most important tasks in the country. Because the area has increased the effects of drought due to the sensitivity of the area and rainfall shortage, it causes changes in the flow and sediment regime, water resources, agriculture, and so on. Since plants react more precisely to the specific climate changes of the environmental water changes, historically, they have been regarded as important indicators to identify the type of climate and its changes. In the recent years, as the satellite images are regularly and accurately separated from the surface, the growing role of vegetation cover associated with drought conditions is more assumed. The NDVI index was first introduced in 1973 and is the most comprehensive indicator ever used in many studies. This index can well reflect changes in the areas with more rangeland coverage. Also, given that the water has a lower NDVI than other surface coatings, therefore, the areas that are covered with the water can be detected by the changes in their NDVI values before and after the flood. 2- Methodology First, in order to identify the trends and droughts in the region, the hydrometric and climatic data (1955-2011) were analyzed. Based on the annual flow diagram of the Qasr-e-Shirin station, the diagrams of rainfall and precipitation were identified using the 45-year moving average index in the river to identify the maximum points of the land as well as the wet points, and subsequently, the data from the satellite imagery of 1987 (first year of landing), the maximum drought of 2005, the maximum wetland of 2003, and those of 2015 were used to collect the satellite images from the hydrometric chart. Subsequently, multispectral images were taken to produce a green index. Using the controlled classification method and the statistical parameter, the least space was used to classify the images. Finally, using the observations, measurements and field observations, with the accuracy of 90 meters at 5 points around the river, the findings of the extracted images were corrected and controlled. 3- Results          An investigation of the hydrologic regime of the basin in the drought and wet periods indicates changes in the river flow and the impact of the climate and environmental factors on it. Despite the continuity of the river flow and the same rainfall (264 mm) during the two periods, the average of the mean and the basin water levels decreased sharply and the flow of the river at the location of the river divisions has changed from a volatile to a seasonal, which has led to the blockage of the river by the sediment yields from them. Using the NDVI index, land use has been classified into cropping water, rain, and pastures. A comparison of the classified images shows that 70% of the vegetation degradation is related to the rangelands and marginal lands of the river and river basins, which are natural in the region. The remaining 30 percent of the vegetation degradation is related to aquaculture and dry-land farming. The research findings from the discovery of changes through the illustrations reveal the role of the climate change in the Alvand river basin. 10188 ha of the damaged land is due to the drought. In other areas, and especially around the river, the changes in the coverage elucidate a negative trend. In the rivers of Khosravi plain, 8001 ha of the pastures around the river is destroyed. 4- Discussion & Conclusions The results of this study showed that the vegetation level in the region has changed during the research period throughout the studied years. The NDVI index showed a desirable correlation with the moving average drought index Therefore, it is suggested that the method described, that is, the combination of the climatic and hydrological elements and the vegetation indices derived from the satellite images, which have been investigated and proved by various researchers, have been repeatedly correlated with the drought to investigate it by the vegetation index in the rangeland and semi-arid areas of the country. The results of this study showed that the NDVI index could be a favorable alternative to moving average, and in the hydrological drought studies, the semi-arid and rangeland areas can be trusted.