Investigating the Effect of Vegetation on the Occurrence of Dust Phenomenon (Case Study: Hormozgan Province)

Vegetation is the first and the most important producer of any ecosystem and reflects many factors in the ecosystem, so by studying the relationship between its changes and other factors such as dust, the interaction of factors can be understood. The purpose of this study was to investigate the effect of vegetation cover and its relationship with the occurrence of optical depth of air (AOD) caused by dust events in Hormozgan province during the study period of 2000 to 2020 by using NDVI, SAVI vegetation indexes and other climatic variables.

The present research investigated the relationship between dust and vegetation. For this purpose, aerosol optical depth (AOD), which is one of the most widely-used indicators for analysis and monitoring of suspended particles in the atmosphere, was used. To determine the dust areas, the AOD index was normalized; then the numerical value of AOD> 0.5 was considered as the high amount of dust. For studying the trend of AOD change, by coding in the Google Earth Engine environment(GEE), the image of MODIS dust products from 2000 to 2020 (20-year period) was extracted and subsequently the dust time series diagram was prepared. According to this chart, the month and year that had the highest level of dust during this period were determined. The NDVI and SAVI indexes during the years (2000-2020) were coded for an average of three months (June, July and August) in the Google Earth engine environment and all satellite images from Landsat 5, 7 and 8 sensors were extracted with a resolution of 30 meters. Then the correlation between vegetation indices and climatic variables with AOD index were calculated.

The results showed that June, July and August had the highest AOD index during the study period and the lowest level was in December of each year. The largest AOD occurred in July during the study period. The most severe case occurred in July 2018 with a magnitude of 0.69 and the lowest case was in December 2000. The charts illustrated that the largest level of dusts occurred in 2003, 2011 and 2018 and the lowest levels occurred in 2000, 2002 and 2017 in Hormozgan province. The results showed that, among the climatic indicators, evapotranspiration, minimum temperature and wind speed had the highest correlation with AOD index. Also, from 2000 to 2020, the rate of evapotranspiration and temperature increased with a slight slope, and due to the direct relationship between these factors and the AOD index, the AOD index increased during the period. The results of correlation between SAVI and AOD showed that there was a slight or weak relationship between them and the correlation index between NDVI and AOD showed a moderate to strong relationship.

The results of the study indicated that the changes in the AOD index in the twenty-year period (2000-2000) have no significant trend and have been mostly influenced by climatic factors and vegetation density in Hormozgan province. Also, the results of studying the spatial pattern of AOD in the province showed that the distribution of AOD in the south to south-west is more than the south-eastern regions of the province. The results showed that the condition of surface vegetation was significantly related to the amount of AOD and their levels increased with decreasing vegetation. Studies have shown that in areas of the province where vegetation is sparse and barren compared to other parts of the province more AOD exist. Also in years when vegetation is less dense due to reduced rainfall, the intensity of dust has increased in these years. The results showed that 2018, 2008 and 2003 years showed the dustiest days and 2000 and 2017 showed the least dusty days per year. According to the results, the highest amount of AOD occurred in the south and southwest of Hormozgan province, which matched the results of NDVI index in these areas according to which vegetation is in the class of 0.1-0 (without cover). This confirms the results of the relationship between dust and vegetation.