بررسی خشکی زایی و احتمال ایجاد کانون های ریزگردی در سواحل جنوب شرقی دریای خزر دوره زمانی 1995 لغایت 2022 (مقاله علمی وزارت علوم)
درجه علمی: نشریه علمی (وزارت علوم)
آرشیو
چکیده
دریای خزر بزرگ ترین دریاچه بسته جهان، در سال های اخیر با نوسانات شدیدی در سطح آب مواجه شده است که عمدتاً به دلیل تغییرات اقلیمی و بهره برداری بی رویه از منابع آبی است. این تغییرات به کاهش سطح آب و پسروی چشمگیر خط ساحلی، به ویژه در سواحل جنوب شرقی و گسترش مناطق خشک منجر شده است. این پژوهش روند خشکی زایی و احتمال شکل گیری کانون های گرد و غبار در این منطقه را بین سال های 1995 تا 2022 تحلیل می کند. برای این منظور، داده های ماهواره ای لندست و شاخص های پوشش گیاهی (NDVI) و پوشش آبی (NDWI) استفاده شد. تحلیل های آماری نشان می دهند که سطح آب دریای خزر از سال 1995 به طور مداوم کاهش یافته و به ویژه بین سال های 2020 تا 2022 به پایین ترین سطح خود در دوره 185ساله ثبت ابزاری تراز دریا رسیده است. نتایج نشان می دهد که این کاهش سطح آب باعث تغییرات عمده در پوشش گیاهی منطقه شده است. در مناطقی مانند خلیج گرگان، پیشروی پوشش گیاهی مانع از تشکیل کانون های گرد و غبار شده است، درحالی که در تالاب گمیشان به دلیل کمبود پوشش گیاهی، احتمال ایجاد کانون های ریزگرد بیشتر است. تحلیل ها همچنین نشان می دهد که از سال 2011 روند خشکی زایی تسریع یافته و پوشش گیاهی به ویژه در مناطق علفی و بوته ای افزایش درخور توجهی داشته است. به طورکلی، نتایج حاکی ازآن است که درصورت ادامه روند کاهش تراز آب دریای خزر، خشکی زایی در مناطق ساحلی افزایش خواهد یافت و مناطق بیشتری از تالاب گمیشان با خطر تبدیل شدن به کانون های ریزگرد روبه رو خواهند شد.Assessment of Shoreline Retreat and Dust Source Potential in the Southeastern Caspian Sea Coastline from 1995 to 2022
The Caspian Sea, the world's largest enclosed lake, has experienced significant fluctuations in water levels in recent years, primarily driven by climate change and unsustainable human exploitation of water resources. These changes have resulted in declining water levels, substantial coastal retreat—especially along the southeastern shores—and expansion of arid regions. This study examined the trends of desertification and the potential emergence of dust sources in the area from 1995 to 2022. For this analysis, we utilized Landsat satellite data along with vegetation (NDVI) and water (NDWI) indices. Statistical analyses revealed a consistent decline in the water level of the Caspian Sea since 1995, reaching its lowest point in the 185-year instrumental record, particularly between 2020 and 2022. The findings indicated that this decrease in water levels had led to significant changes in local vegetation. In regions, such as the Gorgan Gulf, the advance of vegetation had mitigated the formation of dust sources, whereas in Gomishan Wetland, insufficient vegetation cover had heightened the potential for dust source formation. Furthermore, analyses showed that since 2011, the trend of desertification had accelerated with notable increases in vegetation, particularly in herbaceous and shrub areas. Overall, the results suggested that if the downward trend in the water level of the Caspian Sea persists, desertification in coastal areas will intensify and more regions of Gomishan Wetland will face an increased risk of becoming dust sources. Keywords : Shoreline Retreat, Dust Sources, Caspian Water Level Fluctuations, Gomishan Wetland, Gorgan Bay. Introduction The Caspian Sea, the world’s largest enclosed lake, has undergone significant fluctuations in water levels in recent years, primarily due to climate change and unsustainable exploitation of water resources. Factors, such as reduced precipitation, rising temperatures, and excessive extraction from the Volga River—which supplies approximately 80% of the Caspian’s water—have exacerbated these fluctuations. As a result, water levels have declined, leading to substantial shoreline retreat, particularly along the southeastern coast, and expansion of arid zones. These changes have triggered severe environmental and ecological consequences, including accelerated desertification, shoreline erosion, and an increased risk of dust hotspots, especially in areas like Gorgan Bay and Gomishan Wetland. This study aimed to investigate the trends of desertification and the potential formation of dust sources along the southeastern coastline of the Caspian Sea from 1995 to 2022. Materials & Methods This study utilized satellite imagery from Landsat 5, 7, and 8, along with water level data from the Caspian Sea sourced from reputable institutions, including the Caspian National Studies Center, ECMWF, and DAHITI. The satellite imagery was processed using the Google Earth Engine platform, where vegetation (NDVI) and water (NDWI) indices were calculated to analyze shoreline changes and desertification trends. The NDVI was classified into 4 categories: water bodies, barren land, grassy-shrubby vegetation, and forests, while the NDWI was employed to delineate water-land boundaries. The coastal area was segmented into 6 zones based on slope and width to evaluate regions prone to desertification. To assess changes in water levels, the Standard Normal Homogeneity Test (SNHT) was applied to detect abrupt shifts and the Mann-Kendall test was utilized to confirm the significance of trends. The expansion of vegetation cover and the potential for dust hotspots in newly exposed lands were analyzed using statistical methods and 5-year interval maps. Research Findings The results indicated that the water level of the Caspian Sea had steadily declined since 1995, reaching its lowest point at -28.5 m in 185 years between 2020 and 2022. This decline had intensified from 2006, with a notable drop of 64 cm during the period from 2020 to 2022. The Standard Normal Homogeneity Test (SNHT) confirmed abrupt changes in water levels, showing reductions of 102 cm from 1995 to 2022 and 88 cm from 2005 to 2022, while the Mann-Kendall test validated a significant downward trend. These changes had resulted in shoreline retreat covering 460.51 km² with the most pronounced impacts in Gomishan Wetland (225.71 km²) and the western parts of Gorgan Bay (202.03 km²). NDVI analysis revealed that from 1995 to 2010, desertification and vegetation changes had been minimal with annual increases of 4.54 km² in barren land and 4.71 km² in grassy-shrubby vegetation. However, since 2011, desertification had accelerated, with grassy-shrubby vegetation expanding by 20.84 km² and barren land by 5.84 km² annually. In Gorgan Bay, particularly in its western regions and the Caspian-Gorgan intersection, rapid vegetation expansion—such as an 80.88 km² surge post-2013—had mitigated the formation of dust hotspots. Conversely, Gomishan Wetland, which had experienced a greater increase in barren land (70.769 km² post-2013) compared to vegetation (59.778 km²), demonstrated a higher potential for dust source formation. Statistical analyses confirmed that vegetation changes were significant and often abrupt in most zones, while changes in barren land were less statistically significant in areas, such as the northern shore of Gorgan Bay. Discussion of Results & Conclusion The decline in Caspian Sea water levels and the resulting shoreline retreat have significantly impacted southeastern coastal ecosystems. In Gorgan Bay, the rapid expansion of grassy-shrubby vegetation in newly exposed areas has reduced the risk of dust hotspots, challenging earlier predictions of dust source formation in this region. Conversely, in Gomishan Wetland, the prevalence of barren land over vegetation cover increases its vulnerability to becoming a dust source, posing a threat to surrounding areas. These differences can be attributed to variations in coastal slope, soil characteristics, and ecosystem resilience. If the decline in water levels continues, desertification is likely to intensify, particularly in Gomishan Wetland, thereby elevating the risk of dust source formation. This study highlighted the critical importance of restoring vegetation cover with resilient native species, such as grasses and shrubs, and implementing sustainable water management practices in the Caspian basin to prevent the emergence of dust hotspots. Continuous monitoring and protective measures are essential for maintaining the ecological balance of the Caspian coast.
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