تعیین خطر بیابان‌زایی راهکار مناسبی جهت اولویت‌بندی یک محدوده برای مدیریت و کنترل بیابان‌زایی است. یک عامل تعیین کننده خطر بیابان‌زایی، استفاده از شاخص احتمال آسیب پذیری پوشش گیاهی (PVVI) است. جهت انجام این کار، در این تحقیق از شاخص‌های LST و شاخص EVI به ترتیب از محصولات MOD11A2 و MOD13A2 سنجنده MODIS جهت برآورد شاخص‌های‌ TCI و VCI به منظور برآورد شاخص VHI در استان یزد و در بازه زمانی 2001 تا 2019 استفاده شد. شاخص VHI که شدت خشکسالی پوشش گیاهی را نشان می‌دهد در پنج کلاس طبقه‌بندی شد. سپس درصد احتمال وقوع هریک از طبقات محاسبه و در وزن هر طبقه که بر اساس شدت خشکسالی عددی بین صفر تا 4 بود ضرب شد و در نهایت با جمع مقادیر به دست آمده برای هر طبقه، شاخص احتمال آسیب پذیری پوشش گیاهی محاسبه گردید. نتایج نشان داد که به طور کلی در مناطق غربی، شرقی و بخش‌های جنوبی استان یزد احتمال خطر تخریب پوشش گیاهی و در نتیجه بیابان‌زایی بیشتر از مناطق دیگر است. بیشترین احتمال وقوع کلاس بدون خشکسالی مربوط به شهرستان ابرکوه (34/68=VHI) و کمترین مقدار آن مربوط به شهرستان اردکان (59/53=VHI) است. شهرستان ابرکوه با 03/14 درصد کمترین و شهرستان اردکان با 02/46 درصد بیشرین مساحت را در کلاس زیاد شاخص PVVI را دارند. همچنین نواحی کویری و اراضی بدون پوشش مانند کفه ابرکوه در کلاس خطر بیابان‌زایی کم قرار گرفت که دلیل آن می‌تواند عدم توانایی اکولوژیکی این منطقه در احیا شدن باشد. به طور کلی ارزیابی نتایج به دست آمده در این پژوهش نشان داد که شاخص PVVI، توانایی این را دارد که بیابان‌های واقعی را ازز مناطقی که دارای احتمال خطر بیابانزایی هستند، تفکیک کند. پرونده مقاله

Desertification is one of the main ecological and environmental problems in Iran. It is remarkably urgent to monitor and assess this phenomenon and its impact on ecosystem components such as vegetation covers to prevent and combat desertification. This study aimed to evaluate the Change Vector Analysis (CVA) method’s efficiency in assessing and monitoring desertification in Dehshir Plain, Yazd province, central Iran. For this purpose, the imagery of Landsat 5 from 1994 to 1998 (T1) and Landsat 8 from 2014 to 2018 (T2) were selected in June and July period on the Google Earth Engine platform. Then, the Bare Soil Index (BSI) and Normalized Difference Vegetation Index (NDVI) were calculated for the two periods. The direction and the strength of changes in NDVI and BSI were determined based on the CVA method. The results showed that 27.27% and 38.65% of the region surfaces were in degraded and rehabilitated states, respectively. The severity of degradation was higher in the central and northern parts of the study area. Adversely, rehabilitation has increased in the south, west, and east parts of the region. The results of CVA and field reality showed that this method could well reveal changes in ecosystem components over time, which is due to the comparison of two different periods using land cover indicators such as plants and soil. Our finding suggested that the CVA was an appropriate method for monitoring and assessing the desertification phenomenon and determining the area under degradation or rehabilitation in arid regions. پرونده مقاله

Vegetation cover is one of the living components of terrestrial ecosystems and plays an important role in many ecosystem processes that are strongly influenced by climatic events. Thus, meteorological droughts can significantly affect the vegetation cover, especially in arid and semi-arid regions where vegetation is more sensitive to environmental conditions. This study was conducted with the aim of analyzing the effects of meteorological droughts on the vegetation cover in different Land Use Land Cover (LULC) types in Khorasan Razavi province, Iran. For this purpose, the correlation and Linear Regression (LR) between Standardized Vegetation Index (SVI) and meteorological drought indices including Precipitation Evapotranspiration Index (SPEI) and Standardized Precipitation Index (SPI), with 3, 6 and 12-month time scales, were investigated for the period of 2001-2020. Based on the results, it was found that SVI values were negative in the years 2001, 2006, 2008, 2011, 2014 and 2015, in all LULC types, while in 2010, moderate rangeland experienced the most severe drought. The decreasing trend of SVI (increasing vegetation drought) was mostly observed in the southern parts of the province. The correlation between SVI and 6-month SPEI occupied a wider area than the other time scales (23.07%). The highest correlation between SVI and 12-month SPI was distinguished in dense forest, sparse forest, and poor rangeland, and occupied a wider area across the province (24.08%). Moreover, the highest (1.13) and lowest (0.75) changes in the regression coefficient of variations of SVI with multitemporal SPEI and SPI were belonged to moderate forest and agricultural land, respectively. Based on the results of this study, SPEI and SPI showed completely different values in various LULC types. Therefore, any types of indicators should be separately considered to study the terrestrial ecosystems in order to better identify areas affected by meteorological drought. پرونده مقاله