پایش تاثیر خشکسالی بر تغییرات پوشش گیاهی با استفاده از تصاویر ماهواره‌ای (مطالعه موردی: حوزه آبخیز ایلام)

خشک‌سالی رخدادی طبیعی است که تاثیر قابل توجهی در کشاورزی، اقتصاد و در نتیجه ابعاد مختلف زندگی مردم در مناطق خشک و نیمه‌خشک دارد. پوشش گیاهی یکی از اجزای زندۀ اکوسیستم است و نقش مهمی در بسیاری از فرایندهای اکوسیستمی دارد که به‌شدت تحت‌تاثیر رخدادهای اقلیمی، از جمله خشک‌سالی، است. در پژوهش حاضر، وضعیت تغییرات پوشش گیاهی در رابطه با شاخص‌ خشک‌سالی در محدودۀ حوضۀ آبخیز ایلام (شهرستان ایلام) بررسی شد. در این تحقیق، از آمار 30‌سالۀ (13671397) بارش ایستگاه‌های سینوپتیک (شهرستان‌های ایلام، مهران، دهلران، سرابله، ایوان، دره‌شهر و آبدانان) برای محاسبۀ شاخص بارش استاندارد (spi) و از داده‌های تصاویر ماهواره ای لندست (سنجنده‌های oli، etm و tm) برای تهیۀ شاخص تفاضل پوشش گیاهی استاندارد (ndvi) و بررسی تغییرات پوشش گیاهی منطقۀ مورد مطالعه، استفاده شد. واکاوی ارتباط بین شاخص خشک‌سالی (spi) و شاخص پوشش گیاهی (ndvi) با استفاده از روش هم‌بستگی پیرسون بین لایۀ رستری پوشش گیاهی و خشک‌سالی انجام شد. نتایج نشان داد که بر اساس شاخص spi محاسبه‌شده، در سال‌های 1382و 1387 پدیدۀ خشک‌سالی با شدت های بیشتری نسبت به سایر دوره های مورد بررسی (13671397) اتفاق افتاده است. نقشۀ دسته‌بندی پوشش گیاهی حاصل از شاخص ndvi در طول سال‌های 1367 تا 1397 بیانگر کاهش سطح پوشش گیاهی از گذشته تا حال بوده، به‌طوری که این کاهش بیشتر در دستۀ پوشش گیاهی متراکم (از 15.959 هکتار در سال 1367 به 6492 هکتار در سال 1397) رخ داده است. بر اساس یافته‌های تحقیق حاضر، نتیجه گیری می‌شود که تغییرات پوشش گیاهی در طول زمان با شدت خشک‌سالی ارتباط مستقیمی دارد که مدیران و تصمیم‌گیران عرصۀ منابع طبیعی، به‌منظور مدیریت پوشش گیاهی، باید به این موضوع مهم توجه کنند.

Monitoring the Drought Effects on Vegetation Changes using Satellite Imagery (Case Study: Ilam Catchment)

Drought is a natural phenomenon that has a significant impact on agriculture and also influences different aspects of people’s lives in both arid and semiarid regions. Vegetation cover is one of the living components of the ecosystem and plays an important role in many ecosystem processes that are strongly affected by climatic events such as drought. In the present study, the status of vegetation changes in relation to drought index has been investigated in the Ilam catchment (Ilam city). In this research, the 30year precipitation of synoptic stations of Ilam, Mehran, Dehloran, Sarableh, Ivan, Darrehshahr and Abdanan were applied to calculate the standardized precipitation index (SPI). The Landsat satellite images were used to extract the standard vegetation difference index (NDVI) and also to investigate the detection of vegetation changes in the study area. Besides, the relationship between the SPI drought index and NDVI vegetation index was performed by the Pearson correlation method between raster layers of NDVI and SPI. The results of the SPI index showed that the drought in the years 2003 and 2008 occurred with more intensity than other years. Moreover, the vegetation classification map obtained from the NDVI index showed a decreased trend in the level of vegetation from 1988 to 2018 which is occurred mostly in the dense vegetation category (15959 ha in 1988 and 6492 ha in 2018). Based on the results of the present study, it is concluded that the changes in vegetation over time is directly related to the severity of drought, which should be considered by managers and decisionmakers in the natural resources for vegetation management.Extended Abstract1IntroductionDrought is one of the factors that destroys natural ecosystems such as deforestation, desertification, and rangeland destruction. One of the predictions of droughtrelated climate change is to affect species extinction and change vegetation productions. Drought can alter plant natural conditions for survival, reducing overall plant populations and ecosystem productivity, and even threatening regional biodiversity. Since Ilam province is located in the arid and semiarid region in Iran; therefore, it is very important to be aware of the climatic situation, its changes and also its effects on vegetation. Hence, the main purpose of this study is to investigate the changes in vegetation in relation to drought indicators in Ilam catchment. 2Materials and MethodsIn this research, the study area is Ilam catchment which is located in Ilam province. This catchment is one of the vast watersheds of the province with an area of about 11,800 hectares. Within this catchment, the cities of Ilam and Chavar as well as rural centers are located with a total population of about 280,000 people. The average rainfall and temperature of the study area are about 575.5 mm and 16.7 ° C, respectively, so the study area is defined as a semiarid climate according to the de Martonne climatic classification (de Martonne, 1926). In the present study, the standard precipitation index (SPI) was used to investigate the occurrence and severity of climatic drought. For this purpose, the precipitation data from seven synoptic stations of Ilam, Ivan, Sarableh, Dehloran, Darhshahr, Abdanan, and Mehran were used. The SPI index is a normalized value with a mean of zero and a standard deviation of one. Positive SPI values ​​indicate wetness greater than the average precipitation, but, negative values ​​show dry conditions less than average precipitation. Furthermore, Landsat satellite data were used to study the changes in coverage in the study period (19882018) and its relationship with the standard precipitation index. After extracting the NDVI map, vegetation classification methods were used to separate different vegetation classes. Then, the trend of their changes during the study period was investigated. The NDVI index values ​​are in the range of 1 and +1, which tends to be one for dense vegetation. To achieve the research objectives, a Rasterbased regression analysis was used to obtain the regression relationship between SPI and NDVI indices.3 Results and DiscussionThe results showed that in 2013 and 2018, the value of the SPI index for most of the studied stations indicates a negative number showing the occurrence of drought stress in these areas. On the other hand, in 2019 and 2014, the value of the SPI index in all studied stations was positive, which indicates more rainfall than the longterm rainfall of each station. The classification of the NDVI index during the given time shows a decrease in the level of vegetation from the past to the present occurred mostly in the dense vegetation category. The correlation results between NDVI and SPI indices were positive for all five years. In all studied years, the SPI index with NDVI index had an almost high correlation coefficient, so the highest correlation was in 2008 with a correlation coefficient of 0.48 and the lowest correlation was in 1988 with a correlation coefficient of 0.36. Therefore, it seems that the above results are sufficient to monitor the drought situation in Ilam province.4 ConclusionThis study aimed to investigate the trend of vegetation change and climatic drought and to find the relationship between these changes. The study of drought periods with SPI index in the study area indicates the increase of droughts and their frequency. The findings from the NDVI vegetation index show a decrease in vegetation level, especially in the dense cover of the region. On the other hand, there was a significant relationship (α <0.05) between drought and vegetation index. In other words, with increasing drought (reduction of available plant moisture), plant growth and production will be decreased. Similar research is carried out in this area. For example, Baagideh et al. (2011) showed that the quarterly SPI index had the highest correlation with vegetation changes using the NDVI index and chose this index as the basis of their calculations. Therefore, vegetation management should be considered by managers and decisionmakers in the field of natural resources, especially in periods of drought.