تحلیل تغییرات مکانی-زمانی لندفرم ها و کاربری اراضی در بیابان‌زایی دشت یزد-اردکان با استفاده از الگوریتم حداکثر احتمال

بررسی و تحلیل اطلاعات مکانی و زمانی لندفرم‌ها و کاربری اراضی با استفاده از داده‌های چند زمانه از جمله مباحثی است که در مطالعات بیابان‌زایی و تحلیل روند تغییرات آن حائز اهمیت است. پژوهش حاضر با استفاده از تکنولوژی سنجش‌ازدور، به شناسایی و تحلیل تغییرات لندفرم‌های ژئومورفولوژی و کاربری اراضی در بخشی از دشت یزداردکان پرداخته است. بدین منظور از تصاویر سنجنده های tm و oli ماهواره landsat مربوط به سال‌های 1987 و 2016، در بازۀ زمانی 30 ساله استفاده شد. ابتدا تصحیحات لازم بر روی تصاویر انجام گردید و سپس بر اساس نقشه‌های توپوگرافی، ژئومورفولوژی و تصاویر گوگل ارث، لندفرم‌های ژئومورفولوژی و طبقات کاربری اراضی منطقه مورد مطالعه تعریف و نمونه‌های آموزشی انتخاب گردید. سپس تصاویر با الگوریتم حداکثر احتمال طبقه‌بندی و نقشه لندفرم‌ها و کاربری اراضی تهیه شد. دقت تصاویر طبقه‌بندی نیز با ضریب کاپا و دقت کلی محاسبه شد. درنهایت به‌منظور شناخت وضعیت تغییرات لندفرم‌ها از روش آشکارسازی تغییرات پس از طبقه‌بندی استفاده شد. نتایج نشان داد که لندفرم‌های موجود بروی دشت سر پوشیده به‌جز تپه‌های ماسه‌ای مواج دارای روند نزولی تا سال 2016 بودند و این در حالی است که مناطق شهری و اراضی زراعی و باغات و جنگل‌های دست کاشت که بروی دشت سر پوشیده بودند، طبق تغییرات کاربری اراضی دارای روند صعودی بوده‌اند؛ و به‌طورکلی این نتایج نمایانگر آن است که در محدوده مورد مطالعه در طی 30 سال، نه‌تنها بر وسعت اراضی بیابانی افزوده نشده است، بلکه از محدوده اراضی بیابانی کاسته شده و بر دیگر کاربری‌ها افزوده شده است. همچنین در شرق و غرب منطقه مرز لندفرم‌ها تغییر کرده است.

Analyzing the Spatialtemporal Changes of Landforms and LandUse in the Desertification Process of YazdArdakan plain Using Maximum Likelihood Algorithm

1. Introduction Desertification is associated with the process of destroying and demolishing the natural ecosystems in ultraarid, arid, semihumid arid regions, leading to reduced biomass production and the appearance of soil degradation or erosion effects. More than 80% of Iran is located in ultraarid, arid, semihumid arid regions of the country with fragile ecological conditions and are influenced by desertification phenomenon. In order to study the desertification and to analyze the trend of its changes, a wide range of spatial and temporal information such as geomorphology and landuse are needed. In fact, analyzing their changes is the process of identifying differences in the status of an object or a phenomenon by observing it at different times, which provides a basis for better understanding of human relationships and interactions with natural phenomena for managing and making better use of resources. In environmental management, the most common contributions of geomorphologists are to provide a map of landscapes with emphasis on its selected features, to understand the nature and causes of landscape changes, and to organize them according to trend predictions during the formation processes and the transformation of landforms. Human 's specific use of territory as land use is also an important consideration in land tenure assessment. These applications change at time intervals. In arid and semiarid regions, these changes usually lead to increased desertification. The identification of deserts ' landforms and landuse, their classification and mapping at various time intervals are considered as a method for change detection in these areas. Today, using remote sensing (RS) instead of the visual interpretation to identify landforms and land use and their change detection is a necessity. One of the steps in identification of change detection is the classification of remote sensing data. There are various methods for the classification of satellite images. These methods can be used for separation of different geomorphological landforms and landuses and their variations. Each method has its own advantages and limitations. The choice of the research method in geomorphology depends on the purpose of the study and the available data. Today, the use of satellite data and remote sensing techniques are considered as a modern method in geomorphological studies. The most common classification methods can be referred to as the maximum likelihood classification. Other classification methods such as the minimum distance, Mahalanobis distance, and neural network have attracted much attention. The main objective of this study is to investigate and analyze the temporal variations of geomorphologic landforms and landuse in desertification part of YazdArdakan Plain. For this purpose, time series satellite observations of Landsat sensor were used from 1986 to 2016 (over 30 years). 2. Materials and Methods In this research, a part of the ArdakaanYazd basin is considered for desertification analysis due to the issues and problems related to land destruction and the critical focus of wind erosion. This district, according to the latest political divisions, includes central zone and Khezerabad zone of Sadoogh county in Yazd. The area of the study area is 1563.11 km2. The research method in this study is analytical survey. To study the changes of desert 's landforms and land uses, two satellite imageries of TM and OLI of Landsat satellite of 1987 and 2016 were used. First, the radiometric and atmospheric corrections was performed using Flaash algorithm, and then the geomorphological landforms and land use were introduced and the training samples were selected by field observations, topography and geomorphology maps, and Google Earth images. To classify the landforms and land use, the supervised classification of maximum likelihood algorithm was used. Then, the accuracy of classified maps was evaluated using the overall accuracy and the Kappa coefficient metrics. Finally, to evaluate the changes of landforms and landuses, the post classification method was used. To analyze the database, ENVI 5.3, ArcGIS 10.4.1 and Excel 2013 software were used. 3. Results and Discussion In the present study, 15 landforms were identified in the study area with regard to field observations, available resources and geomorphologic map of YazdArdakan plain. Totally, 15 landforms were identified in the study area, including Alluvial Fan, Glacis Pediment Plain, Clay Pan, Glacis Dennoyage Plain, Inselberg, Glacis Epandage Plain, Kalut, Erg (barchans, longitudinal dunes, barchanoid), hills, mountains, Salt Dome, Sebkha and Sand Sheet. In addition, 8 landuse classes were identified in the study area. After the image correction, the geomorphological and land use maps were prepared. Finally, to investigate the nature of the changes, the comparison method of post classification was used, which was applied on maximum likelihood algorithm. Then, the changes of landforms were calculated in terms of its area and percentage. The results showed that the dominant class was the Glacis Epandage Plain in the both satellite images of 1987 and 2016. Then, hills with 17/58 percent of the total area are ranked as the highest area in 1987, while this class had downward trend with 11/58 percent in 2016. In 1987, Barkhan class had the lowest area with 0/17 percent, whereas this class had downward trend with 0/11 percent in 2016. Changes in land use classes over a 30year period in terms of area and percentage indicate that the predominant class in 1987 was the lowdensity rangelands of the range of 255%, which, remains the prevailing class in this year even with the decrease in the area of this class in 2016. The saline lands with 22312.63 hectares of the entire region are of the next order of the most area in 1987 but have the decreasing process to 1.14% in 2016. In 1987, the urban class occupied 0.46% of study area with a growth rate of 2.33% by 2016. The construction of forested trees for deforestation also shows a growth rate of 15.36% over 30 years. 4. Conclusions The results of the landforms and land use classification showed that the maximum likelihood algorithm offers a detailed classification method. The area and the percentage of landforms and land use changes over 30 years showed that landforms such as Barchan, Clay Pan, Longitudinal Dunes, Barchanoid and Kalut had a downward trend, because they were located in the context of the development of the city. The results showed that Sabkha area has followed a downward trend over 30 years. The comparison of our results with the results of previous studies showed that the increase of wells in YazdArdakan plain has helped cultivate the large areas of saline lands. Therefore, the natural and human factors were involved in changing the desert landform and land use in the study area. It should be noted that this study shows that the separation and analysis of the landform variations, such as the Salt Dome and Inselberg created in long time intervals, cannot be easily accomplished using the maximum likelihood algorithm. Because they were of ascendant trend during 30 years according to the outcomes, and this is because of similarity of their adjacent spectral reflections in which case the pixels of these landforms have been replaced by more pixels. However, according to the results, separating and analyzing the changes of the sandy hills is possible with the maximum similarity algorithm method during 30 years (from 1986 to 2016). In general, the results of this research were in the line of the previous studies that confirm above statement.