عنوان مقاله [English]
About one quarter of world’s deserts are covered with quick sands, whereby, sand fields are the most common landforms. The movements of the sand fields are considered as a threat to the roads, natural resources, urban areas, agriculture and infrastructure.Factors such aspoverty of vegetation, increasing of drought due to global warming have led to the dynamic of sand fields with different speeds in manydirections that threat the transportation, health, economic and human activities. Thus, the spatial-temporal monitoring of sand fields dynamic behavior and identifying their directions of development are of great importancein the management of dry regions and conservation of natural resources. Therefore, the aim of this research is the Multi-temporal monitoring of sand field dynamic behavior in the west of Damghanplaya from 1972 to 2016, in the form of three 15-year period through data and remote sensing methods.
Materials and Methods
Damghanplaya Basin with an area of 18070.918 km2issituated between Toroud-ChahShirin Horst and the Alborz Mountains with an elevation of 2319 and 3884 meters respectively. Its general slope is towards the center of Damghan desert with an elevation of 1028m. Damghan playa is a tectonic-sedimentary hole, which is presently influenced by different geomorphic and climatic morphogenetic processes. Because of the vegetation and precipitation shortage,the wind morphogenetic systems dominate other processes. Thus, several types of wind erosion landforms can be observed in this region. The study area is the western erg of Damghanplaya with an area of 71.155 Km2 which is situated in Damghan Basin in the north of Iran’s great central desert. The region is located between latitudes 35° 51´ to 35° 58´ N and longitudes 54° 13´ to 54° 25´ E. This is an applied research and its methodology is a combination of remote sensing analyses. In this regard, topographic maps with a scale of 1: 25,000, geological maps with a scale of 1: 100,000 and Google Earth’s satellite images were used first to determine the position of the study area. Then, spatial database was completed through receiving Landsat satellite images during the period 1972 to 2016. Sinceseveral series of remote sensing satellite images belonging to multiple time periods are needed for monitoring the dynamic behavior of the sand field, four series of Landsat satellite images, MSS, TM , ETM+ and OLI sensors related to three 15 year periods of 1972,1987, 2002 and 2016 respectively, were used in this research. The aforementioned images were obtained from the Landsat satellite archive on the American geological organization website (http://earthexplorer.usgs.gov/). Then,color combinations, IHS transformation, and supervised classification of Maximum Likelihood methods were used to enhance the spatial area of the sand field, and the method of images difference and the calculation of the changing classes level were used to examine the type and trend of the changes..
Findings and Results
The results show that the maximum and minimum area of the sand filed are observed in 2002 and 2016 with an area of 92.2641 and 49.2803 km2 respectively. The results of change detection show that there are three types of changes including increasing, decreasing, and no-changes. As it can be observed,the maximum area of the classes of change belongs to the no change class that the periods of 1972 to 1987 and 2002 to 2016 with the amounts of 58.3506 and 48.2841 km2 respectively,have the highest and lowest areas, while, the minimum area of the classes of change belongs to the class of incremental changes that the periods of 1987 to 2002 and 2002 to 2016 have the highest and lowest areas with the amounts of 38.2833 and 1.0359 km2 respectively. The maximum and minimum areas of decreasing class of changes belong to the periods of 2002 to 2016 and 1987 to 2002 with the amounts of 43.9829 and 14.2693 km2 respectively. In this regard, the no-change and increasing change classes with the standard deviation of 5.0445 and 19.4699 respectively, have the minimum and maximum range of changes during the entire period of 44 years.
The results obtained fromstudying thetemporal trend of changes indicate the existence of a decreasing trend in the no-change and increasing change classes, and also the existence of an increasing trend in the class of decreasing changes.Descending trend of no-change class is uniform and continuous. In contrast, the trend ofincreasing and decreasing classes of changehas a periodic jump in the second time period (1987-2002), but their overall trend is almost uniform.
Discussion and Conclusion
Western erg of Damghanplaya has decreased by approximately 6.7225 km2 in 1987 compared to 1972. Most of this reduction has occurred in the southwestern and eastern parts of the sand field. The southwestern contraction of the erg is in accordance with the pediment and the sand harvesting area, the causes ofwhich are the sand transfer by local winds blowing from the southwest to the northeast, as well as the formation of the desert pavementfacies. In contrast, the eastern contraction of the erg is due to the increase in moisture content from the Haj Aligholiplaya and the increase in humidity caused by agricultural lands adjacent to the erg. In the second period, the trend was completely reversed and the sand field was expanded in 2002 by approximately 17.3659 and 24.0885 km2in 2002 compared to the years 1972 and 1987 respectively. This period is considered to be the most risky periods in terms of environmental hazards. In this period, major spatial expansion of the erg has taken place to the east and especially to the northeast. This expansion can be due to the increased drought severity and the continuation of dry periods and the release ofthe agricultural lands in some cases. In the third period, the situation has improved and the dynamic of sand has reduced, so that the extent of sand field has decreased in 2016 by 25.6178, 18.8952 and 42.9837 km2compared to the years 1972, 1987 and 2002 respectively, which represents the negative balance in the erg. In other words, the amount of the sand entering the erg is far less than that of the sand going out. In terms of location, the contraction of this period on the margins of the ergextends continuously and almost uniformly, but the largest contraction isobserved in the eastern, northeastern and southwestern parts. This decrease is due to the implementation of desert greening plans in the form of quick sands stabilization projects by planting Haloxylon. This indicates the positive and successfulfunction and role of desert greening projects. Also, due to the favorable natural and climatic conditions, the species of Haloxylon has been able to regenerate naturally in the area under cultivation. This has had a positive impact on the stabilization of quick sands and the reduction of erg changes.