Monitoring Changes in Sand Dunes by Analyzing Satellite Images and Wind Station Data (Case Study: Zahak Erg Region in the East of Sistan Plain)

Aeolian sand transport is a complex process influenced by many variables including wind conditions (Lancaster, 1985; Anderson & Haff, 1988; Gillette et al., 2001; Zou et al., 2001; Liu et al., 2005), grain size and sand surface moisture (Jackson, 1998; Wiggs et al., 2004), surface crusting (Leys & Eldridge, 1991), topography (Iversen & Rasmussen 1994; Hesp et al., 2005), and vegetation cover (Buckley, 1987; Kuriyama et al., 2005). The importance of sand dunes studies is due to their impacts on water and soil resources, flora and fauna, human infrastructure, and roads. Sand drifting can lead to losing agricultural lands, burying residential buildings, railways, highways, and other infrastructures in many areas of the desert (Zhu et al., 1980; Lei et al., 2003; Dong, 2004). Bagnlod (1981) has done the first study on the movement of sand dunes. In recent years, many studies have been done on the cognition of sand dune processes, which include the study of the winds and sands migration in different dunes. Needless to say that significant signs of progress have been obtained. In this regard, there is no doubt that the remote sensing technique and its capabilities, as well as the optimal time sequence of satellite imagery in mapping erg areas, have fundamental performances. Using these images, a substantial area of sand dunes can be examined in a short period of time, and then we can talk about the identification of active dunes, their expansion, and relocation.

To study the changes and migration of sand dunes, this study was conducted in two stages. At the first stage, to evaluate changes in the range of sand dunes in the study area, Landsat images were used for the years 2001 and 2019. Moreover, to detect the changes in the Zahak Erg range the ENVI software was used. The results of this section can be important in the overall assessment of the area. Additionally, in order to detect the trend of changes in those parts where significant shifting occurred, we used either Landsat or the Google Earth images with different time intervals. It worth mentioning that for geo-referencing the Google Earth images of recent years (2006-2018) the Stitch Map Software was used. Moreover, to draw the sand rose, the Sand Rose software was used. Sand rose is the Graph of Portable sand by wind energy, which was used by Fryberger and Dean (1979) for the first time. In order to draw this Graph, winds that were faster than the erosion velocity threshold were developed into vector units as sand drift potential.

Mobility is the most important characteristic of sand dunes. In this regard, special attention should be paid to the importance of wind in changing the sand dunes morphology. In addition, studies of sand dunes migration provide basic knowledge about wind processes and sand transfer values. In this study, monitoring and detection of the sand dunes relocation rate have been considered based on the use of Landsat images (for the years 2001-2019). At the first stage, the regions that have been faced changes were identified using the change detection technique for the entire range of Erg. Based on the output map, marked areas with the red color experienced the maximum changes, and also, in this section, sand dunes have been more developed. The differences between the two images within the sand dunes area were estimated at about 23 km2. However, the pattern of sand dunes migration represents relocation from the Northwest to the Southeast. At the next stage of the study, to understand the change rate and relocation trend, Google Earth images were used (years 2006 and 2018). Furthermore, the region with significant changes has been selected for a closer look. Moreover, using Stitch Map software, the georeferenced images were extracted. It worth mentioning that, from the five selected zones on the Erg surface, a total of 368 cases of sand dunes were quantitatively analyzed. According to the results, the greatest amount of migration was found for zone 3 with the amount of 24.72 m, in contrast, the lowest value was calculated at about 14.16 m for zone 5. In addition, the migration average rate for a period of 12 years was calculated. Here, the maximum migration belonged to zone 3 which is 2.06 meters per year. While the minimum one allocated to zone 5 was 1.18 meters per year.

In this study, the authors monitored changes in sand dunes using satellite imagery data and wind data of meteorological stations of Zahak Erg. In this regard, the sand dunes activity rate, dunes migration pattern, and also factors affecting their intensity and patterns were identified. The results of the 368 studied sand dunes in different parts of the erg surface indicated displacement with a northwest-southeast trend, which is consistent with the results of data analysis for wind pattern. But the average amount of displacement of 1.53 meters per year for a period of 12 years (2006 to 2018) with the results of data from Zabol wind station showed the amount of sand carrying more than 300 tons per meter for a year. The changes in wind speed at this station were inconsistent and it was shown that the maximum value obtained was before the stabilization of the active sand dunes. In addition, smooth sands move on the surface of the earth, independent of sand dunes, which cannot be measured by images.