Expansion and Contraction of Iranian Ergs (Case Study: Rig Zarrin in Central Iran)

The ergs located inside the topographic holes are considered as static ergs. Nevertheless, the conditions for the development and expansion of these ergs are suitable in most of the arid regions of central Iran. The most important of these conditions are low rainfall, dry weather, severe regional and local winds, and the availability of a large amount of fine-grained sedimentary materials. Time series analysis of Landsat satellite images during 1977–2017 showed that Rig Zarrin has been developed both in terms of surface (about 40 km2) and height (about 4 meters in the middle sections). The results showed that the horizontal and height development of this erg was intermittent. Topography and more importantly, moisture, the two main factors which control the activity of sand dunes. Since 1998, long-term drought has dominated central Iran and Yazd province. Examination of the images during this time period showed a large expansion of sand surface from 2000 to 2017. But, the level of sand has declined dramatically between 1987 and 1993 due to wet period of the province. Therefore, the area of Rig Zarrin is expanded and contracted by the influence of yearly or periodic moisture and drought.  Also, in terms of height development, moisture has been the main controlling factor. So, in droughts, the height of the erg sand dunes has significantly increased and in wet years it decreases or reaches a low or minimal level. The existence of complex sand dunes with the height of more than 200 meters in the central parts of the erg shows that the erg becomes the center of thermal low pressure, especially in the warm season, and causes the formation of local convergent winds. Seasonal change in the direction and intensity of the wind has changed the direction and amount of movement of sand dunes, and the change in the displacement patterns of pressure has led to a change in the pattern of migration of the erg.