عنوان مقاله [English]
نویسندگان [English]چکیده [English]
The occurrence of dust storms is a common phenomenon in arid and semi-arid areas. Dust storm is related to erosion, transport and displacement and sedimentation of dust and fine-grained soil particles which is influenced by the activity of strong wind. Since the occurrence of severe storms can cause changes in the earth's surface, studies on changes in energy balance, earth's surface temperature, vegetation and spectral changes in land surfaces during such storms can be useful in recognition of the effects of storms on the ground. The spectral reflection and thermal remote sensing data are very useful in the reconstruction and modeling of land surface parameters that are not simply visible and measurable or studied on a large scale. This study examines changes in the quantity of surface albedo, surface radiation power, surface temperature, vegetation index and water vapor, before and after the storm in different units of the earth's surface and its use.
Materials and Methods
In this study, data received from the two satellites, Terra and Aqua of MODIS which includes a calibrated level and location data for 36 bands, with an accuracy of one kilometer has been used to study the land surface temperature, surface albedo, emission, normalized difference index of vegetation NDVI, water vapor, dust and cloudiness phenomena. The days under review were April 24, 28 and 29 and May1 in 2014. On April 28 in 2014 the region witnessed a severe storm at 20 m/s (72 km/s) that have been registered in Naeen Synoptic Stations at the above date.
Discussion and Results
Comparing the Albedo graph shows the Albedo in the day before the storm was slightly smaller than the storm before it occurred. The greater of Albedo in post-hurricane days prior to the days before the storm may be probable due to the removal of surficial layer of soil affected by high velocity wind in the stormy day and the other factor is that in the days after the storm, due to the difference in time and the ascending trend of radiation in the spring solar radiation is likely to be higher these days hence the increase in surface Albedo after the storm is due to the decrease in moisture content of the surficial soil. Comparison of radiation power or discharge of the area's use in the days studied shows that radiation power of land uses before the storm was more than the storm the next day (with the exception of the Playa which is less) and in all three days, the inverse relationship with the Albedo diagram shows that the least surface leakage was in the salt marsh, desert and Playa area and was the largest in aquifer and scattered forests. The lower the radiation power of the users after the storm than before the storm could indicate a change in the surface conditions of the earth, such as a change in surface moisture, because after the storm, soil drought conditions intensify. The comparison of the surface temperature graph shows that the highest surface temperature was observed before and after the storm in the sandy hills and desert lands, and the lowest temperature in the scattered forests and Playa. Comparison of the water vapor diagram does not show regular changes before and after the storm. Changes in the Vegetation Index indicate that before the storm, this index was higher in all uses than in the two days after the storm, while at least every three days, the least amount of this index was related to Playa and the highest was for scattered forests and Agricultural sectors.
Most of the land uses are rangelands (rich, medium and poor) covering about 60% of the area. Most of the dust centers occurred related to the poor pasture usage, which are usually prone to dust due to lack of vegetation. In terms of the rate of dust emission, the most emission rate is related to clay surfaces that cover a small area of the area, which indicates the relationship of the area with the emission rate of dust. Geomorphological map includes 3 units (mountain, pediment plain and Playa plains). The pediment plain cover a wide range of arid regions, and in this research most of the area (71.5%) is related to this unit. The highest number of dust source sites and dust emission rates occurred in the pediment plain, Playa plains, and mountains, respectively, indicating that the plains are susceptible to the occurrence of dust in the studied area.
The results indicate that 81 percent of the dust source area were observed in pediment plain area geomorphologically, covering about 71.5 percent of the area. In other geomorphological landscapes, dust centers are located in the Playa unit (10.7% in 17.5% of the area) and in the mountains (6.4% in 14% of the area) respectively. The results of land use survey showed that the most number of dust harvesting centers occurred in poor rangelands (35.3%) and dryland lands (27.6%), which covers 35.3% of the total area.
The results of this study indicate that storm occurrence in dry and desert region of Gavkhoni and southeast of Isfahan has an impact on the optical and biophysical properties of surface such as surface Albedo, radiation power, surface temperature and vegetation index. This effect is influenced by the type of soil and land features or geomorphological conditions were different. The severity of these changes varies according to the type of land features and its use, and the most affected areas are those that lack vegetation such as playa, desert lands and sand dunes. Also, changes in the reflection of land features indicate that the Playa unit, the sand dunes and the hills, the salt marsh and desert have the most reflection variations before and after the storm, and are centers for surface soil extraction, winding, and dust generation. The study of the direction of the slope and topography shows that the eastern and northeastern slopes have more reflective changes than the western and southwestern slopes, probably due to the fact that the soil is softer in these slopes and facilitates the harvest by the wind.