Evaluation of subsidence rate of Hamedan-Bahar plain and its relationship with environmental parameters

Subsidence is the collapse or subsidence of the earth's surface that occurs on a large scale for a variety of reasons. The phenomenon of subsidence has affected many parts of the world, including Iran, and in recent years has been raised as one of the main issues and challenges. In Iranian subsidence, groundwater decline has been considered as the main factor, because there is a direct relationship between subsidence and the rate of groundwater decline in different regions. In fact, the prevailing arid climate in most of the interior of Iran and the concentration of industrial, agricultural and drinking water consumption on groundwater resources, has provided a good infrastructure for the occurrence of this phenomenon. One of the areas that are exposed to subsidence is Hamedan-Bahar plain. The geographical and climatic location of these plains has caused them to have semi-arid conditions in terms of climate, and this has caused the amount of precipitation in this region to be much lower than in the western and northern regions. In these cases, this region is facing a shortage of surface water, and therefore the utilization of groundwater in this region is much higher than the amount of their nutrition. Therefore, one of the most important factors in over-exploitation of groundwater in the region is unfavorable climatic conditions. The trend of over-abstraction of groundwater in the region has caused the groundwater level of these plains to decline sharply in recent years, which has led to the risk of subsidence as the most important risk. Geomorphology in the plains of Hamedan province. Due to the importance of the issue, in this study, the subsidence status in the Hamedan-Bahar plain has been investigated using radar interferometry and SBAS time series method.

Materials and methods
In this research, descriptive-analytical methods have been used to achieve the desired goals. Research data include Sentinel 1 radar images (73 radar images during the period 16/01/2015 to 14/01/20120), Landsat 8 satellite image (dated 13/06/2020 in order to prepare land use maps). The digital model is 30 m high SRTM, geological map 1: 100000 also information about piezometric wells in the area. Research tools also include GMT (for radar interference and SBAS time series), ENVI (for land use mapping), ARCGIS (for fuzzy logic and AHP), and expert choice (layer weighting). In the first stage, using the radar interferometry method and SBAS time series, the subsidence of the region has been evaluated and in the second stage using 6 parameters including slope, distance from river, type of land use, type of lithology, geomorphological units and groundwater decline and integrated model of fuzzy logic and hierarchical analysis (AHP), areas prone to subsidence have been identified.

Discussion and results
In this research, after preparing the images, first the necessary preprocessions have been done on the images. After performing the preprocessions, based on the time base of the images, the desired pair of images has been selected for radar interference. After preparing the interferogram maps, a map of the subsidence rate of the area during the study period has been prepared. Based on the results, the study area during a period of 5 years (from 16/01/2015 to 14/01/2010), faced with 281 mm of subsidence, based on which it can be said that the study area It has 56 mm of annual subsidence. The results of the evaluations show that the maximum annual subsidence in the region with 95 mm was related to the years 2015 to 2016, and after that the amount of subsidence has been decreasing, and finally during 2018 to from 2019 and 2019 to 2020, it has been reduced to 42 mm per year. Also in this study, using 6 parameters, areas prone to subsidence have been identified. According to the prepared map, the middle areas of the study area, which includes the urban areas of Hamedan, Bahar, Lalejin and Salehabad, due to the type of use, type of lithology, low slope, location in the plain and alluvial fan unit, as well as high water resources Groundwater in these areas has a high potential for occurrence and intensification of subsidence in the future.

Conclusion
The results indicate that the Hamedan-Bahar plain has a great potential for subsidence so that according to the results of this plain during a period of 5 years (from 16/01/2015 to 14/01/20120), with 281 Mm of subsidence has been encountered, based on which it can be said that the study area has 56 mm of subsidence per year. Examination of the time trend of subsidence in the region shows that the maximum amount of annual subsidence in the region with 95 mm was related to the years 2015 to 2016, and after that the amount of subsidence has a decreasing trend, and finally during the years 2018. By 2019 and 2019 by 2020 it has been reduced to 42 mm per year. The study of the spatial trend of subsidence also indicates that the highest rate of subsidence has been in the downstream areas of Hamadan and the adjacent areas of Bahar and Salehabad. Also, the potential assessment results of subsidence-prone areas indicate that the middle areas of the study area, which includes the urban areas of Hamedan, Bahar, Lalejin and Salehabad, due to the type of use, type of lithology, low slope, location in the plain unit. The alluvial fan, as well as the large decline in groundwater resources in these areas, has a high potential for occurrence and intensification of subsidence in the future. Due to the fact that these areas had the highest rate of subsidence in calculating the subsidence of the region, so there is a correlation between the results of radar images and zoning