Introduction
Natural hazards cause significant human and financial losses worldwide each year. Land subsidence is one of these hazards, and its importance has increased in recent years due to the occurrence of droughts in cities, especially in metropolises. This phenomenon has occurred in many plains of Iran due to over-exploitation of groundwater resources. The rate and spatial patterns of subsidence can change over time. Therefore, accurate measurement and predictive tools for monitoring and studying it are necessary. Radar interferometry (InSAR) is one of the new and powerful tools for determining the extent of land subsidence. This method is very effective and useful as a suitable and cost-effective solution in areas lacking leveling infrastructure. In fact, by providing high-precision satellite images, this technique enables the monitoring of the destructive phenomenon of land subsidence and plays a vital role in managing and controlling land subsidence. Given that the amount of groundwater withdrawal strongly affects the extent and distribution of subsidence, this research aimed to monitor annual land subsidence and evaluate its relationship with changes in groundwater levels in the Kermanshah aquifer. For this purpose, the temporal trend of land subsidence and fluctuations in groundwater levels during the same period were investigated and analyzed.
Materials and Methods
In this research, the land subsidence rate in the Kermanshah aquifer was monitored and investigated using the Differential Interferometric Synthetic Aperture Radar (DInSAR) method. For this purpose, seven series of Sentinel A-1 radar images were used to examine the subsidence rate during the 2015-2021 period. Image analysis was performed in the Windows operating system environment using the SNAP software. Then, to estimate the maximum groundwater level decline in the Kermanshah aquifer, data from 38 piezometric wells throughout October 2015-2021 were used. The spatial information and the average groundwater level of the observation wells were entered into the ArcGIS software, and groundwater level variation maps in the study area were calculated using the IDW interpolation method. Subsequently, using the land subsidence maps and the annual groundwater level changes, a map of the average seven-year land subsidence rate and groundwater level changes was prepared. These maps were used for a more detailed and comprehensive analysis of the trend of changes and the mutual effects between the annual subsidence rate and groundwater level changes. In the present study, the relationship and correlation between the annual subsidence rate and the groundwater level change trend, both in urban and agricultural areas and in the entire aquifer domain, were thoroughly investigated and evaluated.
Results and Discussion
The results of the Kermanshah aquifer study in the period 2015 to 2021 indicate significant fluctuations in the subsidence rate and changes in groundwater levels. In the early years of the study period, the subsidence rate in the stress-prone areas of the aquifer increased sharply, so that in 2015-2016 this rate ranged from 5.6 to -14.4 cm/year. This amount of subsidence occurred simultaneously with an unprecedented decline in groundwater levels of up to 12.6 meters during the same year.However, from 2016 to 2021, a trend of improvement in the aquifer's condition was observed, so that the subsidence rate decreased to 6.2 to -11.9 cm/year, and the groundwater level changes were limited to -0.19 to 11.8 meters per year. Detailed examination shows that the northern and western parts of the aquifer experienced the highest subsidence rates, averaging -3.4 to -6.3 cm/year, while the central and southern regions faced a decrease of 0 to -1.8 cm/year. Additionally, in the agricultural lands in the north of the aquifer, the groundwater level decline reached an average of 50 cm per year. Data analysis showed that the coefficient of determination between the average subsidence rate and the average groundwater level changes in urban and agricultural lands were 0.13 and 0.074, respectively, and for the entire aquifer area, it was 0.082, indicating a lack of strong correlation between these two variables in both land uses.
Conclusion
Radar imagery studies of the Kermanshah aquifer from 2015 to 2021 have revealed serious challenges related to declining groundwater levels and land subsidence. The results show a decreasing trend in the subsidence rate, from -14.4 cm/year in 2015-2016 to -11.9 cm/year in 2020-2021, with the highest rates observed in the northwestern and western parts of the aquifer. Concurrently, groundwater levels declined, with a maximum drop of -12.6 m recorded in 2015-2016 and a minimum of -0.19 m in 2019-2020. The maximum groundwater level decline was seen in the central and northern parts, while the southern and urban areas experienced the least decline. However, the correlation between subsidence rates and groundwater changes in urban and agricultural areas was low, suggesting that various factors, including faults, aquifer characteristics, and past groundwater changes, may have contributed to the land subsidence. The differences in the trends of subsidence and groundwater decline across the aquifer indicate the complexity of the issue. The study aims to provide a comprehensive understanding of the surface and subsurface changes in the Kermanshah aquifer. The findings can assist decision-makers in infrastructure and regional development planning, as well as in better managing groundwater challenges and addressing the land subsidence phenomenon through appropriate solutions. The present study provides valuable insights into the hydrogeological dynamics of the Kermanshah aquifer, which can help policymakers and water resource managers develop effective strategies to mitigate the impacts of groundwater depletion and land subsidence in the region. |
