Land subsidence from interferometric SAR and groundwater patterns in the Qazvin plain, Iran

Qazvin plain as one of the important plains in northwest Iran in terms of industrial and agricultural activities, reveals evidence of land subsidence in the last decade. We used a descending ENVISAT dataset during 2003-2006 and two ascending and descending Sentinel-1 tracks acquired during 2014-2016 to study the subsidence in the area. Our time-series analysis of the ENVISAT data reveals that the area is subsiding at an average rate of 40 mm yr(-1) with a maximum value of 50 mm yr(-1) in the line of sight direction. Moreover, our result for the Sentinel-1 ascending and descending tracks shows that the average subsidence rate reaches 65 mm yr(-1) in the line of sight direction. Comparing the obtained mean velocity maps of the two datasets indicates the growing subsidence pattern in the northwest-southeast direction inside the plain. The investigation of the water source reveals that the overexploitation of aquifers caused an approximately 17 m decline in the groundwater level from 2001 to 2011. This drastic drop in the groundwater level has resulted in progressive subsidence over time. The comparison between the extent of subsidence from the image processing and the water level depletion contour map in the Qazvin plain depicts that subsidence is occurring in areas with a high decrease in underground water level. This study suggests that, besides decreasing the number of deep wells as well as their outflow water, the types of agricultural production related to water consumption and irrigation methods in an area such as Iran can play an important role in decreasing the stress on aquifers.