JWSS - Journal of Water and Soil Science

Land subsidence as a hydrogeomorphology event is currently occurring dangerously in many plains of the country due to uncontrolled groundwater extraction from water bearing layers, and accordingly monitoring and studying this phenomenon seems to be necessary. In this study, land subsidence rate of the Najafabad aquifer was determined through the Differential Radar Interferometry (DInSAR) processing of the ASAR and PALSAR radar data and the results were validated by comparying with the differential leveling and groundwater level drowdown data. Processing of the ASAR sensor data estimates the land subsidence in the Najafabad plain at an average annual subsidence rate of 6.7 cm and a total of 41 cm during 6 years period and processing of the PALSAR data suggests an annual rate of 7.7 cm and total subsidence of 30 cm during 4 years period. Most of the occured displacements are related to the Tiranchi, Koushk, Ghahderijan, Goldasht and Falavarjan cities. The simultaneous groundwater level data with acquisition date of the radar satellite images between 2002 and 2014 shows a drawdown ranging from 0.5 to 46.5 meters in the south and east to north of the Najafabad city consistent with the estimated land subsidence areas.The DInSAR processing of the PALSAR data has led to a more accurate results with higher spatial resolution. Results of the radar data processing can be employed for the hazard zonation directly utilized for management and planning of control and preventive measures.

This study evaluated groundwater-level variations in the Isfahan-Borkhar aquifer and examined their relationship to land subsidence. Representative hydrographs, groundwater-level maps, depth-distribution maps for the wettest and driest months of the study period, and a 20-year groundwater-level variation map were prepared and compared with the land subsidence map within the aquifer area. The results indicated that subsidence zones were mainly located in areas experiencing considerable groundwater-level decline. The occurrence of critical subsidence zones in regions with a high density of pumping wells and agricultural lands further confirmed this relationship. In addition, the alluvial-thickness map of the study area was prepared and compared with the land subsidence map to investigate the relationship between the spatial distribution of subsidence zones and alluvial thickness. The findings indicated that land subsidence is associated not only with groundwater-level decline but also with other factors, including alluvial thickness and soil texture. In other words, even under substantial groundwater-level decline, significant land subsidence is unlikely where alluvial thickness and the proportion of compressible fine-grained sediments are low.