Land subsidence in the Nile Delta of Egypt observed by persistent scatterer interferometry

Land subsidence is a common problem in vulnerable deltas. The Nile Delta is no exception. The impacts of land subsidence are heightened by the economic, social and historical importance of the delta to Egypt. A major debate has evolved in the past two decades concerning whether the land surface of the Nile Delta is subsiding. The debate is certainly problematic in light of the fact that current measures of subsidence across the delta are rough estimates at best. To date, knowledge of subsidence rates in the delta is limited to long-term geologic averages that assume spatial uniformity and temporal consistency. In this study, we apply persistent scatterer interferometry ( PSI) to measure the magnitude and monitor the spatial and temporal variations of land subsidence in the Nile Delta, during 1993-2000, using synthetic aperture radar interferometric data of 5.66 cm wavelength. The average measured rates of local subsidence in two major cities in the delta, namely Mansura and Greater Mahala, are -9 and -5 mm year(-1), respectively. The observed deformation features imply that subsidence in both cities is controlled mainly by local groundwater processes. Our PSI measurements indicate that no regional subsidence has occurred in either city between 1993 and 2000. The slight regional subsidence that is expected to occur over time due to the natural compaction of deltaic sediments most likely has been masked by surface displacements caused by seasonal oscillations in the groundwater level.