Comparison of groundwater induced Land subsidence in London and Delhi using PSInSAR
Authors: Agarwal Vivek , Kumar Amit , Gomes Rachel Louise , Marsh Stuart 
Published: March 2021

&lt;p&gt;Unrestrained urbanisation and rapid land use land cover changes can impact underlying aquifer systems, resulting in the instances of land subsidence. Thus, monitoring of groundwater induced land movement is an important part of environmental information systems and helps maintain the safety and economics of a city. Interferometric Synthetic Aperture Radar (InSAR) can facilitate monitoring of land movement and observed boreholes can facilitate groundwater monitoring. In this study, we used Sentinel-1 radar images to obtain land movement using Persistent Scatterer InSAR (PSInSAR) technique in the ENVI SARscape software package. The land movement has been studied between October 2016 and October 2020, using 98 SAR images for Delhi and 100 SAR images for London. This is the first time that such a comparison has been made between these two great cities.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;The land movement InSAR velocity maps for both these cities showed long-term, decreasing, complex, non-linear patterns in the spatial and temporal domain, with few areas of heave and a fair amount of subsidence. The land movement varied between -18 mm/year to +20 mm/year for Delhi and -10 mm/year to +9 mm/year for London. The underground metro construction played an important role in controlling the land movement pattern of Delhi. Its Phase III metro line was mostly built between the years 2015 and 2020 with 28 underground stations, 11 route extension and 3 new lines, namely Pink, Magenta and Grey lines. Similarly, construction of the northern line extension, the Channel Tunnel Rail Link and the Lee tunnel directly affected the land movement pattern of London. In addition, the ground movement was compared to observed groundwater values obtained from various boreholes across both these cities. The extraction and recharge of groundwater to meet the demands of an ever-increasing population directly affected the land movement patterns in both cities. It was observed that when large volumes of groundwater are extracted, then it leads to land subsidence, and when groundwater is recharged, then surface uplift is witnessed. The reasons for this subsidence pattern are consistent for both these cities in a few places, while they are completely different at some other locations.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Delhi has been declared as groundwater critical zone by the government of India, while London is not under critical zone. Delhi is one of the most exploited city with regards to groundwater, owing to its urban fabric and ever-increasing population, and these results reflect that. A similar pressure is exerted on London&amp;#8217;s groundwater by its ever-increasing population, which is not recognised by a critical status but is borne out by these results. Along with the groundwater extraction, sub-surface geology, underground construction, and metro extensions all contribute to form a complex land movement pattern. This study can serve as a guideline to government agencies in identifying the areas and extent of groundwater induced land subsidence, so that they can take proper steps to mitigate it.&lt;/p&gt;