# Monitoring of Ground Movement and Groundwater Changes in London Using InSAR and GRACE
## 5 Discussions
### Relationship between Ground Movement and Groundwater Change  
The ground movement obtained from PSInSAR largely depicts uplift in northern London and subsidence in southern London. Additionally, GRACE highlights that there is more groundwater loss from southern London as compared to northern London during our study period. The two results compliments each other as they represent the same water variations through compaction (InSAR) and mass variation (GRACE). However, there are some anomalies while comparing GRACE results with observed groundwater values for aquifer scale regions and it is difficult to derive a direct relationship between them. The main restriction of GRACE to manage groundwater has been described by [PERSON] [55] and it specifies that it requires great modesty to view GRACE as a water management tool at aquifer scale. Nevertheless it provides a \"big picture\" view that is appropriate input for setting broad regional or national policies [53], a view for which our study provides further evidence. Hence, to further study the relationship between ground movement and groundwater change we have taken observed groundwater data from boreholes provided by the EA. The EA publishes a \"management of the London basin chalk aquifer\" status report each year and we have extracted the data for key boreholes from graphs provided in the report for the year 2010. The contour map for change in groundwater levels for London between the years 2000 and 2010 provided by the EA is consistent, and agrees well with the PSInSAR subsidence map of London obtained using Envisat data (Figure 9). The overall land movement pattern suggests an uplift in north London and subsidence in south London. This is consistent with the EA contour map for groundwater, which shows that there is an overall increase in groundwater level for the northern part and decrease in the southern part of the study area. The average water level rise is in the order of 7 m in north-west London and 4 m in central London, which corresponds to an average uplift of 31 and 20 mm, respectively. It can be seen from Figure 9a that water level is decreasing to form a zone of water level fall crossing from south to east London, and there is prominent land subsidence in the corresponding area. Therefore, we can conclude that extraction or recharge of groundwater for a particular area directly contributes to the land subsidence or uplift of the corresponding area. Additionally, there might be a threat of saline intrusion in east London, in the area where the chalk outcrops around the River Thames from Greenwich to Woolwich. It can be observed from the contour map that groundwater flow is in a north-westerly direction, and thus water extraction could capture saline water if it enters the aquifer. Thus, there were restrictions applied to water abstraction licenses in this area [11]. Figure 9 validates that groundwater level directly influences the ground movement pattern. To further study both the phenomena, individual boreholes have been analysed (location marked in Figures 1 and 3). The time-series of groundwater variation for boreholes are compared to time-series of subsidence patterns at the corresponding location. The land movement is an indirect effect of groundwater change, and hence it is smaller in magnitude than groundwater change. In addition, a time lag is expected in land movement when compared to the groundwater change. To obtain the subsidence at the location of a particular borehole,  
Figure 9: **(a) Contour map for groundwater level change for London between 2002 and 2010, obtained from “management of the London basin chalk aquifer” status report, EA, 2010 [11]. (b) PSInSAR ground movement map for London between 2002 and 2010. The area marked with a blue oval in (a,b) highlights an increase in groundwater and land uplift, respectively, for north London; while the area marked with a red oval highlights a decrease in groundwater and land subsidence for south London.