# Monitoring of Ground Movement and Groundwater Changes in London Using InSAR and GRACE
## 4 Results
### GRACE Results  
The primary aim of the GRACE mission was to map gravity variations and infer corresponding hydrological redistributions. The small timeframe mass variations occurring at or near the Earth's surface are much greater than those taking place deep inside the Earth [52], and hence GRACE anomalies can be related to hydrology after removing the contribution of atmospheric masses from air pressure data. The spatial and temporal groundwater variations from GRACE results are shown in Figure 6a,b, respectively. Only four representative points could be observed for our study area, owing to the poor spatial resolution of GRACE. The spatial variations map (Figure 6a) was made by IDW interpolation of the representative points. The temporal series (Figure 6b) was made by averaging all the points in Figure 6a. Figure 5: Standard deviation distribution of average subsidence velocity (mm/year). Figure 6: _Cont._  
From the spatial variations map (Figure 6a), the total average groundwater over the observed time period seems to be decreasing from the north-western region to the south-eastern region. In Figure 6a, the red zones signifies more groundwater depletion as compared to the blue zones. From the temporal series (Figure 6b), it can be observed that the major contribution to Terrestrial water Storage (TWS) is from groundwater and the contribution from soil moisture and surface runoff is relatively small, thus TWS can be taken as a good representation of groundwater. The slope of best-fit line for the groundwater variation is negative, which indicates that, overall, there is groundwater depletion with time. This overall groundwater is reducing mainly due to the urban fabric of London city. The ever-increasing population exerts pressure on groundwater resource, which leads to its depletion. However, temporal rise and fall in groundwater can be seen, which can be attributed to seasonal and annual changes. A general trend which can be observed is rise in groundwater in winter months around January-February, and fall in groundwater in summer months around June-August. Table 4 summarises the results from spatial groundwater variations from GRACE. On averaging groundwater change over Figure 6a, a value of \(-4.547\) cm over London for an area of 1584 km\({}^{2}\) is observed. The uncertainties associated with this was calculated from accompanied TWS standard deviations and showed an average value of \(\pm 2.1\) cm. Thus, a total of 72.024 million cubic meter (MCM) groundwater deficit is estimated from January 2003 to December 2010, which gives an average value of 9.003 MCM/year. This is approximately 17 times less than the groundwater mass loss for Mexico around the same time period [25]. In the early 1990s, the Environment Agency (EA) implemented the General Aquifer Research, Development and Investigation Team (GARDIT) strategy to control the groundwater level rise in the chalk aquifer under central London resulting from the previous relocation of industries from central London. GARDIT strategy issued licenses to abstract groundwater from London and by the early \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Study Area** & **Average (Spatial)** & **Average Volumetric** & **Average Yearly GW** \\
**Extent** & **GW Change** & **(Spatial) GW Change** & **Change** \\ \hline
1584 km\({}^{2}\) & \(-4.5471\) cm & \(-72.024\) MCM & \(-9.003\) MCM \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Groundwater variations from GRACE. Figure 6: (**a**) Spatial variation of groundwater from GRACE (the red zones signifies larger groundwater depletion as compared to the blue zones). (**b**) Temporal variation of Terrestrial Water Storage (TWS) from GRACE (red line), soil moisture (blue line) and surface runoff (green line) from GLDAS, resulting in groundwater (black line) and linear fit for groundwater (black dotted line). 2000's the rise in groundwater had been stabilized.