# Applications of Satellite Radar Imagery for Hazard Monitoring: Insights from Australia
## 3 Hydrological Hazards
### Drought and Unsustainable Groundwater Extraction
#### 3.2.1 Example Case-Study: Groundwater Extraction and Recharge (Perth, Western Australia)  
In Perth, Western Australia, rapid increases in the volume of extracted groundwater from subsurface aquifers between 2000 and 2005 is correlated with subsidence of up to \(\sim\)7 mm/yr detected from continuous Global Navigation Satellite System (GNSS) measurements [37]. More recently, in August 2017, MAR activities commenced as part of the city's long-term groundwater management strategy. Perth is, therefore, an example where assessments of both groundwater extraction and recharge could be supported by InSAR measurements of subsidence and uplift.  
However, although SAR data have been acquired over Perth since the 1990s from ERS-1/2, ENVISAT ASAR, and ALOS PALSAR [38; 39], short time-series (e.g., Figure 4A) andgaps in time-series (e.g., 12 scenes acquired over a 5-year period: [38]) make assessments of the time-history of displacements challenging. This includes the initial period of Sentinel-1 operations, during which imagery was acquired at 24-day intervals for a 9-month period between August 2015 and May 2016 [40]. The results of these datasets are saturated by atmospheric noise [39] or capture only seasonal displacements, which are shown from levelling to be cm-scale (e.g., [41]).  
An assessment of a longer InSAR time-series is required to determine the recent rate of longer-term (multi-year) subsidence in Perth [40], which from levelling surveys is expected to be \(<\)10 mm/yr [37; 41]. These data are now available from Sentinel-1, which has provided imagery at 12-day intervals since 2016.  
In Perth, subsidence may now be counteracted by the impacts of MAR activities. Ground displacement observations made since the onset of MAR in August 2017 using TerraSAR-X suggest that, since recharge began, the hydraulic pressure in subsurface aquifers has increased enough to cause uplift proximal to the injection wells (Figure 4B). More than \(>\)10 mm of positive LoS displacement is observed between January 2017 and April 2018 (Figure 4B). Ongoing InSAR monitoring, combined with ground-based geodetic infrastructure [15], will be key to the characterisation of the impacts of MAR, including the effects upon long-term subsidence in this region.  
The example of Perth demonstrates the past challenges in using InSAR data to aid Australian groundwater management strategies due to the variable temporal coverage of data. Applications in this setting are consequently limited, whereas InSAR has been applied  
Figure 4: Monitoring ground deformation related to groundwater storage change in Perth, Western Australia, using InSAR analysis of ENVISAT ASAR and TerraSAR-X. (**A**) Map of average LoS displacement rate over an 11-month period using a historical SAR dataset (ENVISAT ASAR). Signals are not consistent with groundwater features or other conceivable causes of ground displacements and are instead dominated by atmospheric phase contributions. (**B**) Map of average LoS displacement rates measured during the first stage of Managed Aquifier Recharge (MAR) activities. Positive LoS displacements are centred around the location of MAR recharge wells (black square). (**C**) Transect that was taken across the x-x’ east-west profile in B, showing relative cumulative LoS displacements on the order of 10 mm over this period.  
extensively worldwide to quantify the impacts of groundwater extraction. Yet observations of local-scale effects in Perth and an example of large-scale (14,000 km\({}^{2}\)) monitoring in the Gippsland Basin, Victoria [42; 43], highlight the potential for better integrating these measurements into groundwater management practices on both local and regional scales, which is now possible with consistent Sentinel-1 data time-series.