# Applications of Satellite Radar Imagery for Hazard Monitoring: Insights from Australia
## 3 Hydrological Hazards
### Drought and Unsustainable Groundwater Extraction  
Much of Australia is characterized by low-precipitation rates in the range of 300-700 mm/yr [29]. Groundwater extracted from subsurface aquifers comprises a large percentage of the available water and ensuring that the balance between groundwater recharge and extraction is maintained is essential to sustain this resource. SAR imagery provides opportunities to assess multiple surface expressions of groundwater dynamics.  
Firstly, information about the discharge and availability of shallow groundwater can be inferred by detecting and monitoring Groundwater Dependent Ecosystems (GDEs: [30]). GDEs are a frequent surface expression of groundwater across Australia, and any impact on groundwater resources will be seen in groundwater-dependant vegetation health (after a time delay that is dependent upon vegetation resilience and aquifer types). Vegetation health metrics are typically derived from multi-spectral EO sensors (e.g., Landsat: [31]), but recent assessments of Australian GDEs demonstrate that SAR imagery could overcome the limitations of multispectral-based approaches. SAR sensing offers temporally stable image time series (unaffected by the cloud cover) and, by taking advantage of radar signal penetration into the canopy, provides information on the vegetation leaf-branch structure (rather than its surface reflectance as is the case of multi-spectral images). By analysing the seasonal variations in time-series of Sentinel-1 SAR intensity and InSAR coherence, Ref. [32] showed that GDE monitoring is, for the first time, possible with SAR imagery on a yearly timescale. The SAR-based GDE mapping showed \(>\)90% similarity to the Global GDE Atlas [31], suggesting that SAR imagery could benefit future monitoring of groundwater-dependent vegetation.  
Secondly, the ground displacement information from InSAR is used to characterize and evaluate groundwater management strategies around the World [18; 33; 34], including in Australia [35]. This is because variations in groundwater storage modify the hydraulic pressure in subsurface mixed-sedimentary aquifers and aquitards, leading to structural changes in compressible sediment layers (the theory of poroelasticity). A hydraulic-pressure decrease results in subsidence, with long-term (i.e., not a transient seasonal trend) subsidence indicative of inelastic (irrecoverable) aquifer compaction, which may result in the permanent loss of porosity within the aquifer and thus a decrease in its ability to store water (e.g., [18]). Conversely, aquifer recharge, including artificial managed aquifer recharge (MAR), causes hydraulic pressure to increase and may result in uplift, although observations of this phenomenon are scarce.  
Monitoring and characterisation of such ground displacements are also necessary in order to mitigate subsequent damage to above- and below-ground infrastructure (including pipes, powerlines, buildings, and roads). For subsidence, ground displacement monitoring is necessary to assess potential impacts upon flood risks, as subsidence can act to amplify relative sea-level rise, risking tidal inundation of areas that were previously above the high-tide line and degrading coastlines. More than 80% of Australia's population live within 50 km of the coast [29], and in low-lying coastal areas, accurate measurements of land subsidence linked to groundwater extraction are valuable for risk assessments of the effects of sea-level rise [36].