# First Results from Sentinel-1A InSAR over Australia: Application to the Perth Basin
## 5 Results
### Multi-Temporal Analysis
#### 5.2.3 Seasonal Deformation Coincident with Wetland Areas  
Subsidence of 20-25 mm/y is identified in three regions over the 0.7-y observation period: Lake Jandabup, Lake Gnangara and Forrestdale Lake (labelled (c1, c2, c3) in Figure 4C, respectively). In each case, displacements are confined to a region up to 2 km wide and are coincident with wetland areas. We investigate these signals using time series of cumulative displacements. Time series from TerraSAR-X and Sentinel-1A show comparable results, although overall, the higher acquisition frequency of TerraSAR-X better captures changes in deformation over time (Figure 6). Where acquisitions from the two satellites are separated by less than three days, we directly compare the Sentinel-1A measurements to TerraSAR-X. In cases where the temporal offset is greater, we compare the Sentinel-1A result to the average of the two closest TerraSAR-X acquisitions. The mean difference between the two datasets is \(\sim\)5 mm at Lake Jandabup and Forrestdale Lake and \(\sim\)3 mm at Lake Gnangara, which in all cases  
Figure 5: Profiles of average displacement rate corresponding to the lines labelled profile x (**A–C**) and profile y (**D–F**) in Figure 4. The top panels (A,D) are profiles through Sentinel-1A data, and the middle panels (B,E) are through TerraSAR-X data. Profiles and errors (grey lines) are calculated using a band 10 pixels wide and the method described in Section 4.4. Profile x shows a broad region of subsidence, whereas profile y reveals adjacent subsidence and uplift as identified in the rectangle in Figure 4. (C,F) show the correlation between the measurements from the two satellites along profiles x and y, respectively. The dashed black line shows a 1:1 correlation. is at least three-times smaller than the magnitude of the maximum displacement measured over the observation period (Figure 6). Figure 6: Time series of cumulative vertical displacement for the three regions of localised subsidence coincident with wetlands identified in Figure 4: (**A**) Lake Jandabup, (**B**) Lake Gnangara and (**C**) Forrestdale Lake. Measurements of hydraulic head are obtained from the Western Australian Department of Water as described in Section 3. Both hydraulic head and displacements are relative to the time of the first InSAR acquisition (27 August 2015) and end on 19 May 2016. Error bars are calculated using the method described in Section 4.4. The inset in (C) is a time series of hydraulic head measured at artesian monitoring bores proximal to Forrestdale Lake, but over a longer time period to demonstrate the longer term seasonal pattern. The temporal evolution of cumulative displacements at all three wetland areas is similar, and we compare the InSAR time series to changes in hydraulic head in the unconfined aquifer measured at nearby artesian monitoring bores and obtained from the Western Australian Department of Water (Section 3). The groundwater data in Figure 6 are the average of hydraulic head measured at all bores within 3 km of each wetland. Groundwater level data and InSAR displacements show close agreement over time (Figure 6), confirming that the InSAR measurements capture surface changes correlated with seasonal variations in hydraulic head [79]. Hydraulic head and ground displacements decrease between the first acquisition in late August 2015 and late March 2016, coincident with the Southern Hemisphere spring and summer months and the period of low rainfall in the Perth region (Figure 6). The signal then begins to reverse, most notably at Lake Jandabup (Figure 6A), with increasing water levels and positive or neutral ground displacement. The maximum relative subsidence is \(\sim\)23 mm at Lake Jandabup, \(\sim\)20 mm at Lake Gnangara and \(\sim\)24 mm at Forrestdale Lake. At Lake Jandabup, there is \(\sim\)7 mm of relative uplift that occurs between March and the time of the last acquisition in May. Lake Jandabup, Lake Gnangara and Forrestdale Lake are located within clay/swamp surface deposits (Figure 2).