# Spatial Distribution and Monitoring of Land Subsidence Using Sentinel-1 SAR Data in Java, Indonesia
## 3 Results
### Spatial Distribution and Land Subsidence Rate
#### 3.1.6 Brebes and Tegal  
Brebes (6.8765\({}^{\circ}\) S, 108.5713\({}^{\circ}\) E) and Tegal (6.8816\({}^{\circ}\) S, 109.1346\({}^{\circ}\) E) are cities situated in Central Java Province. In these regions, LS reached alarming rates of 70 mm/year and 60 mm/ year, respectively (see Figure 8(b.1, b.2). Despite this significant concern, research on LS in Brebes and Tegal remains limited, resulting in an inconclusive explanation of the factors and causes of the issue. The morphology of Brebes and Tegal is dominated by alluvial plains and volcanic zones in the northern and southern parts, respectively [17].  
In Brebes, the area affected by LS is rather extensive, primarily affecting densely populated residential areas and agricultural zones, particularly rice fields, as shown in Figure 8(c.1). This extensive subsidence poses risks not only to the infrastructure, but also to the livelihoods of the residents who rely on agriculture. However, the Tegal area is characterized by residential neighborhoods and aquaculture zones, as shown in Figure 8(c.2).  
Figure 6: LS in Bandung: (**a**) LS rate in mm/year in the Bandung area; (**b.1**) LS time series retrieved from InSAR from 2017 to 2023; and (**b.2**) the field condition of LS in the region highlighted by the black box (**b**).  
Groundwater irrigation in the Brebes and Tegal districts has been offered as a primary water supply solution for agriculture during the dry season. Since 2008, the local government has considered groundwater as one of the sources for irrigation network systems in the area. In 2019, the groundwater irrigation network was established in 17 locations, covering 425 ha of agricultural land [43]. However, studies on the influence of using groundwater for irrigation on LS in Indonesia are limited. A similar situation was reported in Bandung [37]. This study indicates that large-scale groundwater exploitation for agricultural activities has led to a significant decline in the groundwater table.  
Information about groundwater irrigation in Tegal is also limited. However, a similar regulation was published by the local government in 2008, stating that groundwater is one of the sources for the irrigation network in Tegal Regency [44].  
The practice of developing groundwater irrigation on Java Island has been increasing in recent years due to limited surface water supply, particularly during the dry season [45]. Although this practice is common in many countries, overexploitation of groundwater can lead to significant depletion of the groundwater table, reduced stream flow, declining water quality, and triggering LS hazards [46].  
Figure 7: LS in Cirebon: (**a**) LS rate in mm/year in Cirebon; (**b**) LS time series retrieved from InSAR from 2017 to 2023 in Indramayu (**b.1**), Cirebon City (**b.2**), and Cirebon Regency (**b.3**); land use of the region highlighted in black boxes areas, (**a.1**) which correspond to residential housing and agricultural area in Indramayu (**c.1**), (**a.2**) residential and business area (**c.2**) in Cirebon City, and (**a.3**) residential housing in Cirebon Residence (**c.3**).