Sinking Yangon: Detection of subsidence caused by groundwater extraction using SAR interferometry and PSI time-series analysis for Sentinel-1 dataAuthor
Van der Horst, T.
Rutten, M. (mentor)
Inhabitants and ecosystems in delta areas are becoming increasingly vulnerable to the effects of subsidence, triggered both by natural causes and anthropogenic causes. Yangon is a city in the Irrawaddy delta in Myanmar where little is known about the true extent of this hazard, while its effects can potentially harm millions of its inhabitants. The aim of this research was, therefore, to assess if any subsidence is occurring, and to determine if groundwater extraction could be the cause of it. The city Yangon has a large urban extent and is expanding rapidly towards the north, west and east. In the current water supply system, nearly half of the domestic water use is supplied from reservoirs managed by the city council while the other half is obtained by other means. From the latter group, close to 80% extracts this water from aquifers beneath their home. This amounts to two million inhabitants of Yangon extracting their domestic water from the subsoil, that is excluding industrial water use. To assess if subsidence is occurring, surface deformation measurements were performed using Synthetic Aperture Radar interferometry (InSAR) with data from the recently launched Sentinel-1. This technique relies on the calculation of phase differences between two imaging radar acquisitions which were ultimately related to differences in surface motion on an interval scale. The direct derivation of surface deformation from two radar acquisitions was limited by noise interference from atmospheric signals and the measuring of temporally-unstable land surface. To overcome these limitations, a persistent scatterer interferometry (PSI) algorithm was used that selects stable scattering pixels with low noise contributors from a stack of interferograms through which deformation could be extracted. Resulting from the InSAR and PSI analysis, this thesis presents remotely sensed surface-level displacement using InSAR with Sentinel-1 data. Ascending and descending SAR image stacks over Yangon were processed into interferograms with the InSAR processing software, Doris, and next, the time series and line-of-sight velocities of phase stable pixels were extracted using the PSI algorithm StaMPS. The line-of-sight velocities that were decomposed into vertical and horizontal motion, reveal that parts of the city are subsiding at rates over 9 cm/y. Currently, these values cannot be validated due to the unavailability of ground-based measurements, but the consistency in the independent acquisition tracks supports their quality. To relate the surface deformation with subsidence, the domestic extraction of groundwater was mapped on the townships of Yangon. This involved combining various data sources among which a survey on water demand which was performed specifically for the benefit of this research. Although the relation between subsidence and groundwater extraction is more complex than simply their co-occurrence, this method was chosen to get a first order estimate of their causal relation. Additionally, there was not enough information available to employ a more extensive or complex approach. Through mapping water extractions onto the townships of Yangon, extraction rates up to 9mm/y were found. The spatial patterns of measured subsidence and estimated groundwater extraction do not correlate and therefore no indication for a causal relation between the two could be established. Future research should first be directed at identification of the mechanisms involved causing surface deformation. An extension on this research could be done with the suggested improvements on the extraction estimation, but assessing subsidence caused by groundwater pumping through a more extensive approach might be required. Though a relation between surface deformation and groundwater extraction is not established in this thesis, a recommendation is given to mitigate negative effects caused by groundwater pumping.
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