|Paper title||Using SBAS and PS-InSAR techniques to unravel land deformation and characterise the groundwater resources of N’Djamena, Chad|
|Form of presentation||Poster|
Groundwater plays a critical role for ecosystems and is a vital resource for humankind, providing one-third of freshwater demand globally. When groundwater is extracted unsustainably (ie. groundwater extraction exceeds groundwater recharge over extensive areas and for an extended period of time), groundwater levels inevitably decline and can lead to aquifer depletion, which can pose a risk to the sustainability of urban developments and any groundwater-dependent activities. In an ever-changing world, it is increasingly important to effectively manage our aquifer systems to ensure the longevity of the groundwater resources.
This work is undertaken as a partnership between the University of Pavia and the ResEau-Tchad project (www.reseau-tchad.org), with a focus on the urban area of N’Djamena, the fast-growing capital city of Chad. Groundwater contained within the phreatic and semi-confined aquifers underlying the city acts as the main source of water for a population of around one million inhabitants. With an annual growth rate of 7%, the reliance on groundwater for drinking and agricultural purposes is becoming more important. As a result, there is an increasing pressure on urban sanitation infrastructures that have failed to meet the current demand. Additionally, in recent years this area has experienced frequent flooding which is linked to the overflow of the Chari and Logone rivers and increased extreme precipitation events, which may be exacerbated by land subsidence induced by groundwater overexploitation.
Through the use of Advanced Differential Interferometric Synthetic Aperture Radar (A-DInSAR) techniques, land displacement in N’Djamena and the surrounding area has been spatially and temporally quantified for the first time. The current work aims to present two different InSAR processing techniques, Persistent Scatterers (PS) and Small BAseline Subset (SBAS), in a comparative way and also a preliminary analysis of spatial-temporal correlations between deformation measurements and groundwater levels to evaluate a possible cause and effect relationship.
InSAR is a technique that provides a measurement of ground deformation which, in the context of groundwater management, is controlled by the physical parameters of the aquifer such as soil compressibility, thickness, and storativity. Thus, while InSAR results enable large-scale, high resolution measurements of land displacement (on the scale of millimetres), InSAR-derived data itself is not directly quantitative without lithological knowledge of the subsurface. Therefore, the methodology developed to interpret the InSAR data and characterise the groundwater resources of N’Djamena is based on a multidisciplinary approach that integrates limited, in-situ hydrogeological measurements, including groundwater levels collected during a monitoring regime conducted from June 2020 to July 2021, along with the development of a three-dimensional subsurface lithological model based on the collection of available borehole logs and fieldwork validation.
To generate measurements of land displacement, both the PS and SBAS InSAR techniques have been applied to detect surface deformations in N’Djamena and its surrounding area. The PS-InSAR approach analyses interferograms generated with a common master image to produce a signal that remains coherent from one acquisition to another by exploiting temporally stable targets. Alternatively, the SBAS approach relies on small baseline interferograms that maximize the temporal and spatial coherence. In this work, both techniques have been applied in the study area using two time-series of descending and ascending Sentinel-1 Synthetic Aperture Radar images obtained from April 2015 to May 2021. The PS-InSAR technique mainly focuses on the urban area to obtain a high density of PSs, enabling more accurate land deformation measurements. The PS-InSAR vertical deformation rate ranges from -13 mm/yr to 21 mm/yr, while the SBAS values are in the range of -71 mm/yr to 32 mm/yr. The difference in velocity ranges can be explained by the different spatial coverage achieved by the two processing techniques, as the SBAS method provides results even over non-urban areas, which is where the higher displacement rates are estimated. The deformation rate maps obtained from the PS-InSAR and SBAS results are compared from a quantitative and qualitative point of view, taking into account the different types of movement derived from the techniques. The land deformation depicted for the urban area by the two processing techniques indicates a similar pattern of displacement (similar areas of subsidence and uplift). Although the pattern of displacement indicated by the two datasets is similar, the average velocity values obtained with PS-InSAR tend to be noisier than the ones derived using the SBAS technique, particularly when the SBAS time-series shows non-linear deformation trends.
The approach used in this work exploits advanced satellite-based Earth Observation techniques in order to gain further insight into the behaviour of the aquifer system in a region where hydrogeological monitoring is still largely absent. It is anticipated that the findings will help to improve the characterisation of the aquifer and groundwater resource management in the city of N’Djamena and could be further exploited for strategic decisions in sanitation risk management.