Mapping Ground Subsidence

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Subsidence of land surfaces has increasingly become a greater concern around the world as groundwater has continued to be depleted due to increase water demand while surface water depletion means that populations increasingly look to use subsurface water.

The lowering of Earth’s land surfaces promises to be a major problem for urban and rural regions and mapping these threats accurately may prove important to plan, prevent, and minimize this threat. 

Using a literature review and spatial analysis, it is evident that some 34 countries are at major risk in the coming decades for major subsidence of their land surface. In particular, a recent mapping effort that looks at every 30 arc seconds resolution across the globe identified that very flat areas with unconsolidated sediments accumulated in alluvial basins or coastal plains are the most vulnerable.

Land subsidence in California, due to groundwater withdrawal. Fissures near Lucerne Lake (dry) in San Bernardino County, Mojave Desert, California. View to the southwest, with 5-gallon bucket for scale.
Land subsidence in California, due to groundwater withdrawal. Fissures near Lucerne Lake (dry) in San Bernardino County, Mojave Desert, California. View to the southwest, with a white 5-gallon bucket for scale. Photo: Loren Metzger, USGS. Public domain, May 2001.

These areas are also experiencing rapid population growth around the world. These regions are also often flood-prone and some 635 million inhabitants are potentially most at risk for a major subsidence event in the next two decades.[1]

Approaches to Mapping Subsidence

To map subsidence potential, scientists have investigate a variety of factors. Among the better approaches, geographical time-slice weighted regression (GTSWR) and geographical temporal weighted regression (GTWR) can be used to develop maps based on relevant input data.

Using groundwater drawdown data can be applied as input data to demonstrate regions that have the greatest threat from subsidence. In a recent study in China, it was found that groundwater extraction acts as a strong predictor of the degree of subsidence observed.[2]

Mapping Subsidence Using Synthetic Aperture Radar

Scientists have searched for methods that best measure land subsidence changes. One method that has proven useful is using Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR).

Areas shown in blue on this map are subsiding, with darker blue areas sinking faster than lighter blue ones. The areas shown in dark red are rising the fastest. The map was created by comparing thousands of scenes of synthetic aperture radar (SAR) data collected between 2007 and 2011 and 2014 and 2018.
Areas shown in blue on this map are subsiding, with darker blue areas sinking faster than lighter blue ones. The areas shown in dark red are rising the fastest. The map was created by comparing thousands of scenes of synthetic aperture radar (SAR) data collected between 2007 and 2011 and 2014 and 2018. Map: NASA with data from Blackwell, Em, et al. (2020).

The method uses radar data, such as Synthetic Aperture Radar (SAR), that can measure subtle differences between data captured. Metrics such as maximum deformation rates, deformation time series, thermal expansion component of different observations can be determined using developed algorithms and time-series data.

With this approach, scientists are better able to determine the rate of subsidence experienced in a given area, helping to determine potential threats, such as sinkhole development, before these events occur.[3] 

Studies are also using different older and newer satellites to combine different datasets to better assess subsidence change. One study has applied Envisat, Sentinel-1, and Gravity Recovery and Climate Experiment (GRACE) data.

In Lagos, Nigeria, it was observed that subsidence varied between −2 mm and −87 mm per year. The high rates of subsidence are observed and most pronounced around coastal zones and in areas where heavy or large structures are built on landfills. The high rate of subsidence is also contributing to the area’s seasonal flooding, aquifer contamination, and saline water intrusion observed in recent years.[4] 

While most scientific works have focused on the negative effects of subsidence observed around the world, some positive news is evident. One study in California did show that groundwater substitution, conservation efforts and campaigns, and managed aquifer-recharge can mitigate subsidence rates, helping to prevent dangerous changes to land surfaces.[5] (Related: California is Sinking Faster than Previously Thought)

Subsidence is a major and increasing problem in regions that are more greatly depending on groundwater resources. While scientists have increasingly improved their use of satellite-based monitoring and spatial analysis using available field data, these studies have mainly indicated that subsidence is getting worse in many countries.

A recent study in California, however, suggests better ways we can manage subsidence by taking efforts that recharge depleted water sources and managing resources more efficiently. Applying such mitigation efforts could help some of the devastating impacts of rapid subsidence from occurring. 

References

[1]    For more on a global mapping study looking at subsidence, see:  Herrera-García, G., Ezquerro, P., Tomás, R., Béjar-Pizarro, M., López-Vinielles, J., Rossi, M., Mateos, R.M., Carreón-Freyre, D., Lambert, J., Teatini, P., Cabral-Cano, E., Erkens, G., Galloway, D., Hung, W.-C., Kakar, N., Sneed, M., Tosi, L., Wang, H., Ye, S., 2021. Mapping the global threat of land subsidence. Science 371, 34–36. https://doi.org/10.1126/science.abb8549.

[2]    For more on methods and approaches that best capture and map land subsidence, see:  Ali, M.Z., Chu, H.-J., Burbey, T.J., 2020. Mapping and predicting subsidence from spatio-temporal regression models of groundwater-drawdown and subsidence observations. Hydrogeol J 28, 2865–2876. https://doi.org/10.1007/s10040-020-02211-0.

[3]    For more on using imagery and SAR data to measure subsidence, see:  Kumar, S., Kumar, D., Chaudhary, S.K., Singh, N., Malik, K.K., 2020. Land subsidence mapping and monitoring using modified persistent scatterer interferometric synthetic aperture radar in Jharia Coalfield, India. J Earth Syst Sci 129, 146. https://doi.org/10.1007/s12040-020-01413-0.

[4]    For more on the study on Lagos, Nigeria and its subsidence using geoditic satellite data, see:  Ikuemonisan, F.E., Ozebo, V.C., 2020. Characterisation and mapping of land subsidence based on geodetic observations in Lagos, Nigeria. Geodesy and Geodynamics 11, 151–162. https://doi.org/10.1016/j.geog.2019.12.006.

[5]    For more on a study on improving subsidence in California, see:  Sneed, M., Brandt, J.T., 2020. Mitigating Land Subsidence in the Coachella Valley, California, USA: An Emerging Success Story. Proc. IAHS 382, 809–813. https://doi.org/10.5194/piahs-382-809-2020.

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