Global South cities have a crucial opportunity to learn from costly mistakes elsewhere on land subsidence: Leonard Ohenhen
In January 2023, over 800 houses developed cracks overnight in Joshimath, a Himalayan town in Uttarakhand’s Chamoli district, forcing terrified families to evacuate the hilly terrain.
But land subsidence is not restricted to just the developing world.
Leonard Ohenhen, Assistant Professor, Department of Earth System Science, University of California, Irvine, talked to Down to Earth about the concerns of land subsidence even in rich countries of the world due to overdependence on groundwater. Excerpts:
Sushmita Sengupta (SS): Which cities in the world are prone to land subsidence?
Leonard Ohenhen (LO): Most coastal cities are prone to subsidence because coastal areas naturally sink over time due to natural compaction of sediments. However, in some cities where intensive groundwater extraction occurs, the rates are dramatically accelerated beyond natural levels. Notable examples include cities like Jakarta, Mexico City, Tehran, Bangkok, Tokyo, Ho Chi Minh City, Dhaka, Lagos, etc. In the United States, subsidence has been documented in cities including Houston, New Orleans, Norfolk, and Los Angeles. The common thread is the combination of natural coastal vulnerability or intense groundwater pumping, often in areas with clay-rich sediments.
SS: Will climate change make these cities more vulnerable to land subsidence?
LO: Absolutely. Climate change intensifies the problem through multiple pathways. Increased drought frequency drives greater groundwater demand, while rising temperatures increase water consumption. This creates a dangerous feedback loop where climate adaptation efforts often increase groundwater dependence, accelerating subsidence that makes communities more vulnerable.
SS: What are the natural and anthropogenic reasons for such land subsidence?
LO: Natural factors include glacial isostatic adjustment (GIA), where land continues to settle following the retreat of ice sheets thousands of years ago; geological conditions such as natural consolidation of soft, recently deposited sediments; tectonic activity including fault movement and regional geological settling; and natural compaction of deltaic and coastal sediments over time. While natural processes cause land subsidence, anthropogenic activities often accelerate background subsidence. Examples of anthropogenic processes include excessive groundwater extraction, which causes aquifer systems to compact, particularly clay layers that act like sponges — when water is removed. Other processes include oil and gas extraction and mining activities. While natural subsidence typically occurs at rates of millimetres per year, human-induced subsidence can reach several centimetres annually.
SS: Your recent paper Land subsidence risk to infrastructure in US metropolises, published this June in Nature talks about land subsidence in US cities due to high groundwater extraction. How dependent are US cities on groundwater supply? Between unconfined and confined aquifers, which one is more exploited?
LO: The dependence of urban water supply systems on groundwater in the US varies substantially by geography, climate, and urban development patterns. In arid and semi-arid regions such as California’s Central Valley, parts of Texas, Arizona, and the High Plains, groundwater serves as a critical and often primary source of municipal, agricultural, and industrial water supply. In some cities, such as Houston, Phoenix, and Las Vegas, long-term aquifer withdrawals have historically exceeded natural recharge, making groundwater a structurally embedded yet unsustainable component of the urban water budget.
As for whether confined or unconfined aquifers are more exploited, this is not uniform across the US and largely depends on the hydrogeologic setting of each region. In our analysis of 13 cities with available groundwater data, confined aquifers showed the strongest correlation between groundwater-level declines and vertical land motion (Pearson ρ = 0.87), indicating both their heavier exploitation and mechanical sensitivity. However, this reflects the subsample of urban centres where confined systems are present and monitored, not a blanket national trend. In general, however, confined aquifers are more susceptible to land subsidence. This is because when water is withdrawn from confined systems, especially those with compressible interbedded clays, the resulting decrease in pore pressure increases the effective stress, leading to compaction of the aquitard (or clay) layers. However, this is less so in unconfined aquifers.
SS: How was the correlation between land subsidence and groundwater extraction established in your research? How was the sampling done?
LO: In our study, we established the correlation between land subsidence and groundwater extraction using an integrated analytical framework that combined spatial, temporal, and probabilistic approaches. Sampling was conducted across 13 of the 28 US cities for which groundwater-level data were available, encompassing 90 wells classified as tapping confined, unconfined, or unknown aquifers. Only wells located within 50 metres of valid Interferometric Synthetic Aperture Radar (InSAR) observations were retained to ensure spatial co-location between geodetic and hydrologic measurements.
To evaluate spatial correlation, we compared the average vertical land motion (VLM) rates derived from InSAR with the groundwater-level trends estimated using Theil-Sen regression. These trends reflect the rate of change in hydraulic head over the study period (2015-2021). We observed a strong spatial correlation for confined aquifers, with a Pearson correlation coefficient of 0.87 and R² of 76 per cent, indicating that changes in land surface elevation closely tracked groundwater-level declines. In contrast, weaker correlations were found for unconfined (ρ = 0.43) and unknown aquifers (ρ = 0.13), likely due to greater heterogeneity in aquifer properties and delayed mechanical responses.
We further investigated the temporal dynamics by comparing groundwater-level time series and InSAR-derived displacement time series at each well location. Using lagged Pearson correlation analysis, we quantified the synchronicity between hydraulic changes and land surface deformation. The confined aquifers showed the highest average correlation (ρ = 0.5 ± 0.2), statistically significant at the 99 per cent confidence level. This result reflects the strong, and often delayed, mechanical coupling between pore-pressure decline and aquitard compaction in confined systems. By contrast, temporal correlations were notably lower and more variable in unconfined and unknown systems, underscoring the complexity of shallow aquifer responses and the potential influence of surface recharge and seasonal variability.
To characterise the dependence structure between groundwater decline and subsidence in a probabilistic framework, we employed copula-based joint modeling in five cities where confined aquifers exhibited high correlations. We fitted multiple copula families (Gaussian, t, Clayton, Frank, and Gumbel) and selected the best-fitting model based on Bayesian Information Criterion. These models allowed us to compute the conditional probability of vertical displacement exceeding a threshold (e.g., –1 mm) as a function of normalised groundwater-level change. The results showed that in cities such as New York and San Diego, subsidence became highly probable when groundwater levels fell below half the historical minimum. These findings reinforce the conclusion that confined aquifers, where present and exploited, exhibit both stronger mechanical coupling and greater vulnerability to deformation due to anthropogenic stress.
SS: Do you think satellite-based models are best for understanding land subsidence? How have such models helped you?
LO: Satellite-based models, particularly those using InSAR, have become indispensable tools for understanding land subsidence, especially in urban and regional-scale contexts. However, while they offer unique strengths, they are best viewed as complementary to in situ geodetic and hydrological techniques rather than replacements. InSAR excels in spatial resolution. It enables the generation of high-resolution, synoptic maps of vertical land motion at a scale that is unachievable with ground-based instruments alone. This capacity to monitor broad and inaccessible areas consistently over time is crucial for identifying infrastructure risks and for informing regional-scale planning and mitigation efforts. However, satellite methods have limitations, particularly in terms of temporal continuity and subsurface attribution. They provide measurements at fixed revisit intervals (e.g., 6 to 12 days for Sentinel-1) and may suffer from signal decorrelation in vegetated or snow-covered regions. For understanding fine-scale temporal dynamics, ground-based instruments offer critical advantages. For example, extensometers can measure vertical displacement at sub-millimeter precision with hourly or even higher temporal resolution. These instruments are ideal for resolving elastic and inelastic compaction in aquitards and for quantifying the lag between groundwater-level change and land surface response. Similarly, continuous GPS stations provide high-frequency measurements and are well suited for integrating hydrologic and tectonic signals over long periods. Yet, such ground-based methods require sustained maintenance, calibration, and infrastructure investments. They are also spatially limited because they are typically installed in select monitoring wells or urban centres. This means that while they offer detailed insight into local dynamics, they cannot capture regional subsidence variability or provide a comprehensive assessment of hazard exposure across entire metropolitan areas. In practice, the most effective strategies integrate satellite and ground-based data. In our own work, we combined InSAR-derived subsidence with groundwater-level time series from wells and GNSS data to establish spatial and temporal correlations. This hybrid approach enabled us to map deformation patterns, relate them to aquifer behavior, and evaluate risk to infrastructure with both breadth and depth. Therefore, while satellite-based models are exceptionally well suited for spatial characterisation and regional hazard monitoring, they are most powerful when used in conjunction with high-resolution temporal observations from ground-based systems.
SS: What are the major impacts of land subsidence in the US?
LO: Land subsidence in the US can cause impacts ranging from environmental, infrastructural, economic, and public safety domains. First, subsidence exacerbates flood risk. As land elevation decreases relative to sea level, areas become more susceptible to both tidal inundation and storm surge. Second, it causes damage to infrastructure. Differential settlement, particularly when parts of the ground sink unevenly, this can lead to cracking and distortion of buildings, roads, bridges, pipelines, and railways. Such damage is often gradual and difficult to detect until it becomes severe, increasing long-term maintenance costs and raising the risk of structural failure. Subsidence has also disrupted utilities by distorting water and sewer lines, leading to service interruptions and repair needs. Third, land subsidence contributes to land surface deformation that complicates land-use planning. It can alter surface gradients, impairing gravity-fed drainage systems and causing the unintended redirection of water flows, which may trigger localised flooding, wetland loss, or soil instability. Lastly, there are economic and social impacts. The cumulative cost of subsidence-related damage including repairs, flood protection enhancements, land rehabilitation, and insurance claims can be substantial. Moreover, subsidence often disproportionately affects economically disadvantaged communities living in vulnerable areas, intensifying issues of environmental justice.
SS: How should the framework and strategies at city level be developed to address the problem of land subsidence in the US?
LO: Cities need integrated, proactive frameworks that combine continuous monitoring, regulatory enforcement, sustainable water management, and land-use adaptation to effectively address land subsidence. Establishing high-resolution geodetic monitoring systems enables early detection of deformation hotspots and supports data-driven decision-making.
The response to subsidence often follows two main paths: mitigation and adaptation. Mitigation focuses on preventing or slowing subsidence by stabilising groundwater levels and reducing anthropogenic stress. This includes implementing managed aquifer recharge (MAR) programs, diversifying water sources through recycled water, rainwater harvesting, and strengthening regulatory controls on groundwater extraction. Well permitting, coordinated basin-level management, and enforcement mechanisms are essential components of an effective mitigation strategy. Adaptation aims to reduce the impact of ongoing or unavoidable subsidence on infrastructure and communities. This involves updating building codes and engineering standards to account for ground deformation, retrofitting critical infrastructure in high-risk zones, and designing foundations that accommodate differential settlement. Early warning systems should be established to monitor ground motion and angular distortion, triggering timely maintenance or relocation measures where necessary. Importantly, adaptation strategies must also incorporate equity considerations to avoid disproportionately affecting already vulnerable populations.
SS: As you know, cities in the Global South extensively depend on groundwater. What should these cities adopt to avoid land subsidence?
LO: Global South cities have a crucial opportunity to learn from costly mistakes elsewhere by implementing proactive measures before over-extraction becomes entrenched. Key strategies include establishing early groundwater monitoring networks using both in-situ sensors and satellite-based systems to detect aquifer stress and land deformation. Regulation and permitting systems should be implemented to maintain sustainable yield thresholds and prevent irreversible compaction. Cities should invest in diversified water portfolios including surface water infrastructure, wastewater recycling, rainwater harvesting, and locally adapted managed aquifer recharge schemes such as recharge basins or rooftop-to-recharge systems. Subsidence risk must be incorporated into land-use planning and infrastructure design from the outset. However, implementation must be adapted to local realities. In many communities where groundwater is the only feasible water source, blanket extraction moratoriums are neither practical nor equitable. Instead, adaptive governance models are needed that promote responsible use through phased regulation, public education, and community-supported recharge interventions. The goal is sustainable groundwater management that meets growing urban demands without compromising the geological foundations these cities depend upon.