The technical achievement at the heart of the study lies in its methodology. The researchers drew on interferometric synthetic aperture radar, or InSAR, a technique that compares phase differences between radar images of the same location captured at different times by the European Sentinel-1 satellites. Because the radar signal travels to the ground and back, even millimeter-scale shifts in the surface alter the phase of the returning wave. By combining thousands of such acquisitions in a small-baseline time-series framework, the team constructed high-resolution maps of what they call relative local land subsidence for every city in their sample, all processed with a single, uniform pipeline.

That consistency matters. Earlier global assessments relied on heterogeneous data sources and varying methods, making it difficult to compare one city with another or to prioritize interventions. Previous regional studies had documented dramatic cases—Jakarta’s sinking districts, Mexico City’s century-long descent, the subsiding metropolises of coastal China and Iran’s depleting aquifer basins—but a globally coherent baseline was missing. The new work fills that gap, providing velocity estimates, coherence metrics, and uncertainty measures for all 465 cities, and offering them through an interactive visualization platform built on Google Earth Engine, alongside downloadable maps and publicly released processing code.

One of the study’s central methodological challenges is the distinction between relative and absolute motion. InSAR measures ground movement relative to a reference point, and because radar satellites orbit the Earth rather than sit on it, the resulting velocities can carry systematic biases related to satellite orbits, atmospheric delays, and the choice of reference frame. To address this, the team aligned their relative local land subsidence maps to a global vertical land motion product derived from a worldwide network of GNSS stations. The correction anchors the satellite measurements to an absolute terrestrial reference frame, allowing subsidence in, say, Manila to be compared directly with subsidence in Houston.

Instructively, the researchers found that the broad spatial patterns of subsidence remained largely consistent after this vertical land motion alignment—although the correction introduced additional uncertainty. This is a valuable lesson for the field: relative measurements, carefully referenced, can capture the geography of sinking ground even without absolute anchoring, but the absolute version is essential when subsidence must be combined with sea-level rise projections to estimate flood risk. For coastal cities, where the ocean sets a fixed datum, the difference between relative and absolute subsidence is not academic; it determines how quickly dry land becomes inundated land.

The geographic findings are striking. Southeast Asia emerges as one of the most severely affected regions, a result consistent with decades of local studies implicating intensive groundwater extraction from compressible deltaic sediments. West and Central Asia also stand out, where aquifer depletion beneath arid, rapidly growing cities has produced some of the fastest sinking rates ever measured. North America’s inclusion among the hotspots reflects both well-documented cases, such as Houston’s ongoing descent tied to groundwater pumping and oil extraction, and newer analyses revealing that sizable stretches of the United States coastline are quietly subsiding, compounding the threat of rising seas.

Equally important is what the study reveals in places that have rarely been measured. South America, Africa, and Russia—regions historically under-represented in subsidence literature—show clear signals of ground deformation in the new global maps. This matters because subsidence hazard has often been assumed to be a problem of specific geographies: deltas, coastal plains, and arid zones with heavy irrigation. The new dataset suggests the phenomenon is far more widespread, and that cities without monitoring infrastructure or historical records may nonetheless harbor significant, unrecognized risk. Filling these blind spots is precisely what a uniform global survey can do that scattered national studies cannot.

Beyond mapping where the ground is sinking, the study breaks new ground by tracking how subsidence is changing over time. Using changepoint detection and piecewise regression techniques, the team identified marked deceleration in many cities across Southeast and East Asia—a signal that, in some places, management interventions may be working. Tokyo’s famous turnaround, achieved through strict groundwater regulation after decades of sinking, has long served as the canonical example that subsidence can be halted. The new evidence of deceleration across multiple Asian cities hints that similar dynamics—reduced pumping, artificial aquifer recharge, surface water substitution such as China’s south-to-north water diversion project—may be leaving measurable fingerprints in the satellite record.

Yet the picture is not uniformly hopeful. The researchers found that substantial urban areas within those same regions continue to show accelerating subsidence, meaning that city-level averages can conceal neighborhoods where the ground is falling faster than ever. This spatial heterogeneity is one of the strongest arguments for high-resolution monitoring: a single number for a megacity cannot capture the difference between a district resting on stable bedrock and one sinking into soft lacustrine clay. The trajectory analysis, which distinguishes accelerating, decelerating, and stable zones within individual cities, gives planners a tool for targeting adaptation measures—flood defenses, building retrofits, water policy—where they are needed most.

The physical mechanism underlying most of this sinking is well understood. When groundwater is pumped from sand and gravel aquifers interbedded with clays and silts, water pressure that once supported the sediment grains drops, and the fine-grained layers compress. Crucially, much of this compaction is inelastic: once the aquifer skeleton collapses, it does not rebound when pumping stops, meaning lost aquifer storage capacity is permanently forfeited. Add the weight of dense urban construction, natural sediment compaction in deltas, drainage of organic soils, and in some places tectonic motion, and cities face a compound hazard in which sinking land amplifies every other consequence of climate change, from storm surge to river flooding.

What makes this study likely to resonate far beyond the geodesy community is its timing and its openness. With coastal populations growing and climate-driven sea-level rise accelerating, the intersection of sinking land and rising water is one of the defining risk multipliers of the century. By releasing the full subsidence maps, the code that produced them, and an interactive global viewer, the research team has transformed what was once scattered specialist knowledge into a shared public resource. City governments, insurers, and engineers can now locate themselves on a common global baseline, and the deceleration signals documented in parts of Asia offer something rare in climate adaptation: evidence that determined policy can visibly bend the curve of a hazard measured from orbit.

Subject of Research: Global satellite-based assessment of urban land subsidence and its changing trajectories in 465 major cities

Article Title: A global view of urban land subsidence and its changing trajectories

Article References: Wang, L., Ao, Z., Tao, S., Hu, X., Hu, X., Ran, Y., Wang, G., Zhao, K., Bai, H., Xiao, J., Xue, B., Zhao, X., Zhang, F., Jiang, K., Lin, Y., Huang, Y., Gao, Y., Jiang, T., Peng, Z., … Fang, J. (2026). A global view of urban land subsidence and its changing trajectories. Nature Cities. https://doi.org/10.1038/s44284-026-00527-8

Image Credits: AI Generated

DOI: 10.1038/s44284-026-00527-8

Keywords: land subsidence, InSAR, urban geohazards, Sentinel-1, groundwater extraction, vertical land motion, sea-level rise, Nature Cities, remote sensing, sinking cities, aquifer compaction, global mapping

The scale of the analysis is extraordinary. Drawing on more than two million rainfall–runoff events, the researchers assessed the behavior of more than 80,000 catchments across 97 countries. A catchment, in hydrological terms, is the area of land that drains to a particular point in a river network, and understanding how it converts rain into streamflow is one of the oldest problems in the discipline. What makes this study unusual is its focus. Rather than mapping where rivers are or how much water they carry, the team mapped how consistently landscapes perform that conversion from one storm to the next. The answer, it turns out, varies dramatically across the globe in ways that follow identifiable climatic and physical patterns.

The researchers sorted catchments into three behavioral classes. In simple catchments, a given amount of rain delivers roughly the same amount of water to the river every time, producing a predictable, repeatable response. In complex catchments, similar amounts of rainfall can generate very different runoff responses, with the outcome shaped by antecedent conditions and by which parts of the landscape are actively supplying water to the stream during any given storm. Between these extremes sits a third, intermediate group, which the researchers found can respond to rain in more than one stable way, perhaps two or three distinct modes, and switches among them depending on circumstances. The geographic verdict was stark: complex behavior covered 87 percent of the land area evaluated, some 121 million square kilometers, while simple catchments accounted for just 1.5 percent.

That asymmetry carries real consequences. Simple catchments are so rare worldwide that they register almost as curiosities, concentrated along the Coast Mountains of Canada’s British Columbia and the U.S. Pacific Northwest, with smaller pockets in the northern and western United Kingdom, parts of Ireland and northern Spain, and in Tasmania and New Zealand. Forested catchments near Bamfield on Vancouver Island’s west coast, where rain arrives almost every week in winter, exemplify this rare class, and this stretch of coast holds one of the largest concentrations of simple catchments anywhere in the study. Complex catchments, by contrast, dominate across Africa and much of Asia, across most of France, Spain, eastern Germany and Denmark, across the central United States and across most of South America. Intermediate catchments are common across the eastern United States, where landscapes appear to hold several stable response modes in reserve.

What separates the predictable few from the unpredictable many? The strongest single factor was not how much rain a region receives but how often meaningful rainfall occurs over time. Where rain arrives regularly, soils, subsurface stores and the pathways connecting the landscape to the stream remain in similar states from one storm to the next, so a comparable input of rain is more likely to produce a comparable output of runoff. Where rain is less persistent, a catchment repeatedly swings between wet and dry conditions. A storm arriving after a dry spell may largely soak into soils and subsurface storage while sending relatively little water to the river, whereas an identical storm arriving when the landscape is already saturated can send a far larger share of that rain straight downstream. The rhythm of rainfall, in other words, sets the variability of response.

Dr. Ameli framed the finding as a challenge to a long-standing mental model in hydrology. We often imagine a catchment as a pipe, he noted, in which a certain amount of rain in produces a predictable amount of runoff out. But most catchments behave more like dynamic systems with a memory, in which how much of today’s rainfall becomes runoff depends both on conditions created by earlier weather and on which parts of the landscape supply water to the river during the storm. It is not simply the amount of rain that matters, he explained; it is also the rhythm of rainfall. Persistent rain keeps the landscape in a more consistent state, while alternating wet and dry conditions allow the same catchment to behave very differently from one event to the next.

Climate did not act alone. The researchers also found that a region’s overall water availability strongly distinguishes simple from complex catchments, and they examined this using established measures including an aridity index, which compares how much water the atmosphere can draw off a landscape with how much rain actually falls on it, and the long-term balance between rainfall and the water lost to evaporation or taken up by plants. Catchments with simpler, more consistent behavior were generally found in wetter and more humid conditions, while complex catchments were more commonly associated with drier settings, where less water remains once evaporation and plant use are accounted for. Taken together, these results suggest that overall water availability establishes the broad climatic setting, and that rainfall rhythm determines whether the landscape is likely to remain in a stable hydrologic state or alternate among different states.

The physical character of a catchment added a further layer of refinement at local and regional scales. Although climate emerged as the dominant influence, terrain mattered too. Simple catchments tended to occur in steeper landscapes, where slopes can move water rapidly and maintain more direct connections between hillslopes and stream channels, contributing to a more consistent runoff response from one storm to the next. This helps explain why the steep, rain-drenched mountains of the Pacific Northwest and coastal British Columbia host some of the world’s most predictable catchments, while gentler terrain in seasonally variable climates produces the complex, state-dependent behavior that dominates the global map.

The practical payoff of the work lies in modelling and prediction. The study and its accompanying products give hydrologists a common language for comparing catchments in different parts of the world: two catchments on opposite sides of the planet may convert rain to runoff in the same way, and the classification makes that visible at a glance. It also offers a practical guide to how a catchment should be represented in a computer model. A simple catchment can be captured by a straightforward model, while a complex one demands a model that allows different parts of the landscape to take the lead at different times, tracking how active runoff zones shift as conditions change. For flood forecasters, water managers and anyone charged with predicting how a river will respond to the next storm, that distinction could be decisive.

To make the results immediately usable, the team has released an interactive global map of the classification along with a free web application that returns a classification for any user-supplied catchment boundary, and both are publicly available. For a field in which much of the world’s stream network is ungauged, meaning no instrument continuously records its flow, a tool that infers behavioral class from climate and landscape attributes alone extends the reach of hydrological knowledge far beyond the instrumented record. The research, published as a peer-reviewed observational study in Nature Water under the title describing a global classification of hydrologic functional diversity in gauged and ungauged catchments, appeared on 11 September 2026, and its authors declare no competing interests. Its central message is likely to resonate well beyond hydrology: the landscapes that deliver our water are not passive pipes but systems with memory, and reading that memory correctly may be the key to anticipating the floods of the future.

Subject of Research: Global classification of rainfall–runoff behavior and hydrologic functional diversity in catchments

Article Title: Why most rivers don’t respond to storms the same way twice

Article References: Why most rivers don’t respond to storms the same way twice. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: hydrology, catchments, rainfall-runoff, flooding, Nature Water, runoff prediction, aridity index, climate, watersheds, flood modelling, University of British Columbia, global mapping