Dear colleagues and friends,
A dear colleague sends us this article written by Hannah Bird, published on March 7, 2024 by PHYS.ORG and translated by us for this space. Let's see what it's all about...
Land subsidence is a geological hazard caused by the sudden or gradual (from years to decades) settlement of the land surface due to the removal of material from the subsoil. This can be due to a variety of factors, both natural (such as earthquakes, volcanic activity and compaction of unconsolidated fine-grained sediments) and anthropogenic (for example, mining and groundwater extraction). It poses a major problem in urban areas where it can cause the collapse of buildings and damage to infrastructure that can pose a danger to life and a problem of resource management.
There is currently an increase in soil subsidence in densely populated areas. A key factor that generates these subsidence is the extraction of groundwater stored in the porous space of underground layers both for human consumption and for irrigation in agriculture, which leads to the compaction of subsoil sediments, generating subsidence and subsidence of the soil above them.
This is the main focus of new research, published in Geophysical Research Letters, in which researchers identified a significant positive correlation between the rate of groundwater extraction and soil subsidence, meaning that these areas should be an important focus of water resource management to alleviate this geological risk.
Dr. Tsimur Davydzenka, a researcher at the Colorado School of Mines, and his colleagues, Dr. Pejman Tahmasebi and Professor Nima Shokri, used deep learning artificial intelligence to predict the scale of Earth's global subsidence. Explaining the importance of their research, Drs. Davydzenka and Tahmasebi explained: “Earth subsidence is a destructive phenomenon that damages infrastructure and aquifers, as well as putting human lives at risk. Demographic growth has played an undeniable role in the extraction of resources that has led to these collapses.”
“As a result of our research, we realized that there are several studies in different countries that explore local soil subsidence, but there is not enough research on a global scale. Nor do these maps provide any information about the magnitude of the collapse, which is more important than the simple description of these events with the limitation, as mentioned, that they have a regional scale. Using modern, data-based modeling techniques, we present the first global map of subsidence rates, which can be used as a basis for generating groundwater management policies and helping to mitigate these subsidence.”
The researchers used existing databases of land subsidence studies and remote sensing to generate a database in the order of 46,000 subsidence scenarios. This, together with a selection of 23 climatic, geographical and topographic conditions (including rainfall, soil composition, sediment thickness and slopes), were used to train a machine learning model, which was then able to estimate the total area of land at risk of subsidence and of the population in these areas.
The study determined that more than 6.3 million km2 of the Earth's surface (~ 5% of the world's total land surface) are susceptible to subsidence rates considered significant enough to cause damage and require mitigation strategies, which are greater than 5 mm/year. This follows from previous work that had suggested that 12 million km2 of land surface experienced subsidence rates of 430 mm/year. Of these more than 6.3 million km2, 231,000 km2 were identified in urban areas, where population density shows that ~2 billion people (25% of the world's population) are in these high-risk areas.
The machine learning model determined that groundwater extraction was the main predictor of land subsidence, followed by earthquake seismic activity, then environmental conditions (i.e., lack of precipitation) affecting groundwater recharge, the thickness of sedimentary units (larger units have more space for final compaction), the average temperature of the warmer months (important for arid and semiarid regions susceptible to subsidence), the content of soil clay and population density.
Since groundwater extraction is the main concern, Drs. Davydzenka and Tahmasebi suggest ways in which dependence on the world's population could change in the future. “To minimize dependence on groundwater, strategic measures include promoting water use efficiency, implementing strict regulatory frameworks, and encouraging agricultural practices that optimize water consumption. In addition, investing in water recovery and recycling technologies can increase water availability without relying excessively on groundwater.
“The gradual elimination of groundwater extraction could be complemented by taking advantage of alternative sources such as treated wastewater, rainwater collection and stormwater management. However, the transition should also consider the spatial limitations of reservoirs and the economic viability of seawater desalination. By integrating conservation practices, using innovative technology and diversification of water sources, a sustainable water supply ecosystem can be established, mitigating the environmental and socio-economic challenges associated with excessive groundwater extraction.”
Regarding the sediments most affected by subsidence, 3.8 million km2 of unconsolidated sediments (10% of the global extension) were identified as the primary risk, with the highest subsidence rates of 320.6 mm/year. Cultivated land constituted the largest threatened area, with 2.1 million km2 worldwide (12.2% of the world's cultivated land), while subtropical highlands and temperate oceanic climates experienced higher subsidence rates greater than 50 mm/year.
In general, South Asia is considered to have the largest area of land threatened with subsidence (2.2% of its total area experiences subsidence rates greater than 50 mm/year), as well as the largest number of people affected by these (20 million). Other countries with subsidence rates greater than 50 mm/year include the Philippines, Iran, Costa Rica, Indonesia and Uzbekistan.
While this research provides an important global map of land subsidence that will help companies, farmers and local authorities in higher-risk areas to plan for challenges they may experience in the future, further adjustment of the model is required. As such, Drs. Davydzenka and Tahmasebi state that it's “certainly possible” that future models will have a resolution high enough for individual local authorities to use the data for mitigation strategies.
For example, given the importance of groundwater extraction in the data set, including greater detail on the depth of extraction, the type of aquifer, the gap between extraction and the decrease of the water table, as well as the interaction of the oil and gas industry (which contributes to 4.36% of current subsidence records), are necessary steps to improve this vital work. As population growth increases our dependence on groundwater and climate change intensifies droughts, the impact of diminished groundwater on land subsidence will continue to be an increasingly pressing problem in the years to come.
