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Accelerated relative sea-level rise (SLR) rate is driven by by changing climate and vertical land motion (VLM) at the world's coastlines. Today, many coastal environments and communities, such as river deltas, wetlands, and cities experience accelerated land subsidence rates due to human-induced processes such as groundwater extraction and coastal populations ( >500M people worldwide) experience, on average, a four times higher relative sea-level rate than the global mean average. Although subsidence is the dominant force driving relative SLR worldwide, its effect is often still overlooked and/or not fully integrated into global and regional sea-level rise projections. As a result, SLR impact assessments underlying coastal adaptation plans for many governments around the world underestimate future relative SLR rates and consequent flood risks. It is more important than ever for the global scientific community on coastal land subsidence to unite, in a similar fashion as the IPCC has come into existence in the 80's, to combat the growing global challenge of land subsidence. For this purpose the International Panel on Land Subsidence (IPLS) (www.IPLSubsidence.org) was recently launched. The IPLS initiative welcomes all experts from disciplines related to coastal land subsidence, elevation dynamics and relative sea-level rise. The IPLS is envisioned to grow in the coming years connecting the different research communities working on coastal VLM, to become a global focal point of scientific knowledge on coastal land subsidence and create consistent contemporary rate and projections of coastal VLM for the world’s coastlines. The IPLS aims to connect the different research communities working on VLM and elevation change in the coastal zone, consolidate knowledge and identify gaps. The IPLS’s first milestones are to 1) present the First Global Assessment Report on Land Subsidence, including 21st-century projections of land elevation, to inform governments, scientific communities and the public worldwide and 2) propose a consistent framework to combine VLM and sedimentation dynamics across scales and disciplines, 3) properly integrate contemporary and projected VLM and coastal elevation change into IPCC's AR7. The IPLS welcomes experts from all related disciplines to join the initiative and become part of this global effort.

Saltwater intrusion in coastal aquifers is a global problem recently worsened by anthropogenic activities (e.g., aquifer overexploitation, hydraulic reclamation and drainage of low-lying areas) and climate change effects (e.g., severe droughts, sea level rise) that contribute to reduce groundwater natural recharge, water quality, and agricultural production. Many low-lying coastal plains facing the Adriatic Sea are strongly affected by saltwater intrusion with serious consequences on agricultural activities and tourism that may become dramatic in a relatively short time due to climate change. In this framework, this work aims to identify monitoring strategies to characterize the process of saltwater intrusion under the effects of climate change and recommend appropriate countermeasures in two Adriatic low-lying coastal plain: south of the Venice Lagoon (north-eastern Italy), and at the Neretva River mouth (south-eastern Croatia).Geomorphologic, stratigraphic, hydrogeologic, and agricultural data were collected to characterize the aquifer system at both sites and assess the effects of seawater intrusion on agricultural productivity. Saltwater intrusion was monitored and analysed through monitoring systems that provide qualitative and quantitative information on the processes influencing groundwater and surface water dynamics within the two coastal systems. Moreover, laboratory physical models were developed to serve as benchmarks for the numerical models used to simulate the field results. Numerical modelling reliably implements boundary and initial conditions defined in-situ on both sites, simulates existing states, specifies different scenarios, and predicts salinization dynamic changes caused by climate changes.The results of the research activities include the development of specific tools for the management of agriculture-related activities and freshwater resources in coastal areas including vulnerability assessment, mitigation plans, and countermeasures against salt contamination. These results were obtained by integrating the findings gained on both sites, considering differences and peculiarities of the specific areas that are representative of many low-lying plains located on both sides of the Adriatic coast.This study has been funded by the contribution from the EU cofinancing and the Interreg Italy–Croatia Cross Border Collaboration (CBC) Programme 2014–2020 (Priority Axes: Safety and Resilience) through the European Regional Development Fund as a part of the projects MoST (AID: 10047742) and SeCure (AID: 10419304).

The coastline of the African Gulf of Guinea hosts large river deltas, coastal wetlands and mangrove ecosystems. It is in places densely populated, with many capital megacities with millions of inhabitants such as Lagos, Abidjan and Accra situated at the coast. And population projections for these cities suggest a continued staggering increase in the coming decades. Also, many of the economic activities are located along the coastline. For example, Nigerian coastal areas are home to 85% of the nation industry and to more than 100 million people. On the other hand, the coastland retains pristine transitional environments formed during the Holocene, well recognized for the great biological richness and the high level of endemism. These pressures and foreseen growth make these lowly elevated coastal areas particularly vulnerable to relative sea-level rise (SLR).Regional or local impact studies of climate-change induced global sea-level rise are scarce and, when available, do not account for vertical land motion, i.e. land subsidence. Coastal land subsidence is critically under-quantified at global scale and the Gulf of Guinea region is no exception to this. Meanwhile, examples elsewhere in the World show coastal economic development and population growth can accelerate land subsidence to rates that are magnitudes larger than global SLR. With the fast-paced developments taking place along the Gulf of Guinea’s coastline, land subsidence poses an unknown but potentially large hazard to this region.Here, we present the first findings in our work to assess coastal subsidence in this fast-changing region of the World, through regional and local case studies and satellite assessments. By building on the recent advancements in global elevation data, we aim to update coastal elevation estimates and identify low lying hotspots with high vulnerability to relative SLR. Ultimately, we aim to project coastal elevation change for this important region and, in case of human-induced land subsidence, show how much relative SLR can potentially be avoided by changing to sustainable use of natural resources. Hence, we may be able to alter the fate of deltas, wetlands and coastal cities by quantifying and addressing land subsidence as early as in this fast growing region of the world.

Globally, land subsidence caused by groundwater pumping is a common phenomenon. Numerous subsurface processes, both natural and anthropogenic, contribute to its occurrence. In coastal regions, severe land subsidence exacerbated by groundwater pumping is particularly detrimental. On top of that, coastal areas affected by natural compaction, river delta consolidation processes, and additional exposure to drainage-induced aquifer-system compaction are especially susceptible to flooding and salinization due to the steadily rising sea level triggered by climate change.The Mekong delta, one of the world's largest deltas, is densely populated and crucial for agricultural production. The delta is low-lying and has a high rate of natural compaction, whereas human activities accelerate land subsidence. A numerical model of groundwater-extraction-induced aquifer-system compaction was developed in 2017 to demonstrate the effects of 25 years of groundwater extraction on land subsidence in the delta. The model encompassed the time range from 1991 to 2016 using geological, hydrogeological, geomechanical, and remote sensing data. In 2020, the model was updated to include a surface water network. Six scenarios were developed to simulate potential future pathways of hydraulic head evolution and aquifer-system compaction in the Mekong delta from 2019 to 2100.Our research aims to enhance the reliability of the existing numerical model of groundwater extraction-induced aquifer-system compaction in the Mekong delta, given the significance of such scenarios in the development of policies to mitigate the negative effects of groundwater pumping. Our research focuses on four steps.First, a novel subsurface model representation.The 3D subsurface model of the Mekong delta was developed using ten hydrogeological cross-sections derived from 96 geological borehole logs interpolated linearly. This resulted in a subsurface model consisting of 15 layers, including seven aquifers, seven aquitards, and a phreatic top layer. The goal of the current study is to develop a new schematisation of the aquifer system within the Mekong delta based on 522 borehole logs and to investigate the spatial variability of the aquifer system using advanced geostatistical tools.Second, a hydrogeological schematization enhancement.In the current schematisation, aquitards are discretized as a single layer, resulting in the inability to simulate delayed groundwater pressure propagation within the aquitard. Several additional models with refined aquitard discretization are constructed and compared to evaluate the effect.Third, the quantification of the influence of deterministic modelling on compaction.The hydrogeological model is deterministically parameterized and calibrated using hydraulic head time series. Utilizing stochastic modelling of hydrogeological parameters, the impact of this deterministic modelling approach on simulated compaction is determined.Fourth, a hydrogeological and geomechanical parameters consistency improvement.The previous hydrogeological and geomechanical model parameterizations are inconsistent since the groundwater model and the geomechanical module were initially parameterized and calibrated independently. To address this issue, an iterative procedure is used to calibrate storage and compression indexes consistently for each individual model layer. This is accomplished by utilising groundwater head datasets recorded by 358 piezometers and land subsidence datasets retrieved by InSAR from 2006 to 2010 and 2016 to 2019.

Land subsidence is one of most severe geohazards caused by excessive groundwater pumping which has gained increasingly interest over the last decades. Various numerical models have been developed to address this mechanism and support the groundwater policy-makers. In addition, the performance of these models has highlighted the importance of uncertainty quantification related to the unknown hydro-mechanical parameters. Bayesian inversion provides a powerful tool to statistically infer these uncertain parameters by incorporating the numerical model and the available observations. However, Bayesian scheme relies on the sampling algorithm to explore the parameter space. Such exploration requires enormous computational cost in which one realization of numerical model is already costly. Hence, parallel research focuses on surrogate models which aim to fast and accurately approximate the numerical solution with limited training realizations. In this study, Bayesian inversion is facilitated by substituting a coupled groundwater flow-geomechanical model with a surrogate model based on the sparse grid approach. More specifically, a 3D coupled variably-saturated groundwater flow-geomechanical model is first performed to describe the pressure head variation and deformation to the groundwater extraction from 1960 to 2012 in Alto Guadalentín Valley aquifer, Spain. Then, sparse grid method is used to construct the surrogate models which approximate the input/output mapping of the numerical simulator. Lastly, Monte Carlo Markov Chain yields the uncertainties of hydraulic conductivity and compressibility by assimilating the piezometric head records and displacement measurements obtained from Interferometric Synthetic Aperture Radar (InSAR) technique. Our preliminary results demonstrate that the surrogate model has high and fast performance on approximating the state variables in which misfits is negligible with respect to the measurement noise. Bayesian inversion can improve the characterization of parameters of interest whose posterior distributions are significantly constrained comparing to the prior distributions. Moreover, the numerical outcomes with calibrated parameters show a good fit with the available observations. In summary, the illustrated framework takes advantage of novel techniques from various aspects, including monitoring, numerical modeling, statistical analyses and provides a reliable and efficient way to infer properties of aquifer systems with ongoing water pressure depletion.

Aseisimic earth fissures are complex consequences of groundwater withdrawal and natural hydrogeologic conditions. This paper aims to improve the understanding of the mechanism of earth fissuring and investigate the relative importance of various factors to fissure activity, including bedrock geometry, piezometric depletion, compressibility and thickness of the exploited aquifer. For these purposes, a test case characterized by an impermeable and incompressible rock ridge in a subsiding basin is developed, where stress/displacement analyses and fissure state are predicted using an interface finite element model. Three different methods for global sensitivity analysis are used to quantify the extent of the fissure opening to the aforementioned factors. The conventional sampling based Sobol' sensitivity analysis is compared to two surrogate based methods, the general polynomial chaos expansion based Sobol' analysis and a feature importance evaluation of a gradient boosting decision tree model. Numerical results indicate that earth fissure is forming in response to tensile stress accumulation above the ridge associated to pore pressure depletion, inducing the fissure opening at land surface with further downward propagation. Sensitivity analysis highlights that the geometry of bedrock ridge is the most influential feature. Specifically, the fissure grows more when the ridge is steeper and closer to the land surface. Pore pressure depletion is a secondary feature and required to reach a certain threshold to activate the fissure. As for this specific application, the gradient boosting tree is the most suitable method for its better performance in capturing fissure characteristics.

Anthropogenic land subsidence can be evaluated and predicted by numerical models, which are often built over deterministic analyses. However, uncertainties and approximations are present, as in any other modeling activity of real-world phenomena. This study aims at combining data assimilation techniques with a physically-based numerical model of anthropogenic land subsidence in a novel and comprehensive workflow, to overcome the main limitations concerning the way traditional deterministic analyses use the available measurements. The proposed methodology allows to reduce uncertainties affecting the model, identify the most appropriate rock constitutive behavior and characterize the most significant governing geomechanical parameters. The proposed methodological approach has been applied in a synthetic test case representative of the Upper Adriatic basin, Italy. The integration of data assimilation techniques into geomechanical modeling appears to be a useful and effective tool for a more reliable study of anthropogenic land subsidence.

The regular monitoring of the relative position of a sequence of radioactive bullets shot through the well of a vertical borehole can provide in-situ measurements of deep rock compaction. Developed in the '70s, this technology has experienced a growing interest in the '90s, but in recent years, its use and relevance in land subsidence management above producing hydrocarbon reservoirs have been often debated. The present communication analyses the state of the art of the radioactive marker technique and provides a critical review on the role that these measurements might play in the future evolution of land subsidence monitoring and modelling.

The numerical prediction of land subsidence above producing reservoirs can be affected by a number of uncertainties, related for instance to the deep rock constitutive behavior, geomechanical properties, boundary and forcing conditions, etc. The quality and the amount of the available observations can help reduce such uncertainties by constraining the numerical model outcome and providing more reliable estimates of the unknown governing parameters. In this work, we address the numerical simulation of land subsidence above a producing hydrocarbon field in the Northern Adriatic, Italy, by integrating the available monitoring data in the computational model with the aid of Data Assimilation strategies. A preliminary model diagnostic analysis, i.e. the χ2-test, allows for identifying the most appropriate forecast ensemble. Then, a Bayesian approach, i.e. the Red Flag technique, and a smoother formulation, i.e. the Ensemble Smoother, provide a significant reduction of the prior uncertainties. The experiment developed on a real-world gas field confirms that the integration of monitoring observations with classical geomechanical models is a valuable approach to improve the reliability of land subsidence predictions and to exploit in a systematic way the increasing amount of available measurement records.

The extensive loss of land elevation and the consequent exposure to flood hazards are seriously threatening the long-term survival of the Mississippi Delta. Shallow compaction of the top soil is one of the major components contributing to the relative sea level rise. In the last decades, more subsidence measurements have become available and recent studies demonstrate that compaction of Holocene strata is dominant. Here we propose a novel application aimed at modeling the present-day shallow compaction due to consolidation processes in the top soil. Soil compaction is properly computed and accounts for the large soil grain motion and the delayed dissipation of pore-water overpressure. The grain motion is described by means of a Lagrangian approach with an adaptive mesh where the grid nodes follows the accretion/compaction processes. Model calibration is obtained from stratigraphic and geochrology information collected at the Myrtle Grove Subsidence Superstation, where a nearly 40 m-deep sediment core that penetrates the entire Holocene succession allows testing model results over long (millennial) timescales. Model validation with available observations from rod surface-elevation table – marker horizon (RSET-MH) data enables the model to predict future scenarios.

A critical issue concerning geomechanical safety for UGS (underground gas storage) in compartmentalized reservoirs is fault reactivation. Indeed, the displacement (land subsidence, land upheaval) and the stress fields caused by the seasonal injection and production of CH4 into and from deep reservoirs is peculiar. The need of improving our understanding of compartmentalized reservoir behavior and to define safe bounds for the pressure fluctuation in order to prevent undesired land movements and induced seismicity is becoming even more important. This also in view of the expected energy transition when large amount of green energy will potentially be stored and recovered through UGS of compressed air or hydrogen. In this framework, an in-depth modelling investigation has been carried out for the typical UGS geological setting and operations in The Netherlands. The specific goals of the study are the following: (i) explaining the possible mechanisms responsible for seismic events unexpectedly recorded during UGS phases; (ii) understanding which are the critical factors (e.g. the geological configuration, the geomechanical properties, and the reservoir operations) that increase the probability of fault reactivation during the various UGS stages; and (iii) advancing possible guidelines for safe UGS operations. This contribution summarizes the main outcomes obtained by the modelling simulations: the combinations of factors causing fault reactivation during primary production (PP) are also more prone to generate fault failure during cushion gas injection (CG) and UGS. In fact, fault activation during PP leads to a stress redistribution and a new (deformed) “equilibrated” configuration that is newly loaded, in the opposite direction, when the pressure variation changes the sign because of CG and/or UGS. Finally, the various combinations have been ranked to highlight the conditions where the fault system is most likely reactivated during CG and UGS operations: the initial stress regime of the system, the geomechanical properties of the fault, and dislocation of the reservoir compartments are the major influencing drivers to fault instability.

The use of numerical models for land subsidence prediction above producing hydrocarbon reservoirs has become a common and well-established practice since the early '90s. Usually, uncertainties in the deep rock behavior, which can affect the forecast capability of the models, have been taken into account by running multiple simulations with different constitutive laws and mechanical properties. Then, the most uncertain parameters were calibrated to reproduce available subsidence measurements. The objective of this work is to propose a novel methodological approach for land subsidence prediction and uncertainty quantification by integrating the available monitoring information in numerical models using ad hoc Data Assimilation techniques. The proposed approach allows to: (i) train the model with the available data and improve its accuracy as new information comes in, (ii) quantify the prediction uncertainty by providing confidence intervals and probability measures instead of deterministic outcomes, and (iii) identify the most appropriate rock constitutive model and geomechanical parameters. The methodology is tested in synthetic models of production from hydrocarbon reservoirs. The numerical experiments show that the proposed approach is a promising way to improve the effectiveness and reliability of land subsidence models.

Relative sea-level rise (RSLR), i.e. sea-level rise due to climate changes combined with land subsidence, is one of the processes that is most severely threatening the coastal systems around the world. The Venice coastland forms the major low-lying area in Italy and encompasses a variety of environments, such as farmlands, estuaries, deltas, lagoons and urbanized areas. Valuable ecosystems, historical heritages and economic activities are located in this area. Since most of the territory lies at a ground elevation below or slightly above the mean sea-level, also a few mm yr−1 of land subsidence can seriously impacts on the Venice coastal system. In this study, we present an analysis of the vulnerability to RSLR considering an uneven land subsidence distribution, with an application on the Venice coastland. The analysis is delineated at the regional scale by an index-based model and a proper coupling of various thematic layers, such as high spatial resolution land subsidence data retrieved by satellite SAR interferometry, ongoing and projected sea-level rise trends, and morpho-physiographic setting of the coastland.

Tidal marshes are fundamental ecosystems to be preserved and restored to maintain their vital services to the environment and human life. For this reason, many restoration projects have been implemented in the Lagoon of Venice (Italy) to reestablish former tidal marshlands. One fundamental point of the marsh restoration design is the determination of its long-term elevation. This is crucial for the ecological functioning of the system as well as the ability of the landform to keep pace with a rising sea level. Past marsh reconstruction projects have not always been successful. Significant areas become permanently submerged by the sea only few years after their construction and/or vegetation cover remains more patchy and less biodiverse than on natural marshes. Two design parameters which have not received sufficient attention in restoration projects are autocompaction of nourishment sediments and subsidence of underlying strata. To this aim, the planned elevation at the end of the nourishment phase, i.e. the volume of sediments used to build-up the marsh, must take into consideration nourishment autocompaction and land subsidence of the underlying lagoon bottom caused by the nourishment load. To enable monitoring of these dynamics of elevation change, we developed a novel Nourishment Elevation Change (NEC) station to investigate compaction and subsidence of an artificial marsh under development in the central basin of the Lagoon of Venice. Each NEC station is made of four steel bars set into the lagoon subsurface down to a 2-m depth. Their role is to keep a central steel pole free to move vertically with respect to its specific foundation level. The foundation consists of a plate resting either on the top of the pristine lagoon bottom or an anchor inserted into the subsurface to a depth of interest, e.g., 1 m. As the nourishment areas become inaccessible after its development, the pole is marked with a black-and-white striping to be able to measure its movements from a distance and equipped with a horizontal plate on top of the pole. A monitoring network consisting of 10 NECs was established in the artificial marsh area of about 61.000 m2 before the nourishment. The NEC station elevation is monitored with a mm-accuracy topographic intersection technique using a total station. Two stable benchmarks positioned in a nearby existing marsh are used as reference. The maximum distance between the NECs and the benchmarks amounts to 300 m. In addition, the NECs are monitored using aerial drone photogrammetry. The change over time of the distance between NEC top plate and the marsh platform allows quantifying the nourishment autocompaction. The topographic intersection surveys have been ongoing every two weeks since the nourishment started in October 2021. Over the first month of sediment filling a maximum subsidence of about 7 cm has been measured by the NEC station located closest to the nourishment pipe. The other NECs, which are not yet affected by sediment deposition, remained stable. The NEC monitoring system seems promising and will provide quantitative information on the elevation dynamics of newly created artificial marshes.

Depositional landforms, such as tidal marshes and deltas, formed by sediment deposition over the last centuries to millennia. They are complex, vulnerable, and dynamic systems with important roles from environmental and human points of view. The survival of these lowlying landforms is threatened by multiple stressors, e.g. sea level rise and reduction in sediment supply. In addition to these external factors, natural compaction of the sedimentary bodies may have an important role on the elevation dynamics because of the large porosity and compressibility that characterize the shallow deposits. A three-dimensional (3D) finite element simulator (NATSUB3D) has been recently developed to model the long-term dynamics of transitional landforms. The model couples a 3D groundwater flow module to compute over-pressure dissipation with a 1D compaction module based on the elasto-plastic Terzaghi theory. NATSUB3D properly accounts large deformations thanks to an accreting/compacting mesh that follows the grain movements (Lagrangian approach). The NATSUB3D formulation is updated to account for viscous deformations using the NEN-Bjerrum constitutive relationship. Indeed, creep may represent an important process in fine unconsolidated deposits forming Holocene coastal landforms. With the new constitutive model, soil deformation is effective stress and time dependent. The new simulator has been applied on 1D synthetic cases mimicking long-term accretion of sedimentary columns. Hydro-geomechanical properties typical of sediment classes composing depositional landforms are used. A sensitivity analysis has been performed on sedimentation rates and secondary compression coefficients, which are the main parameters affecting the viscous deformation, leading to significantly different elevation dynamics. In the simulation of these processes (i.e., the formation of a sedimentary landform), the overconsolidation ratio (OCR), which is the geomechanical parameter most difficult to quantify and highly impacting soil compaction, can be simply set equal to 1 for the newly formed soil layer. Indeed, OCR is then properly updated by the model itself because the simulation follows the soil deformation since time of sediment deposition, with soil experiencing compaction because new sedimentation occurs on the landform top.

Earth fissure associated with groundwater pumping is a severe geohazard jeopardizing several subsiding basins generally in arid countries (e.g., Mexico, Arizona, Iran, China, Pakistan). Up to 15 km long, 1–2 m wide, 15–20 m deep, and more than 2 m vertically dislocated fissures have been reported. A common geological condition favoring the occurrence of earth fissures is the presence of shallow bedrock ridge buried by compacting sedimentary deposits. This study aims to improve the understanding of this mechanism by evaluating the effects of various factors on the risk of fissure formation and development. Several parameters playing a role in the fissure occurrence have been considered, such as the shape of the bedrock ridge, the aquifer thickness, the pressure depletion in the aquifer system, and its compressibility. A realistic case is developed where the characteristics of fissure like displacements and stresses are quantified with aid of a numerical approach based on finite elements for the continuum and interface elements for the discretization of the fissures. Modelling results show that the presence of bedrock ridge causes tension accumulation around its tip and results in fissure opening from land surface downward after long term piezometry depletion. Different global sensitivity analysis methods are applied to measure the importance of each single factor (or group of them) on the quantity of interest, i.e., the fissure opening. A conventional variance-based method is first presented with Sobol indices computed from Monte Carlo simulations, although its accuracy is only guaranteed with a high number of forward simulations. As alternatives, generalized polynomial chaos expansion and gradient boosting tree are introduced to approximate the forward model and implement the corresponding sensitivity assessment at a significantly reduced computational cost. All the measures provide similar results that highlight the importance of bedrock ridge in earth fissuring. Generally, the steeper bedrock ridge the higher the risk of significant fissure opening. Pore pressure depletion is secondarily key factor which is essential for fissure formation.

Costal reclaimed farmlands are commonly threatened by saltwater intrusion and peat-driven salinity, resulting in low and unstable agricultural productions. Climatic variables have a great effect on soil moisture and salinity influencing crop production during the various growing seasons. For this reason, monitoring soil water and salinity dynamics in the root zone during the crop growing season is fundamental to conceive mitigation strategies (e.g., precision irrigation techniques). To this end, a monitoring network was installed in an agricultural field located at the southern margin of the Venice Lagoon. Three soil-stations were placed along the main sandy paleochannel crossing the farmland southwest to northeast (stations S1, S2, and S3), while stations S4 and S5 were placed in two silty-loamy areas with high peat content. Each station was equipped with three T4e tensiometers (UMS GmbH, Munchen, Germany) at 0.3, 0.5, and 0.7 m, four Teros 12 sensors (METER Group, Inc., Pullman, WA, USA) measuring volumetric water content, temperature, and electrical conductivity (ECb) at 0.1, 0.3, 0.5, and 0.7 m. In addition, a 2 m deep piezometer was installed to monitor groundwater electrical conductivity (ECw) and depth to the water table. Soil samples were collected on each monitoring location and analyzed for texture, bulk density (BD), soil organic carbon (SOC), electrical conductivity (EC 1:5), pH, and cation exchange capacity (CEC). Moreover, a weather station was installed in the experimental field to accurately monitor the local meteorological conditions during the 2019 and 2020 growing seasons. The soil monitoring dataset shows that ECb increases with depth at all locations. Moreover, rainfall events higher than 10 mm/day caused an increase in the ECb at all layers and stations. The monitoring stations inside the paleochannel showed lower ECb if compared to station S4 and S5, probably due to the highest hydraulic conductivity and, consequently, the highest leaching capacity. S5 was characterized by the highest peat content and showed the highest salinity in both soil and groundwater. In general, soil ECb and groundwater ECw showed similar behavior in 2019 and 2020, except for S4 and S5 that were saltier in 2019. These preliminary analyses demonstrated a strong influence of rainfall events on salinity behavior and highlights how climatic variables, soil heterogeneity, and saltwater intrusion at depth play an important role in the complex salinity dynamics within the root zone.

Many studies show the sensitivity of our environment to manmade changes, especially the anthropogenic impact on atmospheric and hydrological processes. The effect on Solid Earth processes such as subsidence is less straightforward. Subsidence is usually slow and relates to the interplay of complex hydro-mechanical processes, thus making relations to atmospheric changes difficult to observe. In the Dead Sea (DS) region, however, climatic forcing is strong and over-use of fresh water is massive. An observation period of 3 years was thus sufficient to link the high evaporation (97 cm/year) and the subsequent drop of the Dead Sea lake level (− 110 cm/year), with high subsidence rates of the Earth’s surface (− 15 cm/year). Applying innovative Global Navigation Satellite System (GNSS) techniques, we are able to resolve this subsidence of the “Solid Earth” even on a monthly basis and show that it behaves synchronous to atmospheric and hydrological changes with a time lag of two months. We show that the amplitude and fluctuation period of ground deformation is related to poro-elastic hydro-mechanical soil response to lake level changes. This provides, to our knowledge, a first direct link between shore subsidence, lake-level drop and evaporation.

The Po River delta is characterized by a large system of shallow lagoons, wetlands, and reclaimed lands. Like many other deltaic regions, the Po Delta has largely subsided due to natural processes and anthropogenic activities, with a strong impact on its geomorphological evolution and significant socio- economic consequences. This paper aimed at highlighting the evolution of the study area over the last 120 years, analysing the land-uses changes in relation to the land subsidence, and using these findings to discuss the main management issues of this fragile low lying area in the decades to come. The analysis of the land use evolution from 1892 to 2015 and the information derived from the metrics measurement suggest a simplification of the land use with a decrease of the saltmarsh extent and an enlargement of the agricultural cover. The analysis of land subsidence, mainly due to gas-bearing groundwater withdrawal, confirms a strong impact on the delta territory, and in particular on its geomorphological asset. In the early 1900s the elevation of the delta plain ranged between −1.0 and +1.6 m above msl, while in 2007 it reduced between −2.8 to +0.3 m above msl. The land reclamation projects have largely increased the farmland usable for crop production but the concurrent loss of land elevation has resulted in a territory extremely difficult to be managed. Because of its elevation, the delta is particularly vulnerable to weather and marine events. As a consequence, the present management of the area is mainly aimed at maintaining the agricultural productivity. Unfortunately, if the human impacts will continue with the actual trend, the stability of the natural ecosystems, such as the wetlands, which represent important socioeconomic realities, will be further undermined. Surely, the conservation of these natural environments should be a priority in the future land use management.

Tidal marshes are vulnerable and dynamic ecosystems with essential roles from protection against marine storms to biodiversity preservation. However, the survival of these environments is threatened by external stressors such as increasing mean sea level, reduction in sediment supply, and erosion. Tidal marshes are formed by deposition over the last centuries to millennia of sediments transported by surface water and biodegradation of organic matter derived from halophytic vegetation. Therefore, the sediment at the surface is characterized by high porosity and their large consolidation potential plays an important role in the future elevation dynamics, which is often not fully recognized.Here we propose a novel three-dimensional numerical model to simulate the long-term dynamics of tidal marshes. A 3D groundwater flow equation in saturated conditions is implemented to compute the over-pressure dissipation with the aid of the finite element (FE) method, whereas the sediment consolidation is computed according to Terzaghi's theory.A Lagrangian approach is implemented in the FE numerical model to properly consider the large soil deformation arising from the deposition of highly compressible material. The hydro-geomechanical properties, that depend on the intergranular effective stress, are highly non-linear.The model takes advantage of a dynamic mesh that simulates the evolution of the landform elevation by means of an accretion/compaction mechanism: the elements deform in time as the soil consolidates and increase in number as the new sediments deposit over the marsh surface. The deposition is treated as input to the consolidation model and can vary in space and time.The model is applied to simulate the long-term evolution of realistic tidal marshes in terms of accretion and consolidation due to the coupled dynamics of surficial and subsurface processes.

Most of the world major deltas are threatened by relative sea level rise, i.e. land subsidence and sea level rise, caused by a combination of anthropogenic pressures and natural processes. This study focuses on the natural components of land subsidence directly and indirectly related to the Holocene delta stratigraphy. Firstly, subsidence is caused by natural compaction of the Holocene sediments following deposition over time under their own weight. Secondly, subsidence is caused by the visco-elastic deformation of the Earth crust driven by cumulative load of the Holocene delta (so-called Sediment Isostatic Adjustment). These two processes are obviously connected and call for a proper evaluation of the weight of (the Holocene portion of) a delta. This requires a proper quantification of specific weight and degree of compaction of Holocene deposits with depth to arrive at a first-order assessment of Holocene delta weight. This study proposes an innovative methodology to address the following two questions: 1) What is the proper weight of a (Holocene) delta? 2) How much have deposits been compacted since their deposition during Holocene delta formation? Our approach integrates knowledge and data on deltaic depositional environments, stratigraphic information, geomechanical properties and other characteristics of the Holocene sequence. The developed approach is applied to eight major deltas worldwide selected from a larger database according to the availability of lithostratigraphic and geomechanical information. The analysis is conducted at the scale of an entire delta, thus required the upscaling and interpolation of datasets generally available from a few wellbores only. Lithostratigraphic data is combined with a backwards modelling procedure to decompact the Holocene delta sequence to their decompacted thickness to provide a proper estimation of their weight, which takes into account the (computed) in-situ compaction degree. The results show a large variability in compaction and specific weight distribution for the different deltas which underscores the substantial role of natural compaction on delta evolution.

Land subsidence associated with groundwater withdrawal is often an underestimated geological hazard that may produce important damage to buildings and infrastructure, change flood risk in some areas, and cause loss of groundwater storage capacity. In the current framework of global climate change, the increasing agricultural and urban use of groundwater resources is a growing problem, especially in arid and semiarid areas. Because monitoring subsidence in these areas is important for management, but early detection is difficult due to slow displacement rates, we developed global groundwater induced land subsidence probability maps. Global land subsidence probability was calculated by applying statistical methods to a set of susceptible geographical, environmental and geological properties based on known, documented subsidence affected areas. Highest values of subsidence probability are concentrated over flat areas composed of unconsolidated sediments, and in agricultural or urban areas subject to prolonged dry periods. Including water scarcity and groundwater use data resulted in an estimation of a proxy land subsidence hazard. Calculated probability does not imply that all the high value areas are currently incurring land subsidence, but it can alert policymakers and groundwater managers to areas that have potential exposure to subsidence hazards and warrant monitoring. The complete results of this work are published in Science Policy Forum section under the title “Mapping the global threat of land subsidence” DOI: 10.1126/science.abb8549