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In this study, the potential of using electromagnetic induction (EMI) and in situ regional calibration for predicting the electrical conductivity of saturated soil paste extract (EC e ) over relatively large spatial and temporal scales in a salt-affected agricultural area in northern Egypt was evaluated. The apparent electrical conductivity (EC a ) was measured using a CMD2 electromagnetic induction instrument in three agricultural plots with EC e values ranging from 5.29 to 115.00 dS m −1 . The EC a data were inverted to generate electromagnetic conductivity images (EMCIs) to provide the spatial distribution of the true soil electrical conductivity (σ) with depth (i.e., 90 cm). Based on the observed EC e , a regional calibration equation was developed. Its prediction ability was validated by performing new measurements (i.e., a second measurement campaign) in the same plots one month later and in four new plots with similar soil conditions (i.e., a third measurement campaign) approximately one year after the first campaign. The four new plots were located approximately 1.5 km away from the three original plots. The results showed that the developed regional calibration equation accurately predicted the EC e with an R 2 of 0.95. The prediction ability was also high, with an RMSE and R 2 of approximately 8.70 dS m −1 and 0.92, respectively, for the validation subsets. Thus, the derived regional calibration equation is reliable and can be used for mapping soil salinity over relatively large spatial and temporal scales in this region without any further calibration. As soil is classified as moderately to severely saline, the proposed technique can be used to improve agricultural management by providing reliable assessments of soil salinity in the study area and other similar agrosystems.

Coastal groundwater dynamics and solute transport are influenced by multiple factors including land reclamation, evaporation, and tidal fluctuations. However, their quantitative impacts on groundwater flow and salinity distribution in large‐scale muddy tidal flats remain insufficiently understood. This study combines field measurements and numerical simulations to investigate hydraulic heads and groundwater salinity over a spring‐neap tidal cycle in a muddy intertidal transect at Jiaozhou Bay, China. Results show that seepage‐face evaporation significantly increases groundwater salinity toward land, with its intensity modulated by seasonal variability. Based on field observations, we propose a new concept of tidal run‐up, which highlights its role in elevating landward hydraulic heads during rising and high tides. Land reclamation significantly alters groundwater flow patterns, transforming the original multiple circulation cells into a single, large cell in which inland groundwater discharges across the entire intertidal zone. Salinity distribution is jointly controlled by inland freshwater input, evaporation, and tidal forcing. Numerical simulations reveal that land reclamation reduces both outflow and inflow fluxes, and the outflow fluxes along the aquifer‐ocean interface decrease from 12.1 m2/d before reclamation to 7.7 m2/d after reclamation during the observation period, with evaporation contributing 56% and 31% of the total outflow flux, respectively. These findings enhance the understanding of groundwater flow and solute transport in large‐scale muddy tidal flats, and offer important implications for biogeochemical processes in intertidal zones.

The interaction between the Loop Current System (LCS) and the Gulf of Mexico (GoM)'s freshwater circulation is investigated during flood and drought years. We combine satellite‐derived sea surface salinity (SSS), ocean currents, and river discharge rates to examine patterns of salinity distribution throughout the entire GoM basin. Saline waters were reported intruding up the Mississippi River during drought years, so a close examination of these years is conducted. The relationship between the Loop Current (LC) and freshwater distribution are further investigated using a stepwise linear regression model. The relative impacts of river discharge and surface advective freshwater fluxes are quantified, and it is concluded that basin‐wide SSS depends significantly on the discharge from the Atchafalaya River, which leads by 4 months. Interannual salinity variability in the GoM is found to depend on the combination of discharge magnitude and timing relative to the position of the LCS. Plain Language Summary The Loop Current System (LCS) is a large, persistent, and dynamic flow that moves saline water in, around, and out of the GoM. On the Northern coast, freshwater is deposited by the Mississippi and Atchafalaya River System. When this freshwater encounters the LCS, it can change the distribution of surface salinity for the entire GoM. When there is less freshwater entering the GoM, the Loop Current (LC) can move Northward and likely deposit saltier water into the Mississippi River. In the opposite case, excessive amounts of freshwater can push the LC away from the coast and create a direct pathway out of the GoM. Through this research, we aim to determine the relative relationship between these mechanisms during flood and drought years. We find that river input is the most important in determining salinity patterns, followed by the amount of water coming into the Gulf from the south then lastly freshwater flux away from the river mouths. Additionally, we find that variability in the amount of freshwater input from 1 year to the next can affect the state of the LCS, further enhancing or dampening the expected salinity patterns. Key Points Salinity patterns driven by the interaction of the Loop Current (LC) and freshwater input during flood and drought years Quantification of the relative impacts of freshwater input variability input and LC circulation on basin‐wide salinity patterns Anomalous basin‐wide salinity patterns can be enhanced or dampened based on timing of LC evolution

An idealized width-averaged model is employed to study the influence of wind stress on subtidal salt intrusion and stratification in well-mixed and partially stratified estuaries. We show that even in mild conditions, wind forcing can influence the estuarine salinity structure in a substantial way. By studying the role of wind forcing on dominant salt transport balances and associated salt transport regimes, we unify and clarify ambiguous observations from previous authors regarding the influence of wind stress: the response of the estuarine salinity structure to wind forcing is different depending on the underlying dominant salt transport balance, which in turn was found to determine whether wind-induced salinity shear, wind-induced modulation of the longitudinal salt distribution, or wind-induced mixing dominates.

Anthropogenic climate change is projected to intensify the global hydrological cycle, posing substantial risks to human societies. However, monitoring these changes through direct observations remains challenging, particularly over the oceans. Since long‐term shifts in the hydrological cycle are expected to alter ocean salinity distribution, understanding the processes governing its evolution is essential. Salinity distribution is known to result from a balance between freshwater fluxes, which broaden the distribution, and mixing processes, which narrow it. Using a novel diagnostic based on the mean salinity variance budget applied to the Estimating the Circulation and Climate of the Ocean (ECCO), we estimate that the large‐scale salinity flux—primarily driven by the seasonal cycle—contributes approximately 23% to this mixing. Our framework also enables us to understand the regional balances, and to identify the regions where these balances are most significant. Our results suggest that accurately representing the seasonal salinity cycle in ocean and climate models is important for simulating the ocean salinity distribution.

Coastal groundwater dynamics and solute transport were influenced by multiple factors including aquitards, tides, evaporation, and slope breaks in coastal aquifers. However, quantification of the impacts of these factors on groundwater flow and salinity distribution remains a challenge. In this study, both field observations and numerical modeling were applied to investigate hydraulic heads and groundwater salinity in a tidal flat with large‐scale seepage faces at Laizhou Bay, China. Results showed that seepage‐face evaporation increased groundwater salinity landward and promoted groundwater and salt exchange within the intertidal zone significantly in comparison to the case without evaporation. Seawater infiltrated the aquifer on the left of the slope break and discharged on the right, forming a groundwater circulation cell, which notably influenced leakage flow between unconfined and confined aquifers through the aquitard. The aquitard prevented approximately 85% of inland freshwater discharge near the slope break, resulting in the formation of two atypical freshwater discharge tubes in the upper and middle intertidal zones. Two additional groundwater circulation cells developed in the lower intertidal zone due to the spring‐neap tidal cycle. The outflow and inflow fluxes over a spring‐neap tidal cycle were numerically estimated to be 1.46 and 1.27 m2/d, respectively, with evaporation accounting for 45% of the outflow flux. These findings provide significant insights for further investigations on groundwater dynamics and solute transport in multi‐layered coastal aquifers, and have strong implications for biogeochemical processes within the intertidal zone. Plain Language Summary The coastal aquifer serves as a crucial connection between terrestrial and marine systems, with groundwater flow and salt transport in coastal regions influenced by factors such as topographic variations (e.g., slope break), tides, aquitards (low‐permeability layers among permeable layers), and evaporation. Quantification of these complex processes is a challenge. Here, we combined field observations and numerical simulations to quantify the effects of slope break, tides, aquitard, and evaporation on groundwater flow paths and salinity distribution beneath a tidal flat. It was found that evaporation may significantly increase groundwater salinity landward, and promoted the mass exchange between groundwater and seawater on the tidal flat surface. The combined effects of slope break, spring‐neap tidal cycle, and aquitard notably altered the pathways of groundwater flow and solute transport in coastal aquifers. These may profoundly influence the biogeochemical conditions in multi‐layered coastal aquifers, with important implications for coastal management and environmental protection. Key Points Seepage‐face evaporation significantly increases groundwater salinity and promotes groundwater/salt exchange within the intertidal zone Two new freshwater discharge tubes and three groundwater circulation cells develop due to the aquitard, spring‐neap tidal cycle, slope break Significant exchanges of groundwater and solutes occur between unconfined and confined aquifers via leakage under the slope break

Small estuaries in Mediterranean climates display pronounced salinity variability at seasonal and event time scales. Here, we use a hydrodynamic model of the Coos Estuary, Oregon, to examine the seasonal variability of the salinity dynamics and estuarine exchange flow. The exchange flow is primarily driven by tidal processes, varying with the spring–neap cycle rather than discharge or the salinity gradient. The salinity distribution is rarely in equilibrium with discharge conditions because during the wet season the response time scale is longer than discharge events, while during low flow it is longer than the entire dry season. Consequently, the salt field is rarely fully adjusted to the forcing and common power-law relations between the salinity intrusion and discharge do not apply. Further complicating the salinity dynamics is the estuarine geometry that consists of multiple branching channel segments with distinct freshwater sources. These channel segments act as subestuaries that import both higher- and lower-salinity water and export intermediate salinities. Throughout the estuary, tidal dispersion scales with tidal velocity squared, and likely includes jet–sink flow at the mouth, lateral shear dispersion, and tidal trapping in branching channel segments inside the estuary. While the estuarine inflow is strongly correlated with tidal amplitude, the outflow, stratification, and total mixing in the estuary are dependent on the seasonal variation in river discharge, which is similar to estuaries that are dominated by subtidal exchange flow.

Arctic sea ice has been declining over past several decades with the largest ice loss occurring in summer. This implies a strengthening of the sea ice seasonal cycle. Here, we examine global ocean salinity response to such changes of Arctic sea ice using simulations wherein we impose a radiative heat imbalance at the sea ice surface, inducing a sea ice decline comparable to the observed. The imposed perturbation leads to enhanced seasonal melting and a rapid retreat of Arctic sea ice within the first 5–10 years. We then observe a gradual freshening of the upper Arctic ocean that continues for about a century. The freshening is most pronounced within the central Arctic, including the Beaufort gyre, and is attributed to excess surface freshwater associated with the stronger seasonal sea ice melting, as well as a greater upper-ocean freshwater storage due to changes in ocean circulation. The freshening of the Nordic Seas can also occur via a distillation-like process in which denser saline waters with increased salinity are exported to the subtropical/tropical North Atlantic by meridional overturning circulation. Thus, enhanced seasonal sea ice melting in a warmer climate can lead to a persistent Arctic freshening with large impacts on the global salinity distribution.

While seawater intrusion (SWI) in coastal aquifers has been extensively investigated, most studies focused on unfrozen conditions. In many cold regions, frozen ground is common, yet its impact on SWI remains poorly understood. Here, we demonstrate that the formation of frozen ground alters salinity distribution in coastal unconfined aquifers, based on laboratory experiments and numerical simulations. Under steady‐state conditions with subzero ground temperature, frozen ground creates a confined flow path between itself and the saltwater wedge. This configuration accelerates terrestrial groundwater flow and induces a pronounced seaward retreat of the saltwater wedge. Moreover, lower ground surface and groundwater temperatures enhance this effect by promoting frozen ground formation. These findings have important implications for understanding salinity dynamics in coastal cold‐region aquifers with varying frozen ground.

A universal law of estuarine mixing is derived here, combining the approaches of salinity coordinates, Knudsen relations, total exchange flow, mixing definition as salinity variance loss, and the mixing–exchange flow relation. As a result, the long-term average mixing within an estuarine volume bounded by the isohaline of salinity S amounts to M ( S ) = S 2 Q r , where Q r is the average river runoff into the estuary. Consequently, the mixing per salinity class is m ( S ) = ∂ S M ( S ) = 2 SQ r , which can also be expressed as the product of the isohaline volume and the mixing averaged over the isohaline. The major differences between the new mixing law and the recently developed mixing relation based on the Knudsen relations are threefold: (i) it does not depend on internal dynamics of the estuary determining inflow and outflow salinities (universality), (ii) it is exactly derived from conservation laws (accuracy), and (iii) it calculates mixing per salinity class (locality). The universal mixing law is demonstrated by means of analytical stationary and one-dimensional and two-dimensional numerical test cases. Some possible consequences for the salinity distribution in real estuaries are briefly discussed. Since the mixing per salinity class only depends on the river runoff and the chosen salinity, and not on local processes at the isohaline, low-mixing estuaries must have large isohaline volumes and vice versa.