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Submarine groundwater discharge (SGD) contributes to the physical and chemical characters of coastal waters by discharging nutrients and contaminants, significantly impacting regional marine ecosystems and contributing to ocean chemical budgets. However, such groundwater discharge varies dramatically across scales and is often not comparable due to different model assumptions and field designs. We used a hydrologic model with integration of fundamental surface and subsurface processes to simulate the coastline level fresh SGD for the Crete Island in the Mediterranean Sea. The modeled hydrological processes suggested that fresh SGD substantially contributes to water flow entering the Mediterranean Sea (2.3 × 108 m3/yr), amounting to 31% of river discharge and 14% of precipitation. Spatially, fresh SGD varied from 2.4 m3/yr/m to 13.4 × 104 m3/yr/m, with an average of 2.6 × 103 m3/yr/m. The local maxima were commonly associated with river mouths reflecting larger hydraulic gradients and higher permeable structures. Temporally, fresh SGD was impacted by episodic precipitation in a delayed and prolonged pattern. We found that fresh SGD variability at the coastline segment level was compared to point measurements and fresh SGD magnitudes summered up to the catchment level were consistent with global products. Our results suggest the coupled surface–subsurface hydrologic modeling approach is a promising strategy to quantify and partition large-scale water budgets down to point observations that typically do not capture the full range of fresh SGD dynamics.

Excess export of nitrate to streams affects ecosystem structure and functions and has been an environmental issue attracting world-wide attention. The dynamics of catchment-scale solute export from diffuse nitrate sources can be explained by the activation and deactivation of dominant flow paths, as solute attenuation (including the degradation of nitrate) is linked to the age composition of outflow. Previous data driven studies suggested that catchment topographic slope has strong impacts on the age composition of streamflow and consequently on in-stream solute concentrations. However, the impacts have not been systematically assessed in terms of solute concentration levels and variation, particularly in humid catchments with strong seasonality in meteorological forcing. To fill this gap, we modeled the groundwater flow and nitrate transport for a cross-section of a small agricultural catchment in Central Germany. We used the fully coupled surface and subsurface numerical simulator HydroGeoSphere to model groundwater and overland flow as well as nitrate concentrations. We computed the water ages using numerical tracer experiments. To represent various topographic slopes, we additionally simulated ten synthetic cross-sections generated by modifying the mean slope from the real-world scenario while preserving the land surface micro-topography. Results suggest a three-class response of in-stream nitrate concentrations to topographic slope, from class 1 (slope > 1:60), via class 2 (1:100 < slope < 1:60), to class 3 (slope < 1:100). Flatter landscapes tend to produce higher in-stream nitrate concentrations within class 1 or class 3, however, not within class 2. Young streamflow fractions and nitrate concentrations decrease sharply when flatter landscapes are not able to maintain fast preferential discharge paths (e.g. seepage). The variation of in-stream concentrations, controlled by degradation variability rather than by nitrate source variability, shows a similar three-class response. Our results improve the understanding of nitrate export in response to topographic slope in temperate humid climates, with important implications for the management of stream water quality. [ABSTRACT FROM AUTHOR]

A new high-performance numerical model (Frehg) is developed to simulate water flow in shallow coastal wetlands. Frehg solves the 2D depth-integrated, hydrostatic, Navier–Stokes equations (i.e., shallow-water equations) in the surface domain and the 3D variably-saturated Richards equation in the subsurface domain. The two domains are asynchronously coupled to model surface-subsurface exchange. The Frehg model is applied to evaluate model sensitivity to a variety of simplifications that are commonly adopted for shallow wetland models, especially the use of the diffusive wave approximation in place of the traditional Saint-Venant equations for surface flow. The results suggest that a dynamic model for momentum is preferred over diffusive wave model for shallow coastal wetlands and marshes because the latter fails to capture flow unsteadiness. Under the combined effects of evaporation and wetting/drying, using diffusive wave model leads to discrepancies in modeled surface-subsurface exchange flux in the intertidal zone where strong exchange processes occur. It indicates shallow wetland models should be built with (i) dynamic surface flow equations that capture the timing of inundation, (ii) complex topographic features that render accurate spatial extent of inundation, and (iii) variably-saturated subsurface flow solver that is capable of modeling moisture change in the subsurface due to evaporation and infiltration.

Abstract Floods introduced by rainfall events are responsible for tremendous economic and property losses. It is important to delineate flood prone areas to minimize flood damages and make mitigation plans. Recently, there has been recognition of the need to understand the risk from groundwater flooding caused by the emergence of groundwater at the ground surface. Mapping groundwater flood risk is more challenging compared to river flooding, especially in karst systems, where subsurface preferential flowpaths can affect groundwater flow. We developed a coupled surface-subsurface modelling framework resolving spatial information and hydrologic data to assess the groundwater flood risk at a karstic watershed: Koiliaris River Basin, Greece. The simulated groundwater table was used to delineate groundwater flooding. Modelled results revealed the role of faults in groundwater flooding generation. We anticipate the coupled surface-subsurface approach to be a starting point for more sophisticated flooding risk assessment, including magnitude and temporal duration of groundwater flooding. Highlights • Coupled surface-subsurface modelling of a mountainous watershed. • Modelling result explicitly shows spatial-temporal dynamics of groundwater flooding. • Faults increase the area of groundwater flooding and reduce response time. • Groundwater flooding causes more than 10 times higher agricultural loses than river flooding. • This study has highlighted the value of coupled hydrologic models in groundwater flooding assessment. [ABSTRACT FROM AUTHOR]

• The accuracy of the numerical model is verified by physical test data. • The influence of geometric parameters of the model on exchange degree is revealed. • The temperature response law of the hyporheic zone driven by spur dike is analyzed. The study of hyporheic exchange and the temperature response law driven by spur dikes is helpful to promote the protection of ecological river environments. Taking the river channel course with a spur dike as the research object, the numerical simulation method was used to study the influence of the river width narrowing rate (L/B), spur dike angle (θ), surface water velocity (U), and hydraulic conductivity (K) on the hyporheic exchange characteristics and the temperature response of the hyporheic zone. Research shows that the degree of hyporheic exchange and the spatial range of the temperature response of the hyporheic zone are positively correlated with the river width narrowing rate, surface water velocity and hydraulic conductivity and negatively correlated with the spur dike angle. The temperature response law of the hyporheic zone is affected by spur dikes in different spatial locations in the following ways, from the cross-section, the upstream response area of the spur dike deepens near the dam bank, while the downstream response area deepens on the other side of the spur dikes, and the downstream response area affected by spur dikes is significantly larger than the upstream response area in the vertical range. From the side section, the response area of the curved boundary appears around the spur dike in the central section of the river, and the curvature radius of the response area of the curved boundary around the spur dike in the side section near the dam is smaller. From the longitudinal section, the sensitivity of the temperature response of the shallow section is significantly stronger than that of the deep section. Consequently, we found that the hyporheic exchange law driven by spur dikes. We offer recommendations for future studies seeking to choose the reasonable structure of the spur dam in practical engineering, and optimize the design of the spur dam. [ABSTRACT FROM AUTHOR]

A new high-performance numerical model (Frehg) is developed to simulate water flow in shallow coastal wetlands. Frehg solves the 2D depth-integrated, hydrostatic, Navier–Stokes equations (i.e., shallow-water equations) in the surface domain and the 3D variably-saturated Richards equation in the subsurface domain. The two domains are asynchronously coupled to model surface-subsurface exchange. The Frehg model is applied to evaluate model sensitivity to a variety of simplifications that are commonly adopted for shallow wetland models, especially the use of the diffusive wave approximation in place of the traditional Saint-Venant equations for surface flow. The results suggest that a dynamic model for momentum is preferred over diffusive wave model for shallow coastal wetlands and marshes because the latter fails to capture flow unsteadiness. Under the combined effects of evaporation and wetting/drying, using diffusive wave model leads to discrepancies in modeled surface-subsurface exchange flux in the intertidal zone where strong exchange processes occur. It indicates shallow wetland models should be built with (i) dynamic surface flow equations that capture the timing of inundation, (ii) complex topographic features that render accurate spatial extent of inundation, and (iii) variably-saturated subsurface flow solver that is capable of modeling moisture change in the subsurface due to evaporation and infiltration. [ABSTRACT FROM AUTHOR]