Inland Boundary Research Articles

Temporal and spatial evolution of residual saltwater contamination in coastal subterranean reservoirs

This study numerically investigated the dynamic process of residual saltwater contamination in subterranean reservoirs (aquifer upstream of subsurface physical barrier) for the first time. The results indicated that the groundwater level elevation resulting from subsurface physical barriers induced a small hydraulic gradient in the subterranean reservoir, with residual saltwater convection driven by the density contrast between saltwater and freshwater. This led to persistent inland intrusion of residual saltwater, causing a gradual expansion of the contamination area (i.e., concentration higher than 0.25 g/L) and a decreased area of high-salinity zone. The area of low concentration residual saltwater (0.25 g/L) initially increased and then decreased over time. For large-scale aquifers, the proportion of the maximum contaminated area to the total aquifer area increased from 0.05 to 0.17. Subsequently, the low-concentration saltwater was considered as a relative high-concentration for ambient freshwater, which released salts into the surroundings and the contamination area reduced to 0.0003 at 160000 d. In small-scale aquifers, residual saltwater transport was affected by inland boundary, and the low-concentration saltwater reached inland boundary at approximately 2000 d, which limited its horizontal expansion and resulted in the polluted area reaching an inflection point early relative to large-scale aquifers. The salt transfer from high-concentration residual saltwater to low-concentration saltwater was closely related to aquifer parameters of hydraulic conductivity, dispersivity, and molecular diffusion coefficient. These findings suggest that residual saltwater contamination should not be overlooked when assessing the efficiency of subterranean reservoirs.

Sea-level rise impacts on groundwater: exploring some misconceptions with simple analytic solutions

Sea-level rise (SLR) causes groundwater salinisation and water-table rise. The impacts these processes can have on water security, agricultural production and infrastructure are becoming widely recognised. However, some misconceptions relating to SLR impacts on groundwater have been observed among students, which may interfere with further learning and the application of science principles to everyday life. These misconceptions include the following: (1) water-table rise will equal SLR; (2) inland movement of the interface causes the rise in the water table under SLR; (3) seawater intrusion (SI) caused by SLR is small compared to SI caused by pumping. These misconceptions are explored with the aid of simple analytic solutions and a Jupyter Notebook. It is shown that: (1) water-table rise is only equal to SLR above the interface under flux-controlled inland boundary conditions; (2) water-table rise under SLR is not caused by SI, but rather is caused by the change in levels at the coastal boundary; (3) SI caused by SLR is a considerable risk, especially under the head-controlled conditions, which will become more common when land is drained to counter the effects of groundwater shoaling.

Estimating the Maximum Safe Pumping Rate in Sloping Unconfined Coastal Aquifers

AbstractUnconfined coastal aquifers with a sloping aquifer bed are ubiquitous. However, most analytical solutions previously developed to estimate pumping‐induced seawater intrusion considered a horizontal aquifer bed. In this study, we first developed a steady‐state analytical solution to quantify the maximum safe pumping rate in sloping unconfined coastal aquifers with a fixed‐flux inland boundary condition, using the single potential approach. The analytical solution corrected by an empirical factor is validated against the numerical simulation, which reproduces the corresponding numerical results well. It is found that coastal aquifers with a higher aquifer bed toward inland (i.e., a positive sloping angle) yield a higher maximum safe pumping rate than that of coastal aquifers with a horizontal aquifer base, since the elevated freshwater head prevents seawater intrusion and enhances groundwater pumping. Specifically, for the aquifer bases with the sloping angle of 0.01 and −0.01, the maximum safe pumping rate is increased by 42.6% and decreased by 48.4%, respectively, in comparison to the case of a horizontal aquifer base, suggesting that neglecting the aquifer base slop can result in a significant error in estimating the maximum safe pumping rate. Moreover, the sensitivity analysis reveals that the maximum safe pumping rate increases with a lower hydraulic conductivity and a larger dispersivity. Our results provide valuable insights into the effect of the aquifer base slope on the maximum safe pumping in coastal aquifers, and analytical solutions developed can serve as a powerful tool for quick assessment of pumping‐induced seawater intrusion in sloping unconfined coastal aquifers.

Analytical solutions for maximum fresh groundwater extraction in stratified coastal aquifers

Existing analytical solutions for defining maximum freshwater extraction in coastal aquifers mostly presume a homogeneous aquifer. In this study, analytical solutions for the pumping-induced interface toe position and the maximum pumping rate of a single well in stratified coastal aquifers are developed, based on the potential theory and the analytical solution proposed by Rathore et al. (2020), where aquifer stratification is concisely characterized by the total transmissivity and the transmissivity centroid elevation (hereafter “TCE”). Three typical aquifer domain scenarios are examined, including semi-infinite, infinite-strip, and rectangular coastal aquifers. The sensitivity analysis based on the proposed analytical solutions demonstrates that in all aquifer domain scenarios, a lower TCE leads to a more seaward interface toe and a larger maximum pumping rate, while the impact of total transmissivity is complicated, depending on the type of the inland boundary condition. The effect of inland and lateral boundary conditions on the maximum pumping rate is enhanced in the stratified aquifers of lower total transmissivity and TCE. Importantly, the boundary effects can be eliminated by properly defining the size of the stratified aquifer domain, providing valuable guidance for the design of numerical models and laboratory experiments that are often of finite sizes. Our analytical solutions can serve as a simple tool for the first-order assessment of allowable freshwater extraction in stratified coastal aquifers.

Effects of Unsaturated Flow on Salt Distributions in Tidally Influenced Coastal Unconfined Aquifers

AbstractUnsaturated flow influences both the seawater extent under steady‐state conditions and the propagation of tides in coastal aquifers. However, its effects on salt distributions in tidally influenced coastal aquifers are little investigated. The present study used numerical simulations and data from laboratory experiments to analyze the effects of unsaturated flow on density‐dependent solute transport in coastal unconfined aquifers. The effects of the inland boundary condition (i.e., constant‐head or constant‐flux) were tested. Compared to a stable sea level, the results show that unsaturated flow has a more pronounced influence on salt distributions in coastal unconfined aquifers when tides are considered, regardless of the type of inland boundary condition. Neglect of unsaturated flow effects leads to expansion of the upper saline plume (USP), shrinkage of the saltwater wedge (seaward movement of saltwater wedge), and overestimation of water and salt exchange across the aquifer‐ocean interface. This is caused by a lower head in the nearshore area during high‐tide periods with the unsaturated zone effects removed. Thus, without the unsaturated zone, stronger head gradients within the nearshore aquifer occur at high tide, leading to stronger tidally driven seawater infiltration and hence a larger USP. Counterintuitively, ignoring unsaturated flow effects leads to greater average inland head over a tidal period, which shifts the saltwater wedge seaward. It is concluded that unsaturated zone effects should not be neglected for modeling tide‐affected seawater intrusion, especially if quantification of near‐shore conditions is important.

Effects of aquitard windows on groundwater fluctuations within a coastal leaky aquifer system: An analytical and experimental study

Low-permeability aquitards are important sedimentary units that protect the underlying confined aquifers from contamination by seawater and anthropogenic sources in coastal leaky aquifer systems. Therefore, the integrity of the aquitard is of crucial importance. However, the aquitard integrity may be vulnerable to horizontal heterogeneities in the form of aquitard windows. In this study, an innovative analytical model was developed to describe groundwater flow in a coastal leaky aquifer system with horizontal heterogeneities in hydraulic properties of either the aquitard or the confined aquifer. Consequently, the effects of aquitard windows, which manifest as horizontally local heterogeneities in hydraulic conductivity and/or specific storage, on the tidal wave propagation can be considered in the analysis. Analytical solutions were derived for tidal-head or constant-flux inland boundary conditions. The derivation process avoided previous methodologies that involved the assembly of large-size matrices. The newly developed analytical model can cover multiple existing analytical models that are associated with the assumptions of homogeneity in hydraulic properties of the aquifer and the aquitard, infinite horizontal extent of the coastal leaky aquifer system, impermeability of the aquitard, or ignorance of aquitard storativity. Analytical investigations revealed that high-permeability aquitard windows acting as preferential-flow conduits tended to dampen the tide-induced head propagation within the underlying confined aquifer. Moreover, an aquitard with a higher storativity (but the same permeability) and of considerable size can result in an apparent propagation bias. The newly developed analytical model in this study was further corroborated through sandbox experiments. Both piezometer measurements and parameter estimates indicated the non-negligible effects of high-permeability aquitard windows on the tidal wave propagation. The sandbox investigation also highlighted the importance of geostatistical inverse modeling in characterizing local heterogeneities within the confined aquifer and in detecting the location and size of aquitard windows, when measurement data of tide-induced aquifer head fluctuations were provided.

Analytical solutions for seawater intrusion and the maximum pumping rate of a well in unconfined coastal aquifers bounded by L-shaped coastlines

Determining the maximum pumping rate (i.e., the maximum allowable pumping rate that does not cause seawater intrusion into the well under the steady-state condition) of a well is the key for sustainable management of coastal aquifers, as overexploitation of coastal aquifers could lead to the widespread occurrence of aquifer salinization (i.e., seawater intrusion). In this study, we develop steady-state analytical solutions to describe seawater intrusion and the maximum pumping rate of a well located in unconfined coastal aquifers bounded by L-shaped coastlines. Rectangular coastal aquifers bounded by two orthogonal constant-head boundaries (i.e., two coastlines) and two orthogonal no-flow inland boundaries are assumed in conceptual models, where replenishment of aquifers is via surface recharge. Analytical solutions are derived by the potential theory and the conformal mapping method, and validated by variable-density flow numerical simulations. It is found that analytical solutions based on the sharp-interface approximation overestimate seawater intrusion and underestimate the maximum pumping rate. An acceptable, conservative maximum pumping rate is obtained by using the correction factor proposed by Lu and Werner (2013). Moreover, the sensitivity of the well location on the maximum pumping rate is investigated, showing that an optimal well location exists. Analytical solutions developed in the current study provide a convenient tool for assessing pumping effects in coastal aquifers bounded by L-shaped coastlines, servicing to various management objectives.

Abiotic and biotic factors controlling the dynamics of soil respiration in a coastal dune ecosystem in western Japan

In this study, we examined the abiotic and biotic factors controlling the dynamics of soil respiration (Rs) while considering the zonal distribution of plant species in a coastal dune ecosystem in western Japan, based on periodic Rs data and continuous environmental data. We set four measurement plots with different vegetation compositions: plot 1 on bare sand; plot 2 on a cluster of young Vitex rotundifolia seedlings; plot 3 on a mixture of Artemisia capillaris and V. rotundifolia; and plot 4 on the inland boundary between the coastal vegetation zone and a Pinus thunbergii forest. Rs increased exponentially along with the seasonal rise in soil temperature, but summer drought stress markedly decreased Rs in plots 3 and 4. There was a significant positive correlation between the natural logarithm of belowground plant biomass and Rs in autumn. Our findings indicate that the seasonal dynamics of Rs in this coastal dune ecosystem are controlled by abiotic factors (soil temperature and soil moisture), but the response of Rs to drought stress in summer varied among plots that differed in dominant vegetation species. Our findings also indicated that the spatial dynamics of Rs are mainly controlled by the distribution of belowground plant biomass and autotrophic respiration.

Approximate analytical solutions for assessing the effects of unsaturated flow on seawater extent in thin unconfined coastal aquifers

This study develops approximate analytical solutions for seawater extent in unconfined coastal aquifers considering unsaturated flow, and assuming steady-state, sharp-interface conditions, for both constant flux (flux-controlled aquifers) and constant head (head-controlled aquifers) inland boundary conditions. These analytical solutions were verified with numerical simulations of variable-saturation and variable-density flow. The results show that neglecting unsaturated flow underestimates the steady-state seawater extent, particularly for flux-controlled aquifers. This occurs because the vadose zone transmits part of the freshwater flux towards the sea, leading to a lower watertable and, consequently, a more landward seawater extent. Larger capillary fringes accompanying finer-grained sediments (and smaller pore sizes) cause more landward locations of the interface toe for both flux-controlled and head-controlled aquifers. The ratio of the capillary fringe thickness to the saturated zone thickness controls the relative difference in the interface toe location between cases with and without unsaturated flow. When this ratio is 17.3%, the relative difference in the interface toe location is 30% for the flux-controlled aquifer (base scenario adopted in the current study). The presented analytical solutions provide improved predictions of the steady-state seawater extent in thin unconfined aquifers, particularly for those with relatively large capillary fringe thicknesses.

Along‐Shore Movement of Groundwater and Its Effects on Seawater‐Groundwater Interactions in Heterogeneous Coastal Aquifers

AbstractStudies of coastal groundwater dynamics often assume two‐dimensional (2D) flow and transport along a shore‐perpendicular cross‐section. We show that along‐shore movement of groundwater may also be significant in heterogeneous coastal aquifers. Simulations of groundwater flow and salt transport incorporating different geologic structure show highly three‐dimensional (3D) preferential flow paths. The along‐shore movement of groundwater on average accounts for 40%–50% of the total flowpath length in both conduit‐type (e.g., volcanic) heterogeneous aquifers and statistically equivalent (e.g., deltaic) systems generated with sequential indicator simulation (SIS). Our results identify a critical role of three‐dimensionality in systems with connected high‐permeability geological features. 3D conduit features connecting land and sea cause more terrestrial groundwater flow through the inland boundary and intensify water exchange along the land‐sea interface. Therefore, conduits increase the rate of SGD compared to equivalent homogeneous, SIS and corresponding 2D models. In contrast, in SIS‐type systems, less‐connected high‐permeability features produce mixing zones and SGD nearer to shore, with comparable rates in 3D and 2D models. Onshore, 3D heterogeneous cases have longer flowpaths and travel times from recharge to discharge compared to 2D cases, but offshore travel times are much shorter, particularly for conduit‐type models in which flow is highly preferential. Flowpath lengths and travel times are also highly variable in 3D relative to 2D for all heterogeneous simulations. The results have implications for water resources management, biogeochemical reactions within coastal aquifers, and subsequent chemical fluxes to the ocean.

Applying Airborne LiDAR to Map Salt Marsh Inland Boundaries

The determination of rates and stocks of carbon storage in salt marshes, as well as their protection, require that we know where they and their boundaries are. Marsh boundaries are conventionally mapped through recognition of plant communities using aerial photography or satellite imagery. We examined the possibility of substituting the use of 1 m resolution LiDAR-derived digital elevation models (DEMs) and tidal elevations to establish salt marsh upper boundaries on the New Brunswick coasts of the Gulf of St. Lawrence and the Bay of Fundy, testing this method at tidal ranges from ≤2 to ≥4 m. LiDAR-mapped marsh boundaries were verified with high spatial resolution satellite imagery and a subset through field mapping of the upland marsh edge based upon vegetation and soil characteristics, recording the edge location and elevation with a Differential Geographic Positioning System. The results show that the use of high-resolution LiDAR and tidal elevation data can successfully map the upper boundary of salt marshes without the need to first map plant species. The marsh map area resulting from our mapping was ~30% lower than that in the province’s aerial-photograph-based maps. However, the difference was not primarily due to the location of the upper marsh boundaries but more so because of the exclusion of mudflats and large creeks (features that are not valued as carbon sinks) using the LiDAR method that are often mapped as marsh areas in the provincial maps. Despite some minor limitations, the development of DEMs derived from LiDAR can be applied to update and correct existing salt marsh maps along extensive sections of coastlines in less time than required to manually trace from imagery. This is vital information for governments and NGOs seeking to conserve these environments, as accurate mapping of the location and area of these ecosystems is a necessary basis for conservation prioritization indices.

Effects of Multiconstituent Tides on a Subterranean Estuary With Fixed-Head Inland Boundary

While tides of multiple constituents are common in coastal areas, their effects on submarine groundwater discharge (SGD) and salinity distributions in unconfined coastal aquifers are rarely examined, with the exception of a recent study that explored such effects on unconfined aquifers with fixed inland freshwater input. For a large proportion of the global coastline, the inland areas of coastal aquifers are topography-limited and controlled by constant heads. Based on numerical simulations, this article examines the variation of SGD and salinity distributions in coastal unconfined aquifers with fixed-head inland boundaries at different distances from the shoreline (i.e., 50, 100, 150, and 200 m). The results showed that the fluctuation intensity of freshwater input was enhanced as the inland aquifer extent decreased, e.g., the range of tide-induced fluctuations in freshwater input increased by around 5 times as the inland aquifer extent decreased from 200 to 50 m. The frequency spectra of the fluctuations of SGD and salinity distributions showed that the coastal aquifer of a shorter inland aquifer extent smoothed out fewer high-frequency tidal constituents but enhanced interaction among different tidal constituents. The interaction among tidal constituents generated new low-frequency signals in the freshwater input and salinity distributions. Regressions based on functional data analysis demonstrated that the inland freshwater input and salinity distributions at any given moment were related to the antecedent (previous) tidal conditions weighted using the probability density function of the Gamma distribution. The influence of the antecedent tidal conditions depended on the inland aquifer extent.

Influence of inland freshwater influx on the natural desalination of coastal aquifers with a cutoff wall

Cutoff walls are commonly used as an alternative to the desalination of coastal aquifers in many coastal regions. Current researches suggested that the high-velocity freshwater flow at the bottom cutoff wall opening was critical to prevent seawater intrusion (SWI). However, the role of the inland freshwater influx on coastal salinization is ignored. The groundwater cycle driven by the inland freshwater influx can be considered as a natural desalination process of coastal aquifers. Field-scale numerical models with the specified-head inland boundary and specified-flux inland boundary were used to investigate the effect of cutoff walls on the desalination of coastal aquifers. The results delineated that (1) both the freshwater flow velocity at the wall opening and inland freshwater influx significantly influence the seawater invasion length. (2) In contrast with the freshwater flow velocity at the wall opening, the inland freshwater influx is indeed the key factor controlling the desalination of coastal aquifers. The increase of inland freshwater influx narrows the freshwater-seawater mixing zone and reduces the groundwater salinization area. These results provide a comprehensive explanation on the hydrodynamic mechanism of cutoff walls and a reference for the groundwater resources management in coastal regions.

Impact of Jetty Configuration Changes on the Hydrodynamics of the Subtropical Patos Lagoon Estuary, Brazil

Coastal infrastructure alterations, such as jetty expansions, are designed to provide improvements to natural dredging and safety of marine access and to maximize the management and efficiency of ports. Furthermore, these alterations have the potential to cause significant environmental changes to estuaries and adjacent coastal areas. Here, the hydrodynamics of Pathos Lagoon was investigated before and after the jetty alterations, where the jetty was increased by approximately 10–18% and the mouth width was reduced by 15%. The TELEMAC-3D numerical model was calibrated and validated using the field data, and then simulated for characteristic low and high extreme discharge years for the old and new jetty configurations. Results showed a flow reduction of approximately 20% both in the ebb and flood conditions in the new configuration, which was accompanied by a slight change in the propagation angle of the western jetty current. Reduction of the saltwater intrusion was registered during both the high and low discharge conditions with the new jetty configuration. During the high discharge periods with NE winds, saltwater intrusion did not reach the previous estuarine inland boundary. During the period of low discharge with SW wind, salinity did not reach further than 180 km inland. Reduced saltwater intrusion was estimated landwards and in the shallow embayments. The horizontal stratification structure of the salinity changed, with the partial centralization of the flow in the access channel. The observed hydrodynamic changes from the infrastructure modifications could affect the estuarine ecosystem by increasing the sediment retention, reducing the transport of marine organisms and water properties into the estuary. This study contributes not only to the understanding of hydrodynamic changes but also to the potential optimization of estuarine and coastal management strategies.

A Semianalytical Method to Fast Delineate Seawater‐Freshwater Interface in Two‐Dimensional Heterogeneous Coastal Aquifers

AbstractThe freshwater‐seawater interface is one of the most important regions in coastal aquifer systems, delineating the subsurface into zones with distinct fluid density and biogeochemical properties. Heterogeneity in hydraulic conductivity is inherent to geological formations, resulting in distinct interface profiles. Currently available analytical solutions are limited to homogeneous and stratified aquifers, and numerical simulations of variable‐density flow and transport are the only option to characterize the interface in a two‐ or three‐dimensional heterogeneous aquifer. This study presents a novel semianalytical method to fast delineate the steady seawater‐freshwater interface in a two‐dimensional, confined, heterogeneous aquifer with a constant flux inland boundary and indistinct seepage face at the vertical coastal boundary. The heterogeneous domain is conceptualized as a series of thin columns of stratified aquifers with the horizontal flow in each of them. The proposed approach is based on the concept of local transmissivity parameters and shows high accuracy in the interface delineation. Leveraging the ability to delineate the interface rapidly along with the Monte Carlo approach, we numerically investigate the stochastic behavior of the interface profile. The uncertainty in seawater intrusion is heavily influenced by the degree of heterogeneity and weakly influenced by the scale of heterogeneity. To homogenize the aquifer, we found that geometric mean generally underestimates the seawater intrusion and cannot be used as an effective parameter. We also showed that the near‐coast, near‐top region of the aquifer is the most influential region and should be characterized at a higher resolution to reduce uncertainties in estimating seawater intrusion.

Saltwater Transport under the Influence of Sea-Level Rise in Coastal Multilayered Aquifers

Guo, Q.; Zhang, Y.; Zhou, Z., and Zhao, Y., 2020. Saltwater transport under the influence of sea-level rise in coastal multilayered aquifers. Journal of Coastal Research, 36(5), 1040–1049. Coconut Creek (Florida), ISSN 0749-0208.The dynamic behavior of groundwater flow and salt transport in coastal multilayered aquifers is affected by sea-level rise (SLR). Two sets of laboratory experiments were completed that took into account SLR (seawater rising from 85 to 105 cm for scenario 1 and inland head decreasing from 100 to 80 cm for scenario 2). Groundwater flow and salt transport in the multilayered aquifers were observed during the experiment. SEAWAT software was used to validate the processes of seawater transport in the coastal multilayered aquifer under the influence of SLR. The hydrogeological parameters were identified by fitting the observed values of groundwater level and salinity in the model. The influences of aquifer heterogeneity, SLR, and boundary condition on solute mixing of saltwater and freshwater were described. Results showed that the transient toe penetration xtoe and area of the seawater wedge increased initially and then tended to stabilize as the seawater level increased in each stage of both scenarios. The spreading rate of the seawater wedge during the process of releasing water in scenario 2 was faster than that of storing water in scenario 1, although the difference between sea level and inland boundary head was the same in the two scenarios. Sensitivity analysis indicated that the estimated parameters were reasonable. Moreover, the variation of the seawater wedge could not be reproduced well when the hydraulic conductivities and saltwater density of the upper and lower layers were increased or decreased. The analysis provided insights into how SLR and inland water head influence seawater intrusion rate and area.

Interactions between topsoil properties and ecophysiological responses of mangroves (Avicenniamarina) along the tidal gradient in an arid region in Qatar

This study investigated the interactions between topsoil properties and ecophysiological responses of Avicennia marina along the tidal gradient in an arid region in Qatar. In February 2017, three plots were established, each at a distance of 0 m D0 , 50 m D50 , and 100 m D100 from the inland boundary of a mangrove forest. Soil samples were collected at 0-10-cm depth in each plot to determine the chemical properties, and the density of seedlings, saplings, and trees was measured. Moreover, above- AGB and below-ground biomass BGB were calculated using an allometric equation for A. marina with the measured diameter at breast height in February 2017. As an indicator of salt stress, chlorophyll fluorescence parameters were measured in October 2017. Salinity 45.60 ppt and exchangeable sodium percentage ESP; 29.02% at D100 were significantly highest. AGB was higher at D100 41.44 Mg ha-1 than at D0 0 Mg ha-1 and D50 7.33 Mg ha-1 , and BGB was higher at D100 44.91 Mg ha-1 than only at D0 0 Mg ha-1 . There was no significant difference in the density of seedlings, saplings, or trees or the chlorophyll fluorescence parameters among the plots. Salt stress was not induced despite the hypersalinity at this site, since A. marina growing in an arid climate can endure strong salinity. Soil pH was highest at D0, followed by at D50 and D100. Organic matter, total nitrogen, available phosphorus, and cation exchange capacity were significantly higher at D100 than at D0 and D50. Higher concentrations of nutrients on the seaward side might result from the tidal gradient and a large input of organic matter and low soil alkalinity.

A universal multifractal approach to assessment of spatiotemporal extreme precipitation over the Loess Plateau of China

Abstract. Extreme precipitation (EP) is a major external agent driving various natural hazards in the Loess Plateau (LP), China. However, the characteristics of the spatiotemporal EP responsible for such hazardous situations remain poorly understood. We integrate universal multifractals with a segmentation algorithm to characterize a physically meaningful threshold for EP (EPT). Using daily data from 1961 to 2015, we investigate the spatiotemporal variation of EP over the LP. Our results indicate that (with precipitation increasing) EPTs range from 17.3 to 50.3 mm d−1, while the mean annual EP increases from 35 to 138 mm from the northwestern to the southeastern LP. Further, historically, the EP frequency (EPF) has spatially varied from 54 to 116 d, with the highest EPF occurring in the mid-southern and southeastern LP where precipitation is much more abundant. However, EP intensities tend to be strongest in the central LP, where precipitation also tends to be scarce, and get progressively weaker as we move towards the margins (similarly to EP severity). An examination of atmospheric circulation patterns indicates that the central LP is the inland boundary with respect to the reach and impact of tropical cyclones in China, resulting in the highest EP intensities and EP severities being observed in this area. Under the control of the East Asian monsoon, precipitation from June to September accounts for 72 % of the total amount, and 91 % of the total EP events are concentrated between June and August. Further, EP events occur, on average, 11 d earlier than the wettest part of the season. These phenomena are responsible for the most serious natural hazards in the LP, especially in the central LP region. Spatiotemporally, 91.4 % of the LP has experienced a downward trend in precipitation, whereas 62.1 % of the area has experienced upward trends in the EP indices, indicating the potential risk of more serious hazardous situations. The universal multifractal approach considers the physical processes and probability distribution of precipitation, thereby providing a formal framework for spatiotemporal EP assessment at the regional scale.

Expanding Freshwater Lenses Adjacent to Gaining Rivers Through Vertical Low‐Hydraulic‐Conductivity Barriers: Analytical and Experimental Validation

AbstractFreshwater lenses within riparian zones of some arid and semiarid settings assist in maintaining the health of riparian ecosystems. We propose an approach for expanding freshwater lenses in saline aquifers adjacent to gaining rivers through the addition of a vertical barrier of low‐hydraulic‐conductivity (low‐K) parallel to the river bank. Sharp‐interface analytical solutions for the lens shape and water table distribution are developed to examine the effectiveness of the proposed method and are verified using sand tank experiments and numerical simulations. The sensitivity analysis is used to apply the method to parameters typical of the Lower River Murray (South Australia) and its floodplain aquifers. The results show that the barrier can create significant freshwater lenses in head‐controlled systems, whereas the barrier may lead to lens shrinkage in flux‐controlled systems due to saline water table rise. That is, the effectiveness of the barrier is highly dependent on the inland boundary condition. The analytical solution presented herein can be used to efficiently predict the riparian freshwater lens extent in response to engineered barriers, adding to existing techniques for studying and modifying riparian freshwater lenses.

Groundwater response to tidal fluctuations in a leaky confined coastal aquifer with a finite length

AbstractThe study on the hydraulic properties of coastal aquifers has significant implications both in hydrological sciences and environmental engineering. Although many analytical solutions are available, most of them are based on the same basic assumption that assumes aquifers extend landward semi‐infinitely, which does not necessarily reflect the reality. In this study, the general solutions for a leaky confined coastal aquifer have been developed that consider both finitely landward constant‐head and no‐flow boundaries. The newly developed solutions were then used to examine theoretically the joint effects of leakage and aquifer length on hydraulic head fluctuations within the leaky confined aquifer, and the validity of using the simplified solution, which assumes the aquifer is semi‐infinite. The results illustrated that the use of the simplified solution may cause significant errors, depending on joint effects of leakage and aquifer length. A dimensionless characteristic parameter was then proposed as an index for judging the applicability of the simplified solution. In addition, practical application of the general solution for the constant‐head inland boundary was used to characterize the hydraulic properties of a leaky confined aquifer using the data collected from a field site at the Seine River estuary, France, and the versatility of the general solution was further justified.