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    A desktop study of the groundwater resources of Cousine and Cousin Islands, Seychelles : Funded by MSP GEF and prepared for Nature Seychelles
    (Flinders University, 2007-12) Werner, Adrian D; van den Akker, Ben; Jakovovic, Danica; Jacob, Anis; Bestland, Erick Anthony
    The groundwater resources of Cousine and Cousin Islands has previously not been investigated, although these resources are accessed via bore pumping to support tourism, residents, and visitors to the islands, in terms of domestic use, drinking water, lawn irrigation and other anthropogenic requirements. It is also likely that groundwater plays an important role in the health of the highly valued ecosystems of these two islands, given the existence of springs and other surface water bodies that depend on groundwater influxes. This report describes a desktop study of the groundwater resources of Cousine and Cousin Islands, and aims to address a number of issues, summarised as: 1. Describe general considerations of importance in the management of groundwater resources of Cousine and Cousine Islands 2. Collect and collate existing knowledge and data on the groundwater resources of Cousine and Cousin Islands 3. Within data availability constraints, development groundwater models of the islands to explore various water supply questions relating to mainly to the sustainability of groundwater extraction 4. Provide advice on water treatment options, hydrological monitoring, and areas of future investigation The report provides an overview of the water resources of small islands. The existence of fresh groundwater on small islands is mostly controlled by island size and climate, but also topography, geology, vegetation and soil type. A review of existing guidelines on island water resources indicated that Cousin and Cousine Island are classified as “very small islands” in the typology of UNESCO (1991). Islands of size similar to Cousine and Cousin Islands are considered by the authors of the UNESCO (1991) guidelines to be unlikely to contain significant fresh groundwater resources, which are expected to occur as a very thin wedge overlying saline (seawater) groundwater. It is therefore somewhat surprising that fresh groundwater supplies are abstracted from both islands in non-trivial quantities. The data collection phase of the study captured information relating to physiography, topography, climate, vegetation and land use, soils and geology, hydrogeology and groundwater use, and some groundwater quality observations. This information was used to develop conceptual models of the islands’ hydrogeology, and these served to develop computer models of the groundwater systems. The available dataset pertaining to the islands’ aquifers was considered to be deficient for the development of predictive management models, and further information needs to be obtained through field investigation, monitoring, sampling and analysis before reliable modelling results, sufficient to guide groundwater management decision, can be provided. Nonetheless, a groundwater modelling investigation was undertaken to assist in guiding management decisions and to demonstrate the capability of contemporary modelling methods for future studies. The Cousine Island groundwater model is a state-of-the-art three-dimensional groundwater flow and seawater intrusion model, which is based on cutting-edge modelling software (i.e. the MODHMS code) and GIS-based modelling techniques (e.g. Groundwater Vistas software). The model simulates the density effects of seawater-freshwater interaction, and uses interpolated surfaces to explicitly represent the hydrostratigraphic units of sand and granite, of which the island is predominantly comprised. Models were developed that simulate either long-term (548 years) or time-variant (i.e. climatic variations) conditions, and were used to evaluate both pumping and no pumping groundwater conditions. Simulations using the Cousine Island model demonstrated that pumping from several shallow bores is more likely to yield lower salinity groundwater than that obtained from a single deeper bore. This outcome is well aligned to “skimmer well” approaches adopted in similar settings of fresh groundwater overlying seawater. The Cousine Island study also demonstrated the potential for climatic variability in both water quality and watertable level, and showed that it is quite likely that a large variability in groundwater salinity could be expected from one year to the next. Simulations of well pumping, at rates similar to those historically recorded on Cousine Island, indicate that groundwater pumping has indeed induced a landward movement of saline groundwater towards the points of extraction. Further modelling is required to better optimise rates of extraction and sites of future bore construction. The model of Cousin Island adopted an approach that was more closely aligned to the limited available information, and more routine methods were applied (e.g. MODFLOW and PMWIN software). A “single-layer” approach (i.e. two-dimensional) was adopted and only groundwater flow was modelled in the absence of salt-related density effects. The results of the Cousin Island modelling study also provided some indication of the benefit of using multiple points of shallow groundwater extraction, although the proximity to the shoreline also proved to be a critical consideration. A review of water treatment options for the island water was discussed. It is clear that existing water chemistry and microbiological information are insufficient to properly plan an optimal water treatment strategy for each island. Further, more clarity of water use needs, in terms of quality and quantity, is required, especially for Cousin Island. A preliminary overview of water treatment options identified that such treatments as pH correction, filtration, reverse osmosis and/or disinfection could be considered to provide for improved water quality to users from Cousine Island, while it was not possible to predict specific water treatment works for Cousin Island with the current available information. However, it is recommended that proper analysis of water quality be undertaken before the purchase of water treatment infrastructure is further considered. Finally, the report outlines a preliminary proposal for future investigation of the islands groundwater resources and water quality, aimed at completing the study predominantly through field investigations, thereby allowing for more reliable estimates of sustainable groundwater use and for the provision of more specific advice relating to water treatment options.
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    Using geochemistry to discern the patterns and timescales of groundwater recharge and mixing on floodplains in semi-arid regions
    (Elsevier, 2019-01-24) Cartwright, Ian; Werner, Adrian D; Woods, Juliette A
    Floodplains may alternate between discharge zones for regional groundwater and areas of recharge from river water during high-flow events. Understanding the mechanisms and timescales of recharge on floodplains is important for their management and for the protection of fragile ecosystems. The floodplains of the River Murray host important ecosystems, particularly remnant eucalypt forests that are vulnerable to changes in inundation, rising regional water tables, and salt accumulation. This study addresses floodplain recharge and groundwater mixing on the Pike and Katarapko floodplains of South Australia. At Pike, 3H activities of groundwater in the low hydraulic conductivity Coonambidgal Formation that crops out on the floodplain are ∼0.25 TU. 3H activities of groundwater in the underlying higher hydraulic conductivity Monoman Formation decrease from ∼1 TU near the contact with the Coonambidgal Formation to <0.02 TU at >15 m depth. Groundwater 14C activities are between 40 and 95 pMC but are less well correlated with depth. The 3H or 14C activities do not vary systematically with distance from the surface water channels on the floodplains. These observations imply that groundwater recharge at Pike is dominantly through the floodplain rather than through the channel banks. In contrast to the regional groundwater where total dissolved solids (TDS) concentrations are commonly >35,000 mg/L, the TDS of groundwater on the floodplain is locally <500 mg/L. A correlation between 3H activities and TDS and the presence of groundwater with relatively low 14C activities but above detection 3H activities implies that recently recharged waters have mixed with regional groundwater in the floodplain sediments. The 3H activities in these mixed waters implies that mixing occurs over a few years. By contrast, at Katarapko, the highest 3H activities in the Monoman Formation groundwater (up to 2.35 TU) are closer to the Murray River, implying that recharge through the bank may occur. These contrasting patterns of recharge probably reflect local topographic controls. Understanding the recharge-discharge relationships are vital for managing proposed floodplain inundation programs aimed at improving ecosystem health.
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    Combined geophysical and analytical methods to estimate offshore freshwater extent
    (Elsevier, 2019-06-22) Knight, Andrew C; Werner, Adrian D; Irvine, Dylan
    Offshore fresh groundwater is increasingly suggested as a potential water resource for onshore human demands. In many cases, onshore pumping already draws significant fresh groundwater from offshore. However, offshore aquifers and the extent of offshore freshwater are usually poorly characterised due to data scarcity. This study combines geophysical data, hydraulic information and a first-order mathematical analysis to investigate offshore freshwater extent in the Gambier Embayment (Australia). A large seismic data set, combined with onshore and offshore bore-log geological profiles, are used to explore the regional offshore hydro-stratigraphy. Aquifer hydraulic parameters and onshore heads are obtained from onshore investigations. A novel application of Archie’s law, geophysical data and onshore hydrochemical data provide useful insights into the salinity profiles within four offshore wells. These are compared to steady-state, sharp-interface estimates of the freshwater extent obtained from a recently developed analytical solution, albeit using simplified conceptual models. Salinities derived from resistivity measurements indicate that in the south of the study area, pore water with total dissolved solids (TDS) of 2.2 g L-1 is found up to 13.2 km offshore. Offshore pore-water salinities are more saline in the northern areas, most likely due to thinning of the offshore confining unit. The analytical solution produced freshwater-saltwater interface locations that were approximately consistent with the freshwater-saltwater stratification in two of the offshore wells, although analytical uncertainty is high. This investigation provides a leading example of offshore freshwater evaluation applying multiple techniques, demonstrating both the benefit and uncertainty of geophysical interpretation and analytical solutions of freshwater extent.
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    Dispersion effects on the freshwater–seawater interface in subsea aquifers
    (Elsevier, 2019-05-29) Solorzano-Rivas, Silvia; Werner, Adrian D; Irvine, Dylan
    Recent recognition of the widespread occurrence of freshwater beneath the ocean has renewed interest in approaches to understand and predict its extent. The most straightforward methodologies are based on the sharp-interface approximation, which neglects dispersive mechanisms. The understanding of dispersion effects on freshwater extents in coastal aquifers is based almost entirely on onshore aquifer situations. This study explores dispersion in offshore coastal aquifers, in terms of the steady-state freshwater extent, seawater circulation and freshwater discharge, through numerical experimentation. Results show that increasing dispersion causes a seaward shift in the interface toe location, as expected, whereas the interface tip shows a non-monotonic relationship with dispersion that depends on the contrast between aquifer and aquitard hydraulic conductivities. Higher dispersion leads to enhanced seawater recirculation rates and freshwater discharge, as opposed to non-monotonic relationships obtained previously for onshore aquifers. The mixing zone at the toe widens as dispersion increases, similar to onshore cases, whereas the mixing zone at the tip has a surprisingly non-monotonic relationship with dispersion. The dispersion relationships revealed in this study can be explained by counteractions between dispersion, density and advective forces, and refraction across the aquifer-aquitard interface, which in combination produce offshore aquifer behaviour that differs, in some ways, to the manner in which onshore aquifers respond to dispersive processes. Consequently, previous empirical corrections to sharp-interface methods (to account for dispersive effects) applied to onshore coastal aquifers are ineffective in their application to offshore settings.
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    Rebuttal to “The case of the Biscayne Bay and aquifer near Miami, Florida: density-driven flow of seawater or gravitationally driven discharge of deep saline groundwater?” by Weyer (Environ Earth Sci 2018, 77:1-16)
    (Springer, 2018-10-11) Provost, Alden M; Werner, Adrian D; Post, Vincent Eduard Alexander; Michael, Holly A; Langevin, Christian D
    A recent paper by Weyer (Environ Earth Sci 2018, 77:1–16) challenges the widely accepted interpretation of groundwater heads and salinities in the coastal Biscayne aquifer near Miami, Florida, USA. Weyer (2018) suggests that the body of saltwa-ter that underlies fresh groundwater just inland of the coast is not a recirculating wedge of seawater, but results instead from upward migration of deep saline groundwater driven by regional flow. Perhaps more significantly, Weyer (2018) also asserts that established hydrologic theory is fundamentally incorrect with respect to buoyancy. Instead of acting along the direction of gravity (that is, vertically), Weyer (2018) claims, buoyancy acts instead along the direction of the pressure gradient. As a result, Weyer (2018) considers currently available density-dependent groundwater flow and transport modeling codes, and the analyses based on them, to be in error. In this rebuttal, we clarify the inaccuracies in the main points of Weyer’s (2018) paper. First, we explain that Weyer (2018) has misinterpreted observed equivalent freshwater heads in the Biscayne aquifer and that his alternative hypothesis concerning the source of the saltwater does not explain the observed salinities. Then, we review the established theory of buoyancy to identify the problem with Weyer’s (2018) alternative theory. Finally, we present theory and cite successful benchmark simulations to affirm the suitability of currently available codes for modeling density-dependent groundwater flow and transport.
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    A conceptual study of offshore fresh groundwater behaviour in the Perth Basin (Australia): Modern salinity trends in a prehistoric context
    (Elsevier, 2018-10) Morgan, Leanne K; Werner, Adrian D; Patterson, Aine E
    Fresh groundwater is thought to occur off the coast of Perth, Western Australia, in the confined Leederville and Yarragadee aquifers. Onshore hydraulic heads suggest that offshore groundwater may be augmenting onshore groundwater extraction, which is a critical component of Perth’s water supply.
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    Boundary condition nomenclature confusion in groundwater flow modelling
    (Wiley, 2019-04-18) Jazayeri, Amir; Werner, Adrian D
    To solve the partial differential equations of groundwater flow, the information about head (h) and/or head gradient (∇h) must be specified along the boundaries of a model domain. The descriptors of different boundary condition (BC) types are drawn from founding mathematicians mainly of the 19th century (Cheng and Cheng 2005). Mathematically, there are five different BC types, including: Dirichlet (Type 1), Neumann (Type 2), Robin (Type 3), Cauchy and Mixed (Liu 2018). These names are sometimes used in communicating the BCs of groundwater flow models, and therefore, correct association between nomenclature and the mathematical form of BCs is important for properly communicating model characteristics.
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    Efficacy and Efficiency of Multivariate Linear Regression for Rapid Prediction of Femoral Strain Fields during Activity
    (Elsevier, 2018-12-11) Ziaeipoor, Hamed; Martelli, Saulo; Pandy, Marcus G; Taylor, Mark
    Multivariate Linear Regression-based (MLR) surrogate models were explored to reduce the computational cost of predicting femoral strains during normal activity in comparison with finite element analysis. The musculoskeletal model of one individual, the finite-element model of the right femur, and experimental force and motion data for normal walking, fast walking, stair ascent, stair descent, and rising from a chair were obtained from a previous study. Equivalent Von Mises strain was calculated for 1000 frames uniformly distributed across activities. MLR surrogate models were generated using training sets of 50, 100, 200 and 300 samples. The finite-element and MLR analyses were compared using linear regression. The Root Mean Square Error (RMSE) and the 95th percentile of the strain error distribution were used as indicators of average and peak error. The MLR model trained using 200 samples (RMSE < 108 µε; peak error < 228 µε) was used as a reference. The finite-element method required 66 s per frame on a standard desktop computer. The MLR model required 0.1 s per frame plus 1848 s of training time. RMSE ranged from 1.2% to 1.3% while peak error ranged from 2.2% to 3.6% of the maximum micro-strain (5020 µε). Performance within an activity was lower during early and late stance, with RMSE of 4.1% and peak error of 8.6% of the maximum computed micro-strain. These results show that MLR surrogate models may be used to rapidly and accurately estimate strain fields in long bones during daily physical activity.
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    Transport and fate of viruses in sediment and stormwater from a Managed Aquifer Recharge site
    (Elsevier, 2017-10-27) Sasidharan, Salini; Bradford, Scott A; Simunek, Jiri; Torkzaban, Saeed; Vanderzalm, Joanne
    Enteric viruses are one of the major concerns in water reclamation and reuse at Managed Aquifer Recharge (MAR) sites. In this study, the transport and fate of bacteriophages MS2, PRD1, and ΦX174 were studied in sediment and stormwater (SW) collected from a MAR site in Parafield, Australia. Column experiments were conducted using SW, stormwater in equilibrium with the aquifer sediment (EQ-SW), and two pore-water velocities (1 and 5 m day−1) to encompass expected behavior at the MAR site. The aquifer sediment removed >92.3% of these viruses under all of the considered MAR conditions. However, much greater virus removal (4.6 logs) occurred at the lower pore-water velocity and in EQ-SW that had a higher ionic strength and Ca2+ concentration. Virus removal was greatest for MS2, followed by PRD1, and then ΦX174 for a given physicochemical condition. The vast majority of the attached viruses were irreversibly attached or inactivated on the solid phase, and injection of Milli-Q water or beef extract at pH = 10 only mobilized a small fraction of attached viruses (<0.64%). Virus breakthrough curves (BTCs) were successfully simulated using an advective–dispersive model that accounted for rates of attachment (katt), detachment (kdet), irreversible attachment or solid phase inactivation (μs), and blocking. Existing MAR guidelines only consider the removal of viruses via liquid phase inactivation (μl). However, our results indicated that katt > μs > kdet > μl, and katt was several orders of magnitude greater than μl. Therefore, current microbial risk assessment methods in the MAR guideline may be overly conservative in some instances. Interestingly, virus BTCs exhibited blocking behavior and the calculated solid surface area that contributed to the attachment was very small. Additional research is therefore warranted to study the potential influence of blocking on virus transport and potential implications for MAR guidelines.
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    Evaluation of management scenarios for potable water supply using script-based numerical groundwater models of a freshwater lens
    (Elsevier, 2019-02-20) Post, Vincent Eduard Alexander; Galvis, Sandra C; Sinclair, Peter J; Werner, Adrian D
    Challenges in balancing freshwater demands and the long-term availability of freshwater from small island aquifers warrants responsive management, whereby groundwater conditions guide decisions about pumping rates to avoid well salinization. We evaluate responsive freshwater lens management for the first time, through transient, three-dimensional, dispersive modelling of Bonriki Island (Kiribati). Both responsive- and fixed-management scenarios are explored, including a novel pumping redistribution strategy. Modelling results reveal that responsive management offers superior lens protection, particularly during droughts. Pumping redistribution produced lower salinities but greater lens depletion. All scenarios indicate that the Bonriki lens will continue to decline, consistent with previous shorter-timeframe projections. Lower lens storage losses are attainable by abstracting groundwater at the maximum acceptable salinity, contrary to traditional strategies of seeking the lowest available salinities. The methodology developed in this research provides a blueprint for investigating responsive, “monitor-and-react” management scenarios, which we advocate as best practice for balancing freshwater demands with long-term lens security.
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    The role of surface charge and pH changes in tropical soils on sorption behaviour of per- and polyfluoroalkyl substances (PFASs)
    (Elsevier, 2019-04-05) Oliver, Danielle P; Li, Yasong; Orr, Ryan; Nelson, Paul; Barnes, Mary; McLaughlin, Michael J; Kookana, Rai S
    This study investigated the effect of surface charge on the sorption of perfluorooctane sulfonic acid (PFOS), perfluorooctanoic acid (PFOA) and perfluorohexane sulfonic acid (PFHxS) onto 7 tropical soils as a function of pH. The net surface charge became less negative with decreasing pH (from 7.5 to 3.5) in all soils. The rate of change in net surface charge varied from −0.6 to −2.8 (cmol/kg)/pH unit. The effect on sorption behaviour of PFASs was variable among soils. For two soils, the average sorption increased 54- and 45-fold for PFOS, 33- and 9-fold for PFOA, and 39- and 400-fold for PFHxS, across the pH range 7.5 to 3.5. Sorption in another sandier soil showed negligible change with decreasing pH. Sorption in the other soils did not change significantly until the pH decreased to approximately 5.5. The soils with high contents of sesquioxides (Fe and Al oxides) showed the most marked increase in sorption with decreasing pH. This study demonstrated that in addition to hydrophobic interactions with OC and other processes, electrostatic interactions are also important in the sorption process for these chemicals in soils. In acidic, variably charged tropical soils there is the possibility that any PFOS, PFOA or PFHxS sorbed to the soils may become desorbed if management practices (e.g. liming) raised soil pH.
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    Groundwater chemistry and the Gibbs Diagram
    (Elsevier, 2018-08-09) Marandi, Andres; Shand, Paul
    The ‘Gibbs Diagram’ represents some of the key processes controlling surface water chemistry. This review highlights that the processes listed on the Gibbs Diagram may not be applicable for assessing processes controlling groundwater chemistry. We discuss the importance of geochemical processes governing groundwater chemistry in the Gibbs Diagram framework. We show that the processes represented on the Gibbs Diagram—originally developed for surface waters—are unlikely to represent key processes controlling the chemistry of most groundwater systems.
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    Development of an integrated model for the Campaspe catchment: a tool to help improve understanding of the interaction between society, policy, farming decision, ecology, hydrology and climate
    (International Association of Hydrological Sciences, 2018-06-05) Iwanaga, Takuya; Zare, Fateme; Croke, Barry; Fu, Baihua; Merritt, Wendy; Partington, Dan J; Ticehurst, Jenifer; Jakeman, Anthony
    Management of water resources requires understanding of the hydrology and hydrogeology, as well as the policy and human drivers and their impacts. This understanding requires relevant inputs from a wide range of disciplines, which will vary depending on the specific case study. One approach to gain understanding of the impact of climate and society on water resources is through the use of an integrated modelling process that engages stakeholders and experts in specifics of problem framing, co-design of the underpinning conceptual model, and discussion of the ensuing results. In this study, we have developed such an integrated modelling process for the Campaspe basin in northern Victoria, Australia. The numerical model built has a number of components: Node/link based surface water hydrology module based on the IHACRES rainfall-streamflow model Distributed groundwater model for the lower catchment (MODFLOW) Farm decision optimisation module (to determine irrigation requirements) Policy module (setting conditions on availability of water based on existing rules) Ecology module (determining the impacts of available streamflow on platypus, fish and river red gum trees) The integrated model is component based and has been developed in Python, with the MODFLOW and surface water hydrology model run in external programs, controlled by the master program (in Python). The integrated model has been calibrated using historical data, with the intention of exploring the impact of various scenarios (future climate scenarios, different policy options, water management options) on the water resources. The scenarios were selected based on workshops with, and a social survey of, stakeholders in the basin regarding what would be socially acceptable and physically plausible options for changes in management. An example of such a change is the introduction of a managed aquifer recharge system to capture dam overflows, and store at least a portion of this in the aquifer, thereby increasing the groundwater resource as well as reducing the impact of existing pumping levels.
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    Impacts of Different Onset Time El Niño Events on Winter Precipitation over South China
    (MDPI, 2018-09-20) Fan, Lingli; Xu, Jianjun; Guan, Huade
    Winter precipitation over South China tended to be much higher than normal for the spring El Niño events during 1979–2016. For the spring El Niño events, the meridional and zonal circulations served as a bridge, linking the warmer sea surface temperature (SST) in the eastern equatorial Pacific (EEP) and South China winter precipitation. This possible physical process can be described as follows: During boreal winter, a positive SST anomaly in the EEP was concurrent with strong anomalous convection activity over South China via anomalous Walker circulation, an anomalous Hadley Cell along 110°–130° E, and a zonal westward teleconnection wave train pattern at 700 hPa in the Northern Hemisphere. In addition, an anomalous pumping effect at 200 hPa contributed to the convective activity. Meanwhile, the western Pacific subtropical high moved southwards and strengthened at 500 hPa, and abnormal southwesterly winds brought plentiful water vapor to South China at 850 hPa. All these factors favored an increase in precipitation over South China. For the summer El Niño events, the aforementioned anomalies were weaker, which resulted in a precipitation close to normal over South China.
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    Examination of the ecohydrological separation hypothesis in a humid subtropical area: Comparison of three methods
    (Elsevier, 2019-02-18) Luo, Zidong; Guan, Huade; Zhang, Xinping; Xu, Xiang; Dai, Junjie; Hua, Mingquan
    The ecohydrological separation between soil water sources for plant water uptake and groundwater recharge has been recently examined in various climate zones primarily based on isotopic composition of water. The existence of the ecohydrological separation has profound implications for mechanistic ecohydrological modeling and water resource management. However, it is still unclear when and where the ecohydrological separation occurs, especially in humid regions. In this study, high frequency sampling of precipitation, bulk soil water, groundwater and twig xylem water for hydrogen and oxygen isotope composition measurement was conducted in a humid subtropical site in the central southern China from March 2017 to April 2018. We examined evidence of the ecohydrological separation with three methods (dual-isotope space, line-conditioned excess (lc-excess), and the piecewise isotope balance (PIB) method). The results show that the isotopic composition of plant xylem and bulk soil water are not distinguishable from those of precipitation water on the dual-isotope space due to a weak evaporation effect at the study site, indicating that there is no evidence of the ecohydrological separation. However, the other two methods support the ecohydrological separation in this humid area, with the results from the PIB method revealing more temporal details. The present study suggests that the ecohydrological separation can happen in subtropical humid climate. It is more likely to occur in spring and winter at the study site when plant-accessible water pool has been replenished by antecedent precipitation, while ecohydrological connection seems to occur during winter snowmelt. With the limitations of three methods, the caution should be taken when only one method is applied in examining the ecohydrological separation in such an environment.
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    Uncertainty analysis for seawater intrusion in fractured coastal aquifers: Effects of fracture location, aperture, density and hydrodynamic parameters
    (Elsevier, 2019-02-04) Koohbor, Behshad; Fahs, Marwan; Ataie-Ashtiani, Behzad; Belfort, Benjamin; Simmons, Craig Trevor; Younes, Anis
    In this study we use polynomial chaos expansion (PCE) to perform uncertainty analysis for seawater intrusion (SWI) in fractured coastal aquifers (FCAs) which is simulated using the coupled discrete fracture network (DFN) and variable-density flow (VDF) models. The DFN-VDF model requires detailed discontinuous analysis of the fractures. In real field applications, these characteristics are usually uncertain which may have a major effect on the predictive capability of the model. Thus, we perform global sensitivity analysis (GSA) to provide a preliminary assessment on how these uncertainties can affect the model outputs. As our conceptual model, we consider fractured configurations of the Henry Problem which is widely used to understand SWI processes. A finite element DFN-VDF model is developed in the framework of COMSOL Multiphysics®. We examine the uncertainty of several SWI metrics and salinity distribution due to the incomplete knowledge of fracture characteristics. PCE is used as a surrogate model to reduce the computational burden. A new sparse PCE technique is used to allow for high polynomial orders at low computational cost. The Sobol’ indices (SIs) are used as sensitivity measures to identify the key variables driving the model outputs uncertainties. The proposed GSA methodology based on PCE and SIs is useful for identifying the source of uncertainties on the model outputs with an affordable computational cost and an acceptable accuracy. It shows that fracture hydraulic conductivity is the first source of uncertainty on the salinity distribution. The imperfect knowledge of fracture location and density affects mainly the toe position and the total flux of saltwater entering the aquifer. Marginal effects based on the PCE are used to understand the effects of fracture characteristics on SWI. The findings provide a technical support for monitoring, controlling and preventing SWI in FCAs.
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    Urban Land Systems: An Ecosystems Perspective
    (MDPI, 2018-01-11) Kopecka, Monika; Nagendra, Harini; Millington, Andrew
    We live in an urbanizing world. Since 2008, more than half of humanity lives in cities, both large and small, and old and new. We also live in a world that is becoming even more urbanized, it is expected that by 2050, 66 per cent of the world’s population will live in cities [1]. The process of urbanization, accompanied by the rapid expansion of cities and the sprawling growth of metropolitan regions over the world, is one of the most important transformations of a natural landscape. In the context of land systems science, contemporary urbanisation is a set of land-use change processes and the various contemporary cityscapes are the resulting land systems. Population growth increases urban footprints with consequences on biodiversity and climate. Much of the explosive urban growth has been unplanned and conflicting land-use demands often arise as land is a limited resource. Increased requirements for living space and intensive landscape utilization constitute two of the principal reasons for the environmental change, with significant impacts on quality of life and ecosystems. This special issue of LAND explores urban land dynamics with particular regard to ecosystem structure, and discusses consequent environmental changes and their impacts. The studies cover a wide range of countries and contexts, and draw on a number of disciplinary methods and interdisciplinary approaches from the social and natural sciences. The papers have been authored by 41 researchers from 29 institutions in countries worldwide: from Australia, Bangladesh, China, India, Iraq, Italy, Japan, Nigeria, Philippines, Saudi Arabia, Slovakia, Spain, Thailand, the United Kingdom, and the USA.
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    Vulnerability mapping of coastal aquifers to seawater intrusion: Review, development and application
    (Elsevier, 2018-12-30) Parizi, Esmaeel; HosseiniHosseini, Seiyed Mossa; Ataie-Ashtiani, Behzad; Simmons, Craig Trevor
    In this study, a review of the overlay/index methods served for delineation of vulnerable zones in coastal aquifers affected by SWI is provided. Then, a more realistic presentation of the vulnerability mapping of coastal aquifers to SWI through modified GALDIT index method by incorporating the influential factors on SWI is established. The modifications on GALDIT method including incorporating the seaward hydraulic gradient (i) instead of the height of groundwater level above sea level (L) (so-called GAiDIT), and considering hydraulic gradient (i) as an additional parameter to the GALDIT (so-called GALDIT-i). Three GALDIT, GAiDIT, and GALDIT-i methods were evaluated with data from three coastal confined and phreatic/confined aquifers located in the south of the Caspian Sea, northern Iran. While no highly vulnerable zone was recognized by GALDIT method across three studied aquifers, averagely 43.4% and 50.5% of aquifers area were defined as highly vulnerable zones by GAiDIT and GALDIT-i, respectively. Furthermore, the final vulnerability maps obtained by GALDIT-i and then GAiDIT indicates higher correlation by three groundwater quality indices specific to SWI including ( = 0.72 and 0.63) and ( = 0.69 and 0.62) and also the distribution of TDS in groundwater ( = 0.71 and 0.61) compared with GALDIT ( = 0.33, 0.42, and 0.36, respectively). The values of vulnerability index obtained by GALDIT-i and GAiDIT are more strongly correlated with the length of SWI into the aquifer () based on Strack's analytical approach than GALDIT ( = 0.52, 0.36, and 0.32, respectively). The results of sensitivity analysis indicated that the hydraulic gradient, height of groundwater level above sea level, aquifer type, and existing status of seawater intrusion has the greatest impact on the groundwater vulnerability across the studied aquifers by GALDIT-i and GAiDIT methods. Results also indicated that serving the influential parameters in GALDIT methods regarding the hydrological and anthropogenic characteristics across the aquifer provide a more realistic characterization of the SWI. This modification leads to an accurate aquifer vulnerability mapping to SWI in aquifers characterized by transient anthropogenic drivers (e.g. pumping) which can be served as a promising tool for decision-making to properly assess and manage risk.
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    Co-extinctions annihilate planetary life during extreme environmental change
    (Springer Nature, 2018-11-13) Strona, Giovanni; Bradshaw, Corey J A
    Climate change and human activity are dooming species at an unprecedented rate via a plethora of direct and indirect, often synergic, mechanisms. Among these, primary extinctions driven by environmental change could be just the tip of an enormous extinction iceberg. As our understanding of the importance of ecological interactions in shaping ecosystem identity advances, it is becoming clearer how the disappearance of consumers following the depletion of their resources — a process known as ‘co-extinction’ — is more likely the major driver of biodiversity loss. Although the general relevance of co-extinctions is supported by a sound and robust theoretical background, the challenges in obtaining empirical information about ongoing (and past) co-extinction events complicate the assessment of their relative contributions to the rapid decline of species diversity even in well-known systems, let alone at the global scale. By subjecting a large set of virtual Earths to different trajectories of extreme environmental change (global heating and cooling), and by tracking species loss up to the complete annihilation of all life either accounting or not for co-extinction processes, we show how ecological dependencies amplify the direct effects of environmental change on the collapse of planetary diversity by up to ten times.
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    Modelling size constraints on carbonate platform formation in groundwater upwelling zones
    (Springer Nature, 2018-11-29) Keppel, Mark N; Post, Vincent Eduard Alexander; Love, Andrew James; Werner, Adrian D; Clarke, Jonathan D A; Halihan, Todd
    Carbonate depositional systems related to groundwater upwelling are ubiquitous around the world and form ecologically and culturally important features of many landscapes. Spring carbonate deposits record past climatic and hydrological conditions. The reconstruction of past processes using spring carbonate proxies requires fundamental understanding of the factors that control their geometry. In this work, we show that the spatial extent of spring carbonate platforms is amenable to quantitative prediction by simulating the early growth stage of their formation for the iconic mound springs in the central Australian outback. We exploit their well-defined, circular geometry to demonstrate the existence of two size-limiting regimes: one controlled by the spring flow rate and the other by the concentration of lattice ions. Deviations between modelled and observed size metrics are attributable to diminishing spring flow rates since formation, enabling assessment of the relative vulnerability of springs to further hydrological change.