Groundwater Connectivity Research Articles

Brief communication: The hidden labyrinth: deep groundwater in Wright Valley, Antarctica

Abstract. Since the 1960s, a deep groundwater system in Wright Valley, Antarctica, has been the hypothesized source of brines to hypersaline Don Juan Pond and Lake Vanda, both of which are rich in calcium and chloride. Modeling studies do not support other possible mechanisms, such as evaporative processes, that could have led to the current suite of ions present in both waterbodies. In 2011 and 2018, an airborne electromagnetic survey was flown over Wright Valley to map subsurface resistivity (down to 600 m) in exploration of liquid water. The surveys revealed widespread unfrozen brine in the subsurface near Lake Vanda, Don Juan Pond, and the North Fork of Wright Valley. While our geophysical survey can neither confirm nor deny deep groundwater connectivity between Lake Vanda and Don Juan Pond, it does point to the potential for deep valley-wide brine, likely within the Ferrar Dolerite formation.

Global Recharge Data Set Indicates Strengthened Groundwater Connection to Surface Fluxes

AbstractGroundwater is an invaluable global resource, but its long‐term viability as a resource for consumption, agriculture, and ecosystems depends on precipitation recharging aquifers. How much precipitation recharges groundwaters varies enormously across Earth's surface, yet recharge rates often remain uncertain. Here we use a global synthesis of field‐estimated recharge across six continents to show that globally recharge first‐order follows a simple function of climatic aridity. We use this relationship to estimate long‐term recharge in energy‐limited systems outside of permafrost regions. Our aridity‐based recharge estimates are consistent with the global field data but, on average, double previous estimates of global models. Our higher recharge estimates are likely caused by preferential groundwater recharge and discharge occurring at grid scales finer than global models. The higher recharge estimates suggest that more groundwater contributes to evapotranspiration and streamflow than previously represented by global hydrological models and global water cycle diagrams.

Hydro Economic Asymmetries and Common‐Pool Overdraft in Transboundary Aquifers

AbstractThe common‐pool nature of groundwater resources creates incentives to over pump that contribute to their rapid global depletion. In transboundary aquifers, users are separated by a territorial border and might face substantially different economic and hydrogeologic conditions that can alternatively dampen or amplify incentives to over pump. We develop a theoretical model that couples principles of game theory and groundwater flow to capture the combined effect of well locations and user asymmetries on pumping incentives. We find that heterogeneities across users (here referred to as asymmetries) in terms of either energy cost, groundwater profitability or aquifer response tend to dampen incentives to over pump. However, combinations of two or more types of asymmetry can substantially amplify common‐pool overdraft, particularly when the same user simultaneously faces comparatively higher costs (or aquifer response) and profitability. We use this theoretical insight to interpret the emergence of the Disi agreement between Saudi Arabia and Jordan in association with the Disi‐Amman water pipeline. By using bounded non‐dimensional parameters to encode user asymmetries and groundwater connectivity, the theory provides a tractable generalized framework to understand the premature depletion of shared aquifers, whether transboundary or not.

Terrestrial evidence for ocean forcing of Heinrich events and subglacial hydrologic connectivity of the Laurentide Ice Sheet.

During the last glacial period, the Laurentide Ice Sheet (LIS) underwent episodes of rapid iceberg discharge, recorded in ocean sediments as “Heinrich events” (HEs). Two competing models attempt to describe the stimulus for HEs via either internal ice sheet oscillations or external ocean-climate system forcing. We present a terrestrial record of HEs from the northeastern LIS that strongly supports ocean-climate forcing. Subglacial carbonate precipitates from Baffin Island record episodes of subglacial melting coincident with the three most recent HEs, resulting from acceleration of nearby marine-terminating ice streams. Synchronized ice stream acceleration over Baffin Island and Hudson Strait is inconsistent with internal ice sheet oscillations alone and indicates a shared ocean-climate stimulus to coordinate these different glaciological systems. Isotopic compositions of these precipitates record widespread subglacial groundwater connectivity beneath the LIS. Extensive basal melting and flushing of these aquifers during the last HE may have been a harbinger for terminal deglaciation.

A structured framework to interpret hydro-climatic and water quality trends in Mediterranean climate zones

Many regions of the world have experienced rapid hydro-climatic changes in recent decades. Mediterranean areas have especially experienced prolonged droughts, changes to runoff, and low-flow periods. Many methods and assumptions for assessing trends in rainfall and runoff are available, however, a comprehensive framework is required that can consider variations at large temporal and spatial scales, long-term persistence (LTP), seasonal patterns, and multiple change points. An analysis framework has been developed in R and applied to a network of 107 rainfall gauges (with about 100 yrs data) and 90 streamflow-water quality gauges (30 – 50 yrs data) across South-west Western Australia, a region with a typical Mediterranean climate. Trends were estimated using the Original Mann–Kendall (OMK) family of tests including using the Mann-Kendall Test under the Scaling Hypothesis (MKLTP) and Seasonal Mann–Kendall (SMK). Single and multiple change points were calculated by the non-parametric Lanzante’s test (LAT), E-Agglomerative algorithm (ECP), and Energy Divisive (EDP). Trend tests show annual rainfall has decreased by 2.6 mm/yr in the Western areas and increased by 0.9 mm/yr in the Eastern areas with corresponding change points in the 1940 s-1960 s and from 1995 to 2000 although 30 % of the annual rainfall and 100 % of the standardized precipitation index (SPI) time series were found to be under LTP. A regional flow trend analysis shows catchments are yielding much less runoff in more than 90 % of gauged stations. The runoff in perennial and ephemeral streams has shown a decline with the rate of 6 (54 %) and 2 (50 %) mm/yr, respectively. Runoff and baseflow decline occurred as step changes lagged after rainfall, and the rate of decline in baseflow was more than for runoff, suggesting that groundwater connectivity with stream inverts has fallen substantially. Stream salinities increased by 64 mg/L/yr (or 15 %) overall which was highly significant. The framework provides a structured approach to analyse changes in rainfall, runoff and stream salinity for a Mediterranean climate that is projected to become even warmer and drier.

Water stable isotopes reveal a complex rainfall to groundwater connectivity in central Honduras

The inter-mountainous region of central Honduras has been experiencing abrupt urban drinking water shortages during the last decade. Land use fragmentation and rainfall deficits have rapidly reduced surface water quality and quantity in this region. Here we present a 3-yr (2018–2020) tracer study within the headwaters of the Choluteca River basin (2949 km2). We sampled rainfall (weekly N = 156; daily N = 270), drilled wells (N = 166; up to ~300 m depth), boreholes (N = 70; ~4–12 m depth), and springs (N = 128) to assess the spatiotemporal connectivity between rainfall and mean groundwater recharge elevations (MREs). Clear W-shaped incursions characterized rainfall isotopic seasonality from the dry to the wet season. Air mass back trajectory analysis revealed three primary moisture sources: 73 % (east, Caribbean Sea), 17 % (southwest, Pacific Ocean), and 10 % (north; Gulf of Mexico). Groundwater sources exhibited a strong meteoric origin with evidence of secondary evaporation evolution, characterized by low d-excess values. MREs for the drilled wells ranged from 821 to 2018 m asl with a mean value of 1570 ± 150 m asl. Seasonal isotopic variability during dry-wet transitions and the influence of rapid infiltration limited the performance of the MRE method in springs and boreholes. MREs coincided primarily with coniferous forests, pasture, and crop areas, within regions of moderate to high transmissivity. These results are intended to guide the mapping and delineation of critical recharge areas in central Honduras to enhance municipal water regulations, effective environmental protection, and long-term conservation practices.

Concurrent 5. Presentation for: Multiple tracers for dis-connectivity of shallow aquifers, alluvium, and coal seam gas wells in the Great Artesian Basin

Presented on Tuesday 17 May: Session 5 The potential for connectivity between water supply aquifers and gas reservoirs raises community, government, and scientific concerns. Methane can occur naturally, making it difficult to determine whether water bore methane levels are being influenced by nearby gas operations. This poses a challenge in the Surat Basin, where coal seam gas production operates alongside groundwater using industries (including feedlots, agriculture, mines). Water and gas samples were taken from water bores and coal seam gas (CSG) wells in the Walloon Coal Measures and from overlying aquifers (nominally, the Springbok, Gubberamunda, Orallo, and Mooga sandstones) and the Condamine Alluvium, for stable isotopes of gases, groundwater and dissolved inorganic carbon, as well as strontium isotopes. Most of the sampled water bores had isotopic signatures distinct from CSG wells, though a minority from gassy Springbok Sandstone and Walloon Coal Measure water bores could not be distinguished from CSG wells. In those few cases, neither connectivity or dis-connectivity could be confirmed. Alluvium and shallow aquifer samples have higher R36Cl values distinct from the older CSG production waters, as is the case with most 14C measurements. Waters from the Condamine River indicate potential surface water connectivity with the alluvium. The use of multiple tracers has shown that groundwater in some aquifers can be differentiated from groundwater in the coal seam gas reservoir and hence are useful tools in identifying where groundwater connectivity occurs. Understanding this connectivity forms another line of evidence to improve impact prediction models on a regional scale as well as providing information on connectivity in local groundwater investigations. To access the presentation click the link on the right. To read the full paper click here

From microbes to mammals: Pond biodiversity homogenization across different land‐use types in an agricultural landscape

AbstractLocal biodiversity patterns are expected to strongly reflect variation in topography, land use, dispersal boundaries, nutrient supplies, contaminant spread, management practices, and other anthropogenic influences. Contrary to this expectation, studies focusing on specific taxa revealed a biodiversity homogenization effect in areas subjected to long‐term intensive industrial agriculture. We investigated whether land use affects biodiversity levels and community composition (α‐ and β‐diversity) in 67 kettle holes (KH) representing small aquatic islands embedded in the patchwork matrix of a largely agricultural landscape comprising grassland, forest, and arable fields. These KH, similar to millions of standing water bodies of glacial origin, spread across northern Europe, Asia, and North America, are physico‐chemically diverse and differ in the degree of coupling with their surroundings. We assessed aquatic and sediment biodiversity patterns of eukaryotes, Bacteria, and Archaea in relation to environmental features of the KH, using deep‐amplicon‐sequencing of environmental DNA (eDNA). First, we asked whether deep sequencing of eDNA provides a representative picture of KH aquatic biodiversity across the Bacteria, Archaea, and eukaryotes. Second, we investigated if and to what extent KH biodiversity is influenced by the surrounding land use. We hypothesized that richness and community composition will greatly differ in KH from agricultural land use compared with KH in grasslands and forests. Our data show that deep eDNA amplicon sequencing is useful for in‐depth assessments of cross‐domain biodiversity comprising both micro‐ and macro‐organisms, but has limitations with respect to single‐taxa conservation studies. Using this broad method, we show that sediment eDNA, integrating several years to decades, depicts the history of agricultural land‐use intensification. Aquatic biodiversity was best explained by seasonality, whereas land‐use type explained little of the variation. We concluded that, counter to our hypothesis, land use intensification coupled with landscape wide nutrient enrichment (including atmospheric deposition), groundwater connectivity between KH and organismal (active and passive) dispersal in the tight network of ponds, resulted in a biodiversity homogenization in the KH water, leveling off today's detectable differences in KH biodiversity between land‐use types. These findings have profound implications for measures and management strategies to combat current biodiversity loss in agricultural landscapes worldwide.

Multiple tracers for dis-connectivity of shallow aquifers, alluvium, and coal seam gas wells in the Great Artesian Basin

The potential for connectivity between water supply aquifers and gas reservoirs raises community, government, and scientific concerns. Methane can occur naturally, making it difficult to determine whether water bore methane levels are being influenced by nearby gas operations. This poses a challenge in the Surat Basin, where coal seam gas production operates alongside groundwater using industries (including feedlots, agriculture, mines). Water and gas samples were taken from water bores and coal seam gas (CSG) wells in the Walloon Coal Measures and from overlying aquifers (nominally, the Springbok, Gubberamunda, Orallo, and Mooga sandstones) and the Condamine Alluvium, for stable isotopes of gases, groundwater and dissolved inorganic carbon, as well as strontium isotopes. Most of the sampled water bores had isotopic signatures distinct from CSG wells, though a minority from gassy Springbok Sandstone and Walloon Coal Measure water bores could not be distinguished from CSG wells. In those few cases, neither connectivity or dis-connectivity could be confirmed. Alluvium and shallow aquifer samples have higher R36Cl values distinct from the older CSG production waters, as is the case with most 14C measurements. Waters from the Condamine River indicate potential surface water connectivity with the alluvium. The use of multiple tracers has shown that groundwater in some aquifers can be differentiated from groundwater in the coal seam gas reservoir and hence are useful tools in identifying where groundwater connectivity occurs. Understanding this connectivity forms another line of evidence to improve impact prediction models on a regional scale as well as providing information on connectivity in local groundwater investigations.

Climate, land cover and topography: essential ingredients in predicting wetland permanence

Abstract. Wetlands in the Prairie Pothole Region (PPR) are forecast to retract in their ranges due to climate change, and potholes that typically contain ponded water year-round, which support a larger proportion of biological communities, are most sensitive to climate change. In addition to climate, land use activities and topography also influence ponded water amounts in PPR wetlands. However, topography is not typically included in models forecasting the impacts of climate change on PPR wetlands. Using a combination of variables representing climate, land cover/land use and topography, we predicted wetland permanence class in the southern Boreal Forest, Parkland and Grassland natural regions of the Alberta PPR (N = 40 000 wetlands). We show that while climate and land cover/land use were strong predictors of wetland permanence class, topography was as important, especially in the southern Boreal Forest and Parkland natural regions. Our misclassification error rates for the gradient boosting models for each natural region were relatively high (43–60) though our learning rates were low (< 0.1) and our maximum tree depths shallow (5–7) to balance bias and overfitting. Clearly, factors in addition to climate, topography and land cover/land use influence wetland permanence class (i.e., basin size, depth, ground water connectivity, etc.). Despite classification errors, our results indicate that climate was the strongest predictor of wetland permanence class in the Parkland and Grassland natural regions, whereas topography was most important in the southern Boreal Forest Natural Region among the three domains we considered.

Tap Water Quality Degradation in an Intermittent Water Supply Area

Decentralized tap water systems are an important drinking water source worldwide. A good quality, high-pressure continuous water supply (CWS) is always the target of any urban settlement. However, tap water in some areas are reported with deteriorated water quality even though treated well before supplying. Such deterioration of tap water quality is reported widely from areas with low water availability and in economically poor countries where water are supplied intermittently (IWS). This study focuses in identifying tap water quality in IWS and causes of water quality degradation using nitrate-nitrogen (NO3-N) as an indicator and stable isotopes of hydrogen (δD) as tracer. Nine water reservoirs and ninety municipal tap water (ten per reservoir) samples were collected during the wet (June–September) and dry (November–February) seasons in the Kathmandu Valley (KV), Nepal. Ten percent of the tap water samples exhibited higher NO3-N than those of their respective reservoirs during the wet season, while 16% exhibited higher concentrations during the dry season. Similarly, the isotopic signatures of tap water exhibited 3% and 23% higher concentrations than those of their respective reservoirs during the wet and dry seasons, respectively. Coupling analysis between NO3-N and δD demonstrates close connection of groundwater and tap water. The results indicate groundwater intrusion as the primary component in controlling tap water quality variations within the same distribution networks during IWS. Meanwhile, the obtained results also indicate probable areas of intrusion in the KV as well as usefulness of δD as a tool in the assessment of tap water systems.

The potential of groundwater-dependent ecosystems to enhance soil biological activity and soil fertility in drylands

Water availability controls the functioning of dryland ecosystems, driving a patchy vegetation distribution, unequal nutrient availability, soil respiration in pulses, and limited productivity. Groundwater-dependent ecosystems (GDEs) are acknowledged to be decoupled from precipitation, since their vegetation relies on groundwater sources. Despite their relevance to enhance productivity in drylands, our understanding of how different components of GDEs interconnect (i.e., soil, vegetation, water) remains limited. We studied the GDE dominated by the deep-rooted phreatophyte Ziziphus lotus, a winter-deciduous shrub adapted to arid conditions along the Mediterranean basin. We aimed to disentangle whether the groundwater connection established by Z. lotus will foster soil biological activity and therefore soil fertility in drylands. We assessed (1) soil and vegetation dynamics over seasons (soil CO2 efflux and plant activity), (2) the effect of the patchy distribution on soil quality (properties and nutrient availability), and soil biological activity (microbial biomass and mineralization rates) as essential elements of biogeochemical cycles, and (3) the implications for preserving GDEs and their biogeochemical processes under climate change effects. We found that soil and vegetation dynamics respond to water availability. Whereas soil biological activity promptly responded to precipitation events, vegetation functioning relies on less superficial water and responded on different time scales. Soil quality was higher under the vegetation patches, as was soil biological activity. Our findings highlight the importance of groundwater connections and phreatophytic vegetation to increase litter inputs and organic matter into the soils, which in turn enhances soil quality and decomposition processes in drylands. However, biogeochemical processes are jeopardized in GDEs by climate change effects and land degradation due to the dependence of soil activity on: (1) precipitation for activation, and (2) phreatophytic vegetation for substrate accumulation. Therefore, desertification might modify biogeochemical cycles by disrupting key ecosystem processes such as soil microbial activity, organic matter mineralization, and plant productivity.

Surface Water Dynamics and Rapid Lake Drainage in the Western Canadian Subarctic (1985–2020)

AbstractThe area and distribution of surface water are shifting rapidly in many regions across the circumpolar Arctic. In this study, we explore the effects of climate and terrain factors on the area of lakes in the Northwest Territories, Canada. We used the Landsat satellite archive to map interannual changes in 5,328 lakes and ponds in the Lower Mackenzie Plain between 1985 and 2020. The high temporal resolution of our dataset allowed us to classify gradual and abrupt changes in lake area and identify rapid drainage events. We used Generalized Additive Models and Random Forests to test the effects of climate and terrain factors on changes in lake area. Despite increases in the area of smaller lakes driven by increasing precipitation, we found that the total lake area has decreased by approximately 1%. Overall, 29% of lakes exhibited an increasing trend in the area, while 11% exhibited a decreasing trend, and the majority of these changes (65%) were non‐linear in nature. Lakes located in fire scars were also 3.8 times more likely to show a decreasing trend in area. Analysis of a large fire indicates that lakes within the burned region exhibited declines in an area that persisted until the end of the study period 20 years after the fire. These declines are likely related to the impact of fire on thaw depth, groundwater connectivity, and the development of new drainage pathways. Our results highlight the importance of rapid drainage and wildfire as drivers of declines in the lake area.

Study on hydrogeochemical connection and water quality assessment of subsidence lake and shallow groundwater in Luling coal-mining area of the Huaibei coalfield, Eastern China

Abstract The surface water from subsidence lakes and shallow groundwater plays an important role for agriculture, industry and local communities in coal mining areas. The connection between the two types of water and quality, however, remains unclear. In this study, 37 samples were collected from a subsidence lake and shallow groundwater around Luling coal mine. The hydrogeochemisty data and formation mechanisms were analyzed to reveal the connection proof. Compared with shallow groundwater, the surface water from the subsidence lake has a higher content of Na+ and HCO3−. For the two types of water samples, Na+ and Mg2+ are two most abundant cations, followed by Ca2+. The contents of anions followed the same order: HCO3− > SO42− > Cl−. The water samples were controlled by NaHCO3 (99.3%) type. The chemical composition of the two types of water were similar, which are all mainly influenced by silicate minerals weathering and ion-exchange interactions, among which ion-exchange interactions were more intense than in shallow groundwater. The recharging source of the two types of water mainly came from atmospheric precipitation. The two waters in the study area, in general, showed the risk of high salt and low-moderate alkali damage. The outcomes provide a better understanding of the subsidence lake and groundwater and it will help utilization of water resources.

Trust and incentives for transboundary groundwater cooperation

International transboundary aquifers provide important water supplies to over 150 countries. Long-term sustainability of these aquifers requires transboundary cooperation and yet only a select few (1%) transboundary aquifers are regulated by a treaty. To better understand the incentives that allow treaties to emerge, we develop a two-player game theoretic model that couples groundwater behavior and economic incentives to represent the social dilemma of transboundary aquifer cooperation. The game incorporates economic incentives and hydrogeological features and highlights the importance of trust to evaluate the benefits and risks of a treaty. We demonstrate the ability of the game to reproduce key features of cooperation in the Genevese aquifer, which is governed by the longest-running and most collaborative transboundary aquifer treaty on record. We analyze the comparative statics of the game to explore the role of groundwater connectivity, alternative water supply, water demand, and trust on the emergence of transboundary treaties. The solution space highlights how economic incentives for cooperation are greatest when the value of water is commensurate with the cost of groundwater abstraction. Cooperation requires high trust in situations characterized by water abundance or scarcity. The model results further indicate how two different types of agreements are likely to emerge. Treaties that limit how much is being pumped have greater potential when countries have access to an alternative water source, whereas treaties that restrict where the aquifer is being exploited have greater potential in water-scarce regions with emerging concerns over groundwater depletion. In addition to helping explain the emergence of existing treaties, this framework offers potential to identify aquifers that may be amenable to cooperation.

Ecohydrological interactions in a boreal fen–swamp complex, Alberta, Canada

AbstractThe Western Boreal Plain (WBP) comprises a diverse array of wetland types; however, swamps are understudied in the WBP relative to other wetlands, despite their ubiquity. We apply an ecohydrological and GIS‐based research approach at a fen–swamp complex in the WBP to characterize the ecohydrological properties of the varying wetland types and relate these interactions to the hydrologic function of the watershed. In this study, we evaluate 3 years of hydrological monitoring data, with additional hydrochemical, vegetation and remote sensing data. In our analyses, we identified five land types: fen, flat peat swamp and peat margin swamp (peatlands), mineral swamp and upland. Flat peat swamp was distinguished from fen using Ducks Unlimited criteria, stating fens cannot have trees >10 m in height. Little difference in water table variability, groundwater connectivity, vegetation composition and water chemistry were found between flat peat swamp and fen, suggesting that for all practical purposes, they can be considered a single unit and tree height alone cannot be used to differentiate these peatland types. In contrast, peat margin swamps exhibited lower and more variable water tables and consistent downward hydraulic gradients and comprised a mixture of peatland and upland vegetation. Peat margin swamps, however, exhibited similar porewater pH, electrical conductivity and base cation concentrations as upland, flat peat swamp and fen, suggesting that they are well connected hydrologically. Peat margin swamps were also found to modulate subsurface water movement between fen and upland (via reduced transmissivity from lower water tables) and therefore act as distinct ecohydrological units.

The impact of stream‐groundwater exchange on seasonal nitrate loads in an urban stream

AbstractUrbanization negatively impacts water quality in streams by reducing stream‐groundwater interactions, which can reduce a stream's capacity to naturally attenuate nitrate. Meadowbrook Creek, a first order urban stream in Syracuse, New York, has an inverse urbanization gradient, with heavily urbanized headwaters that are disconnected from the floodplain and downstream reaches that have intact riparian floodplains and connection to riparian aquifers. This system allows assessment of how stream‐groundwater interactions in urban streams impact the net sources and sinks of nitrate at the reach scale. We used continuous (15‐min) streamflow measurements and weekly grab samples at three gauging stations positioned longitudinally along the creek to develop continuous nitrate load estimates at the inlet and outlet of two contrasting reaches. Nitrate load estimates were determined using a USGS linear regression model, RLOADEST, and differences between loads at the inlet and outlet of contrasting reaches were used to quantify nitrate sink and source behaviour year‐round. We observed a nitrate load of 1.4 × 104 kg NO3− per water year, on average, at the outlet of the urbanized reach while the nitrate load at the outlet of the downstream, connected reach was 1.0 × 104 kg NO3− per water year, on average. We found the more heavily urbanized, hydrologically‐disconnected reach was a net source of nitrate regardless of season. In contrast, stream‐groundwater exchange caused the hydrologically connected reach to be both a source and sink for nitrate, depending on time of year. Both reaches alter nitrate source and sink behaviour at various spatiotemporal scales. Groundwater connection in the downstream, connected reach reduces annual nitrate loads and provides more opportunities for sources and sinks of nitrate year‐round than the hydrologically disconnected stream reach. Mechanisms include groundwater discharge into the stream with variable nitrate concentrations, surface‐water groundwater interactions that foster denitrification, and stream load loss to surrounding near‐stream aquifers. This study emphasizes how loads are important in understanding how stream‐groundwater interactions impact reach scale nitrate export in urban streams.

Chemical Connectivity and Multi-Element Composition of Groundwater in Depressional Wetlands

Chemical Connectivity and Multi-Element Composition of Groundwater in Depressional Wetlands

Assessing the Impact of Irrigation Efficiency Projects on Return Flows in the South-Eastern Murray–Darling Basin, Australia

Improving irrigation efficiency (IE) is an approach used globally to help meet competing demands for water and facilitate reallocation of water between sectors. In the Murray–Darling Basin in Australia, the Australian government has invested heavily in IE projects to recover water for the environment. However, this approach has been seriously questioned, out of concerns that improved IE would reduce irrigation return flows to rivers and therefore offset water recovery. In this study, we use a water balance model to assess the impact of the IE projects on return flows and highlight sensitivities and uncertainties. The model enables the impact on return flows to be assessed on specific IE projects and regional characteristics. Overall, reductions in return flows are estimated to be less than 20% of the total proposed IE savings. The history of IE in the southern MDB has meant that most of the current reductions are in ground return flows. Our estimate is much lower than two previous studies, mainly due to different assumptions being used on groundwater connectivity between irrigation areas and major streams. While the IE projects significantly reduce seepage to groundwater (with off-farm and on-farm projects reducing seepage by 19% and 53% of total savings respectively), not all seepage reductions will translate to a reduction in ground return flows to rivers. A lower estimate is consistent with existing monitoring and groundwater modeling studies. In this paper, the study results are discussed in a broader context of impacts of IE projects on volumes and salinity of streams and groundwater resources.

Astronomical Tide and Storm Surge Signals Observed in an Isolated Inland Maar Lake Near the Coast

Aimed at the explanation of clear tidal signal and storm surge signals in a closed inland lake near the coast (the Huguangyan Lake), this work uses a combined approach with observations and model experiments. Huguangyan Lake is a closed inland freshwater coneless volcanic crater lake near the coast in tropical southern China, less than 5 km from an estuary. It has a diameter of about 1.5 km and relatively deep water of up to 20 m. Bottom pressure was measured from an acoustic Doppler current profiler (ADCP) for 10 days in September 2018 and 10 days in January 2019. The observations encompass the period of Typhoon Mangkhut, which passed the region when it made its landfall. The time series demonstrate clear tidal and subtidal signals. The tidal signal remains even if we exclude the barometric pressure effect. Interestingly, the lake has no surface connection with the ocean. The astronomical tide has an amplitude of about 2 cm. The major tidal signals include the principal solar semidiurnal (S2) and lunisolar (K1) constituents. During the passage of Typhoon Mangkhut, the water level variability inside the lake increased by an order of magnitude (>0.3 m). To examine whether the lake water level change was due to the natural oscillations inside the lake (or seiche), a numerical wind-driven hydrodynamics model was designed using the 3-D Finite Volume Community Ocean Model (FVCOM). The results show that a small first-order seiche can be generated, but only with a time scale of minutes and with a magnitude much smaller than the observed surface elevation changes. This excludes any measurable seiche and the observed surface elevation change inside the lake cannot be wind-driven. Moreover, tides inside the lake are not generated by tidal potential, as the lake is too small for having a locally generated tide. The main result of our study has therefore excluded the local tidal-generating force, wind-driven seiche, and barometric effect, as possible causes of the lake oscillation which has tidal and subtidal signals. The subtidal variation is at least one order of magnitude greater than tides inside the lake and is caused by weather-induced overall coastal ocean water level oscillations transmitted into the lake through groundwater connection. All these lead to the major conclusion that the lake is connected to the coastal ocean through groundwater.