Karst aquifers supply drinking water for 25 % of the world’s population, and they are, however, vulnerable to climate change. This study is aimed to investigate the effects of various monsoons and ...teleconnection patterns on Niangziguan Karst Spring (NKS) discharge in North China for sustainable exploration of the karst groundwater resources. The monsoons studied include the Indian Summer Monsoon, the West North Pacific Monsoon and the East Asian Summer Monsoon. The climate teleconnection patterns explored include the Indian Ocean Dipole, E1 Niño Southern Oscillation, and the Pacific Decadal Oscillation. The wavelet transform and wavelet coherence methods are used to analyze the karst hydrological processes in the NKS Basin, and reveal the relations between the climate indices with precipitation and the spring discharge. The study results indicate that both the monsoons and the climate teleconnections significantly affect precipitation in the NKS Basin. The time scales that the monsoons resonate with precipitation are strongly concentrated on the time scales of 0.5-, 1-, 2.5- and 3.5-year, and that climate teleconnections resonate with precipitation are relatively weak and diverged from 0.5-, 1-, 2-, 2.5-, to 8-year time scales, respectively. Because the climate signals have to overcome the resistance of heterogeneous aquifers before reaching spring discharge, with high energy, the strong climate signals (e.g. monsoons) are able to penetrate through aquifers and act on spring discharge. So the spring discharge is more strongly affected by monsoons than the climate teleconnections. During the groundwater flow process, the precipitation signals will be attenuated, delayed, merged, and changed by karst aquifers. Therefore, the coherence coefficients between the spring discharge and climate indices are smaller than those between precipitation and climate indices. Further, the fluctuation of the spring discharge is not coincident with that of precipitation in most situations. Karst spring discharge as a proxy can represent groundwater resource variability at a regional scale, and is more strongly influenced by climate variation.
Land surface change (LSC) due to human‐caused global environmental changes has considerably affected the development of regional droughts. The standardized wetness index (SWI) developed recently by ...combining the standardized precipitation evapotranspiration index (SPEI) with the evapotranspiration (ET) estimated from the Budyko framework considers the joint effects of climate variability and LSC on the land dryness/wetness conditions. Here, using a 25‐year (1984–2008) monthly gridded terrestrial water budget dataset, a comparative global analysis of the spatio‐temporal variability of drought characteristics (duration D, severity S, peak K, frequency, and drought area) estimated based on SWI and SPEI is presented. The relationship between dryness/wetness (as indicated by SWI and SPEI) and LSC (as indicated by the Normalized Difference Vegetation Index, NDVI) is explored by correlation analysis and the dynamic time warping algorithm (DTW). The results show that SWI is strongly correlated with SPEI for most global regions (except for the extremely arid and extremely humid regions). Both SWI and SPEI show similar directions and magnitudes in the trends of drought characteristics during 1984–2008. However, the drying trends in SPEI are stronger than that of SWI particularly in arid regions, accompanied with larger global drought areas and higher frequencies of extreme drought. Furthermore, the correlation between SWI and LSC is larger than that between SPEI and LSC particularly in arid regions (e.g., northern Africa, Indus, and southern Australia). As SWI accounts for the effects of LSC, it can be inferred that the underestimation of drying trend indicated by SWI relative to SPEI is partially caused by LSC.
SWI and SPEI show similar directions and magnitudes of drought characteristics (D, S, and K) trends during 1984–2008 and increasing trends in global drought area after 1997.
The drying trends in SPEI are stronger than that of SWI particularly in arid regions, leading to larger global drought areas and higher frequencies of extreme drought.
The correlation between SWI and LSC is larger than that between SPEI and LSC especially in arid regions.
In this study, a groundwater flow cycling in a karst springshed and an interaction between two springs, Spring Creek Springs and Wakulla Springs, through a subground conduit network are numerically ...simulated using CFPv2, the latest research version of MODFLOW-CFP (Conduit Flow Process). The Spring Creek Springs and Wakulla Springs, located in a marine estuary and 11 miles inland, respectively, are two major groundwater discharge spots in the Woodville Karst Plain (WKP), North Florida, USA. A three-phase conceptual model of groundwater flow cycling between the two springs and surface water recharge from a major surface creek (Lost Creek) was proposed in various rainfall conditions. A high permeable subground karst conduit network connecting the two springs was found by tracer tests and cave diving. Flow rate of discharge, salinity, sea level and tide height at Spring Creek Springs could significantly affect groundwater discharge and water stage at Wakulla Springs simultaneously.
Based on the conceptual model, a numerical hybrid discrete-continuum groundwater flow model is developed using CFPv2 and calibrated by field measurements. Non-laminar flows in conduits and flow exchange between conduits and porous medium are implemented in the hybrid coupling numerical model. Time-variable salinity and equivalent freshwater head boundary conditions at the submarine spring as well as changing recharges have significant impacts on seawater/freshwater interaction and springs’ discharges. The developed numerical model is used to simulate the dynamic hydrological process and quantitatively represent the three-phase conceptual model from June 2007 to June 2010. Simulated results of two springs’ discharges match reasonably well to measurements with correlation coefficients 0.891 and 0.866 at Spring Creeks Springs and Wakulla Springs, respectively. The impacts of sea level rise on regional groundwater flow field and relationship between the inland springs and submarine springs are evaluated as well in this study.
•Groundwater flow cycling is quantitatively represented by a numerical model.•Seawater/freshwater interface position significantly affects groundwater interaction.•Discrete-continuum CFPv2 model is applied for non-laminar flow in conduits.•Understanding the hydrological process is important to seawater intrusion.•The effect of sea level rise on groundwater flow in a karst aquifer is evaluated.
Drought projections are accompanied with large uncertainties due to varying estimates of future warming scenarios from different modeling and forcing data. Using the standardized precipitation index ...(SPI), this study presents a global assessment of uncertainties in drought characteristics (severity S and frequency Df) projections based on the simulations of 28 general circulation models (GCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5).A hierarchical framework incorporating a variance-based global sensitivity analysis was developed to quantify the uncertainties in drought characteristics projections at various spatial (global and regional) and temporal (decadal and 30-yr) scales due to 28 GCMs, three representative concentration pathway scenarios (RCP2.6, RCP4.5, RCP8.5), and two bias-correction (BC) methods. The results indicated that the largest uncertainty contribution in the globally projected S and Df is from the GCM uncertainty (>60%), followed by BC (<35%) and RCP (<16%) uncertainty. Spatially, BC reduces the spreads among GCMs particularly in Northern Hemisphere (NH), leading to smaller GCM uncertainty in the NH than the Southern Hemisphere (SH). In contrast, the BC and RCP uncertainties are larger in the NH than the SH, and the BC uncertainty can be larger than GCM uncertainty for some regions (e.g., southwest Asia). At the decadal and 30-yr time scales, the contributions for three uncertainty sources show larger variability in S than Df projections, especially in the SH. The GCM and BC uncertainties show overall decreasing trends with time, while the RCP uncertainty is expected to increase over time and even can be larger than BC uncertainty for some regions (e.g., northern Asia) by the end of this century.
Recent studies have extended the applicability of the Budyko framework from the long-term mean to annual or shorter time scales. However, the effects of water storage change ΔS on the overall water ...balance estimated from the Budyko models (BM) at annual-to-monthly time scales were less investigated, particularly at the continental or global scales, due to the lack of large-scale ΔS data. Here, based on a 25-yr (1984–2008) global gridded terrestrial water budget dataset and by using an analytical error-decomposition framework, we analyzed the effects of ΔS in evapotranspiration (ET) predicted from BM at both grid and basin scales under diverse climates for the annual, wet-seasonal, dry-seasonal, and monthly time scales. Results indicated that the BM underperforms in the short dry (wet) seasons of predominantly humid (dry) basins, with lower accuracy under more humid climates (at annual, dry-seasonal, and monthly scales) and under more arid climates (at wet-seasonal scale). When the effects of ΔS are incorporated into BM, improvements can be found mostly at annual and dry-seasonal scales, but not notable at wet-seasonal and monthly scales. The magnitudes of ΔS are positively correlated with the errors in BM-predicted ET for most global regions at annual and monthly scales, especially under arid climates. Under arid climates, the variability of ET prediction errors is controlled mainly by the ΔS variability at annual and monthly time scales. In contrast, under humid climates the effect of ΔS on ET prediction errors is generally limited, particularly at the wet-seasonal scale due to the more dominant influences of other climatic factors (precipitation and potential ET) and catchment responses (runoff).
Monitoring spatial and temporal chemical status of water bodies is crucial to assist environmental policy, identify the chemical fingerprints, and further reduce the source orientated pollutants. ...Elbe River is one of the major rivers affected by anthropogenic activities in vicinity countries. This study assessed the spatiotemporal changes in response to source shift of Cd, Cu, Ni, Pb, and Zn in the suspended particulate matter (SPM) at upstream, midstream, and downstream of the Elbe River reach in Saxony state, Germany. The average contents of trace metals in SPM was found in the order of Zn (676 mg/kg) » Pb (79 mg/kg) > Cu (74 mg/kg) > Ni (48 mg/kg) » Cd (3.2 mg/kg). According to the Mann-Kendall trend test, Cd, Cu, Pb, and Zn showed significant declines over 1998–2016. The results of source apportionment indicate industrial, urban, natural, and historical mining sources influencing the metal contents in the Elbe River of Saxony. The contributions of industrial and urban pollution decreased by 58.2% from 1998 to 2007 to 2008–2016. The contribution of the natural source was steady over the last two decades.
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•Long-term in spatiotemporal changes of metals in the Elbe River had been evaluated.•Most metals showed significant declines over 1998–2016.•Primary source shift of metals was identified by PMF receptor model.•Industrial and urban pollution showed decreasing contribution.
Microbes live throughout the soil profile. Microbial communities in subsurface horizons are impacted by a saltwater–freshwater transition zone formed by seawater intrusion (SWI) in coastal regions. ...The main purpose of this study is to explore the changes in microbial communities within the soil profile because of SWI. The study characterizes the depth-dependent distributions of bacterial and archaeal communities through high-throughput sequencing of 16S rRNA gene amplicons by collecting surface soil and deep core samples at nine soil depths in Longkou City, China. The results showed that although microbial communities were considerably impacted by SWI in both horizontal and vertical domains, the extent of these effects was variable. The soil depth strongly influenced the microbial communities, and the microbial diversity and community structure were significantly different (
p
< 0.05) at various depths. Compared with SWI, soil depth was a greater influencing factor for microbial diversity and community structure. Furthermore, soil microbial community structure was closely related to the environmental conditions, among which the most significant environmental factors were soil depth, pH, organic carbon, and total nitrogen.
Representative elementary volume (REV) is important to characterize dense nonaqueous phase liquids (DNAPLs) during surfactant-Enhanced aquifer remediation (SEAR) period. To investigate the REVs of ...DNAPL in remediation, a perchloroethylene (PCE) SEAR experiment is conducted in a two dimensional (2D) heterogeneous translucent porous media. Light transmission techniques are used to quantify PCE saturation (Soil) and PCE-water interfacial area (AOW). Afterward, corresponding REVs are estimated using a criterion of relative gradient error (εgi) to reveal the change of REVs of DNAPL over the entire remediation period. Results from this work suggest the presence of surface active agents strongly affect the REVs of DNAPL. At the beginning of the SEAR experiment, the frequency of minimum Soil-REV size closely follows a Gaussian distribution in 0.0mm–11.0mm. Simultaneously, the frequency of minimum AOW-REV size is close to a Gaussian distribution in 2.0mm–9.0mm and appears a peak value in 13.0mm–14.0mm. As SEAR experiment proceeds, both the shapes of frequency and cumulative frequency of REV sizes are changed. At the end of SEAR experiment, the frequency of minimum Soil-REV and minimum AOW-REV size tend to Gaussian distributions in 0.0mm–6.0mm and 0.0mm–9.0mm, respectively, which suggest both minimum Soil-REV size and minimum AOW-REV size show decreasing tendency. Continuous quantification of the REVs of DNAPL is realized in this study to reveal the change of REVs influenced by surface active agent. The finding has important significance on improving our understanding of the characteristics of DNAPL in SEAR process, simulating DNAPL remediation and designing appropriate remediation scheme with high-resolution.
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•DNAPL saturation and DNAPL-water interfacial area in remediation process are quantified.•Quantification of DNAPL REVs is influenced by surface active agents.•The presence of surface active agents strongly affects the REVs of DNAPL.•Both minimum Soil-REV and AOW-REV sizes show decreasing tendency.•Frequency of minimum Soil-REV and AOW-REV sizes tends to be Gaussian.
The groundwater system in the Dongting Lake Plain, central-south China, was investigated from the perspective of tectonics. As a typical tectonic basin, the Dongting Lake Basin has a complex basement ...topography shaped by a series of tectonic events since the Cretaceous, which presents a vast depression basin with four extensive uplifts scattered in the plain. The barrier effects of the tectonic uplifts on the groundwater system were inferred based on the basement topographic map and Darcy’s law. They were also validated with the spatial analysis results associated with groundwater chemistry and the flow field. Tectonic uplift can reduce the thickness of the aquifer and, as a result, the flow direction of groundwater may change accordingly. In addition, a stagnant area can easily be formed where the extension area of uplift is large. Seven subsystems are distinguished considering the effects of the tectonic uplifts and major surface-water bodies on groundwater systems. This work is expected to deepen the understanding of the groundwater system of the Dongting Lake Plain on the macro scale and provide a new perspective for the analysis of groundwater systems in other similar plains with complex basement topography.
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