Groundwater Recharge Rates Research Articles

Chronology and characteristics of groundwater along the United Arab Emirates-Oman arid region: a guide for regional sustainability

Estimating groundwater recharge rate is essential for water sustainability of the arid region of the United Arab Emirates that receives most groundwater recharge from the bordering mountain ranges in Oman. Analyses of major ions, stable isotopes (δ2H and δ18O) and the radioactive isotope (14C) and the HydroGeoSphere (HGS) three-dimensional numerical model approach were applied here to investigate groundwater residence time in the aquifers and the recharge rates in the region. Estimate of groundwater recharge ranges from 1.03 to 36.6 mm/year with an average of 5.8 mm/year based on groundwater ages and ranges from 0.23 to 35.3 mm/year with an average of 6.5 mm/year based on HGS modelling. The data provide vital information for making sustainable plans for natural water resources with expected future reduction in rainfall in this region and other arid areas of the Middle East and the world.

Understanding dynamics of groundwater flows in the Mississippi River Delta

Groundwater is a major component of the water cycle in the river-deltaic environment. However, the dynamics of groundwater flows in the river delta remains an open question owing to the complexities in the hydrologic and geological settings. This study employed a hydrogeological modeling approach to investigate groundwater dynamics in the Mississippi River Delta (MRD). A detailed groundwater model for the top 50 m of the MRD was constructed from the Head of Passes (RK 0) to Jesuits Bend (RK 108) using geotechnical borings and topobathymetric maps. Mississippi River stage and tide stage data were the key hydrologic data, which reflect hydrologic forcing (floods, storms, and hurricanes) to the groundwater system. By investigating the 2012 hydrologic year, the study presents several intriguing findings. Groundwater discharge and recharge rates to the river and surrounding bays were estimated 3 or 4 orders of magnitude smaller than Mississippi River discharge rate to the Gulf of Mexico. Nevertheless, the model showed strong surface-groundwater interactions controlled by local hydraulic gradient at the river and bay interfaces during severe hydrologic events. Specifically, during Hurricane Isaac pore water pressure was estimated 4–6 times higher than the normal condition and peak groundwater recharge was reached due to storm surges. Peak groundwater discharge sharply occurred a few days later after the hurricane passed. The study found such a dramatic change in surface-groundwater interactions was due to quick surface water receding and delayed groundwater response. As a consequence, more areas are likely exposed to harmful high pore water pressure and low factor of safety condition, which would destabilize sediments, enhance erosion, and compromise safety of coastal infrastructures such as the ring levees.

Recharge seasonality based on stable isotopes: Nongrowing season bias altered by irrigation in Nebraska

AbstractThe sustainability of groundwater resources for agricultural and domestic use is dependent on both the groundwater recharge rate and the groundwater quality. The main purpose of this study was to improve the understanding of the timing, or seasonality, of groundwater recharge through the use of stable isotopes. Based on 768 groundwater samples collected from aquifers underlying natural resources districts in Nebraska, the isotopic composition of groundwater (δ2H and δ18O) was compared with that of precipitation by (a) mapping the isotopic composition of groundwater samples and (b) mapping a seasonality index for groundwater. Results suggest that for the majority of the state, groundwater recharge has a nongrowing season signature (October–April). However, the isotopic composition of groundwater suggests that in some intensively irrigated areas, human intervention in the water cycle has shifted the recharge signature towards the growing season. In other areas, a different human intervention (diversion of Platte River water for irrigation) has likely produced an apparent but possibly misleading nongrowing season recharge signal because the Platte River water differs isotopically from local precipitation. These results highlight the need for local information even when interpreting isotopic data over larger regions. Understanding the seasonality of recharge can provide insight into the optimal times to apply fertilizer, specifically in highly conductive soils with high leaching potential. In areas with high groundwater nitrate concentrations, this information is valuable for protecting the groundwater from further degradation. Although previous studies have framed nongrowing season recharge within the context of future climate change, this study also illustrates the importance of understanding how historical human intervention in the water cycle has affected groundwater recharge seasonality and subsequent implications for groundwater recharge and quality.

Mapping groundwater resiliency under climate change scenarios: A case study of Kathmandu Valley, Nepal

Groundwater resources of Kathmandu Valley in Nepal are under immense pressure from multiple stresses, including climate change. Due to over-extraction, groundwater resources are depleting, leading to social, environmental and economic problems. Climate change might add additional pressure by altering groundwater recharge rates and availability of groundwater. Mapping groundwater resilience to climate change can aid in understanding the dynamics of groundwater systems, facilitating the development of strategies for sustainable groundwater management. Therefore, this study aims to analyse the impact of climate change on groundwater resources and mapping the groundwater resiliency of Kathmandu Valley under different climate change scenarios. The future climate projected using the climate data of RCM's namely ACCESS-CSIRO-CCAM, CNRM-CM5-CSIRO-CCAM and MPI-ESM-LR-CSIRO-CCAM for three future periods: near future (2010–2039), mid future (2040–2069) and far future (2070–2099) under RCP 4.5 and RCP 8.5 scenarios were bias corrected and fed into the Soil and Water Assessment Tool (SWAT), a hydrological model, to estimate future groundwater recharge. The results showed a decrease in groundwater recharge in future ranging from 3.3 to 50.7 mm/yr under RCP 4.5 and 19–102.1 mm/yr under RCP 8.5 scenario. The GMS-MODFLOW model was employed to estimate the future groundwater level of Kathmandu Valley. The model revealed that the groundwater level is expected to decrease in future. Based on the results, a groundwater resiliency map of Kathmandu Valley was developed. The results suggest that groundwater in the northern and southern area of the valley are highly resilient to climate change compared to the central area. The results will be very useful in the formulation and implementation of adaptation strategies to offset the negative impacts of climate change on the groundwater resources of Kathmandu Valley.

A multi-environmental tracer study to determine groundwater residence times and recharge in a structurally complex multi-aquifer system

Abstract. Despite being the main drinking water resource for over 5 million people, the water balance of the Eastern Mountain Aquifer system on the western side of the Dead Sea is poorly understood. The regional aquifer consists of fractured and karstified limestone – aquifers of Cretaceous age, and it can be separated into a Cenomanian aquifer (upper aquifer) and Albian aquifer (lower aquifer). Both aquifers are exposed along the mountain ridge around Jerusalem, which is the main recharge area. From here, the recharged groundwater flows in a highly karstified aquifer system towards the east and discharges in springs in the lower Jordan Valley and Dead Sea region. We investigated the Eastern Mountain Aquifer system for groundwater flow, groundwater age and potential mixtures, and groundwater recharge. We combined 36Cl ∕ Cl, tritium, and the anthropogenic gases SF6, CFC-12 (chlorofluorocarbon) and CFC-11, while using CFC-113 as “dating” tracers to estimate the young water components inside the Eastern Mountain Aquifer system. By application of lumped parameter models, we verified young groundwater components from the last 10 to 30 years and an admixture of a groundwater component older than about 70 years. Concentrations of nitrate, simazine (pesticide), acesulfame K (ACE-K; artificial sweetener) and naproxen (NAP; drug) in the groundwater were further indications of infiltration during the last 30 years. The combination of multiple environmental tracers and lumped parameter modelling helped to understand the groundwater age distribution and to estimate recharge despite scarce data in this very complex hydrogeological setting. Our groundwater recharge rates support groundwater management of this politically difficult area and can be used to inform and calibrate ongoing groundwater flow models.

인공신경망을 활용한 미계측지역 지하수 함양량 추정 기법의 적용성 평가

국내의 경우 다양한 방법의 지하수 함양량 산정 기법이 적용되고 있으나, 표준화된 방법이 마련되어 있지 않은 실정이다. 지하수위 시계열 자료를 활용한 TSM-hWTF 방법이 개발되어 있으나 지하수 관측소의 개수가 한정적이어서 미계측 지역에 대한 지하수 함양량 산정은 어려웠다. 이를 해결하고자, 207개 지하수 관측 지점의 지형 표고, 경사, 인근 하천, 지질, 토양 등과 같은 각 관측소의 지점 특성 15개 항목을 입력 인자, 각 지점의 지하수 함양율을 출력 인자로 설정한 후 다층퍼셉트론 기법에 의한 인공신경망 모델을 개발하였다. 지하수 함양율의 모델 예측값과 실제값의 상관계수는 0.8 이상의 높은 값을 보였으며, 표토특성과 심토특성이 가장 큰 영향을 미치는 것으로 분석되었다. 홍성군 서부면 양곡리 지역에 대하여 본 모델을 적용하여 추정한 지하수 함양량과 9개 지하수 관측 자료로부터 TSM-hWTF로 추정한 지하수 함양량은 유사한 경향을 보이는 것으로 평가되어 본 인공신경망 모델에 기반한 지하수 함양량 추정이 효과적인 것으로 나타났다.

Evaluation of MAR Methods for Semi-Arid, Cold Regions

Mongolia is a semi-arid, highly continental region with highly variable precipitation and river discharge. The groundwater aquifer located near Ulaanbaatar, the capital city of Mongolia, is the only one source for city water supply consumption, and it is important to ensure that groundwater is available now and in the future. The main watercourse near the capital city is the Tuul River, fed by precipitation in the Khentii Mountains. The semi-arid and cold environment shows high variability in precipitation and river discharge. However, due to absence of precipitation in winter and spring, the riverbed usually runs dry during these times of the year, and weather observations show that the dry period has been extending in recent years. However, in parallel with urban development, the extended groundwater aquifer has shown a clear decline, and the groundwater levels have dropped significantly. Therefore, a groundwater management system based on managed aquifer recharge is proposed, and a strategy to implement these measures in the Tuul River valley is presented in this paper. This strategy consists of the enhancement of natural recharge rates during the wet summer from the northern drainage canal, an additional increase in groundwater recharge through melting the ice storage in the dry period, as well as the construction of underground dams to accumulate groundwater and a surface water reservoir that releases a constant discharge in the outlet. To increase natural recharge rates of groundwater during the early dry period through the melting ice storage period, the MATLAB icing code, which was written for ice storage for limited and unlimited areas, was considered through finite element subsurface FLOW (FEFLOW) simulation scenarios as a water source in ice form on the surface. A study of the artificial permafrost of underground as an ice dam was processed in FEFLOW simulation scenarios for accumulating groundwater resources. The results of these artificial recharging methods were individually calculated, combined, and compared with the surface reservoir, which releases a constant discharge through the dam. In this paper, new ideas are presented involving managed aquifer recharge—MAR methods, and include application to aufeis, a mass of layered ice for groundwater recharge by melting. Additionally, the accumulation of groundwater using artificial permafrost is used as an underground dam. In addition, was considered recharging scenario only with constant release water amount from water reservoir also with all MAR methods together with reservoir combination.

Applying satellite‐derived evapotranspiration rates to estimate the impact of vegetation on regional groundwater flux

AbstractFreshwater resources are at a premium across the world, including many water‐limited areas across Australia. Ongoing water level decline in some groundwater systems suggests that the rate of loss may have reached unsustainable levels . However, identifying which factors are primarily responsible for the trend (lack of rainfall–recharge, presence of vegetation, and groundwater pumping) remains challenging using groundwater observations alone. We applied satellite‐derived estimates of evapotranspiration (ETa), which, when combined with local rainfall data and field‐based groundwater level observations, established a regional water balance spanning 1,500 km2 and seven groundwater lenses over a 10‐year period (2000 – 2010). Assuming that the extent of the freshwater isohaline represents the recharge area for a lens, the water balance suggests that the median annual groundwater recharge rate varied between 226 ± 92 mm year−1 (Mikkira) and −162 ± 194 mm year−1 (Uley East). Uley South is the most regionally significant lens in the system and recorded a median annual recharge rate of 91 ± 182 mm year−1. Overlaying vegetation highlighted the impact of woodland areas on groundwater recharge, where the trees were accessing groundwater to support ETa, provided the water table was <10 m. Areas of grassland demonstrated the highest median groundwater recharge rates of 151 mm year−1, followed by cropping (133 mm year−1) and pasture (95 mm year−1). Exploring the resilience of the groundwater system to variations in extraction (pumping) and woodland coverage suggests that resource managers must consider both systematic losses in order to maintain groundwater equilibrium.

Estimating groundwater recharge on the southern slope of Mount Kilimanjaro, Tanzania

This paper used three methods namely: water-table fluctuation (WTF), soil moisture balance (SMB), and chloride mass balance (CMB) to estimate groundwater recharge in a degraded Kahe catchment located on the southern slope of Mt. Kilimanjaro, Tanzania. Three methods yielded different groundwater recharge rates. Results of the WTF method showed that recharge in the catchment was about 248.4 million m3/year, whereas those of CMB and SMB methods were 156.0 and 132.1 million m3/year, respectively. The estimated recharge rates ranged between 132.1 and 248.4 million m3/year with an average of 191.34 ± 27.80 million m3/year. Differences in the estimated rates can be attributed to the scales of measurements, assumptions in each method, and the quality of the data used. Satellite images taken in between 2000 and 2017 were used to estimate the land-use changes and their impacts on groundwater recharge in the study catchment. Analyzed satellite images showed that over the 17-year period, natural forests and bushes and shrubs decreased by 3.6 and 4.1%, while agricultural land and built-up area increased by 12.8 and 0.8%, respectively. Using SMB method, we found that these land-use changes have contributed to a decrease in groundwater recharge of about 42% between 2000 and 2017 (i.e., from 227.8 to 132.1 million m3/year). The findings from this study are useful for assessing the potential impacts of land-use change on water resources in the catchment.

Multi-proxy speleothem record of climate instability during the early last interglacial in southern Turkey

A stalagmite from Dim Cave in southern Turkey contains a climate record documenting rapid and significant changes in amounts of precipitation between ~132 ka and ~128 ka, during the penultimate glacial – interglacial transition. Some U–Th dates have been compromised by carbonate dissolution but rigorous selection and tuning to δ18O records from other speleothems has generated a robust age model. Growth rate was initially very slow but a rapid increase at ~129 ka was accompanied by strong negative trends in δ18O and δ13C, a combination implying the onset of much wetter conditions. Isotopic values at ~129 ka suggest that groundwater recharge rates and biogenic activity in the soil zone exceeded those of the early Holocene. A significant isotopic enrichment event at ~128 ka, during which there was alternating aragonite and calcite deposition, documents a strong drying event with a duration of ~200 years. A concurrent decrease in 87Sr/86Sr ratios indicates increased groundwater residence times and the cumulative evidence suggests amounts of rainfall fell from well above to slightly below present-day levels. Similar δ18O enrichment events are present in coeval speleothem records from southwest France and the Northern Alps, and these, together with pollen evidence from Italy, Greece and the Iberian margin of drier conditions at this time, imply that a climate anomaly extended across the northern Mediterranean borderlands. The timing, duration and structure of this episode are consistent with marine evidence of strong North Atlantic cooling early in the last interglacial and there is a resemblance to the Holocene 8.2 ka event recorded globally in many proxy-climate archives.

Holocene climate recorded by magnetic properties of lake sediments in the Northern Rocky Mountains, USA

We present two hypotheses regarding the evolution of Holocene climate in the Northern Rocky Mountains that stem from a previously unpublished environmental magnetic record from Jones Lake, Montana. First, we link two distinct intervals of fining magnetic grain size (documented by an increasing ratio of anhysteretic to isothermal remanent magnetization) to the authigenic production of magnetic minerals in Jones Lake bottom waters. We propose that authigenesis in Jones Lake is limited by rates of groundwater recharge and ultimately regional hydroclimate. Second, at ~8.3 ka, magnetic grain size increases sharply, accompanied by a drop in concentration of magnetic minerals, suggesting a rapid termination of magnetic mineral authigenesis that is coeval with widespread effects of the 8.2 ka event in the North Atlantic. This association suggests a hydroclimatic response to the 8.2 ka event in the Northern Rockies that to our knowledge is not well documented. These preliminary hypotheses present compelling new ideas that we hope will both highlight the sensitivity of magnetic properties to record climate variability and attract more work by future research into aridity, hydrochemical response, and climate dynamics in the Northern Rockies.

A GIS-based linear regression modeling approach to assess the impact of geologic rock types on groundwater recharge and its hydrological implication

In this study, the efficacy of the linear regression modeling technique in the company of recharge rate-induced geophysical parameters was implemented through geographical information system to establish the essentiality of geology factor in recharge rate estimate and prediction. The study empirically assessed the in situ and the spatiotemporal variability in the recharge rate in response to the multi-faceted geologic settings in the Perak province, Malaysia. The used data sets were 2D resistivity data, climate data and the ancillary data (surface geologic rock types; [quaternary (QUA); Devonian (DEV); Silurian (SIL); igneous(ING)]. The acquired 2D data were interpreted for delineating vadose zone units having varying geoelectrical parameters consisting of resistivity (ρ) and its thickness (D) (depth to aquifer top). The combined GIS and linear regression techniques were applied to relate the vadose zone geoelectrical parameters with the spatially modeled rainfall–recharge estimate. The obtained results were processed in R statistical software environment to develop GIS linear regression (GLR)-based recharge models for the area underlain geologic rock types. The developed GIS linear regression (GLR)-based recharge models were validated using the models’ parameter significance evaluation and models’ forecasting accuracy assessment approaches. The results of the applied GLR-based recharge models at each 2D location were processed to produce groundwater recharge maps for the rock types. Applying the concept of GIS-based geoelectrical parameters recharge estimate and rainfall–recharge rate–subsurface media estimate approaches, the output of the GLR-based recharge models was quantitatively evaluated to give expected recharge estimates of 242 mm/yr, 248.mm/yr, 268 mm/yr and 224 mm/y and total recharge water values of 377.9 × 106 m3/year, 319 × 106 m3/year, 161.6 × 106 m3/year, 227.3 × 106 m3/year, for the area underlain rock types, namely QUA, DEV, SIL and ING, respectively. Based on these results, the impact of geologic rocks’ factor on the area recharge rate estimates is established. Thus, the consideration of the geology factor is very important in accurate estimation and prediction of the groundwater recharge rate. The developed GLR-based recharge models and the produced groundwater recharge maps are viable decision tools for efficient management of groundwater resources in the study area and any area of similar geologic settings.

Using multiple lines of evidence to map groundwater recharge in a rapidly urbanising catchment: Implications for future land and water management

Understanding how land-use change influences the water cycle is of critical importance to land and water management. Determining the timing, location and rates of groundwater recharge and their relationship to land-use is often challenging, leading to significant uncertainties in water budgets and water cycle planning. In this study, a combination of physical and hydrochemical/isotope techniques were used to estimate and map groundwater recharge rates and identify its key controlling factors in a rapidly urbanising catchment in southeast Australia. The primary objective was to provide qualitative and quantitative information regarding recharge, allowing comparison and monitoring of changes as future urbanisation takes place. The presence of significant tritium in shallow groundwater (>1.0 TU), along with radiocarbon activities > 85 pMC and low salinity (e.g. EC < 600 µs/cm) allowed identification of areas where significant recharge has taken place in recent years. These were strongly associated with elevated topography on the basin margin, and the absence of volcanic clay - the dominant lithology underlying most of the region. This interpretation is supported by time-series analysis of soil moisture profiles, which indicate minimal vertical propagation of precipitation below 1.5 m depth in volcanic clay soils. Estimation of recharge rates was conducted using chloride mass balance and water table fluctuation analysis in water table aquifer monitoring bores. Rates mostly ranged between 1.5 and 50 mm/yr; however, recharge exceeding 100 mm/year was identified in a spatially restricted zone at the edge of the basin. Here, the volcanic lithology is absent and Quaternary sand directly overlies the lower Cainozoic sand aquifer. This area comprises a small percentage of land in the study area (approximately 15%) but is estimated to contribute a large proportion (nearly half) of recharge. The findings underscore the importance of characterising recharge locations and processes to support the protection of groundwater quality and quantity, for example, through careful land-use planning.

An Estimation of Hydrometeorological Drought Stress over the Central Part of India using Geo-information Technology

Drought is a creeping natural hazard commencing from lack of rainfall and generally associated with various climatic aspects. Drought-related water deficiency has severe consequences upon environmental processes and socioeconomic activities. In the past few decades, a number of drought indices have been developed for assessing the extent, onset, duration and intensity of drought. The Bundelkhand region located in the central part of India has been affected by recurrent drought events during the past few decades. This study seeks to examine hydrometeorological drought stress of that area using remote sensing and meteorological indicators, i.e., standardized precipitation index (SPI), hydrology-based rainfall anomaly index (RAI) and standardized water-level index (SWI). Daily rainfall data from Climate Hazards Group InfraRed Precipitation with Stations (CHIRPS) and Tropical Rainfall Measuring Mission (TRMM) were integrated with station-based groundwater datasets (1998–2015) to analyze the hydrometeorological drought condition of the area. In addition, groundwater datasets were used to evaluate the long-term hydrological drought situation and compared with meteorological drought indices. The study reveals a good agreement among all hydrometeorological drought indices distinctly in few years (2002 and 2013). However, the findings were not coherent in all years due to high rate of runoff and poor groundwater recharge. In spite of having normal rainfall, the undulating terrains of this rugged land confine the infiltration process and cause hydrological drought stress in several parts of the area.

Global travertine deposition modulated by oscillations in climate

ABSTRACTTravertine deposits are important records of past fluid flow in the Earth's crust, and document fluid migration through both tectonic activity and changes in climate. While many studies hint at possible relationships between travertine formation and global climate, none have investigated these connections on a global scale. Here we compile 1649 published travertine ages from six continents to test the hypothesis that global and/or regional changes in climate regulate travertine deposition. Peaks in bedded travertine ages occur with main frequencies that correspond to 100‐kyr changes in global climate, where most peaks occur during glacial terminations or interglacial periods, including a large peak that coincides with the Early Holocene climatic optimum. Time–series analysis also suggests a possible connection with 41‐kyr obliquity cycles. At regional scales, many peaks also correspond with local times of high precipitation or wet conditions. This can be attributed to higher groundwater recharge rates, providing the necessary water to form travertine. Many bedded travertine‐depositing systems may therefore be water‐limiting and sufficient CO2 may be present even during times of no travertine deposition. Exceptions to this conclusion are banded vein travertine deposits, which typically form during times of dry climate when water tables are low. Copyright © 2019 John Wiley &amp; Sons, Ltd.

Groundwater conditions related to climate change in the semi-arid area of western Iran

The effect of climate change on future groundwater conditions in the Toyserkan basin in western Iran has been studied. In recent years, overexploitation for agricultural activities has led to water-table decline. Groundwater recharge rate predictions in the study area were obtained from the RCP4.5 Scenario of the 5th Assessment Report of the Intergovernmental Panel on Climate Change and HadGEM2 General Circulation Model. Outputs were downscaled with the RegCM4 Regional Climate Model coupled to the Community Land Model version 4.5 (CLM 4.5). RegCM4 model validation and prediction were attempted for 7 years (1999–2005) and 11 years (2015–2025), respectively. Validation results showed that RegCM4 reasonably simulated daily precipitation and monthly temperature and runoff. Firstly, geological, geophysical and hydrogeological data were used and evaluated to develop the conceptual model. Secondly, a 3D numerical model of groundwater flow was developed in order to describe the groundwater regime and predict the effects of water management strategies. Two scenarios were defined for the prediction period. The first scenario assumes that current exploitation rates will be continued, while the second one assumes a 20 percent decrease in pumping due to increased irrigation efficiency. The results showed a water-table rise from 2015 to 2025, which is heightened by increase in irrigation efficiency.

Water resources management and sustainability over the Western desert of Iraq

In this research, a comprehensive survey of water resources management and sustainability was conducted over the Western desert of Iraq. A modeling was performed using both remote sensing and numerical analytical tools. Field measurements of hydrological and hydrogeological variables were used to assess surface and groundwater resources. The watershed modeling system was used to identify the existing ponds in the region and to determine their topographical and geometric characteristics based on the digital elevation models. In this study, an annual water harvesting rate was determined for each basin of the studied area, and the annual rate of all the ponds was about 8000 m3/km2. New prominent areas were identified in the field of water harvesting in which water can be exploited for various agricultural uses and the establishment of communities. The second part of water resources is groundwater resources where the amount of renewable and non-renewable groundwater in the region was estimated at more than 30 billion cubic meters. The maps obtained from the Geological Survey were used to identify important groundwater aquifers in the region and to prepare a table of promising areas for investment in future strategic projects. A group of preferred areas was identified as part of the medium-term investment plan for different uses and for safe extraction of groundwater around the province at an approximate annual rate of a billion cubic meter which is equal to the annual recharge rate of groundwater.

Quasi-Saturated Layer: Implications for Estimating Recharge and Groundwater Modeling.

This study presents an extension of the concept of “quasi‐saturation” to a quasi‐saturated layer, defined as the uppermost dynamic portion of the saturated zone subject to water table fluctuations. Entrapped air here may cause substantial reductions in the hydraulic conductivity (K) and fillable pore water. Air entrapment is caused by a rising water table, usually as a result of groundwater recharge. The most significant effects of entrapped air are recharge overestimation based on methods that use specific yield (S y), such as the water table fluctuation method (WTF), and reductions in K values. These effects impact estimation of fluid flow velocities and contaminant migration rates in groundwater. In order to quantify actual groundwater recharge rates and the effects of entrapped air, numerical simulations with the FEFLOW (Version 7.0) groundwater flow model were carried out using a quasi‐saturated layer for a pilot area in Rio Claro, Brazil. The calculated recharge rate represented 16% of the average precipitation over an 8‐year period, approximately half of estimates using the WTF method. Air entrapment amounted to a fillable porosity of 0.07, significant lower that the value of 0.17 obtained experimentally for S y. Numerical results showed that the entrapped air volume in the quasi‐saturated layer can be very significant (0.58 of the air fraction) and hence can significantly affect estimates of groundwater recharge and groundwater flow rates near the water table.

Sea Level Rise Impact on Atoll Islands: Implication to SDG 6

Low-lying atoll islands that depend heavily on fresh groundwater for survival are particularly vulnerable to sea level rise (SLR), which calls for appropriate climate action SDG 13. As the sea level rises, the associated increase in surface seawater inundation and subsurface saltwater intrusion will reduce the availability of fresh groundwater due to permanent salinization of groundwater with corresponding thinning of freshwater lens. This paper provides scientific insights on how freshwater lens in atoll islands respond to SLR. Simulations on saturated-unsaturated variable-density groundwater flow with salt transport are performed by the groundwater flow and solute transport model SUTRA (Saturated-Unsaturated Transport) developed by the U.S. Geological Survey. Model simulations and statistical analyses suggest that freshwater lens thickness depends mainly on groundwater recharge rate, island size and aquifer hydraulic conductivity. The impact of various geo-hydrologic parameters on fresh groundwater sustainability is then analyzed to explore feasibility of increasing groundwater recharge through rainwater harvesting, as a mitigation measure. The implication to the achievement of sustainable clean water and sanitation for all (SDG 6) is also discussed.

Prediction of nitrate accumulation and leaching beneath groundwater irrigated corn fields in the Upper Platte basin under a future climate scenario

Understanding the impacts of future climate change on soil hydrological processes and solute transport is crucial to develop appropriate strategies to minimize the adverse impacts of agricultural activities on groundwater quality. To evaluate the direct effects of climate change on the transport and accumulation of nitrate-N, we developed an integrated modeling framework combining climatic change, nitrate-N infiltration in the unsaturated zone, and groundwater level fluctuations. The study was based on a center-pivot irrigated corn field at the Nebraska Management Systems Evaluation Area (MSEA) site. Future groundwater recharge (GR) and actual evapotranspiration (ETa) rates were predicted via an inverse vadose zone modeling approach by using climatic data generated by the Weather Research and Forecasting (WRF) climate model under the RCP 8.5 scenario, which was downscaled from the global CCSM4 model to a resolution of 24 km by 24 km. A groundwater flow model was first calibrated on the basis of historical groundwater table measurements and then applied to predict the future groundwater table in 2057–2060. Finally, the predicted future GR rate, ETa rate, and groundwater level, together with future precipitation data from the WRF climate model, were used in a three-dimensional (3D) model to predict nitrate-N concentrations in the subsurface (saturated and unsaturated parts) from 2057 to 2060. The future GR was predicted to decrease in the study area, as compared with the average GR data from the literature. Correspondingly, the groundwater level was predicted to decrease (30 to 60 cm) over the 5 years of simulation in the future. The nitrate-N mass in the simulation domain was predicted to increase but at a slower rate than in the past. Sensitivity analysis indicated that the accumulation of nitrate-N is sensitive to groundwater table elevation changes and irrigation rates.