Groundwater Level Response Research Articles

Sensitivity of groundwater levels to low-frequency climate variability in a large watershed

Groundwater level (GWL) can vary over a wide range of timescales. Previous studies highlighted that low-frequency variability (interannual (2–8 years) to decadal (>10 years)) originating from large-scale climate variability, represents a significant part of GWL variance. It remains an open question, however, how GWL, including extremes, may respond to changes in large-scale climate forcing, affecting precipitation variability. Focusing on the Seine River basin, this study therefore aims to assess how GWLs respond to changes in interannual to decadal climate variability.We implemented an empirical numerical approach, which enables an assessment of the GWL sensitivity to changes in precipitation variability over a range of timescales (up to decadal), using the Seine hydrosystem as a case study. The approach consists in: i) identifying and modifying the spectral content of precipitation in the low-frequency range; ii) using these perturbed precipitation fields as input in the physically-based hydrological/hydrogeological CaWaQS model for the Seine River basin to simulate the corresponding GWL response; iii) comparing the spectral content, mean, variance and extremes of perturbed GWLs with reference (i.e. unperturbed) GWLs. Two interannual (2–4 yr and 5–8 yr) and one decadal (15 yr) timescales were modified individually by either increasing or decreasing their amplitude by 50 %. This led to six scenarios of perturbed low-frequency precipitation variability, which were subsequently used as CaWaQS inputs to assess the GWL response.Results indicated increased (decreased) GWL up to 5 m when low-frequency precipitation variability increased (decreased) by 50 %. This led to an increased occurrence of groundwater floods (droughts) with increased severity and decreased occurrence of groundwater droughts (floods) with decreased severity, respectively. These results indicate: i) how using biased climate data, in terms of low-frequency variability, leads to large deviations in the GWL simulation, ii) to what extent potential changes in low-frequency climate variability may affect future GWL, and particularly drought and flood occurrence and severity.

Analysis of the Response of Shallow Groundwater Levels to Precipitation Based on Different Wavelet Scales—A Case Study of the Datong Basin, Shanxi

The rise in shallow groundwater levels is typically triggered by precipitation recharge, exhibiting a certain lag relative to precipitation changes. Therefore, identifying the response mechanism of shallow groundwater levels to precipitation is crucial for clarifying the interaction between precipitation and groundwater. However, the response mechanism of groundwater levels to precipitation is complex and variable, influenced by various hydrogeological and geographical conditions, and often exhibits significant nonlinear characteristics. To address this issue, this study employs methods such as continuous wavelet transform, cross wavelet transform, and wavelet coherence to analyze the response patterns of groundwater levels to precipitation at different wavelet scales in the Datong Basin from 2013 to 2022: (i) At short wavelet scales (10.33~61.96 d), the groundwater level dynamics respond almost instantaneously to extreme rainfall; (ii) At medium wavelet scales(61.96~247.83 d), the precipitation-groundwater recharge process shows characteristics of either rapid recovery or significant delay; (iii) At long wavelet scales (247.83~495.67 d), three potential groundwater processes were identified in the Datong Basin, exhibiting long-term lag responses throughout this study period, with lag times of 11.18 days, 148.75 days, and 151.49 days, respectively. Furthermore, the results indicate that the lag response time of shallow groundwater levels to precipitation is not only related to the wavelet scale but also to the identified depth conditions of different groundwater regions, groundwater extraction intensity, precipitation intensity, and aquifer lithology. This study distinguishes the temporal and spatial response mechanisms of shallow groundwater to precipitation at different wavelet scales, and this information may further aid in understanding the interaction between precipitation and groundwater levels.

Hydro‐Mechanical Characterization of a Fractured Aquifer Using Groundwater Level Tidal Analysis: Effect of Pore Pressure and Seismic Dynamic Shear Stresses on Permeability Variations

AbstractGroundwater level tidal analysis is a powerful technique to monitor aquifer's permeability and hence its change over time. Earthquakes are known to affect aquifer's properties, in their vicinity through static stress changes but also further away through dynamic stresses. Most often changes are in the form of permeability increases, but sometimes decreases; the changes can be either permanent or transient. These observations are relatively well documented but the physical processes behind these changes are not well understood. By combining solid‐earth and barometric tidal groundwater level responses in a borehole in a coherent poroelastic theoretical framework, and a bi‐layer hydrogeological model, we recover a 15 years‐long time series evolution of aquifer transmissivity and shear modulus. This study showcases the full potential of the tidal analysis method, coupling pore pressure diffusion and rock deformation, at the frontier of hydrogeology and rock physics. This unprecedented measurement of permeability and shear modulus evolution by tidal analysis reveals, during interseismic period, high sensitivity of this shallow aquifer to effective stress, and thus to pore pressure. Thanks to additional finite element simulation of seismic wave propagation, we explore the different mechanisms affecting permeability and shear modulus in the studied fractured andesite aquifer. This study confirms the predominant role of seismic dynamic stresses, and more precisely of dynamic shear stresses, in the change of permeability following an earthquake.

Dynamic change and driving response of shallow groundwater level based on random forest in southwest Songnen Plain

Study regionThe southwest Songnen Plain in China.Study focus:This study investigated the multi-time groundwater dynamic types and the trends of groundwater level. Random forest model was applied to analyse on driving factors of groundwater level.New hydrological insights:This study selected the following data from four periods in 2005, 2010, 2015, and 2020: groundwater depth, precipitation, evaporation, groundwater extraction, dry field area, and paddy field area. Pearson correlation analysis, Kriging interpolation, Mann-Kendall test, and other methods were employed to determine the groundwater dynamic types and changes in level within the four periods. Pearson correlation analysis shows that the dynamic types of groundwater table change greatly in the four periods, and the correlation between groundwater depth and precipitation decreases first and then increases. Kriging interpolation showed that the groundwater depth was lower in the west, and the depth around irrigation areas was higher. The Mann-Kendall test showed that the groundwater depth changed abruptly over time during the irrigation period and rainy season; the average groundwater depth increased by 1.36 m from 2005 to 2010, but only by 0.09 m from 2010 to 2020. The random forest model of groundwater level dynamics and its driving factors was established, and it was found that the size of the paddy field had the greatest influence on groundwater dynamics, affecting 32.15%.

Island hydrogeology in the tropics: Constraining a 3D variable-density groundwater flow and solute transport model with geophysics

Saltwater intrusion is the greatest risk to coastal community water supplies where they are dependent on fresh groundwater as the main source of supply. For small, fractured bedrock island aquifers, the fresh groundwater lens dynamics and transition zone geometry are complex. This study investigated the impacts of projected increases in groundwater pumping on a fresh groundwater lens to evaluate changes to the lens geometry and localized up-coning from the deeper, more saline aquifers beneath a small bedrock island in the tropics. A combination of traditional hydrogeological datasets and an airborne electromagnetic survey were used to develop a three-dimensional density-dependent groundwater flow and solute transport model using the SEAWAT code. This investigation represents one of very few studies that have taken such an approach. The model was calibrated using observed groundwater hydraulic heads and chloride concentration data, and calculated chloride values based on bulk conductivity measurements determined from inverted geophysical data. A staged calibration approach was adopted, firstly assessing the time-average lens extent and geometry, and secondly considering the seasonal groundwater level response. In the calibration, the geophysical data helped constrain the lens geometry in the absence of hydraulic head and chloride data. The calibrated model was used to test scenarios where groundwater pumping rates were increased above the current demand of 452 m3 d−1, showing that the lens is likely to be stable, i.e., its available storage is not expected to contract excessively, for extraction rates of up to 3,000 m3 d−1. The combined use of geophysical data and a numerical modeling approach was advantageous in investigating the lens characteristics. It also demonstrated how these techniques can be used together to evaluate coastal water resources and to manage water supply risks for coastal communities. The study demonstrated that the freshwater lens can likely support the freshwater demands of the remote community, and is a preferred option compared with high-cost and more complicated options such as seawater desalination and managed aquifer recharge.

Double verification and quantitative traceability: A solution for mixed mine water sources

Accurate identification of mine water sources is one of the keys to safe coal mining. However, conventional water source identification methods rely on groundwater-level responses and hydrochemical characteristics. The identification results often have multiple interpretations, lack verification of the actual situation, and cannot trace source quantitatively for mixed water sources. In this paper, we propose a new double verification and quantitative traceability (DV-QT) method. The double verification consists of verification and the consistency of water source identification results based on conventional methods and verification of the rationality of both the water source identification results and the inrushing water channel results. When the verification fails, the groundwater-level or hydrochemical data shall be analyzed in depth, or the hydraulic connection between groundwater in the channel and water source needs to be determined. The quantitative traceability includes the multivariate mixing and mass balance calculations (M3) that calculate the proportion of water sources. Through an example application, the proposed method identified the causes of the identification error of the traditional hydrochemical characteristic analysis method, that is, the hydrochemical evolution process during water inrush cannot be ignored, and allowed for accurate water source identification and quantitative mine water traceability. The new approach is especially useful for mixed water sources and in mines with complicated hydrogeological conditions.

Characterizing coefficient of permeability based on response of groundwater level to river stage using regional database

Characterizing coefficient of permeability based on response of groundwater level to river stage using regional database

Groundwater level response to precipitation at the hydrological observatory of Pinios (central Greece)

To study groundwater dynamics and their response to precipitation in the Pinios river basin (central Greece), the variation trends and seasonal patterns of fourteen monitored groundwater and two precipitation time series were examined using time series decomposition, autocorrelation, and wavelet analysis. The objective is to perform indirect methodologies based on groundwater level and precipitation data to attain a detailed understanding of the hydrogeological mechanisms of the aquifer system and develop an approach that can be easily employed worldwide. The three approaches confirm local groundwater circulation understanding by dividing the aquifer system into three parts characterised by varying transmissivity and storativity properties. The extended memory effect, ranging between 70 and 230 days suggests great inertia and storage capacity of the examined groundwater system. The wavelet analysis proves that the system acts as a filter which absorbs the precipitation frequencies converting them into GWL signals, as demonstrated by the 256 days periodicity shared by groundwater level and precipitation time series. Generally, GWLs present a hysteresis to precipitation due to the geological setup and local flow conditions and are particularly affected by a seasonal component, as also highlighted by time series decomposition. An evident annual periodicity is also observed in hydrological and hydrogeological time series. Results suggest the effectiveness of the proposed combination of methods in developing hydrological understanding and their general applicability to other regions where at least groundwater level and precipitation time series are available. By utilising such data-driven approaches, hydrogeologists and water resource managers can gain valuable insights into the behavior of aquifers, groundwater flow patterns, and the impact of human activities on groundwater quality and quantity.

Response of the shallow groundwater level to the changing environment in Zhongmu County, China

Response of the shallow groundwater level to the changing environment in Zhongmu County, China

Modeling groundwater-level responses to multiple stresses using transfer-function models and wavelet analysis in a coastal aquifer system

In coastal aquifers, dynamic stresses such as climate forcings, groundwater withdrawals, and ocean tidal fluctuations cause nonlinear responses to groundwater levels. Such responses to the stresses impact groundwater resources and related flooding and infrastructure risks at multiple scales. We used time-series models such as transfer-function models and wavelet analysis to quantify the relative contribution of these stresses to groundwater-level fluctuation in wells from the unconfined and confined aquifers in an Atlantic coastal aquifer. Climate forcings, such as precipitation and temperature, explained most of the groundwater-level variation for wells in the unconfined aquifer, whereas groundwater withdrawals were the dominant driver of groundwater levels for wells in the confined aquifer. The impact of groundwater withdrawals also was detected in several wells in the unconfined aquifer. Although the influence of ocean tides on groundwater levels commonly is observed in coastal aquifers, we found that daily groundwater withdrawals can obscure the semi-diurnal coherence signal of the two series. The magnitude of groundwater-level fluctuation that could be explained solely by tides was minor compared to that explained by climate or withdrawal stresses. Transfer-function modeling showed seasonal withdrawals from wells in confined aquifers had a significant, yet heterogeneous influence on groundwater levels in coastal aquifers, which highlights climate and withdrawals as key compounding stresses in coastal hydrology. This study demonstrates the value of time-series approaches to advance characterization of groundwater systems in areas with limited hydrogeologic parameter information.

Storage in the deep aquifer: Stochastic process in time and frequency domain using groundwater level response to seismic activation of the Tohoku earthquake, Japan, 2011

AbstractThis study delves into the interplay of Rayleigh wave‐groundwater‐aquifer dynamics, utilizing seismic data from designated stations, particularly the magnitude 9 event on March 11, 2011 (02:46:23 PM epicenter), Tohoku earthquake, Japan. Auto spectra uncover significant peaks, linking groundwater level with the vertical motion of Rayleigh waves. High coherence between groundwater levels and seismogram vertical displacements confirms this connection. Coherence analysis validates it at the significant frequency bands. Groundwater storage coefficient estimation aligns with typical values of confined aquifers, ranging from 1.0E‐05 to 1.0E‐03. This consistency applies across different methods and other earthquake events, highlighting seismic impact. The findings enhance understanding of aquifer behavior during seismic events, aiding groundwater management and earthquake readiness in susceptible regions. Further research should expand datasets for coefficient accuracy. Insights advance aquifer behavior comprehension during seismic events, guiding groundwater management and earthquake readiness in prone areas.

Early Warning Indicators of Groundwater Drought in Mountainous Regions

AbstractAquifers in mountainous regions are susceptible to drought. However, the diverse hydroclimatology, the small and responsive aquifers, and the varied nature of interactions between groundwater and surface water lead to complex groundwater level responses that are challenging to interpret for understanding groundwater drought. In this study, generalized additive models (GAMs) are used to explore the sensitivity of summer groundwater levels to various climate and hydrological predictor variables (indicators) in each of a snowmelt‐ and rainfall‐dominated hydroclimatic regime in British Columbia, Canada. A sensitivity analysis explores individual seasonal predictor variables, station versus gridded climate data, teleconnection indices, and time series length. GAMs are then generated for different combinations of predictor variables to identify the best combination for each region. In the snowmelt‐dominated regime, maximum spring temperature, maximum snow water equivalent, and the winter Nino 3.4 index is the best combination of variables for predicting summer groundwater levels. In the rainfall‐dominated regime, maximum spring temperature, winter precipitation, and spring streamflow is the best combination. The unique combinations of predictor variables for each region can be used as early warning indicators for groundwater drought preparedness by water managers prior to the beginning of the summer. The Standardized Groundwater Level Index (SGI) is also effective at indicating which wells had pronounced responses to periods of drought in each region. However, the SGI differed among aquifers of similar type, suggesting other factors such as aquifer response mechanism and groundwater pumping may have an important influence on the SGI.

Practical Approach for Defining the Sustainable Yield of Wells in Low-Permeability Fractured Rocks

Groundwater sustainability is still an open question. Theoretical and practical approaches to the definition of groundwater sustainability were mainly developed on large scales, while few studies are available for its definition at a single well scale, especially in fractured aquifers. This study examines how much the sustainable yield of a well in fractured aquifers depends on the trend of drawdown over time. For this aim, pumping tests conducted in fractured rocks were considered and analytical models were applied to examine the long-term response of groundwater levels in some wells. To define the operational pumping flow of a well in these aquifers, results show that long constant flow rate pumping tests are preferred to step-drawdown tests. The late drawdown–time curve and residual drawdown segment-curves during the recovery, approximated by power, exponential or polynomial equations, represent the first step to extrapolating the long-term drawdown in the well. This prediction combined with the assessment of a drawdown limit in the well (as a function of the saturated thickness of the aquifer) are essential to plan the first operation of the well (flow rate and time of pumping). Subsequently, through the monitoring of this first operation step, the following operation phases can be updated and improved.

Characterisation of the Hydrogeological Properties of the Ntane Sandstone Aquifer Using Co-Seismic and Post-Seismic Groundwater Level Responses to the Mw 6.5 Moiyabana Earthquake, Central Botswana

The 3 April 2017 Mw 6.5 Moiyabana earthquake (Central Botswana) had a significant impact on groundwater levels; a gradual co-seismic increase and a stepwise decline in groundwater levels were observed in response to the earthquake at boreholes MH2 and Z12836, respectively. In this study, we investigated the response of groundwater levels to Earth tides by computing the amplitude and phase shift of the M2 tidal constituent to estimate the temporal variations of the storativity, transmissivity, and permeability of the Ntane sandstone aquifer (the main aquifer system) prior to and after the earthquake event. The storativity and permeability computed for borehole MH2 showed a decrease in magnitude of 3.17432 × 10−4 and 1.85 × 10−13 m2 respectively, indicating that strong ground shaking at borehole MH2 might have consolidated the aquifer material, thus resulting in decreased aquifer permeability. The aquifer coefficient of storativity decreased by 2.85 × 10−4 while permeability was enhanced by 0.047 × 10−13 m2 at borehole Z12836. Enhanced permeability might have resulted from increased/enhanced fracturing of the aquifer, fracture clearing and dynamic shaking.

Гүний Хоолойн газар доорхи усны ашиглалтын нөлөө

Comparative methods and the last decade of daily data were used estimate the relationship between precipitation and groundwater level response, and to analyze the consumption influence on confined aquifers in Gunii khooloi groundwater deposit, Umnugovi province, Mongolia.We have synthesized all the available data derived from the monitoring studies as well as carried out field measurements in this study. As a result of the monitoring study, water level of confined aquifer has been decreasing since 2011 related to the water consumption. There is a small depression cone in the eastern part of the area. According to the water level contour of 2013, groundwater level in confined aquifer near to the pumping well in the central part has been fluctuated and decreased about 10m. But it is observed that water level fluctuation is relatively stabilized from the 2015.

Monitoring groundwater storage in a fractured volcanic aquifer system.

The use of groundwater resources is under increasing pressure to meet the demands of the society. Despite this, the understanding of its availability is still limited when compared to other water reserves. The aquifer studied was the volcanic Serra Geral Aquifer System (SASG) located in a humid subtropical region. This research aims to explore in a pioneering way the large-scale hydrogeological processes taking place at the SASG using a unique and complementary tool of intensive field monitoring, remote sensing data, and hydrological modeling. Results showed a fast response of groundwater levels to rainfall, with an average of 29days of lag times. We also identified areas in the southern region of the basin with higher depletion rates, with GRACE results indicating a negative trend (- 57mmyear-1) associated with a major recharge event that occurred in the region before the period monitored. Also, the interflow calculated using the MGB hydrological model showed that the flow within the limits between the soil and the volcanic rocks (different in hydraulic conductivity) varies spatially, influencing the baseflow of rivers as for slopes and soil capacity to store water.

Persistent groundwater reduction induced by dynamic stresses from the 2019 Ridgecrest earthquake observed within a fractured aquifer near Ash Meadows, Nevada, USA

Groundwater observations in Ash Meadows, Nevada are dedicated to protect its unique ecosystem and to manage groundwater resources. The groundwater levels are sensitive to natural factors such as earth tides and earthquakes, allowing us to study dynamic hydrologic properties. During the 2019 M6.4 and M7.1 Ridgecrest earthquakes, Ash Meadows groundwater levels declined in the fractured carbonate aquifer by 7-9 cm, whereas no coseismic groundwater level response was observed in the overlying (∼280 m) alluvial aquifer. We analyzed the static and dynamic stresses produced by the earthquake, and found static stresses should have produced only coseismic water level increases in the Ash Meadows region. At the same time, dynamic stresses were large enough to induce coseismic water level response and are likely the driving force for groundwater level reductions in the carbonate aquifer. Although tidal analysis does not show an observable long-term change in hydraulic parameters, we observed a temporary diffusivity enhancement in the 6 days following the M6.4 foreshock. In addition, we did not observe a postseismic recovery of water level in the carbonate aquifer. Instead, a subsequent 2-year observation of water levels indicates a persistent decline at a rate of 1.7−4.6×10−5 m/day, which is in direct contrast to a generally increasing water level before the earthquake. As of August 2021, water levels in the carbonate aquifer have declined between 1.4-3.7 cm since the earthquake. Persistent water level declines induced by the passage of seismic waves may modify aquifer properties in the short (days) and long-term (months to years). High resolution groundwater monitoring is needed to better understand the postseismic response in hydrologically sensitive aquifers such as Ash Meadows.

Exploring the response of shallow groundwater to precipitation in the northern piedmont of the Qinling Mountains, China

Groundwater recharge from precipitation is a complex process that often exhibits strong nonlinearity. This process is influenced by multiple hydrogeological and topographical conditions. This study aims to assess the response of shallow groundwater table to precipitation in the northern piedmont of the Qinling Mountains in China from 2005 to 2015 using a combination of the Mann-Kendall (MK) test, continuous wavelet transform, and cross-wavelet transform. The MK test revealed a slight upward trend of precipitation from 2005 to 2015. In addition, 71% of the shallow groundwater monitoring wells exhibited increasing trends in the groundwater table depth. Continuous wavelet transform demonstrated periodic groundwater responses to precipitation. Both precipitation and groundwater table showed significant monthly oscillation of 9–15 months, while the highest wavelet power was detected at the temporal scale of 12 months. According to the results of the cross wavelet transform, lag times between groundwater level responses and precipitation events were 78, 73, and 99 days for P1-PF1, P2-FT1, and P3-ST1 in pluvial fans, first terraces, and second terraces, respectively. Moreover, the results suggested spatiotemporal variations in the lag time, which might be due to the variation in groundwater levels, aquifer lithology, precipitation intensity, and groundwater exploitation intensity. The current study revealed the spatiotemporal response mechanism of the shallow groundwater to precipitation, providing a scientific basis for assessing the regional water cycle processes and ensuring effective groundwater resource management.

Groundwater level response identification by hybrid wavelet-machine learning conjunction models using meteorological data.

Due to its heterogeneous and complex nature, groundwater modeling needs great effort to quantify the aquifer, a crucial tool for policymakers and hydrogeologists to understand the variations in groundwater levels (GWL). This study proposed a set of supervised machine learning (ML) models to delineate the GWL changes in the Zarand-Saveh complex aquifer in Iran using 15-year (2005-2020) monthly dataset. The wavelet transform (WT) procedure was also used to improve the GWL prediction ability of ML models for 3-month horizons using input datasets of precipitation, evapotranspiration, temperature, and GWL. The four well-accepted standalone ML methods, i.e., artificial neural network (ANN), adaptive neuro-fuzzy inference system (ANFIS), group method of data handling (GMDH), and least square support vector machine (LSSVM), were implemented and compared with the hybrid wavelet conjunction models. The methods were compared based on root mean square error (RMSE), mean absolute error (MAE), correlation coefficient (R), and Nash-Sutcliffe efficiency (NSE). Comparison outcomes showed that the hybrid wavelet-ML considerably improved the standalone model results. The wavelet transform-least square support vector machine (WT-LSSVM) model was superior to other standalone and hybrid wavelet-ML methods to predict GWL. The best GWL predictions were acquired from the WT-LSSVM model with input scenario 5 involving all influential variables, and this model produced RMSE, MAE, R, and NSE as 0.05, 0.04, 0.99, and 0.99 for 1month ahead of GWL prediction, while the corresponding values were obtained as 0.18, 0.14, 0.95, and 0.90 for 3months ahead of GWL prediction, respectively.

Lag response of groundwater to changes in water and sediment characteristics in the lower Yellow River, China

• XLD operation decreased groundwater level along the lower Yellow River. • The highest groundwater level was delayed after XLD operation. • Inter-annual changes in groundwater were mainly determined by WL, SSC, and ISC. • Response of groundwater to changes in water–sediment lagged by 2–5 months. The process of water exchange between rivers and groundwater is critical for the circulation of material and energy; however, this process is strongly affected by dam-regulated water and sediment. The lower Yellow River seeps downward throughout its coarse and suspended (perched) riverbed and feeds the surrounding floodplain groundwater. The Xiaolangdi Dam (XLD) and water–sediment regulation scheme (WSRS) have altered the exchange processes between river and groundwater along the lower Yellow River. However, few researchers have examined the response of this river-fed groundwater system to the operation of the XLD and implementation of the WSRS. As such, in this study, we analyzed the responses of groundwater to changes in flow and sediment characteristics based on statistical analysis using historical data, extremely randomized tree models, and distributed lag non-linear models. We found that, since the XLD began operating in 2002, the groundwater level near the lower Yellow River substantially decreased, and the groundwater level in June significantly decreased (at the 0.05 level). The highest groundwater level in the pre-dam period occurred in August, whereas the highest level in the post-dam period occurred in October. Changes in the annual groundwater level and in the mid- and post-WSRS seasons along the lower Yellow River were mainly determined by the water level and incoming sediment coefficient in the river, whereas those in the pre-WSRS season were mainly determined by the water level and suspended sediment concentration. Regarding the changes in groundwater level between the mid- and post-WSRS seasons, we found that suspended sediment concentration was the dominant factor in the pre-dam period, whereas the combined effect of water and sediment was the dominant factor in post-dam period. The response of the monthly groundwater level to changes in water and sediment characteristics lagged by 2–5 months. These results are essential to understanding the interaction between river water and groundwater.