Groundwater Level Response Research Articles

Quantifying the Impact of Lagged Hydrological Responses on the Effectiveness of Groundwater Conservation

AbstractMany irrigated agricultural areas seek to prolong the lifetime of their groundwater resources by reducing pumping. However, it is unclear how lagged responses, such as reduced groundwater recharge caused by more efficient irrigation, may impact the long‐term effectiveness of conservation initiatives. Here, we use a variably saturated, simplified surrogate groundwater model to: (a) analyze aquifer responses to pumping reductions, (b) quantify time lags between reductions and groundwater level responses, and (c) identify the physical controls on lagged responses. We explore a range of plausible model parameters for an area of the High Plains aquifer (USA) where stakeholder‐driven conservation has slowed groundwater depletion. We identify two types of lagged responses that reduce the long‐term effectiveness of groundwater conservation, recharge‐dominated and lateral‐flow‐dominated, with vertical hydraulic conductivity (KZ) the major controlling variable. When high KZ allows percolation to reach the aquifer, more efficient irrigation reduces groundwater recharge. By contrast, when low KZ impedes vertical flow, short term changes in recharge are negligible, but pumping reductions alter the lateral flow between the groundwater conservation area and the surrounding regions (lateral‐flow‐dominated response). For the modeled area, we found that a pumping reduction of 30% resulted in median usable lifetime extensions of 20 or 25 years, depending on the dominant lagged response mechanism (recharge‐ vs. lateral‐flow‐dominated). These estimates are far shorter than estimates that do not account for lagged responses. Results indicate that conservation‐based pumping reductions can extend aquifer lifetimes, but lagged responses can create a sizable difference between the initially perceived and actual long‐term effectiveness.

A Method for Evaluating Coastal Underground Barrier Wall Using Groundwater Tidal Response.

Tidal response methods are usually used to estimate the hydraulic parameters of coastal aquifers. In this study an analytical model for aquifer tidal response was used. An existing analytical solution for tidal response of groundwater levels was extended to evaluate a subsurface barrier wall to prevent saltwater penetration in a coastal aquifer. A field feasibility study was conducted at the Komesu Dam, Japan. Groundwater levels were observed at pairs of sites on the seaward and reservoir sides of the wall. Groundwater-level time series data collected from a reservoir-side site near a horizontal hollow pipe penetrating the wall contained a visible sinusoidal tidal component, whereas data from another reservoir-side site did not. Analysis of these observations on the groundwater tidal response derived hydraulic parameters of the barrier wall between the paired observation sites. Although the parameters derived by the used simple formulas seem only approximate or apparent, the difference of the results for the two pairs indicated that the extended tidal response method can be useful for evaluation of the barrier function of the wall.

Water table response to rainfall and groundwater simulation using physics-based numerical model: WASH123D

Study regionFour districts (Meinong, Qishan, Dashu, and Daliao) of Kaohsiung city, Southern Taiwan Study focusThe understanding of aquifer recharge in terms of water table response to rainfall is of critical importance to groundwater systems management and various endeavors have been made to estimate the amount of recharge using rainfall data. The purpose of this study is to evaluate the groundwater level response to rainfall and determine the recharge potential for shallow aquifers. We showed a simple approach to estimate specific yield (Sy) and hydraulic conductivity (k) as functions of rainfall and water level data. New hydrological insights for the regionCorrelation method is applied to investigate groundwater level response to associated rainfall and it was found that the rise in water table linearly depends on the rainfall amount per event. Results show the annual recharge rates of 244–1472 mm year─1, which represent 12–43% of rainfall in the study area. The estimated k (order of 10─4 to 10─5 m s─1) and Sy (0.20–0.51) were used as prior values to setup groundwater numerical modeling using WASH123D. The real-time case scenario simulation using pumping and rainfall data indicated the reasonable hydrological response of groundwater levels to rainfall. The long-term simulations should be performed with WASH123D to deal with the subjectivity of sustained groundwater pumping and sustainability of aquifers for better groundwater resource planning and management.

Evolution of the groundwater system in the Chihuahua-Sacramento aquifer due to climatic and anthropogenic factors

Abstract Groundwater is the main source of water in arid cities where precipitations are low and not evenly distributed. The combined impact of climate variability and intensive human activities has caused a substantial decline in groundwater levels. Understanding the response of groundwater levels to meteorological and anthropogenic factors is a key step to propose water management alternatives. Meteorological and groundwater data were used to design a multi-step approach to assess the influential factors on the groundwater system in the City of Chihuahua, Mexico. The analysis of historical groundwater levels and climate showed a clear increase in meteorological drought, as well as a groundwater abstraction trend since 1986. Rainfall, groundwater recharge, and groundwater level displayed a significant decrease. Overall, the groundwater depth is continuously increasing with a strong correlation with groundwater abstraction. Despite having a significant trend, the changes in land-cover, groundwater recharge, and meteorological drought were not the main factors inducing the decreased level of water in the aquifer. The continuous abstraction of groundwater from 1986 to 2010 has led to a depletion of groundwater levels from 32 to 92 m. The findings of this study lay a foundation for future water resource management in the study area.

Hydro-geomechanical characterisation of a coastal urban aquifer using multiscalar time and frequency domain groundwater-level responses

Hydraulic properties of coastal, urban aquifers vary spatially and temporally with the complex dynamics of their hydrogeology and the heterogeneity of ocean-influenced hydraulic processes. Traditional aquifer characterisation methods are expensive, time-consuming and represent a snapshot in time. Tidal subsurface analysis (TSA) can passively characterise subsurface processes and establish hydro-geomechanical properties from groundwater head time-series but is typically applied to individual wells inland. Presented here, TSA is applied to a network of 116 groundwater boreholes to spatially characterise confinement and specific storage across a coastal aquifer at city-scale in Cardiff (UK) using a 23-year high-frequency time-series dataset. The dataset comprises Earth, atmospheric and oceanic signals, with the analysis conducted in the time domain, by calculating barometric response functions (BRFs), and in the frequency domain (TSA). By examining the damping and attenuation of groundwater response to ocean tides (OT) with distance from the coast/rivers, a multi-borehole comparison of TSA with BRF shows this combination of analyses facilitates disentangling the influence of tidal signals and estimation of spatially distributed aquifer properties for non-OT-influenced boreholes. The time-series analysed covers a period pre- and post-impoundment of Cardiff’s rivers by a barrage, revealing the consequent reduction in subsurface OT signal propagation post-construction. The results indicate that a much higher degree of confined conditions exist across the aquifer than previously thought (specific storage = 2.3 × 10−6 to 7.9 × 10−5 m−1), with implications for understanding aquifer recharge, and informing the best strategies for utilising groundwater and shallow geothermal resources.

Aquifer Parameters Estimation from Natural Groundwater Level Fluctuations at the Mexican Wine-Producing Region Guadalupe Valley, BC

Determining hydrogeological properties of the rock materials that constitute an aquifer through stress tests or laboratory tests presents inherent complications. An alternative tool that has significant advantages is the study of the groundwater-level response as a result of the pore-pressure variation caused by the internal structure deformation of the aquifer induced by barometric pressure and solid Earth tide. The purpose of this study was to estimate the values of the physical/hydraulic properties of the geological materials that constitute the Guadalupe Valley Aquifer based on the analysis of the groundwater-level response to barometric pressure and solid Earth tide. Representative values of specific storage (1.27 × 10−6 to 2.78 × 10−6 m−1), porosity (14–34%), storage coefficient (3.10 × 10−5 to 10.45 × 10−5), transmissivity (6.67 × 10−7 to 1.29 × 10−4 m2∙s−1), and hydraulic conductivity (2.30 × 10−3 to 2.97 × 10−1 m∙d−1) were estimated. The values obtained are consistent with the type of geological materials identified in the vicinity of the analyzed wells and values reported in previous studies. This analysis represents helpful information that can be considered a framework to design and assess management strategies for groundwater resources in the overexploited Guadalupe Valley Aquifer.

Simulating the Shallow Groundwater Level Response to Artificial Recharge and Storage in the Plain Area of the Daqing River Basin, China

Water shortage and overexploitation of groundwater (GW) have become the key factors restricting the development of the Xiongan New Area (XNA), the environmental management of Baiyangdian Lake, and the social and economic development of surrounding areas. This study used a numerical GW flow model to quantitatively analyze the changes to the shallow GW level and GW reserves of the plain area of the Daqing River Basin over the next 15 years (2021–2035) under different artificial recharge schemes with the south to north water diversion project (SNWDP) acting as the GW recharge source. The results showed increasing GW storage and GW levels and that the regional GW resources are in a positive equilibrium state. The rates of change of the well irrigation supply scheme and the joint river-well irrigation supply scheme in the XNA will increase by 14.56% and 11.04% by 2035 as compared with the current situation. The well irrigation recharge scheme for the XNA was determined to be the most effective when comparing with the effects of the different artificial recharge schemes on the GW levels and recharge. This study provides a reference for the management and protection of aquifers in other areas suffering serious GW overexploitation.

Water level response in wells to dynamic shaking in confined unconsolidated sediments: A laboratory study

This paper focuses on laboratory experiments designed to investigate the influence of dynamic shaking on water level fluctuations in boreholes completed in unconsolidated sediments. A large flume filled with natural sandy sediments was subjected to horizontal dynamic loading at different frequencies (0.5–15 Hz) and accelerations (0.1 g, 0.15 g, 0.25 g and 0.5 g), generated by a shaking table. Thus, the change in characteristics of the water and saturated sediments were investigated, i.e. how water levels and hydraulic properties change with time, are of significant interest. A range of different of water level changes following oscillating pore pressure for hydrostatic and hydrodynamic (flowing) conditions were observed: oscillation, step-rise, step-rise with oscillation, and step-drop with oscillation, similar to those observed in the field after earthquakes. We found that water level declines occur when the imposed frequency is less than 2 Hz, while the magnitude of acceleration shows weak correlation with the water level change when it is less than 0.5 g. Sinusoidal wave propagation reduces hydraulic conductivity first sharply, stabilizes, and is then followed by a constant value with a slight increase. Heterogeneous consolidation, mechanical resonance, compaction, dilation, and re-packing of the grain structure may constitute physically plausible mechanisms for water level fluctuations and hydraulic parameter adjustment. Laboratory experiments can therefore provide an insight and a quantitative process-oriented approach to better understand groundwater level response to seismic waves.

A Method for Integrating Delayed Recharge Flux Through Unsaturated Zones into Analytical and Numerical Groundwater Flow Modeling

AbstractThe temporal delay in the water table's response to precipitation is a common phenomenon. Using a mathematical model, we developed a function that represents the recharge flux through unsaturated zones of varying thickness to model the delayed response of groundwater level due to precipitation. The recharge flux from the developed function was integrated into the analytical and numerical models to estimate the actual groundwater levels in the unsaturated zone. The analytical and numerical models accurately determined the actual groundwater levels. Additionally, calibrated hydraulic parameters from numerical estimates reflected the values measured in the actual aquifer, thereby highlighting the potential application of the developed function to wide‐ranging groundwater problems. Based on the recharge function, a recharge module can be developed for existing flow simulators to enhance groundwater modeling.

Changes in groundwater level and tidal response caused by the Wenchuan earthquake, China

Earthquakes can trigger various hydrological responses such as changes in groundwater level, tidal response, and aquifer permeability, but the relationship between these changes remains unclear. In this study, the changes in groundwater level and tidal response induced by the 2,008 Ms8.0 Wenchuan earthquake, China, are analyzed for epicenter distances of tens of kilometers to more than 1,000 km. The results show that the water levels in 34 wells changed, including step-like, pulse, oscillation and gradual changes. The water levels of 8 out of 17 wells with tidal responses experienced changes in phase shift based on tidal analysis, and the water levels and phase shifts varied in opposite directions. The changes in phase shift indicate that the earthquake changed the horizontal or vertical permeability of the aquifer, or even the groundwater movement. Several possible mechanisms are proposed to account for these changes: breaching of the aquitard, associated with five of the studied wells in which phase shift increased to different extents but water level decreased; reduction in the fracture aperture, accounting for one well whose phase shift changed from positive to negative but water level increased; clogging/unclogging of the fractures, associated with one well with increasing and then decreasing phase shift; lower aquifer permeability or the occurrence of rapid fluctuations in pore pressure, accounting for another well where phase shift gradually decreased but the water level changed little. Each mechanism may deepen understanding of hydrological responses after earthquakes.

Impact of artificial recharge on groundwater and springs: Jinan, China, case study

Baotu spring, known widely as the most famous spring in China, has experienced continuous water level drops, especially during the dry season. Artificial recharge (AR) to groundwater via the Yufu river has been conducted to maintain the required water level to keep regional springs flowing. The purpose of this study is to evaluate the effects of recharge on groundwater. A three-dimensional numerical model was built to simulate groundwater level changes after recharge was applied to the Yufu river. The model was calibrated using hydraulic heads measured at multiple monitoring wells. The results showed that AR is an effective way to maintain the groundwater level in a region where groundwater was excessively exploited for irrigation and industry use. In the studied region, the response of groundwater level to AR near the Yufu river was faster in the west than that in the east as a result of different hydrogeologic conditions. Furthermore, different springs responded differently to AR. For example, the water level increases at the Baotu and Heihu springs were 0.06 m and 0.04 m, respectively, whereas other springs experienced decreases. This implies that a more detailed hydrogeologic characterization is necessary to determine the preferential flow paths and low permeability layers in order to maximize the effect of AR for sustainable groundwater management.

Technical note: Disentangling the groundwater response to Earth and atmospheric tides to improve subsurface characterisation

Abstract. The groundwater response to Earth tides and atmospheric pressure changes can be used to understand subsurface processes and estimate hydraulic and hydro-mechanical properties. We develop a generalised frequency domain approach to disentangle the impacts of Earth and atmospheric tides on groundwater level responses. By considering the complex harmonic properties of the signal, we improve upon a previous method for quantifying barometric efficiency (BE), while simultaneously assessing system confinement and estimating hydraulic conductivity and specific storage. We demonstrate and validate this novel approach using an example barometric and groundwater pressure record with strong Earth tide influences. Our method enables improved and rapid assessment of subsurface processes and properties using standard pressure measurements.

Characterising hillslope–stream connectivity with a joint event analysis of stream and groundwater levels

Abstract. Hillslope–stream connectivity controls runoff generation, during events and during baseflow conditions. However, assessing subsurface connectivity is a challenging task, as it occurs in the hidden subsurface domain where water flow can not be easily observed. We therefore investigated if the results of a joint analysis of rainfall event responses of near-stream groundwater levels and stream water levels could serve as a viable proxy for hillslope–stream connectivity. The analysis focuses on the extent of response, correlations, lag times and synchronicity. As a first step, a new data analysis scheme was developed, separating the aspects of (a) response timing and (b) extent of water level change. This provides new perspectives on the relationship between groundwater and stream responses. In a second step we investigated if this analysis can give an indication of hillslope–stream connectivity at the catchment scale. Stream water levels and groundwater levels were measured at five different hillslopes over 5 to 6 years. Using a new detection algorithm, we extracted 706 rainfall response events for subsequent analysis. Carrying out this analysis in two different geological regions (schist and marls) allowed us to test the usefulness of the proxy under different hydrological settings while also providing insight into the geologically driven differences in response behaviour. For rainfall events with low initial groundwater level, groundwater level responses often lag behind the stream with respect to the start of rise and the time of peak. This lag disappears at high antecedent groundwater levels. At low groundwater levels the relationship between groundwater and stream water level responses to rainfall are highly variable, while at high groundwater levels, above a certain threshold, this relationship tends to become more uniform. The same threshold was able to predict increased likelihood for high runoff coefficients, indicating a strong increase in connectivity once the groundwater level threshold was surpassed. The joint analysis of shallow near-stream groundwater and stream water levels provided information on the presence or absence and to a certain extent also on the degree of subsurface hillslope–stream connectivity. The underlying threshold processes were interpreted as transmissivity feedback in the marls and fill-and-spill in the schist. The value of these measurements is high; however, time series of several years and a large number of events are necessary to produce representative results. We also find that locally measured thresholds in groundwater levels can provide insight into the connectivity and event response of the corresponding headwater catchments. If the location of the well is chosen wisely, a single time series of shallow groundwater can indicate if the catchment is in a state of high or low connectivity.

Unsaturated Zone Flow Processes and Aquifer Response Time in the Chalk Aquifer, Brighton, South East England.

The Chalk aquifer is one of the main sources of water in South East England. The unsaturated zone in the aquifer plays an important role controlling the time and magnitude of recharge and is major pathway for contaminant transport to the water table. A range of previous work has addressed flow processes in the Chalk unsaturated zone, but physical understanding is still incomplete. Here we present the results of a study on flow mechanism in the Chalk unsaturated zone using a combination of statistical analysis and novel laboratory methods. The study was undertaken at three sites (North Heath Barn [NHB], Pyecombe East [PE], and Preston Park [PP]) on the Chalk of the Brighton block, South East England. Daily and hourly time series data of groundwater level and rainfall were correlated. The results show that a slower groundwater level response to rainfall occurs during dry seasons (summer and autumn) when the amount of effective rainfall is less than 4 mm/day, with a thicker and drier unsaturated zone. A faster response occurs during wet seasons (winter and spring) when the daily effective rainfall exceeds 4 mm/day with a thinner and wetter unsaturated zone. Periods of very rapid response (within 15 h) were observed during wet seasons at NHB and PE sites, with unsaturated hydraulic conductivity (Ku ) inferred to reach 839 mm/day. A slower response was observed at an urbanized site (PP) as a result of reduction in direct recharge due to reduced infiltration, due to presences of impermeable infrastructure covering the area around PP borehole. Laboratory measurements of Ku of the Chalk matrix using a geotechnical centrifuge show variation from 4.27 to 0.07 mm/day, according to the level of saturation. Thus, the rapid responses cannot be linked to matrix flow only but indicate the contribution of fracture and karstic flow processes in conducting water.

Relation of groundwater level and rainfalls in the peat swamp forest, burned peatland and mixed plantation areas of Kampar Peninsula, Riau Province

Peatland forest in the Kampar peninsula have converted into various land uses, mainly converted to palm oil plantation. The massive conversion has decreased biodiversity, changed the hydrological cycle, and increased the peatland fire that dramatically decreased the quality of the peatland ecosystem. It is essential to understand the peatland characteristics and its hydrological cycle to develop sustainable peatland management. This study aimed to determine the characteristics of peat soil and the response of groundwater level to rainfall on burned peatland, mixed plantation and primary peat swamp forests in Siak district. The study was conducted from July to December 2018 by using a hydrometeorological station equipped with rainfall and groundwater level sensors. Disturbed and undisturbed soil samples were analyzed to identify the soil characteristics. The response of groundwater level to rainfall was analyzed using simple regression. The results showed that the burned peatland and mixed plantation have increased bulk density, decreased soil porosity, and caused a decrease of water infiltration. Simple regression analysis between the rainfall and the increase of GWL on the primary peat swamp forest resulted in 0.941(P)0.5106 and an R² value 0.6189, meanwhile on the burned peatland resulted in 1.2127(P)0.7818 and an R² value 0.8616, and on the mixed resulted 0.7455 in (P)0.7831 and an R2 = 0.7557.

Response of a Coastal Groundwater System to Natural and Anthropogenic Factors: Case Study on East Coast of Laizhou Bay, China.

With a shifting climate pattern and enhancement of human activities, coastal areas are exposed to threats of groundwater environmental issues. This work takes the eastern coast of Laizhou Bay as a research area to study the response of a coastal groundwater system to natural and human impacts with a combination of statistical, hydrogeochemical, and fuzzy classification methods. First, the groundwater level dynamics from 1980 to 2017 were analyzed. The average annual groundwater level dropped 13.16 m with a descent rate of 0.379 m/a. The main external environmental factors that affected the groundwater level were extracted, including natural factors (rainfall and temperature), as well as human activities (irrigated area, water-saving irrigated area, sown area of high-water-consumption crops, etc.). Back-propagation artificial neural network was used to model the response of groundwater level to the above driving factors, and sensitivity analysis was conducted to measure the extent of impact of these factors on groundwater level. The results verified that human factors including irrigated area and water-saving irrigated area were the most important influencing factors on groundwater level dynamics, followed by annual precipitation. Further, groundwater samples were collected over the study area to analyze the groundwater hydrogeochemical signatures. With the hydrochemical diagrams and ion ratios, the formation of groundwater, the sources of groundwater components, and the main hydrogeochemical processes controlling the groundwater evolution were discussed to understand the natural background of groundwater environment. The fuzzy C-means clustering method was adopted to classify the groundwater samples into four clusters based on their hydrochemical characteristics to reveal the spatial variation of groundwater quality in the research area. Each cluster was spatially continuous, and there were great differences in groundwater hydrochemical and pollution characteristics between different clusters. The natural and human factors resulted in this difference were discussed based on the natural background of the groundwater environment, and the types and intensity of human activity.

Development of an Apparent Recharge Coefficient (ARC) for Estimating Groundwater Storage Changes due to Precipitation Events Using Time Series Monitoring Data

Significant variation in the precipitation events caused by global climate change has made it difficult to manage water resources due to the increased frequency of unexpected droughts and floods. Under these conditions, groundwater is needed to ensure a sustainable water supply; thus, estimates of precipitation recharge are essential. In this study, we derived an apparent recharge coefficient (ARC) from a modified water table fluctuation equation to predict groundwater storage changes due to precipitation events. The ARC is calculated as the ratio of the recharge rate over the specific yield (R/Sy); therefore, it implicitly expresses variation in Sy. The ARC varies spatially and temporally, corresponding to the precipitation events and hydrogeological characteristics of unsaturated materials. ARCs for five monitoring wells from two basins in Korea in different seasons were calculated using a 10-year groundwater level and weather dataset for 2005–2014. Then, the reliability of the ARCs was tested by the comparison of the predicted groundwater level changes for 2015 and 2016 with observed data. The root mean square error ranged from 0.03 to 0.09 m, indicating that the predictions were acceptable, except for one well, which had thick clay layers atop the soil layer; the low permeability of the clay slowed the precipitation recharge, interfering with groundwater level responses. We performed a back-calculation of R from the Sy values of the study areas; the results were similar to those obtained via other methods, confirming the practical applicability of the ARC. In conclusion, the ARC is a viable method for predicting groundwater storage changes for regions where long-term monitoring data are available, and subsequently will facilitate advanced decision making for allocating and developing water resources for residents, industry, and groundwater-dependent ecosystems.

A Probabilistic Model of Aquifer Susceptibility to Earthquake-Induced Groundwater-Level Changes

ABSTRACTA probabilistic model for earthquake-induced persistent groundwater-level response as a function of peak ground velocity (PGV) has been constructed using a catalog of monitoring well observations spanning multiple earthquakes. The regional-scale, multi-site, multi-earthquake investigation addresses the occurrence and absence of hydraulic responses to large earthquakes spanning almost a decade of seismic shaking. Persistent groundwater-level changes, or absences of change, have been quantified in 495 monitoring wells in response to one or more of 11 recent New Zealand earthquakes larger than Mw 5.4 that occurred between 2008 and 2017. A binary logistic regression model with random effects has been applied to the dataset using three predictors: earthquake shaking (PGV), degree of hydrogeological confinement (monitoring well depth), and rock strength (site-average shear-wave velocity). Random effects were included as a partial proxy for variations in monitoring wells’ susceptibilities to earthquake-induced persistent water-level changes. Marginal probabilities have been calculated as a function of PGV and related to modified Mercalli intensity (MMI) levels using a New Zealand-specific MMI–PGV relationship that enables the likelihood of persistent water-level changes to be expressed for MMIs of II–VIII. This study capitalizes on one of the largest catalogs of earthquake hydrological observations compiled worldwide and is the first attempt at incorporating seismic and hydrogeological factors in a common probabilistic description of earthquake-induced groundwater-level changes. This modeling framework provides a more generalizable approach to quantifying responses than alternative metrics based on epicentral distance, magnitude, and seismic energy density. It has potential to enable better comparison of international studies and to inform practitioners making engineering or investment decisions to mitigate risk and increase the resilience of water-supply infrastructure.

The Response of Groundwater Level to Climate Change and Human Activities in Baotou City, China

The response mechanism of groundwater to climate change and human activities in cities within arid and semi-arid regions, such as the Urban Planning Area of Baotou City (UPABC), northern China, is a complicated problem to understand. We analyzed the climate change relationships, including precipitation and temperature, and analyzed changes in human activities, such as groundwater consumption, and then statistically analyzed the main factors affecting groundwater depth. Furthermore, cross-wavelet and wavelet coherence methods were used to analyze the response relationship and hysteresis of groundwater depth to precipitation to better understand the groundwater depth response law. The results showed that the annual precipitation in the UPABC reduction rate was 3.3 mm/10 yr, and the annual average temperature increase rate was 0.43 °C/10 yr, from 1981 to 2017. The unconfined water decrease rate was 0.50 m/yr, and the confined water decrease rate was 0.7 m/yr. The unconfined and confined water depths were affected by precipitation and groundwater exploitation, respectively, with correlation coefficients of 0.58 and 0.57, respectively. The hysteresis of groundwater depth to precipitation was about 9–14 months. However, changes in groundwater depth, especially confined water depth, were greatly affected by groundwater exploitation. This reflected the imbalance in consumption and recharges in the UPABC, highlighting the long-term risk in areas relying on access to this resource. Therefore, arid inland zones of northern China, such as the UPABC, should pay more attention to the rational development of groundwater and strengthen the management and protection of groundwater resources.

Response of Groundwater Levels in a Coastal Aquifer to Tidal Waves and Rainfall Recharge

Groundwater level in coastal aquifers is usually affected by tidal waves and rainfall recharge. Therefore, the objective of this study is to present a mathematical model to account for the effects of tidal waves and rainfall recharge simultaneously. The model is based on the Dupuit–Forchheimer assumptions and is separated into a tidal waves component and rainfall recharge component. A new more general analytical solution for the recharge component is acquired by the generalized integral transform technique. The beach slope, the inclination of an impermeable base of an aquifer, and any randomly distributed rainfall recharge are taken into account in the model. A new finding is that the highest fluctuation in groundwater levels might occur when the range of rainfall recharge is larger than the decay length.