Application of spectral analysis to determine hydraulic diffusivity of a sandy aquifer (Pingtung County,Taiwan)

The study demonstrates spectral relationships in the time–frequency domain for one‐dimensional groundwater flow in aquifers bounded by fluctuating boundaries. By nature, the solutions of spectral equations are non‐linear complex functions. To determine hydraulic diffusivity in the governing equations, it is required that the data are collected from the spectra of water levels at the fluctuating boundaries and observation wells. Hydraulic diffusivity thus can be obtained by an iterative inverse approach. This paper presents an application in Pingtung County of Taiwan to determine the hydraulic diffusivity of a sandy aquifer under confined conditions. Spectral density function of water level obtained from tidal boundaries and observation wells are used to approximate hydraulic diffusivity, which yields an averaged value of 1·26 × 10⁶ m²/h. Copyright © 2004 John Wiley & Sons, Ltd.     

Applicability of spectral analysis to determine hydraulic diffusivity

This study is to evaluate the applicability of estimating the one-dimensional horizontal hydraulic diffusivity of an unconfined aquifer with time-dependent fluctuation of lateral head and vertical recharge boundaries using observed water level spectra. Different models of boundary condition are imposed to evaluate the statistical significance between the calculated hydraulic diffusivity (ξ^′) with the given hydraulic diffusivity (ξ). The auto-spectra of the water level in observation wells tapping the same aquifer are closely related to those at the disturbed boundaries. For an aquifer with a constant hydraulic diffusivity, the water level fluctuation in the monitoring wells is linearly related to the water level spectra observed at the boundaries. The spectral density function of aquifer hydraulic head varies inversely with specific yield (S _y) and directly with recharge. Given small variation in water level spectra at the disturbed boundaries, the water level fluctuation in the aquifer is affected by the recharge condition and the aquifer spectral density function is sensitive to S _y. Using an iterative technique to estimate ξ from 1400 sets of given parameters, 99% of the ξ^′/ξ values deviated within only one order of magnitude with the model length (L) being equal to 1 km and 10 km. For L equal to 100 m, approximately 82% of the ξ^′/ξ population falls within two orders of magnitude. Therefore, spectral analysis of aquifer hydraulic head response can be used to estimate the hydraulic diffusivity of an unconfined aquifer which is affected by periodic variations in recharge and head at boundaries.     

Determination of the conduit structure in a karst aquifer based on tracer data—Lurbach system,Austria

A structure model was used to analyse solute‐transport parameter estimates based on tracer breakthrough curves. In the model system, groundwater flow is envisioned to be organised in a complex conduit network providing a variety of short circuits with relative small carrying capacities along different erosion levels. The discharge through the fully filled conduits is limited owing to void geometries and turbulent flow; thus, a hierarchic overflow system evolves where conduits are (re‐)activated or dried up depending on the flow condition. Exemplified on the Lurbach–Tanneben karst aquifer, the applicability of the model approach was tested. Information derived from multi‐tracer experiments performed at different volumetric flow rates enabled to develop a structural model of the karst network, under constraint of the geomorphological and hydrological evolution of the site. Depending on the flow rate, groundwater is divided into up to eight flow paths. The spatial hierarchy of flow paths controls the sequence of flow path activation. Conduits of the topmost level are strongly influenced by reversible alteration processes. Sedimentation or blocking causes an overflow of water to the next higher conduit. Flow path specific dissolutional denudation rates were estimated using the temporal development of the partial discharge. Copyright © 2012 John Wiley & Sons, Ltd.     

Two‐dimensional flow response to tidal fluctuation in a heterogeneous aquifer‐aquitard system

In alluvial coastal aquifers, finer sediments are preferentially deposited along the downstream direction, so the hydraulic conductivity is generally heterogeneous and changes with distance from the coastline. To investigate the influence of aquifer heterogeneity on seawater‐groundwater interaction, a new two‐dimensional model characterising groundwater flow in an aquifer‐aquitard system was developed assuming that the hydraulic conductivity of the aquifer linearly increases with the distance from the coastline along the inland direction. A closed‐form analytical solution was derived using the separation‐of‐variables method. Comparing the new solution with the numerical solution by comsol Multiphysics (Sweden) based on the finite‐element method, one can see that the new solution agreed with the numerical solution very well except at the early time. We found that both aquitard leakance and the heterogeneity factor (b) could result in the propagation bias. The propagation bias represents the inconsistency between the theoretical calculation and the observed strong attenuation and small time lag between the head and tide fluctuations. The attenuation decreased with perpendicular distance from the coastline (x‐axis), whereas the time lag increased with distance along the x‐axis. The relationship between the time lag and the distance along the x‐axis seemed to be linear when b was 0.001 m^?1, whereas it obeyed a power function when b was greater than 0.01 m^?1. Copyright © 2014 John Wiley & Sons, Ltd.     

Ergodicity and the ‘scale effect’

Solute plume spreading in an aquifer exhibits a ‘scale effect’ if the second spatial concentration moment of a plume has a non-constant time-derivative. Stochastic approaches to modeling this scale effect often rely on the critical assumption that ensemble averages can be equated to spatial averages measured in a single field experiment. This ergodicity assumption should properly be evaluated in a strictly dynamical context, and this is done in the present paper. For the important case of trace plume convection by steady groundwater flow in an isotropic, heterogeneous aquifer, ergodicity does not obtain because of the existence of an invariant function on stream surfaces that is not uniform throughout the aquifer. The implications of this result for stochastic models of solute transport are discussed. © 1997 Elsevier Science Ltd. All rights reserved     

The effect of low‐permeability fault zones on groundwater flow in a compartmentalized system. Experimental evidence from a carbonate aquifer (Southern Italy)

The hydrogeological behaviour of fault zones in carbonate aquifers is often neglected in conceptual and numerical models. Furthermore, no information is available regarding the relationships between piezometric levels when significant compartmentalization occurs due to the occurrence of low‐flow fault zones. The aim of this study was to refine the conceptualization of subsurface flow in faulted carbonate aquifers and to analyse relationships between sub‐basins within a compartmentalized aquifer system in Southern Italy. The interactions between compartments that straddle low‐flow faults were investigated over four hydrologic years using a statistical approach to compare (i) the hydraulic heads within two wells located up‐ and down‐gradient of tectonic discontinuities as well as (ii) the rainfall and piezometric levels. The results of this study suggest that a set of barriers exists between the wells, and, therefore, the total head loss observed between the wells (approximately 80 m) should be distributed across several aquitards, with one aquitard exhibiting a relatively high permeability or low degree of integrity. Due to slight differences in permeability, transient conditions in aquitards can occur over relatively short periods, which is in agreement with the results of the statistical data analysis. Consequently, rather than being caused by pure aquitards, aquifer system compartmentalization likely results from slight differences in the permeability between lower‐permeability fault zones and adjacent higher‐permeability protoliths. Copyright © 2014 John Wiley & Sons, Ltd.     

A simulation of large‐scale groundwater flow and travel time in a fractured rock environment for waste disposal purposes

Two‐dimensional regional groundwater flow was simulated based on a conceptual model of low‐permeability crystalline rocks of the Whiteshell Research Area (WRA) in south‐eastern Manitoba. The conceptual model consists of fracture zones that strike in different directions and dip at various angles in the background rock mass. The thickness and hydraulic properties of the fracture zones in the conceptual model were varied as were the fluid properties and the boundary conditions of the groundwater flow system. The effects of these variations on the groundwater flow pattern and on the convective travel time along pathways from a hypothetical disposal vault at 500 m depth to discharge locations at the ground surface were evaluated. The vault was located in the regional discharge area of the groundwater system. A homogeneous conceptual model of the WRA, having only freshwater flow, formed a groundwater flow pattern with a regional flow system. Local flow systems developed increasingly with the introduction of fracture zones 20 m and 3 m thick, and depth‐dependent fluid density. This indicates a reduction in groundwater residence time by fracture zones and fluid density. Flow pathways were analysed using both a stream‐function and a particle‐tracking technique. The pathways and their lengths from the location of the vault to the surface varied spatially according to the flow patterns. The minimum travel time along these pathways was less than 150 000 and greater than 4 000 000 years in models with and without fracture zones, respectively, indicating that the presence of fracture zones was the major controlling factor. A precise knowledge and refinement of conceptual model parameters is necessary during site selection for waste disposal purposes. Copyright © 2004 John Wiley & Sons, Ltd.     

Determination of longitudinal dispersivity in an unconfined sandy aquifer

Study of the mobility of contaminants in an aquifer is an important issue for the proper remediation of contaminated groundwater. Determination of associated solute transport parameters therefore is essential for investigation of the extent to which groundwater can be contaminated. This study aimed at determining solute transport parameters for an unconfined sandy aquifer at a laboratory scale through various tracer tests using a conservative solute as a tracer. Tracer tests consisted of both well‐tracer tests (single and double wells) and an aquifer tracer test using a plume‐capturing device such as time domain reflectometry (TDR). The results showed that longitudinal dispersivities estimated from the single and double well‐tracer tests were 2·2 cm and 13·5 cm for a travel distance of 9·3 cm and 13·5 cm from the injection point respectively. These results agreed reasonably well with the results of the aquifer tracer test. The solute transport parameters obtained at multiple points in the aquifer through the aquifer tracer test revealed that the dispersivity length was proportional to the travel distance by a factor of 0·3, which was moderately higher than the value of 0·1 given in the literature. Copyright © 2002 John Wiley & Sons, Ltd.     

Characterizing rainfall–groundwater dynamics in a hard‐rock aquifer system using time series,geographic information system and geostatistical modelling

The aim of this study was to investigate rainfall–groundwater dynamics over space and annual time scales in a hard‐rock aquifer system of India by employing time series, geographic information system and geostatistical modelling techniques. Trends in 43‐year (1965–2007) annual rainfall time series of ten rainfall stations and 16‐year (1991–2006) pre‐monsoon and post‐monsoon groundwater levels of 140 sites were identified by using Mann–Kendall, Spearman rank order correlation and Kendall rank correlation tests. Trends were quantified by Kendall slope method. Furthermore, the study involves novelty of examining homogeneity of pre‐monsoon and post‐monsoon groundwater levels, for the first time, by applying seven tests. Regression analysis between rainfall and post‐monsoon groundwater levels was performed. The pre‐monsoon and post‐monsoon groundwater levels for four periods – 1991–1994, 1995–1998, 1999–2002 and 2003–2006 – were subjected to geographic information system‐based geostatistical modelling. The rainfall showed considerable spatiotemporal variations, with a declining trend at the Mavli rainfall station (p‐value < 0.05). The Levene's tests revealed spatial homogeneity of rainfall at α = 0.05. Regression analyses indicated significant relationships (r² > 0.5) between groundwater level and rainfall for eight rainfall stations. Non‐homogeneity and declining trends in the groundwater level, attributed to anthropogenic and hydrologic factors, were found at 5–61 more sites in pre‐monsoon compared with post‐monsoon season. The groundwater declining rates in phyllite–schist, gneiss, schist and granite formations were found to be 0.18, 0.26, 0.21 and 0.14 m year^?1 and 0.13, 0.19, 0.16 and 0.02 m year^?1 during the pre‐monsoon and post‐monsoon seasons, respectively. The geostatistical analyses for four time periods revealed linkages between the rainfall and groundwater levels. Copyright © 2013 John Wiley & Sons, Ltd.     

Groundwater contamination risks from conservative point source pollutants in a future climate

ABSTRACT

The groundwater contamination risk in future climates was investigated at three locations in Sweden. Solute transport penetration depths were simulated using the HYDRUS-1D model using historical data and an ensemble of climate projections including two global climate models (GCMs), three emission scenarios and one regional climate model. Most projections indicated increasing precipitation and evapotranspiration until mid-century with a further increase at end-century. Results showed both increasing and decreasing groundwater contamination risks depending on emission scenario and GCM. Generally, the groundwater contamination risk is likely to be unchanged until mid-century, but higher at the end of the century. Soil and site specific relationships between Δ(P – PET) (i.e. change in the difference between precipitation, P, and potential evapotranspiration, PET) and changes in solute transport depths were determined. Using this, changes in solute transport depths for other climate projections can be assessed.     

Using hydrochemical,stable isotope,and river water recharge data to identify groundwater flow paths in a deeply buried karst system

The deeply buried river‐connected Xishan karst aquifer (XKA) in western Beijing, China, has been suffering from diminishing recharge for several decades, which in turn leads to the disappearing of spring water outflows and continuously lowering of groundwater level in the area. Thus, it is important to correctly recognize the groundwater recharge and flow paths for the sustainable development of the XKA. To investigate these issues, the hydrochemical and isotopic compositions are analysed for both surface water and groundwater samples collected over an area of about 280 km². Results show that (a) the river water is characterized by high Na contents; (b) the δ²H and δ¹⁸O values in the river water are distinctively higher than those of groundwater samples, after experiencing the long‐time evaporative enrichment in the upstream reservoir; (c) the Sr concentrations and ⁸⁷Sr/⁸⁶Sr ratios of groundwater clearly indicated the interaction between water and carbonate minerals but excluded the water–silicate interaction; and (d) the groundwater samples in the direct recharge area of the XKA have the lowest Na concentrations and the δ²H and δ¹⁸O values. Based on the large differences in the Na contents and ¹⁸O values of groundwater and surface water, a simple two‐component mixing model is developed for the study area and the fractions of the river water are estimated for groundwater samples. We find that the distribution pattern of the river water fractions in the XKA clearly shows a change of directions in the preferential flow path of the groundwater from its source zone to the discharge area. Overall, our results suggest that the recharged surface water can be a useful evidence for delineating the groundwater flow path in river‐connected karst aquifer. This study improves our understanding of the heterogeneity in karst groundwater systems.     

Relationships between hydraulic parameters in a small stream under varying flow and seasonal conditions

Twenty conservative tracer injections were carried out in the same reach of a small woodland stream in order to determine how variation in discharge and leaf accumulation affect stream hydraulic parameters. The injections were made at various discharge rates ranging from 2·6 to 40 l/s. Five of the injections were made during late autumn, when there were large accumulations of leaves in the stream. Estimates of hydraulic parameters were made by fitting a transient storage solute transport model to conservative tracer concentration profiles. Velocity increased almost linearly with increasing discharge, indicating a decline in the Darcy friction factor. Dispersion also increased with increasing discharge, especially for the lower flow injections. The relative size of the storage zone was small (∽0·1). There was no definable relationship between discharge and the relative storage zone size, but the rates of exchange between the storage zone and the main channel increased markedly with increasing discharge. The presence of large accumulations of leaves had a clear effect on the hydraulic characteristics of the stream, producing much higher friction factors, larger storage zone sizes and lower velocity than would have been predicted by discharge alone. Copyright © 1999 John Wiley & Sons, Ltd.     

A novel platform to evaluate the dampening of water and solute transport in an experimental channel under unsteady flow conditions

This paper presents a novel platform to study the dampening of water and solute transport in an experimental channel under unsteady flow conditions, where literature data are scarce. We address the question about what could be the smallest size of experimental platform that is useful for research, project studies, and teaching activities and that allows to do rational experiments characterized by small space occupation, short experimental duration, high measurement precision, high quality and reproducible experimental curves, low water and energy consumption, and the possibility to test a large variety of hydrograph scenarios. Whereas large scale hydraulic laboratories have focused their studies on sediment transport, our platform deals with solute transport. The objectives of our study are (a) building a platform that allows to do rational experiments, (b) enriching the lack of experimental data concerning water and solute transport under unsteady state conditions, and (c) studying the dampening of water and solute transport. We studied solute transport in a channel with lateral gain and lateral loss under different experimental configurations, and we show how the same lateral loss flow event can lead to different lateral loss mass repartitions under different configurations. In order to characterize water and solute dampening between the input and the output of the channel, we calculate dampening ratios based on peak coordinates of time flow curves and time mass curves and that express the decrease of peak amplitude and the increase of peak occurrence time between the input and output curves. Finally, we use a solute transport model coupling the diffusive wave equation for water transfer and the advection–diffusion equation for solute transport in order to simulate the experimental data. The simulations are quite good with a Nash–Sutcliffe efficiency NSE > 0.98 for water transfer and 0.84 < NSE < 0.97 for solute transport. This platform could serve hydrological modellers because it offers a variety of measured parameters (flow, water height, and solute concentration), at a fine time step under unsteady flow conditions.     

Saltwater intrusion estimation in a karstified coastal system using density-dependent modelling and comparison with the sharp-interface approach

Abstract

Saltwater intrusion is a naturally occurring phenomenon that is exacerbated significantly by excessive groundwater exploitation in coastal aquifers. In order to determine the extent of saltwater intrusion in a karstified coastal aquifer in Crete, Greece, a three-dimensional, density-dependent groundwater flow and transport model was developed and compared to the more traditional sharp-interface approach. The karstified medium was modelled using a combination of the equivalent porous medium approach (for lower-order fractures) and a discrete fracture approach (for the main fractures/faults). The model takes into consideration the geomorphologic characteristics of the karstic system, such as the depth and orientation of the fault network, and the diffusion phenomena associated with the variable densities of freshwater and saltwater—parameters that create a complex system, inducing uncertainty in the model. The model results showed that the orientation of the fractures, the pumping activity and the fluid density effects drive the seawater intrusion front asymmetrically inland.

Editor Z.W. Kundzewicz

Citation Dokou, Z. and Karatzas, G.P., 2012. Saltwater intrusion estimation in a karstified coastal system using density-dependent modelling and comparison with the sharp-interface approach. Hydrological Sciences Journal, 57 (5), 985–999.     

Groundwater evolution and mean water age inferred from hydrochemical and isotopic tracers in a tropical confined aquifer

The coastal confined aquifer in the Gulf of Urabá (Colombia) is an important water source for the banana agro‐industry as well as for urban and rural communities. However, the main processes controlling recharge and mixing in the aquifer are still poorly understood. Hydrochemical analyses and stable isotope monitoring were conducted to (a) determine groundwater recharge origin, mean groundwater age, and the main processes governing groundwater chemistry and the potential mixing of marine water and the influence of diffusive processes from the two surrounding aquitard layers. Hydrochemical data indicate that the main processes affecting the dissolved chemical composition include cation exchange, dissolution of carbonated and CO₂, and silicate weathering. δ¹⁸O and δ²H compositions combined with ¹⁴C data highlight the differences in climatic conditions between the recharge zone and the confined section of the aquifer, which is close to the Atlantic Ocean. Groundwater samples with ¹⁴C ages from recent to 28,300 years BP show a depleted isotopic trend ranging from ?6.43‰ to ?9.14‰ in δ¹⁸O and from ?43.2‰ to ?65.7‰ in δ²H. The most depleted δ¹⁸O and δ²H compositions suggest a cooler recharge climate than the current conditions (corresponding to the last glacial period of the late Pleistocene). Depleted δ¹³C values in the total dissolved inorganic carbon indicate the existence of organic material oxidation processes within the geologic formation. These results can be used or transferred to enhance groundwater modelling efforts in other confined coastal aquifers of South America where scarcity of long‐term monitoring data limits water resources planification under a changing climate.     

A two‐dimensional analytical solution for groundwater flow in a leaky confined aquifer system near open tidal water

Groundwater in coastal areas is commonly disturbed by tidal fluctuations. A two‐dimensional analytical solution is derived to describe the groundwater fluctuation in a leaky confined aquifer system near open tidal water under the assumption that the groundwater head in the confined aquifer fluctuates in response to sea tide whereas that of the overlying unconfined aquifer remains constant. The analytical solution presented here is an extension of the solution by Sun for two‐dimensional groundwater flow in a confined aquifer and the solution by Jiao and Tang for one‐dimensional groundwater flow in a leaky confined aquifer. The analytical solution is compared with a two‐dimensional finite difference solution. On the basis of the analytical solution, the groundwater head distribution in a leaky confined aquifer in response to tidal boundaries is examined and the influence of leakage on groundwater fluctuation is discussed. Copyright © 2001 John Wiley & Sons, Ltd.     

Quantification of the hydraulic diffusivity of a bentonite-sand mixture using the water head decrease measured upon sudden flow interruption

This paper presents a new modelling approach to quantify the hydraulic diffusivity of low-permeability unconsolidated porous media under confined saturated-flow conditions in the laboratory. The derived analytical solution for the transient variation of the hydraulic head after flow interruption was applied to experimental data obtained from continuous measurements of the water pressure at two locations in the soil column. Three soil samples made of a mixture of natural bentonite (at different mass fractions) and medium sand were studied during a series of stepwise constant flow rates of water. The numerical results well fit the experimentally measured decrease of the dimensionless hydraulic head. The study shows that the increase of the mass fraction of bentonite in the soil sample from 10 to 30% is accompanied by a strong decrease of the hydraulic diffusivity from 2.4 × 10⁻² to 1.1 × 10⁻³ m² s⁻¹, which is clearly due to the decrease of the hydraulic conductivity of the soil sample. The specific storages obtained for each of the three samples are in the same order of magnitude and seem to decrease with the increase of mass fraction of bentonite. However, they clearly reflect the predominant portion of the compressibility of the porous medium compared with that of water. Compared with reported literature values for clayey soils, the specific storage values in this study are slightly higher, varying within the range of 2 × 10⁻³ to 8.1 × 10⁻³ m⁻¹._. The experimental results also give insight into the limitations of the modelling approach. In the case of low-permeability soils (K < 2 × 10⁻⁶ ms⁻¹) and steady-flow conditions with low Reynolds numbers, for example, Re < 0.003, it is recommended to choose a time duration for flow interruption between subsequent flow rate steps of longer than 5 s. For high-permeability porous media, to increase the precision of the quantified hydraulic diffusivity, it might be useful to select a measuring frequency significantly higher than 1 Hz.     

Seasonal and interannual behaviour of groundwater catchment boundaries in a Chalk aquifer

Groundwater catchment boundaries and their associated groundwater catchment areas are typically assumed to be fixed on a seasonal basis. We investigated whether this was true for a highly permeable carbonate aquifer in England, the Berkshire and Marlborough Downs Chalk aquifer, using both borehole hydrograph data and a physics‐based distributed regional groundwater model. Borehole hydrograph data time series were used to construct a monthly interpolated water table surface, from which was then derived a monthly groundwater catchment boundary. Results from field data showed that the mean annual variation in groundwater catchment area was about 20% of the mean groundwater catchment area, but interannual variation can be very large, with the largest estimated catchment size being approximately 80% greater than the smallest. The flow in the river was also dependent on the groundwater catchment area. Model results corroborated those based on field data. These findings have significant implications for issues such as definition of source protection zones, recharge estimates based on water balance calculations and integrated conceptual modelling of surface water and groundwater systems. Copyright © 2015 John Wiley & Sons, Ltd.     

Research on the diffusion of pore fluid and in-situ hydraulic diffusivity in the epicentral region of Xinfengjiang reservoir earthquakes

According to the fact that the Xinfengjiang reservoir earthquakes are caused mainly by water seepage, this paper using the data ofM _s⩾2. 0 earthquakes, studies the hydraulic diffusivity of the mainshock zone by the expansion of the distribution area of epicenters. It is indicated thatin-situ hydraulic diffusivity during the preshock activity of the Xinfengjiang reservoir region was about 6. 2 m²/s. However, after the main shock, thein-situ hydraulic diffusivity in the main shock region increased by fifty percent, that is to say, to 9. 7 m²/s. During the long period after main shock occurrence thein-situ hydraulic diffusivity was affected by significant anisotropy of the medium and fluctuation of water level. No regularity can be found. In addition, we compare the diffusivity found by experiment with rock samples collected with thein-situ hydraulic diffusivity estimated. It is shown that the diffusivity of rock samples with fractures is about the same as the diffusivity estimatedin-situ. However, the diffusivity of whole rock samples is 3 orders of magnitudes smaller than that estimatedin-situ. Finally, we discuss the limits on the method by the expansion of distribution area of epicenters in the study of reservoir induced seismicity. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 364–371, 1991. This subject is sponsored by the Chinese Joint Seismological Science Foundation.     

Quantifying temporal variations in water resources of a vulnerable middle eastern transboundary aquifer system

Freshwater resources in the arid Arabian Peninsula, especially transboundary aquifers shared by Saudi Arabia, Jordan, and Iraq, are of critical environmental and geopolitical significance. Monthly Gravity Recovery and Climate Experiment (GRACE) satellite‐derived gravity field solutions acquired over the expansive Saq transboundary aquifer system were analysed and spatiotemporally correlated with relevant land surface model outputs, remote sensing observations, and field data to quantify temporal variations in regional water resources and to identify the controlling factors affecting these resources. Our results show substantial GRACE‐derived terrestrial water storage (TWS) and groundwater storage (GWS) depletion rates of ?9.05 ± 0.25 mm/year (?4.84 ± 0.13 km³/year) and ?6.52 ± 0.29 mm/year (?3.49 ± 0.15 km³/year), respectively. The rapid decline is attributed to both climatic and anthropogenic factors; observed TWS depletion is partially related to a decline in regional rainfall, while GWS depletions are highly correlated with increasing groundwater extraction for irrigation and observed water level declines in regional supply wells.