Impact of water withdrawals from groundwater and surface water on continental water storage variations

Humans have strongly impacted the global water cycle, not only water flows but also water storage. We have performed a first global-scale analysis of the impact of water withdrawals on water storage variations, using the global water resources and use model WaterGAP. This required estimation of fractions of total water withdrawals from groundwater, considering five water use sectors. According to our assessment, the source of 35% of the water withdrawn worldwide (4300 km³/year during 1998–2002) is groundwater. Groundwater contributes 42%, 36% and 27% of water used for irrigation, households and manufacturing, respectively, while we assume that only surface water is used for livestock and for cooling of thermal power plants. Consumptive water use was 1400 km³/year during 1998–2002. It is the sum of the net abstraction of 250 km³/year of groundwater (taking into account evapotranspiration and return flows of withdrawn surface water and groundwater) and the net abstraction of 1150 km³/year of surface water. Computed net abstractions indicate, for the first time at the global scale, where and when human water withdrawals decrease or increase groundwater or surface water storage. In regions with extensive surface water irrigation, such as Southern China, net abstractions from groundwater are negative, i.e. groundwater is recharged by irrigation. The opposite is true for areas dominated by groundwater irrigation, such as in the High Plains aquifer of the central USA, where net abstraction of surface water is negative because return flow of withdrawn groundwater recharges the surface water compartments. In intensively irrigated areas, the amplitude of seasonal total water storage variations is generally increased due to human water use; however, in some areas, it is decreased. For the High Plains aquifer and the whole Mississippi basin, modeled groundwater and total water storage variations were compared with estimates of groundwater storage variations based on groundwater table observations, and with estimates of total water storage variations from the GRACE satellites mission. Due to the difficulty in estimating area-averaged seasonal groundwater storage variations from point observations of groundwater levels, it is uncertain whether WaterGAP underestimates actual variations or not. We conclude that WaterGAP possibly overestimates water withdrawals in the High Plains aquifer where impact of human water use on water storage is readily discernible based on WaterGAP calculations and groundwater observations. No final conclusion can be drawn regarding the possibility of monitoring water withdrawals in the High Plains aquifer using GRACE. For the less intensively irrigated Mississippi basin, observed and modeled seasonal groundwater storage reveals a discernible impact of water withdrawals in the basin, but this is not the case for total water storage such that water withdrawals at the scale of the whole Mississippi basin cannot be monitored by GRACE.     

Assessing the restoration time of surface water and groundwater systems under groundwater pumping

Since surface water and groundwater systems are fully coupled and integrated, increased groundwater withdrawal during drought may reduce groundwater discharges into the stream, thereby prolonging both systems’ recovery from drought. To analyze watershed response to basin-level groundwater pumping, we propose a modelling framework to understand the resiliency of surface water and groundwater systems using an integrated hydrologic model under transient pumping. The proposed framework incorporates uncertainties in initial conditions to develop robust estimates of restoration times of both surface water and groundwater and quantifies how pumping impacts state variables such as soil moisture. Groundwater pumping impacts over a watershed were also analyzed under different pumping volumes and different potential climate scenarios. Our analyses show that groundwater restoration time is more sensitive to variability in climate forcings as opposed to changes in pumping volumes. After the cessation of pumping, streamflow recovers quickly in comparison to groundwater, which has higher persistence. Pumping impacts on various hydrologic variables were also discussed. Potential for developing optimal conjunctive management plans using seasonal-to-interannual climate forecasts is also discussed.     

Dynamic attribution of global water demand to surface water and groundwater resources: Effects of abstractions and return flows on river discharges

As human water demand is increasing worldwide, pressure on available water resources grows and their sustainable exploitation is at risk. To mimic changes in exploitation intensity and the connecting feedbacks between surface water and groundwater systems, a dynamic attribution of demand to water resources is necessary. However, current global-scale hydrological models lack the ability to do so. This study explores the dynamic attribution of water demand to simulated water availability. It accounts for essential feedbacks, such as return flows of unconsumed water and riverbed infiltration. Results show that abstractions and feedbacks strongly affect water allocation over time, particularly in irrigated areas. Also residence time of water is affected, as shown by changes in low flow magnitude, frequency, and timing. The dynamic representation of abstractions and feedbacks makes the model a suitable tool for assessing spatial and temporal impacts of changing global water demand on hydrology and water resources.     

Assessment of the long-term safety of radioactive waste disposal: 1. Paleoreconstruction of groundwater formation conditions

Isotopic characteristics (δ²H, δ¹⁸O, ²³⁴U/²³⁸U, Ar, ³H) of natural and technogenically affected waters were determined in the area around the burying grounds of the Siberian Chemical Combine with the aim to assess the circulation conditions of natural waters and the safety of radioactive waste disposal in reservoir beds.     

Effects of shield brine on the safe disposal of waste in deep geologic environments

The salinity of groundwater increases with depth in the Canadian Shield (up to 1.3 kg/L of density). The existence of brine can be critically important for the safe geologic disposal of radioactive wastes, as dense brine can significantly retard the upward migration of radionuclides released from repositories. Static and flushing conditions of the deep brine are analyzed using a U-tube analogy model. Velocity reduction due to the presence of dense brine is derived under flushing conditions. A set of illustrative numerical simulations in a two-dimensional cross section is presented to demonstrate that dense brine can significantly influence regional groundwater flow patterns in a shield environment. It is implied from the results that (1) the existence of Shield brine can be an indicator of a hydrogeologically stable environment, (2) activities near ground surface may not perturb the stable groundwater environment in the deep brine region, and thus, (3) the deep brine region can be considered as a candidate geologic site for the safe disposal of waste. In addition to brine, other issues associated with long-term waste disposal, such as geological, glacial and seismic events, may need to be considered for the safe storage of spent nuclear fuel in a shield environment.     

Geomorphologic control on pollutant retardation at the groundwater–surface water interface

The results of research on the pollutant retardation potential of permeable riverbed sediments in catchments with significant groundwater–surface water (GW-SW) interaction are presented. The fraction of organic carbon and cation exchange capacity of fluvial sediments in various geomorphologic environments have been quantified. Sediments in selected reaches of the rivers Tern and Leith (UK), from the underlying Permian sandstone aquifers, and from along the length of the rivers Severn and Eden into which the Tern and Leith discharge have been investigated. Statistical analyses show significant variation in the geochemistry and pollutant retardation potential of sediments from different geomorphologic features, and between upland and lowland rivers. The sorption potential of fine-grained sediments deposited in pools was greater than sand in runs and coarser deposits in riffles. Similarly, sediments in lowland rivers were found to have a greater retardation potential than those in upland rivers. There was generally greater retardation potential in fluvial sediments of all types than in the underlying aquifers, and in lowland rivers the fluvial sediment retardation potential greatly dominated that of the aquifer. The findings demonstrate the potential for pollutant retardation processes in riverbed sediments of sandstone catchments, and suggest that consideration of retardation processes at the groundwater–surface water interface should be included into environmental risk-assessment studies, in order to better assess and manage the effects of contaminated groundwater discharges to rivers, particularly in lowland catchments. Copyright © 2008 John Wiley & Sons, Ltd.     

生态补水对白洋淀流域地表水和地下水水化学特征的影响

生态补水是维持和改善白洋淀生态环境的重要途径.为研究生态补水对白洋淀水环境的影响,分别在补水前与补水后采集淀水、河水及地下水样品,分析区域地表水和地下水水化学特征.结果表明：（1）白洋淀补水前、后地表水与地下水的水化学组成中Na⁺为主要阳离子,补水后阴离子以HCO₃^-为主,淀区南部地表水电导率高;补水后地表水与地下水Ca²⁺、Mg²⁺和HCO₃^-浓度显著增加,水体电导率降低.（2）补水前地下水为Na-HCO₃型水,地表水主要为Na-Cl·SO₄及Na-Cl·HCO₃类型;补水后地表水与浅层地下水向Ca·Mg-HCO₃型演化,深层地下水水化学类型基本保持不变.（3）生态补水使白洋淀水位升高,淀区水面积增大,缓解了水资源短缺的问题;同时也使浅层地下水水化学组成发生改变,而深层地下水暂未受到影响.生态补水后,受稀释和混合作用的影响,水体Na⁺、Cl^-和SO₄^2-浓度显著下降,Ca²⁺、Mg²⁺及HCO₃^-浓度增加.在白洋淀生态补水中应"先治污,后补水",以减少补水过程中污染物向淀区的运移,还应注意区域地下水位上升过程中的阳离子交换及水岩相互作用,为合理调配生态补水及改善白洋淀生态环境提供科学依据.     

Spatio-temporal patterns of the interaction between groundwater and surface water in plains

The study of the dynamics of anthropic disturbances that have an effect on the hydrological systems in plains requires integral simulation tools for their diagnosis. The objective of this article is, first, to analyse and reproduce the spatio-temporal interactions between groundwater (GW) and surface water, net recharge, GW level, surface run-off, and evapotranspiration in the upper creek basin of Del Azul, which is located in the centre of the province of Buenos Aires, Argentina, and second, to obtain insights to apply the methodology to other similar situations. For this purpose, a model coupling the semidistributed hydrological model (Soil and Water Assessment Tool [SWAT]) and the hydrogeological model (MODFLOW) has been used. A simulation was carried out for a period of 13 years (2003–2015) on a daily scale. The application of the SWAT–MODFLOW coupling gave good results based on the adjustment between the calculated flows and levels, reaching a Nash–Sutcliffe of 0.6 and R²0.6 at the Seminario hydrometric station located at the watershed outlet point. According to the annual average balance, out of the total rainfall, evapotranspiration accounts for 85%, recharge accounts for 10.2%, and surface run-off accounts for 4.8%. Annual and monthly trends of the stream–aquifer interaction were determined, obtaining on average an annual GW discharge of 34 mm and an annual average recharge of the stream to the aquifer of 1.4 mm. Monthly GW discharges are higher in winter–spring (July to December with an average of 3.3 mm) and lower in summer–autumn (January to June with an average of 2.8 mm). The monthly average recharge of the stream towards the aquifer varies from 0.02 to 0.36 mm and is higher in March, May, and August, when water excess is produced in the basin. Through the analysis of coupled modelling, it is possible to analyse and reproduce the spatio-temporal transitions of flow existing between the stream, the hyporheic zone, and the aquifer.     

Beaver dam analogues drive heterogeneous groundwater–surface water interactions

Beaver dam analogues (BDAs) are a cost-effective stream restoration approach that leverages the recognized environmental benefits of natural beaver dams on channel stability and local hydrology. Although natural beaver dams are known to exert considerable influence on the hydrologic conditions of a stream system by mediating geomorphic processes, nutrient cycling, and groundwater–surface water interactions, the impacts of beaver-derived restoration methods on groundwater–surface water exchange are poorly characterized. To address this deficit, we monitored hyporheic exchange fluxes and streambed porewater biogeochemistry across a sequence of BDAs installed along a central Wyoming stream during the summer of 2019. Streambed fluxes were quantified by heat tracing methods and vertical hydraulic gradients. Biogeochemical activity was evaluated using major ion porewater chemistry and principal component analysis. Vertical fluxes of approximately 1.0 m/day were observed around the BDAs, as was the development of spatially heterogeneous zones of nitrate production, groundwater upwelling, and anaerobic reduction. Strong contrasts in hyporheic zone processes were observed across BDAs of differing sizes. This suggests that structures may function with size-dependent behaviour, only altering groundwater–surface water interactions after a threshold hydraulic step height is exceeded. Patterns of hyporheic exchange and biogeochemical cycling around the studied BDAs resemble those around natural beaver dams, suggesting that BDAs may provide comparable benefits to channel complexity and near-stream function over a 1-year period.     

Modeling groundwater–surface water interactions using the Dupuit approximation

Global errors in head and/or discharge may be introduced when groundwater flow to a stream is modeled using the Dupuit approximation. We consider a simple case of steady groundwater flow in the vertical plane to a horizontal stream bed in direct connection with the aquifer, and compare solutions to the exact problem with Dupuit solutions where common representations of the stream are chosen. In all cases considered, adopting the Dupuit approximation introduces global errors into the mathematical model, and the magnitude of the errors depends on the regional flow conditions. This behavior makes calibration of a model difficult and limits the predictive abilities of the model under conditions of changed regional flow. The global errors and their dependence on flow conditions can be minimized, but not eliminated by treating the resistance of a fictitious leaky stream bed as an effective parameter.We propose an alternate Dupuit model of groundwater–surface water interaction and demonstrate, for the case considered, that adding a second effective parameter allows us to eliminate global errors in head and discharge, and eliminate the dependence of the effective values on the flow field. Explicit expressions are provided to evaluate the two effective properties. We propose that the results be used as a general guideline for modeling groundwater–surface water interaction at streams.     

Integration of surface water with groundwater resources aided by numerical modelling

ABSTRACT

The possibility was studied of a gradual extension of the irrigable area of the Tarlac Irrigation System, located in the alluvial plain of Luzon Island (Philippines). The extension would be made by integrating groundwater with surface water supplied from a reservoir already designed. Aquifer exploitation was studied adopting the criterion that groundwater is a very expensive resource to be turned to only in severe drought years. The objectives of the study were twofold: (a) planning the distribution of the wells (to be drilled during the various stages of the irrigation system development) so as to minimize the pumping cost; (b) establishing a policy for the well management in conjunction with the reservoir operation. Different schemes of the combined system management were analysed on the basis of the climatic and hydrological regime over the period 1950–1972. For this period the monthly water requirements for the different crops and the monthly values of aquifer recharge were computed. The economic analyses were performed using present prices of agricultural products and power together with several different hypotheses about future prices. A finite element model of the semiconfined aquifer was postulated and calibrated; the importance of the return flow from irrigation was also tested. Simulations of exploitation schemes provided a detailed forecast of the aquifer response to the irrigation demands.     

Impact of water transfer on interaction between surface water and groundwater in the lowland area of North China Plain

The contradiction between the freshwater shortage and the large demand of freshwater by irrigation was the key point in cultivated lowland area of North China Plain. Water transfer project brings fresh water from water resource‐rich area to water shortage area, which can in turn change the hydrological cycle in this region. Major ions and stable isotopes were used to study the temporal variations of interaction between surface water and groundwater in a hydrological year after a water transfer event in November 2014. Irrigation canal received transferred Yellow River, with 2.9% loss by evaporation during water transfer process. The effect of transferred water on shallow groundwater decreased with increasing distance from the irrigation canal. Pit pond without water transfer receives groundwater discharge. During dry season after water transfer event, shallow groundwater near the irrigation canal was recharged by lateral seepage and deep percolation of irrigation, whereas shallow groundwater far from irrigation canal was recharged by deep percolation of deep groundwater irrigation. Canal water lost by evaporation was 2.7–17.4%. Influence of water transfer gradually disappeared until March as the water usage of agricultural irrigation increased. In the dry season, groundwater discharged to irrigation canal and pond; 2.2–31.6% canal water and 11.3–20.0% pond water were lost by evaporation. In the rainy season (June to September), surface water was fed mainly by precipitation and surface run‐off, whereas groundwater was recharged by infiltration of precipitation. The two‐end member mix model showed that the mixing ratio of precipitation in pond and irrigation canal were 73–83.4% (except one pond with 28.1%) and 77.3–99.9%, respectively. Transferred water and precipitation were the important recharge sources for shallow groundwater, which decreased groundwater salinity in cultivated lowland area of North China Plain. With the temporary and spatial limitation of water transfer effects, increased water transfer amounts and frequency may be an effective way of mitigating regional water shortage. In addition, reducing the evaporation of surface water is also an important way to increase the utilization of transfer water.     

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.     

Effects of ground water exchange on the hydrology and ecology of surface water

Ground water exchange affects the ecology of surface water by sustaining stream base flow and moderating water-level fluctuations of ground water-fed lakes. It also provides stable-temperature habitats and supplies nutrients and inorganic ions. Ground water input of nutrients can even determine the trophic status of lakes and the distribution of macrophytes. In streams the mixing of ground water and surface water in shallow channel and bankside sediments creates a unique environment called the hyporheic zone, an important component of the lotic ecosystem. Localized areas of high ground water discharge in streams provide thermal refugia for fish. Ground water also provides moisture to riparian vegetation, which in turn supplies organic matter to streams and enhances bank resistance to erosion. As hydrologists and ecologists interact to understand the impact of ground water on aquatic ecology, a new research field called "ecohydrology" is emerging.