River Base Flow Research Articles

Synoptic estimates of diffuse groundwater seepage to a spring-fed karst river at high spatial resolution using an automated radon measurement technique

Groundwater (GW) seepage can provide a major source of water, solutes, and contaminants to rivers, but identifying magnitudes, directions and locations of seepage is complicated by its diffuse and heterogeneous distributions. However, such information is necessary to develop programs and policies for protecting ecosystems and managing water resources. Here, we assess GW seepage to the Ichetucknee River, a spring-fed, low gradient, gaining stream in north-central Florida, through automated longitudinal surveys of radon (222Rn) activities at three different flow conditions. A 222Rn mass balance model, which integrates groundwater and spring water end member 222Rn activities and longitudinal 222Rn distributions in river water, shows that diffuse groundwater seepage represents about 16% of the total river baseflow, consistent with previous results obtained from ion (Ca2+, Cl−, SRP and Fe) mass balances and dye tracer methods. During high river stage, the contribution from seepage increases to 18–23% of the river flow. The spatial distribution of GW seepage is more variable in the upper 2.2-km reach of the river than the lower 2.8-km reach, regardless of river flow conditions. The upper reach has a narrower flood plain than the lower reach, which limits evapotranspiration and increases hydraulic gradients toward the river following storm events. Seepage in the lower reach is also limited by hydrologic damming by the receiving river, which inundates the floodplain during high flow conditions, and reduces the hydraulic head gradient. These results demonstrate the variable nature of seepage to a gaining river in both time and space and indicate that multiple synoptic analyses of GW seepage are required to assess seepage rates, determine time-averaged solute fluxes, and develop optimal management policies for riverine ecosystems.

Impact of climate variability and streamflow alteration on groundwater contribution to the base flow of the Lower Zab River (Iran and Iraq)

Overall water resources evaluations including groundwater contributions to river flow are critical for assessing climate change and drought impacts on basin hydrological responses. By utilising precipitation, daily stream flow and simulated river discharge alteration data, this study introduces a simple but comprehensive methodology to better understand the potential impact of river regulation coupled with climate variability and drought on groundwater involvement. The Lower Zab River basin in northern Iraq has been selected as a representative case study to demonstrate the new methodology. Three of the most commonly used base flow separation methods were assessed: Eckhardt algorithm, flow duration curve and Chapman filtering algorithm. In addition, the indicators of hydrologic alteration method and the reconnaissance drought index have been applied. The results demonstrated that some of the underground water responds to precipitation events. A noticeable increase in groundwater contribution has been observed during the hydrological years between 1998–2001 and 2006–2008 as a result of a sharp decline in the average precipitation. However, the opposite has been observed during the hydrological year 1987, which impacted negatively on the basin water resources availability. The reservoir release through the dry periods causes the observed variations in base flow index (BFI) values between the pre- and post-river damming time periods. Considering the BFI long-term seasonal variation, index values started to rise in April and reached their maximum by the end of June. A steady decline has been recorded between August and September.

Microhabitat patterns of soft-bodied benthic algae in a lowland river largely fed by groundwater

In lowland rivers largely fed by groundwater, the aquifer resurgence plays a major role in supporting local plant diversity. However, large knowledge gaps exist with regard the role of groundwater on algal diversity and spatial arrangement. In this context, our main aim was to evaluate the distributive gradients of soft-bodied benthic algae across a mid-size lowland river with a bottom dominated by hard substrate (Oglio River, north Italy). Along the investigated river sector, a clear segregation both for taxa and for environmental data was observed, especially in terms of pH, hydrological variables and nutrient availability. Overall, we recorded an important contribution of groundwater supply in shaping the structure and complexity of algal communities at the microhabitat level (in the meter range) during the river base flow. All these considerations shed new light on the response of algal taxa to a relevant groundwater ingression in a lowland river.

Groundwater and Surface Water Availability via a Joint Simulation with a Double Control of Water Quantity and Ecologically Ideal Shallow Groundwater Depth: A Case Study on the Sanjiang Plain, Northeast China

Joint assessment of groundwater-surface water resources can help develop sustainable regional water management plans for intensive agriculture. In this study, we estimated allowable groundwater and surface water quantities using a water balance model, WetSpass-GMS, for the Sanjiang Plain (10.9 × 104 km2), one of the most important grain production bases in China. We then applied a double control based on the groundwater availability and the concept of an ecologically ideal shallow groundwater depth (EISGD) to three different water use scenarios: (A) continuation of the current water use management; (B) maximal use of water resources under a double control; and (C) irrigation of 266.7 × 104 hectares that are suitable for rice cultivation. We found an annual allowable surface water quantity of 4.71 billion cubic meters for the region and an annual exploitable groundwater quantity of 4.65 billion cubic meters under full consideration of water requirements, i.e., sustaining river base flow, necessary riverine sediment transport, and ecological water supplies for wetlands and reservoirs. Our simulation results showed that for Scenario A, groundwater level in the region would continue falling, and that the groundwater levels in wet, normal and dry years would drop below the EISGD level in 2028, 2023 and 2019, respectively. For Scenario B, groundwater and surface water would be able to support rice paddies of 219.7 × 104 hectares, 212.7 × 104 hectares, and 209.3 × 104 hectares during wet, normal and dry years, respectively. For Scenario C, future demands on groundwater and surface water under wet, dry and normal years would all exceed their allowable supplies. Overall, this study indicates that integrated management plans promoting an increase of surface water use and a reduction in irrigation with groundwater should be developed for sustainable agriculture and ecological preservation on the Sanjiang Plain.

Ecology-oriented groundwater resource assessment in the Tuwei River watershed, Shaanxi Province, China

In arid and semi-arid regions, a close relationship exists between groundwater and supergene eco-environmental issues such as swampiness, soil salinization, desertification, vegetation degradation, reduction of stream base flow, and disappearance of lakes and wetlands. When the maximum allowable withdrawal of groundwater (AWG) is assessed, an ecology-oriented regional groundwater resource assessment (RGRA) method should be used. In this study, a hierarchical assessment index system of the supergene eco-environment was established based on field survey data and analysis of the supergene eco-environment factors influenced by groundwater in the Tuwei River watershed, Shaanxi Province, China. The assessment system comprised 11 indices, including geomorphological type, lithology and structure of the vadose zone, depth of the water table (DWT), total dissolved solids content of groundwater, etc. Weights for all indices were calculated using an analytical hierarchy process. Then, the current eco-environmental conditions were assessed using fuzzy comprehensive evaluation (FCE). Under the imposed constraints, and using both the assessment results on the current eco-environment situation and the ecological constraint of DWT (1.5–5.0 m), the maximum AWG (0.408 × 108 m3/a or 24.29 % of the river base flow) was determined. This was achieved by combining the groundwater resource assessment with the supergene eco-environmental assessment based on FCE. If the maximum AWG is exceeded in a watershed, the eco-environment will gradually deteriorate and produce negative environmental effects. The ecology-oriented maximum AWG can be determined by the ecology-oriented RGRA method, and thus sustainable groundwater use in similar watersheds in other arid and semi-arid regions can be achieved.

Trapping and episodic flushing of suspended sediment from a tidal river

Recent studies suggest that tidal forcing can be as important as gravitational circulation in maintaining an estuarine turbidity maximum (ETM). It is further postulated that a long-term mass balance between the import and export of sediment in an estuary may require episodic large river discharges or ‘freshets’ to flush sediment out of the ETM towards the open sea. In this study, we use a 2-month data set from a mooring in a tidal river that drains into a large drowned-river-valley estuary (Kaipara Harbour, New Zealand) to investigate interactions between tidal-current asymmetry and gravitational circulation. During baseflow river discharge and on spring tides, suspended-sediment transport was directed up-channel (landwards), driven by tidal pumping due to tidal-current asymmetry. During neap tides, the suspended-sediment flux was approximately zero. The data suggest that the bed was not locally erodible and that bed sediments at the site were being supplied by an ETM. The ETM only migrated far enough down channel to be observed at the mooring site during spring tides when the tidal excursion was longer. Suspended sediments were effectively trapped and recycled within the ETM. During and after two freshets, high river discharge displaced saline water from the tidal river, water-column stratification strengthened and the surface and bed current speeds increased. As a result, the advective component of the down-channel directed suspended-sediment flux increased. This provided a transport pathway for sediment out of the otherwise tidally pumped, flood-dominant system. We conclude that largest export of sediments out of the tidal river would potentially occur when a large freshet coincides with an apogean spring tide.

Comment on “Climate and agricultural land use change impacts on streamflow in the upper midwestern United States” by Satish C. Gupta et al.

AbstractIncreasing precipitation and land use/land cover (LU/LC) change have contributed to increasing streamflow and base flow in many Midwestern rivers but the relative importance of causal factors is open to debate. The dominant LULC change in the agricultural Midwest is the emergence of soybean production that occurred in the mid‐ to late‐20th Century that replaced many sod‐based rotations and increased total row crop area devoted to annual maize and soybean crops. Increasing precipitation may be a more important factor for increasing total discharge whereas LULC changes contributed more to base flow changes.

Identifying groundwater recharge connections in the Moscow (USA) sub-basin using isotopic tracers and a soil moisture routing model

Globally, aquifers are suffering from large abstractions resulting in groundwater level declines. These declines can be caused by excessive abstraction for drinking water, irrigation purposes or industrial use. Basaltic aquifers also face these conflicts. A large flood basalt area (1.1 × 105 km2) can be found in the Northwest of the USA. This Columbia River Basalt Group (CRBG) consists of a thick series of basalt flows of Miocene age. The two major hydrogeological units (Wanapum and Grand Ronde formations) are widely used for water abstraction. The mean decline over recent decades has been 0.6 m year−1. At present day, abstraction wells are drying up, and base flow of rivers is reduced. At the eastern part of CRBG, the Moscow sub-basin on the Idaho/Washington State border can be found. Although a thick poorly permeable clay layer exists on top of the basalt aquifer, groundwater level dynamics suggest that groundwater recharge occurs at certain locations. A set of wells and springs has been monitored bi-weekly for 9 months for δ18O and δ2H. Large isotopic fluctuations and d-excess values close to the meteoric water line in some wells are indicating that recharge occurs at the granite/basalt interface through lateral flow paths in and below the clay. A soil moisture routing (SMR) model showed that most recharge occurs on the granitic mountains. The basaltic aquifer receives recharge from these sedimentary zones around the granite/basalt interface. The identification of these types of areas is of major importance for future managed-aquifer recharge solutions to solve problems of groundwater depletion.

Occurrence and flow of groundwater in crystalline rocks of Sardinia and Calabria (Italy): an overview of current knowledge

Crystalline rocks constitute aquifers that are of interest for water supply in many countries. In Italy, the hydrogeological characteristics of crystalline rocks are not well known and the interest in these aquifers is generally scarce. Nevertheless, the large extent of these rocks in Sardinia and Calabria regions and the local scarcity of water resources justify this preliminary analysis. Crossing the information derived by the available data on climate conditions, type of crystalline rock, structural setting, characteristics of wells and springs, base flow of streams and rivers and pumping test results, a preliminary quantification of the potential yield of the crystalline aquifers of both regions was performed. The processing of data involving different aspects confirmed that the crystalline rocks of Sardinia and Calabria form low-permeability aquifers where groundwater circulation occurs in the weathered layer and, mainly, in the most fissured layers closest to the surface. In the heavily fractured zones, it was seen that the groundwater flow is more active and can also occur at a greater depth. Groundwater mainly outflows in the valleys, feeding streams, numerous low discharge springs and surrounding aquifers. Climate, rock covers and tectonic style influence the aquifer yield, which has been evaluated herein to range from 1 to 2 L/s per km2 and from 2 to 12 L/s per km2 in Sardinia and Calabria, respectively. This preliminary characterization shows that there are reasons to deepen the present knowledge on these aquifers in order to optimize the present use of their groundwater resources

Long-term climate sensitivity of an integrated water supply system: The role of irrigation

The assessment of the impact of long-term climate variability on water supply systems depends not only on possible variations of the resources availability, but also on the variation of the demand. In this framework, a robust estimation of direct (climate induced) and indirect (anthropogenically induced) effects of climate change is mandatory to design mitigation measures, especially in those regions of the planet where the groundwater equilibrium is strongly perturbed by exploitations for irrigation purposes. The main goal of this contribution is to propose a comprehensive model that integrates distributed crop water requirements with surface and groundwater mass balance, able to consider management rules of the water supply system. The proposed overall model, implemented, calibrated and validated for the case study of the Fortore water supply system (Apulia region, South Italy), permits to simulate the conjunctive use of the water from a surface artificial reservoir and from groundwater. The relative contributions of groundwater recharges and withdrawals to the aquifer stress have been evaluated under different climate perturbations, with emphasis on irrigation practices. Results point out that irrigated agriculture primarily affects groundwater discharge, indicating that ecosystem services connected to river base flow are particularly exposed to climate variation in irrigated areas. Moreover, findings show that the recharge both to surface and to groundwater is mainly affected by drier climate conditions, while hotter conditions have a major impact on the water demand. The non-linearity arising from combined drier and hotter conditions may exacerbate the aquifer stress by exposing it to massive sea-water intrusion.

Effects of wildfire on runoff generating processes in northern Mongolia

Seasonal runoff generation in a headwater (~100 km2) of the mountainous Sugnugr Basin, situated within the discontinuous permafrost zone of northern Mongolia, was investigated. The role of the active layer and the effects of a recent wildfire on runoff generation were elucidated. On the catchment scale, stormflow events were investigated by streamflow hydrograph analysis and stable isotope investigations. Hydrological response parameters indicate increasing recession periods and delayed flow fractions during summer as active layer thickness increases. Isotopic signature revealed a high ratio of meltwater in river baseflow in early summer and a gradual decline until the beginning of September. During a stormflow event in early summer, a high fraction of event water was determined by two-component hydrograph separation. At a taiga-vegetated hillslope, subsurface flow was observed following precipitation events after a relative increase in upper soil volumetric water content (VWC i ) of 0.05 m3 m−3, demonstrating the retarding characteristics of the organic surface layer. Depending on active layer depth, subsurface flow predominantly occurred in the organic surface layer in early summer and subsequently subsided into the underlying mineral layer as soils thawed. After a wildfire, the water retention capacity of the organic surface layer diminished, as was reflected by a VWC i of 0.01 m3 m−3. Here, stormflow water temperatures indicated quickflow in the mineral layer above a degenerated frost table. The results of the study suggest that wildfires increase stormflow runoff while baseflow declines. Hence, the problem of water shortages in downstream areas during dry weather conditions is exacerbated.

A long‐term study of stable isotopes as tracers of processes governing water flow and quality in a lowland river basin: the upper Thames, UK

AbstractA long‐term study of O, H and C stable isotopes has been undertaken on river waters across the 7000‐km2 upper Thames lowland river basin in the southern UK. During the period, flow conditions ranged from drought to flood. A 10‐year monthly record (2003–2012) of the main River Thames showed a maximum variation of 3‰ (δ18O) and 20‰ (δ2H), although interannual average values varied little around a mean of –6.5‰ (δ18O) and –44‰ (δ2H). A δ2H/δ18O slope of 5.3 suggested a degree of evaporative enrichment, consistent with derivation from local rainfall with a weighted mean of –7.2‰ (δ18O) and –48‰ (δ2H) for the period. A tendency towards isotopic depletion of the river with increasing flow rate was noted, but at very high flows (>100 m3/s), a reversion to the mean was interpreted as the displacement of bank storage by rising groundwater levels (corroborated by measurements of specific electrical conductivity). A shorter quarterly study (October 2011–April 2013) of isotope variations in 15 tributaries with varying geology revealed different responses to evaporation, with a well‐correlated inverse relationship between Δ18O and baseflow index for most of the rivers. A comparison with aquifer waters in the basin showed that even at low flow, rivers rarely consist solely of isotopically unmodified groundwater.Long‐term monitoring (2003–2007) of carbon stable isotopes in dissolved inorganic carbon (DIC) in the Thames revealed a complex interplay between respiration, photosynthesis and evasion, but with a mean interannual δ13C‐DIC value of –14.8 ± 0.5‰, exchange with atmospheric carbon could be ruled out. Quarterly monitoring of the tributaries (October 2011–April 2013) indicated that in addition to the aforementioned factors, river flow variations and catchment characteristics were likely to affect δ13C‐DIC. Comparison with basin groundwaters of different alkalinity and δ13C‐DIC values showed that the origin of river baseflow is usually obscured.The findings show that long‐term monitoring of environmental tracers can help to improve the understanding of how lowland river catchments function. Copyright © NERC 2015. Hydrological Processes © 2015 John Wiley & Sons, Ltd.

The Hydrogeological Model of Latvia LAMO4 as a Tool for Investigating the Processes of Nature. Sources of Groundwater Inflow for the Iecava River

In 2010–2012, the hydrogeological model (HM) of Latvia (LAMO) was established by the scientists of Riga Technical University (RTU). In 2013–2014, LAMO was upgraded. In 2015, the recent version LAMO4 was developed. In LAMO4, the plane approximation step was decreased from 500 meters to 250 meters. The base flows of rivers for HM were adjusted by accounting for data that were provided by the measurements of river streams. LAMO is run by the commercial program Groundwater Vistas (GV), version 6. It contains the MODFOW and MODPATH programs for supporting HM and for tracking groundwater particles, respectively. By using MODPATH, the case of the Iecava river was investigated in order to find the sources that support groundwater inflow into the river (its base flow). Unforeseen results were obtained. Due to the appliance of the large regional HM, it was discovered that some part of the river base flow comes from the areas that are located very far from the river drainage basin. It also was not expected that even within the basin the particle traces were very complex. The results described in the paper serve as demonstration for the usefulness of applying a large complex HM for investigating the processes of nature.

Spatial-temporal Evolution of River Baseflow in Northern Shaanxi

Spatial-temporal Evolution of River Baseflow in Northern Shaanxi

Long-term trends in groundwater recharge and discharge in a fractured bedrock aquifer – past and future conditions

In the context of climate change, it is important to understand possible future projections and historical trends of groundwater recharge, flow, and discharge to surface reservoirs. Knowledge of a vast range of possible conditions is required to fully appreciate the variability of the hydrologic cycle and hence the long-term vulnerability of groundwater-dependent habitats. This research investigates historical trends for a groundwater–surface water interacting system that supports a fragile ecosystem in southern Quebec. A transient model was developed using MODFLOW to simulate site-wide groundwater flow for the study area. The model was used to simulate past hydrogeological conditions (1900–2010) using a new data set of available precipitation (rain and snowmelt) and temperature. This data set was used to simulate the overall groundwater budget and to determine groundwater discharge (river baseflow and spring flow) in the study area. This allows for the quantification of century-long trends in flow data, as well as the extreme maximum and minimum flows over 110 years. Recharge was variable, ranging from 41 to 197 mm/year over the study period. Lower recharge rates from 1950 to 1965 induced marked effects on spring flow. Although the trend is not statistically significant, there appears to be, for the second half of the study period (1966–2010), a tendency towards a reversal to an increase for recharge, hydraulic heads, spring flow and baseflows. A longer time series would be necessary to confirm this tendency. The simulated historical trends are compared with flow projections for future scenarios (2041–2070). The confirmation that the natural system has been subjected to a wide range of climatic conditions over the last century helps to inform about its resilience. This study highlights the utility of groundwater flow modeling using historical climate data sets to gain a better understanding of long-term trends for climate change-related hydrogeological and ecohydrological studies.

Geologic and Geomorphic Controls on the Occurrence of Fens in the Oregon Cascades and Implications for Vulnerability and Conservation

Montane fens are biologically diverse peat-forming wetlands that develop at points of groundwater discharge. To protect these ecosystems, it is critical to understand their locations on the landscape and the hydrogeologic systems that support them. The upper Deschutes Basin has a groundwater flow system that supports baseflow in many rivers, but little is known about the wetland types and groundwater dependence of the thousands of wetlands within the watershed. In 292 randomly selected wetlands, we quantified landscape metrics thought useful for discriminating montane fens from non-peat-forming wetlands. We inspected these wetlands and classified 67 of them as fens. Of the landscape metrics, only geology reliably differentiated fens from other types of wetlands. Nearly all fens develop in low-permeability glacial till found at approximately 1400–1800 m in elevation, and are concentrated in areas mantled by pumice deposits that originated primarily from the eruption of Mt. Mazama approximately 7700 years BP. Stratigraphic and hydrologic factors indicate the fens are supplied by perched aquifers in glacial till, instead of the deeper regional aquifer system. Their hydrogeologic setting makes the fens highly vulnerable to expected changes to recharge associated with climate change, but not to groundwater pumping from the regional aquifer.

Propose An Algorithm To Improve The Accuracy of Snow Covered Mapping Using MODIS Images

Recognition and study of snow reservoirs as the supplier of the base flow of rivers and main outset of freshwater resources in snowy and high basins play an important role in planning and management of water resources usage. However, one of the main problems in snow phenomenon recognition using optical satellite images is to separate clouds and snow. To supper this problem, we use the fact that the cloud does not have a stable geolocation compared to snow. A temporal filter is designed by the combination of Modis Terra and Aqua to remove the cloud pixels. Moreover, different spectral behavior of the cloud in different wavelengths makes it possible to separate it from the snow. A normalized difference cloud index is defined using Modis data to detect and remove the cloud pixels from the image. The pixel-based method is used to extract the snow coverage map of the Northen area of the Fars province using the daily Modis data spanning between 1392 and 1393. In order to evaluate the final results, the data from 14 ground stations as well as Landsat8 OLI image are used as ground truth. The accuracy of 100% was achieved using the first method while the accuracy of the second method by corresponding the pixels of snow coverage maps is estimated as 98. 58%. According to the results and accomplished evaluations, the snow maps generated using the threshold-based method without or with the cloud coverage removed by the application of the proposed method has a high precision. The results can then be easily used in the snowmelt run-off modeling in the water resource and reservoirs management.

Relationship between Sediment Chemistry, Equilibrium Phosphorus Concentrations, and Phosphorus Concentrations at Baseflow in Rivers of the New Zealand National River Water Quality Network.

Stream sediments can act as a source or a sink of dissolved (filtered) phosphorus (P) via abiotic and biotic processes. The cumulative action and magnitude of abiotic processes has been quantified by the equilibrium P concentration at zero net sorption or desorption (EPC). The EPC was determined in 76 large rivers of contrasting climate, topography, and geology across New Zealand. Measurements of EPC (0.004-0.065 mg L) indicated sediments were acting as a source of filtered reactive P (FRP) to the water column. The EPC was related to the proportion of intensive agriculture in the catchment, the concentration of readily available P in the sediment, sediment size, and catchment slope and elevation. Determination of EPC will yield a relative assessment of the sediment's ability to supply P to the water column especially at baseflow. Furthermore, the EPC may be less prone to short-term variation (e.g., diurnal patterns) compared with grab samples. This information will help target efforts to mitigate FRP concentrations in rivers by managing sediment inputs. Additional work is required to determine, for instance, how long an EPC measurement remains valid before new sediment is deposited or existing sediment is scoured.

Groundwater as an emergency source for drought mitigation in the Crocodile River catchment, South Africa

Abstract. Global climate change has received much attention worldwide in the scientific as well as in the political community, indicating that changes in precipitation, extreme droughts and floods may increasingly threaten many regions. Drought is a natural phenomenon that causes social, economical and environmental damage to society. In this study, we assess the drought intensity and severity and the groundwater potential to be used as a supplementary source of water to mitigate drought impacts in the Crocodile River catchment, a water-stressed sub-catchment of the Incomati River catchment in South Africa. The research methodology consists of three parts. First, the spatial and temporal variation of the meteorological and hydrological drought severity and intensity over the catchment were evaluated. The Standardized Precipitation Index (SPI) was used to analyse the meteorological drought and the Standardized Runoff Index (SRI) was used for the hydrological drought. Second, the water deficit in the catchment during the drought period was computed using a simple water balance method. Finally, a groundwater model was constructed in order to assess the feasibility of using groundwater as an emergency source for drought impact mitigation. Results show that the low-rainfall areas are more vulnerable to severe meteorological droughts (lower and upper crocodile). Moreover, the most water stressed sub-catchments with high level of water uses but limited storage, such as the Kaap located in the middle catchment and the Lower Crocodile sub-catchments, are more vulnerable to severe hydrological droughts. The analysis of the potential groundwater use during droughts showed that a deficit of 97 Mm3 yr−1 could be supplied from groundwater without considerable adverse impacts on the river base flow and groundwater storage. Abstraction simulations for different scenarios of extremely severe droughts reveal that it is possible to use groundwater to cope with the droughts in the catchment. However, local groundwater exploitation in Nelspruit and White River sub-catchment will cause large drawdowns (> 10 m) and high base flow reduction (> 20%). This case study shows that conjunctive water management of groundwater and surface water resources is necessary to mitigate the impacts of droughts.

A high-resolution global-scale groundwater model

Abstract. Groundwater is the world's largest accessible source of fresh water. It plays a vital role in satisfying basic needs for drinking water, agriculture and industrial activities. During times of drought groundwater sustains baseflow to rivers and wetlands, thereby supporting ecosystems. Most global-scale hydrological models (GHMs) do not include a groundwater flow component, mainly due to lack of geohydrological data at the global scale. For the simulation of lateral flow and groundwater head dynamics, a realistic physical representation of the groundwater system is needed, especially for GHMs that run at finer resolutions. In this study we present a global-scale groundwater model (run at 6' resolution) using MODFLOW to construct an equilibrium water table at its natural state as the result of long-term climatic forcing. The used aquifer schematization and properties are based on available global data sets of lithology and transmissivities combined with the estimated thickness of an upper, unconfined aquifer. This model is forced with outputs from the land-surface PCRaster Global Water Balance (PCR-GLOBWB) model, specifically net recharge and surface water levels. A sensitivity analysis, in which the model was run with various parameter settings, showed that variation in saturated conductivity has the largest impact on the groundwater levels simulated. Validation with observed groundwater heads showed that groundwater heads are reasonably well simulated for many regions of the world, especially for sediment basins (R2 = 0.95). The simulated regional-scale groundwater patterns and flow paths demonstrate the relevance of lateral groundwater flow in GHMs. Inter-basin groundwater flows can be a significant part of a basin's water budget and help to sustain river baseflows, especially during droughts. Also, water availability of larger aquifer systems can be positively affected by additional recharge from inter-basin groundwater flows.