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Freezing-induced groundwater-level decline is widely observed in regions with a shallow water table, but many existing studies on freezing-induced groundwater migration do not account for freezing-induced water-level fluctuations. Here, by combining detailed field observations of liquid soil water content and groundwater-level fluctuations at a site in the Ordos Plateau, China, and numerical modeling, we showed that the interaction of soil water and groundwater dynamics was controlled by wintertime atmospheric conditions and topographically driven lateral groundwater inflow. With an initial water table depth of 120 cm and a lateral groundwater inflow rate of 1.03 mmd-1 , the observed freezing and thawing-induced fluctuations of soil water content and groundwater level are well reproduced. By calculating the budget of groundwater, the mean upward flux of freezing-induced groundwater loss is 1.46 mmd-1 for 93 d , while the mean flux of thawing-induced groundwater recharge is as high as 3.94 mmd-1 for 32 d. These results could be useful for local water resources management when encountering seasonally frozen soils and for future studies on two- or three-dimensional transient groundwater flow in semi-arid and seasonally frozen regions. By comparing models under a series of conditions, we found the magnitude of freezing-induced groundwater loss decreases with initial water table depth and increases with the rate of groundwater inflow. We also found a fixed-head lower boundary condition would overestimate freezing-induced groundwater migration when the water table depth is shallow. Therefore, an accurate characterization of freezing-induced water table decline is critical to quantifying the contribution of groundwater to hydrological and ecological processes in cold regions. [ABSTRACT FROM AUTHOR]

A coupled regional and local model is required when groundwater flow and solute transport are to be simulated in local areas of interest with a finer grid while regional aquifer boundary and major stresses should be retained with a coarser grid. The coupled model should also maintain interactions between the regional and local flow systems. In the Beijing Plain (China), assessment of managed aquifer recharge (MAR), groundwater pollution caused by rivers, capture zone of well fields, and land subsidence at the cone of depression requires a coupled regional and local model. This study evaluates three methods for coupling regional and local flow models for simulating MAR in the Chaobai River catchment in the Beijing Plain. These methods are the conventional grid refinement (CGR) method, the local grid refinement (LGR) method and the unstructured grid (USG) method. The assessment included the comparison of the complexity of the coupled model construction, the goodness of fit of the computed and observed groundwater heads, the consistency of regional and local groundwater budgets, and the capture zone of a well filed influenced by the MAR site. The results indicated that the CGR method based on MODFLOW-2005 is the easiest to implement the coupled model, capable of reproducing regional and local groundwater heads and budget, and already coupled with density and viscosity dependent model codes for transport simulation. However, the CGR method inherits shortcomings of finite difference grids to create multiple local models with inefficient computing efforts. The USG method based on MODFLOW-USG has the advantage of creating multi-scale models and is flexible to simulate rivers, wells, irregular boundaries, heterogeneities and the MAR site. However, it is more difficult to construct the coupled models with the unstructured grids, therefore, a good graphic user interface is necessary for efficient model construction. The LGR method based on MODFLOW-LGR can be used to create multiple local models in uniform aquifer systems. So far, little effort has been devoted to upgrade the LGR method for complex aquifer structures and develop the coupled transport models. [ABSTRACT FROM AUTHOR]

Unconfined aquifers beneath piedmont pluvial fans are widely distributed in front of mountains and proper for water supply with pumping wells. However, the catchment zone and capture zones of a pumping well in such an unconfined aquifer is not well known. We develop a preliminary simplified model where groundwater flows between a segmental inflow boundary and a discharge boundary of constant head. The catchment zone is delineated from numerical simulation via MODFLOW and MODPATH. Results are expressed with dimensionless variables and lumped parameters to show general behaviors. Sensitive analyses indicate that there are 4 types of the catchment zone according to different connections to the boundaries. The shape of the catchment zone is quantitatively analyzed with typical shape factors. Capture zones with respect to special travel times are identified from travel time distribution in the catchment zone. The modeling results can be applied in the design of water supply wells and delineation of protection zones at a site with similar hydrogeological conditions. [ABSTRACT FROM AUTHOR]

• Sustainable groundwater development is achievable in Beijing plain. • Substituting groundwater abstraction with transferred surface water has stopped depletion. • Managed aquifer recharge is very effective for sustaining groundwater abstraction. • Climate variation has larger impact on groundwater resources than the anticipated future climate change. To cope with the groundwater depletion problem and achieve sustainable groundwater development, groundwater conservation measures and managed aquifer recharge (MAR) have been implemented worldwide. However, knowledge gaps exit how does the aquifer system respond to these interventions differently and if these interventions are adequate to lead to long-term sustainable groundwater development under future climate change. In Beijing Plain, two measures have been implemented: reduction of groundwater abstraction by substituting groundwater abstraction with transferred surface water and implementation of managed aquifer recharge (MAR) in two major rivers. This study aims to assess how do the shallow and deep aquifers respond to these measures and if these measures can lead to long-term sustainable groundwater development in Beijing Plain under future climate change. A 3-D transient groundwater flow model was calibrated and used to simulate groundwater level and budget changes from 2021 to 2050. The monthly groundwater recharge was estimated using the projected monthly precipitation from three downscaled regional climate models under two scenarios (RCP4.5 and RCP8.5). The results show that declines in groundwater head and storage can be reversed with the combined two measures, thereby contributing to achieve sustainable groundwater development. The reduction of abstractions is a deciding measure to reverse the trend of groundwater depletion, especially in the deep confined aquifers, while large scale MAR schemes can restore the cones of depressions in shallow aquifers and maintain the groundwater abstraction. Climate variation has large impacts on groundwater resources, especially, consecutive dry years can cause rapid groundwater storage depletion. The projected monthly precipitation from 2021 to 2050 is not significantly different from the past. Therefore, the projected future precipitation has minor impacts on groundwater resources in the next 30 years. The findings from the study will support the Beijing municipality to maintain the tight control on groundwater abstraction and to implement large-scale MAR schemes in two rivers. This successful example will encourage managers of other heavily exploited aquifers to take similar measures to achieve sustainable groundwater development. [ABSTRACT FROM AUTHOR]

• Groundwater mounds resulting from nearshore recharge is identified in a sandy hillslope. • The thickness of vadose zone determines fluxes across water table along the hillslope. • Nearshore recharge temporarily changes streamlines and increases tortuosity of flowpaths. • Flowpaths near the wetland are mainly controlled by regional GW discharge and strong ET. Groundwater mounds and depressions induced by rainfall and evapotranspiration near wetlands/lakes have been widely observed, but their effects on groundwater flow field are poorly known. Here, a sandy hillslope near a wetland in the Ordos Plateau, China, which has a semi-arid climate, is selected for investigation. After identifying temporary groundwater mounds/depressions based on three water level observation wells, a HYDRUS-2D model is setup to obtain the highly transient daily flow field. The variable thickness of unsaturated zone near the wetland, which controls soil water content and unsaturated hydraulic conductivity, is found to be responsible for groundwater mounds resulting from focused recharge and groundwater depressions resulting from nonuniform evapotranspiration. On nonrainy days, evapotranspiration in the lowland is supported by flowpaths from regional groundwater discharge, and in the slope is supported by a flow cell from the upland to the upper part of the slope, which causes water table recession in the upland. On rainy days, focused recharge in the lowland leads to two temporary local flow cells, which break the regional groundwater discharge flowpaths into two temporary flow cells. The two temporary flow cells toward the upland contribute to water table rises in the uplandMoreover, we find focused recharge in rainy days does not change the overall direction of flowpaths controlled by regional groundwater discharge and strong evapotranspiration, but only increase the tortuosity of flowpaths. By coupling daily atmospheric conditions and regional groundwater discharge, this study has implications for groundwater recharge estimation and future groundwater modelling in arid regions. [ABSTRACT FROM AUTHOR]

As an indispensable component of groundwater circulation, groundwater evapotranspiration rate ( ET G ) estimation using water table fluctuations is a hot research topic in the past decades. However, most existing methods for estimating ET G using either diurnal or seasonal water table fluctuations are based on the assumptions that groundwater recharge equals 0 in the whole period and ET G equals 0 in a specific duration of time. The aim of the current study is to develop a method to estimate ET G when both assumptions do not apply. In the discharge area of a semi-arid catchment in the Ordos Plateau, NW China, it is found out that the water table fluctuations are influenced greatly by recharge, ET G and the vertical inflow rate ( q in ) in the seasonal scale but are controlled by atmospheric pressure instead of evapotranspiration in the diurnal scale, which implies that the existing methods are not applicable. Therefore, we propose a method to estimate ET G and q in using the seasonal water table fluctuations based on the assumptions that q in is stable throughout the year and ET G has a linear relationship to the reference evapotranspiration ( ET 0 ). It is found out that q in is around 1.12 mm/d, and the ratio of ET G to ET 0 is around 0.4. ET G in April through July with the highest ET 0 of the year is found to be around 3 mm/d, which is much larger than q in , thus leads to significant decline in water table. ET G in November with the lowest ET 0 of the unfrozen period is found to be 1.0 mm/d, which is slightly smaller than q in , thus results in a weak increase in water table. The vertical component of regional groundwater inflow is found to exceed precipitation recharge, indicating that regional groundwater inflow plays the major role on maintaining the shallow water table in the discharge area. Groundwater evapotranspiration is also found to exceed precipitation recharge in most time of the unfrozen period, indicating a specified-flux upper boundary with water loss in the discharge area of an arid catchment. The proposed method could be applied to similar study areas and is critical to understand the budget of groundwater. [ABSTRACT FROM AUTHOR]

• We separate the roles of upward discharge and aquitards in flowing artesian zone. • Basinal flow are coupled with well hydraulics to examine hydraulics of flowing wells. • A low K aquitard causes a large flowing artesian zone but low stable discharge rate. • Baseflow reduction is the main source of groundwater derived in flowing wells. Causes and water sources of flowing artesian wells attracted the interest of many hydrogeologists throughout history, however, a quantitative model that satisfactorily considers the roles of topography, groundwater recharge/discharge and aquitards on hydraulics of flowing wells is still lacking. In this study, a three-layer river-valley basin with a recharge boundary is used to obtain the basinal flow field, spatial distribution and transient discharge rates of flowing wells. Even if there is an aquitard separating the unconfined and the confined aquifers, groundwater discharge to the river still plays a critical role in occurrence of flowing wells. An aquitard with lower permeability would enlarge the zone with flowing wells, implying that flowing wells are more likely to occur in basins with continuous aquitards. By comparing flowpaths and inter-aquifer leakages before and after numerically installing a flowing well in a three-layer basin, we find decreased upward leakage in the discharge limb plays a much larger role than increased downward leakage in the recharge limb in the stable discharge rate of a flowing well. This recognition is different from previous studies where the stable discharge rate is supported only by downward leakage from the overlying aquifer. By considering well hydraulics in a basinal background flow field, this study improves understanding of the mechanisms of flowing artesian wells and the interaction of surface water and deep groundwater, indicating that flowing wells are valuable for sampling deep groundwater representative of deep and old groundwater in the discharge area of a basin. [ABSTRACT FROM AUTHOR]

• Three alternative conceptual models were constructed. • Each model was calibrated automatically and evaluated statistically. • Hydrogeological interpretations are the basis for the conceptualisation. • Modelling objectives determine the complexity of the conceptual models. Three alternative groundwater flow models were evaluated for Beijing Plain, China. The first model (AM1) was constructed with the "thin layer approach" in which all 9 model layers, including five aquifers separated by four aquitards, are continuously present in the same model area. The second model (AM2) was constructed with the "quasi-3D approach" in which the hydrogeological formations were classified into five aquifer units consisting of mixed permeable and semi-permeable layers at different depth ranges. The third model (AM3) was constructed with the "true layer approach" in which aquifers and aquitards were defined according to hydrostratigraphic properties, and model layers are absent in the area where corresponding hydrogeological formations intersect bedrocks. All 3 models were calibrated with the parameter optimization method under the steady state flow condition with the same hydrological stresses and observation data. All three models fit to observations well with the similar calibration criteria values. Furthermore, AIC and BIC information criteria could not distinguish three alternative models. Only KIC could identify AM3 as the best model. Major differences of the three alternative models were identified from a hydrogeological perspective. The AM1 model depicted an illusion through contour maps that groundwater was present everywhere in the deep aquifers. The model computed larger vertical leakages because more abstraction rates were assigned improperly in deep aquifers. The AM2 model was able to compute regional groundwater balances and depicted spatial groundwater level variations. However, the AM2 model computed longer groundwater travel times around the wellfield and should not be used for the delineation of the well field protection zones and contaminant transport simulation. The AM3 model could not only compute the regional groundwater balances and describe spatial groundwater distribution in deep confined aquifers, but also delineate the capture zone of the wellfield. It can therefore be used for simulating contaminant transport. Furthermore, the AM3 model is suitable to construct a coupled regional and local flow model for simulating a managed aquifer recharge scheme. [ABSTRACT FROM AUTHOR]