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• Dual source λET model was improved for film mulched cropland with shallow groundwater. • Ignoring film mulching leads to overestimation of λE and then λET. • Responses of the resistances in SW model to λET and its components were quantified. • Average T/ET during maize growing season was as high as 0.78–0.84. • Path analysis revealed the dominated effects of Hc and WTD on maize λET. Evapotranspiration (ET) is a key link in terrestrial water cycle and influenced by complex environmental factors in cropland. Accurate assessment of crop ET over heterogeneous underlying surfaces remains challenging. In this study, the dual source Shuttleworth-Wallace (SW) model was modified to estimate λET (latent heat corresponding to ET) through incorporating the effects of film mulching and shallow groundwater. λET and latent heat from transpiration (λT) were measured in a maize field in northwest China during 2017 and 2018 to assess the performance of the original (SW) and modified (SWM) models. Results indicate that the overestimation of latent heat from soil evaporation (λE) due to ignoring film mulching could be obviously mitigated by SWM. Besides, considering capillary rise from shallow groundwater in SWM improved not only λET estimates but also the accuracy of canopy resistance parameter. The SWM model outperformed the SW model that the slopes of fitting lines for λET and λT increased from 0.60 to 0.79 to 0.77–0.99 at daily scale and Nash-Sutcliffe Efficiency coefficient exceeded 0.86. Relative error and RMSE were decreased to below 0.15 and 19.88 W m−2, respectively. The estimations of λET and λT by SWM were also improved at hourly scale. Seasonal variation and partition of λET between two years were comparable and average T/ET for the entire growing season was about 78 %-84 %, which was largest at the heading stage. Path analysis on the responses of λET to the environmental factors recognized the greatest direct effect of crop height or leaf area index, and identified the significantly direct control of shallow groundwater on λT in this area. The study presented an improved dual source model to better quantify λET, which would benefit water resources management for film mulched croplands with shallow groundwater. [ABSTRACT FROM AUTHOR]

• Rainfall classification aided the understanding of deep soil moisture (DSM) recharge. • Groundwater overwhelmed the role of direct rainfall in recharging DSM. • A four-stage relationship between DSM and groundwater table was identified. • A conceptual model for DSM recharge was proposed. Deep soil moisture (DSM) plays a critical role in vadose zone hydrological processes. However, the recharge processes and their underlying mechanisms remain unclear due to the scarcity of comprehensive deep hydrological data, especially in regions characterized by active interactions between surface and subsurface hydrological processes. To address this knowledge gap, we performed an event-based study using a four-year hydrological monitoring dataset, encompassing rainfall, profile soil moisture, and groundwater along a hillslope at the Red Soil Critical Zone Observatory (CZO) in China. A total of 226 rainfall events were classified into four distinct types (I-IV) using a k -means clustering algorithm. The changes in DSM and groundwater table (GWT) were dependent on the rainfall type. The GWT (0.57–0.97) played a greater role in contributing to the DSM than the direct rainfall (0.48–0.64) based on a structural equation model, which was more pronounced from upper slope to foot slope. DSM recharge pattern can be described by a four-stage conceptual model, as indicated by the relationship between DSM and GWT: a slow and linear increase (stage 1), a small plateau (stage 2), a rapid linear rise (stage 3), and finally a great plateau (stage 4). DSM at the small and great plateau were closely related with the field capacity and saturation, respectively. Rainfall type, vadose zone features, and soil physical properties jointly shaped DSM recharge patterns. These findings can offer valuable insights into understanding the deep hydrological processes of the red soil region as well as regions with similar hydroclimatic conditions. [ABSTRACT FROM AUTHOR]

Highlights • A modified SWAT model is used to balance the pumping intensity against crop yield. • Impacts of irrigation on crop groundwater productivity are identified by simulation. • Effects of the limited exploitation are assessed by water balance and crop yield. • Irrigation schedule is optimized with groundwater-constraint to minimize yield loss. • Trade-off between groundwater depletion and crop yield reduction is explored. Abstract Groundwater overdraft due to extensive irrigation has led to a shallow aquifer depletion crisis in the Haihe River basin. Quantitative simulation of the variations in shallow groundwater and crop yield under different conditions of limited exploitation using a distributed hydrological model is important in the well-irrigated plain of this basin. Based on multiple modeling experiments by a modified Soil and Water Assessment Tool (SWAT) model, three limited irrigation schemes for winter wheat (Triticum aestivum L.) were selected as simulation scenarios. The simulated results using the SWAT model showed that under scenario 1 (applying two rounds of irrigation corresponding to the jointing and heading stages of winter wheat), the average rate of decline in the shallow groundwater table was approximately 2/3 of that under the basic scenario (current irrigation schedule), but the winter wheat yield decreased by 13% compared with the basic scenario. Under scenario 2 (applying one round of irrigation at the jointing stage of winter wheat), the average rate of decline in the shallow groundwater table was approximately 1/4 of that under the basic scenario, but the reduction in the winter wheat yield compared with the basic scenario increased to 28%. The amount of overexploited shallow groundwater in the cropland area decreased from 17.5 × 108 m3 a−1 (under the basic scenario) to 11.0 × 108 m3 a−1 under scenario 1 and 4.5 × 108 m3 a−1 under scenario 2. Under scenario 3 (rain-fed conditions during the winter wheat season), the regional variation in the shallow groundwater table shifted to a recovery trend with an average rate of 0.22 m a−1, which was equivalent to restoring 3.5 × 108 m3 a−1 of shallow aquifer storage in the cropland area. However, the reduction in the winter wheat yield compared with the basic scenario reached 54% under the rain-fed scheme. Considering the trade-off between groundwater conservation and crop production under limited exploitation, linear programming was used to optimize the irrigation schedule at the subbasin scale. As a result, to satisfy the constraint of stopping groundwater drawdown, the average minimal reduction in the winter wheat yield would be 42% under the optimal irrigation schedule. By contrast, to satisfy the constraint of restricting the reduction in the crop yield to within the threshold required to maintain winter wheat self-sufficiency, the minimal rate of decline in the shallow groundwater table would be 0.26–0.52 m a−1 under the optimal irrigation schedule. In addition, the uncertainty in the simulated shallow groundwater variation in this study was acceptable, indicating that the above assessments could provide reasonable references for regional groundwater management. [ABSTRACT FROM AUTHOR]

Highlights • The SWMM is improved to incorporate groundwater feedbacks into LID simulations. • The SWMM-LID-GW is calibrated, validated and compared with the SWMM. • Shallower groundwater results in less exfiltration and more groundwater drainage. • Rainfall and in-situ soil types show less impact on LID in shallow groundwater. • The SWMM-LID-GW outperforms the SWMM for a wide range of groundwater conditions. Abstract Low impact development (LID) practices are effective in restoring natural hydrological processes, but they may be affected by groundwater and pose additional risk to groundwater dynamics and contamination in shallow groundwater conditions. Their performance in shallow groundwater is not well understood due to monitoring difficulty and the deficiency of modeling tools. The Storm Water Management Model (SWMM) is an urban hydrology-hydraulic model that has been widely used to evaluate the performance of LID practices. However, it cannot accurately simulate the hydrological processes of LID practices in shallow groundwater. This study presents the development, calibration, validation, and testing of a modified SWMM (SWMM-LID-GW) that incorporates groundwater feedbacks into LID simulations. The hydrological performance of LID practices under different environmental conditions was simulated, based on which the impacts of initial groundwater table depth, rainfall type, and in-situ soil type were evaluated, and a number of general nonlinear multivariate formulas were proposed to predict the hydrologic performance of LID practices in shallow groundwater environment and support the feasibility analysis of LID practices before project implementation. The reduction of exfiltration and increase of groundwater drainage in shallow groundwater accounted for a significant proportion of the water budget of LID practices. The influence of shallow groundwater was greater for shallower groundwater tables, in less permeable in-situ soils, and for events of higher intensities and longer durations. However, due to the dominant role of shallow groundwater, the impacts of both rainfall type and in-situ soil type on the hydrological performance of LID practices in shallow groundwater were minimal in general, in contrast to the impacts on deep groundwater tables. The SWMM-LID-GW outperformed the current SWMM for a wide range of groundwater conditions, particularly for locations in which the groundwater table frequently rises to 1.0 m or less below ground. [ABSTRACT FROM AUTHOR]

[Display omitted] • Groundwater NO 3 −-N is well predicted using few easily accessible predictors and small datasets. • Electric conductivity, nitrogen fertilization and precipitation are selected as accessible predictors of groundwater NO 3 −-N. • A parsimonious prediction model with good performance is achieved with a small dataset. • Model balances the accuracy of data on intrinsic factors and the accessibility of data on extrinsic factors. Groundwater nitrate poses a health risk to humans, and monitoring groundwater quality is time-consuming and expensive. To improve the efficiency and reduce the costs of monitoring groundwater nitrate, a parsimonious model was developed using easily accessible predictors and small datasets based on random forest (RF), neural network (NN), and support vector machine (SVM) algorithms. Groundwater NO 3 −-N concentrations of 1196 groundwater samples from intensive agricultural areas along with data on 13 predictors were obtained for the construction of prediction models. R2 and RMSE metrics were used to evaluate the models, with an emphasis on a minimal number of predictors and a low data size. The results showed that even with few readily available predictors (conductivity, EC; chemical nitrogen fertilization rate, NF; and precipitation, PPT) and a small dataset (260–700 observations), the three models achieved good precision, with R2 and RMSE values of 0.70–0.80 and 6.87–8.13, respectively. In these prediction models, EC was the primary predictor acting as an intrinsic factor, NF acted as a loading factor to supplement the blind area of EC for NO 3 −-N prediction, and PPT was introduced because of its accessibility to improve predictive performance. The final predictors applied in this study link the two currently dominant prediction methods by combining the high predictive accuracy of the intrinsic predictor with the accessibility of the extrinsic predictor, and the results demonstrate the practicability and applicability advantages of this model in groundwater NO 3 −-N prediction. This model provides technical support for the rapid and low-cost realization of the online monitoring of NO 3 −-N pollution in shallow groundwater in intensive agricultural areas. [ABSTRACT FROM AUTHOR]

• Reduced natural rainfall led to a downgrade of soil moisture and groundwater. • Soil water movements were affected by land use and rainfall events. • Shallow groundwater variations depicted ecological water utilization in check dams. • Processes from rainfall to shallow groundwater were constructed in check dams. Stable regional water resource distribution is essential for ecosystems, yet soil water dynamics and groundwater evolutions of check dams remain many uncertainties under natural rainfall reduction in semi-arid areas. To explore these changeable hydrologic processes, continuous fixed-point observation and water balance models were introduced in nearly-four consecutive hydrological years (2018–2021). The results showed that soil water storage presented a downward fluctuation in each soil layer of all land uses. Moreover, the grassland profile moisture descended slowly and had deeper infiltration depths than others, which might be an ideal land-use type for the reservoir's sustainable utilization in check dams. The results also indicated that surface soil moisture (<50 cm) could rapidly respond to a single rainfall event > 10 mm d−1, whereas deep soil moisture (>50 cm) and shallow groundwater fluctuations were mainly affected by annual cumulative precipitation. Furthermore, a conceptual hydrological model of rainwater falling to the ground, percolating through soil profiles, and reaching groundwater through different vegetation covers in check dams has been proposed. Soil infiltration rates varied under different land use types and depths, ranging from 0.82 to 3.47 cm h−1. The groundwater recharge time from accumulated annual rainfall was 69–250 days, whereas the discharge time due to vegetation water consumption and groundwater lateral movement was 66–774 days. With a decrease in annual rainfall, the recharge time gradually increased, whereas the discharge time gradually decreased. These results inaugurate new perspectives for a deeper understanding of ecohydrological equilibrium in check dams and provide theoretical insights into ecological restoration in semi-arid areas. [ABSTRACT FROM AUTHOR]

The study presents a shallow groundwater isoscape for the Baltic region, which covers the major part of the Baltic Artesian Basin (BAB). BAB is an important palaeogroundwater reservoir, but prior to this study, little has been known about the spatial variability of δD and δ 18 O values in modern precipitation input across the region. To overcome this limitation, we hypothesized that the isotopic composition of shallow groundwater in the BAB could be used as a proxy for the mean weighted annual isotopic composition of local precipitation. However, the results of the study reveal many clear discrepancies between the isotopic composition of precipitation and shallow groundwater in the area. The isotopic composition of shallow groundwater is mostly biased towards isotopically depleted wintertime precipitation. We propose that the formation of shallow groundwater in the BAB area could be largely affected by variations in soil structure and land cover. The derived isoscape based on δ 18 O values of shallow groundwater in the BAB area is characterised by high spatial resolution and can therefore serve as a fairly reliable reference basis for further hydrogeological, ecological and forensic applications. [ABSTRACT FROM AUTHOR]

• Locate shallow groundwater occurrences. • Test site: Western Desert (area: ∼ 680,000 km2) in Egypt. • Integrated (remote sensing, machine learning, GIS, field, geochemistry) approach. • Pressurized groundwater access deep-seated faults and discharge at the near-surface. The focus of this study is to locate shallow groundwater (SGW) occurrences in arid lands using the Western Desert (WD; area: ∼680,000 km2) of Egypt as a test site. The SGW in the study area originated from paleo-precipitation during previous wet climatic periods. In wet periods, fossil groundwater was at higher levels, ascended along high-angle faults, and discharged at the surface. In contrast, at present, the water levels are lower, and the discharge occurs at near-surface elevations. Spring locations were identified as the dependent variable, while the independent variables included remote sensing–based variables and geomorphological features indicative of current or paleo discharge locations, including elevation, slope, curvature, distance to sapping features, soil moisture, NDVI, radar backscatter coefficient, and brightness temperature. Relationships between SGW occurrences (target) and their controlling factors (independent variables) were established using extreme gradient boosting (XGB), support vector machine (SVM), and logistic regression (LR) methods. The trained models were used to map SGW locations across the entire WD. Findings include the following: (1) the XGB yielded the most favorable result in identifying SGW locations (overall accuracy: 0.93) compared to SVM (overall accuracy: 0.88) and LR (overall accuracy: 0.87); (2) areas with a very high probability of SGW occurrences were found in lowlands and proximal to sapping features; (3) the overwhelming majority of the cultivated lands within the southern and central sections of the WD lie within areas identified as high and very high probability SGW locations; (4) our models identify-two previously unrecognized major SGW occurrences, an eastern zone (EZ; length: 800 km; width: 9 to 80 km; area: 43,000 km2) and an east–west trending northern zone (NZ) centered over the Qattara depression (length: 500 km; width: 200 km; area: 62,150 km2); (5) additional criteria were used to refine the modeled (XGB) SGW distribution (southern and central WD: 43,200 km2 to 23,400 km2; EZ: from 21,300 km2 to 17,400, and NZ: from 62,150 to 30,700 km2) including presence of shallow aquifers to accommodate rising Nubian waters, Nubian water salinity (fresh to brackish), and low to moderate thickness (<1 km) of post-Nubian successions that rising waters interact with. The techniques are cost-effective and efficient and could be readily applied to large sectors of Saharan Africa and Arabia, whose landscape and fossil aquifers bear many resemblances in their geologic, climatic, and geomorphic characteristics to the Nubian Sandstone Aquifer System. [ABSTRACT FROM AUTHOR]

Summary The estimation of water and solute transit times in catchments is crucial for predicting the response of hydrosystems to external forcings (climatic or anthropogenic). The hydrogeochemical signatures of tracers (either natural or anthropogenic) in streams have been widely used to estimate transit times in catchments as they integrate the various processes at stake. However, most of these tracers are well suited for catchments with mean transit times lower than about 4–5 years. Since the second half of the 20th century, the intensification of agriculture led to a general increase of the nitrogen load in rivers. As nitrate is mainly transported by groundwater in agricultural catchments, this signal can be used to estimate transit times greater than several years, even if nitrate is not a conservative tracer. Conceptual hydrological models can be used to estimate catchment transit times provided their consistency is demonstrated, based on their ability to simulate the stream chemical signatures at various time scales and catchment internal processes such as N storage in groundwater. The objective of this study was to assess if a conceptual lumped model was able to simulate the observed patterns of nitrogen concentration, at various time scales, from seasonal to pluriannual and thus if it was relevant to estimate the nitrogen transit times in headwater catchments. A conceptual lumped model, representing shallow groundwater flow as two parallel linear stores with double porosity, and riparian processes by a constant nitrogen removal function, was applied on two paired agricultural catchments which belong to the Research Observatory ORE AgrHys. The Global Likelihood Uncertainty Estimation (GLUE) approach was used to estimate parameter values and uncertainties. The model performance was assessed on (i) its ability to simulate the contrasted patterns of stream flow and stream nitrate concentrations at seasonal and inter-annual time scales, (ii) its ability to simulate the patterns observed in groundwater at the same temporal scales, and (iii) the consistency of long-term simulations using the calibrated model and the general pattern of the nitrate concentration increase in the region since the beginning of the intensification of agriculture in the 1960s. The simulated nitrate transit times were found more sensitive to climate variability than to parameter uncertainty, and average values were found to be consistent with results from others studies in the same region involving modeling and groundwater dating. This study shows that a simple model can be used to simulate the main dynamics of nitrogen in an intensively polluted catchment and then be used to estimate the transit times of these pollutants in the system which is crucial to guide mitigation plans design and assessment. [ABSTRACT FROM AUTHOR]

Highlights: [•] A lumped catchment model was developed based on the Boussinesq equation. [•] The model included two groundwater layers with contrasting hydrodynamics and chemistry. [•] The model was calibrated to stream flow and nitrate data from a lowland dairying catchment. [•] Annual stream flow and nitrate load were dominated by fast flow groundwater discharge. [•] Denitrification was estimated to remove 43% of leached nitrate, 80% of this occurring in the fast flow groundwater layer. [Copyright &y& Elsevier]

Summary: Freshwater lenses have been identified as having penetrated the shallow regional saline groundwater beneath the Cooper Creek floodplain near Ballera (south-west Queensland). Piezometers were installed to evaluate the major-element chemistry along a floodplain transect from a major waterhole (Goonbabinna) to a smaller waterhole (Chookoo) associated with a sand dune complex. The floodplain consists of 2–7m of impermeable mud underlain by unconsolidated fluvial sands with a saline watertable. Waterholes have in places scoured into the floodplain. The transect reveals that groundwater recharge takes place through the base of the waterholes at times of flood scour, but not through the floodplain mud. Total dissolved solids rise with distance from the waterhole and independently of the presence of sand dunes. Stable water isotopes (δ2H and δ18O) confirm that recharge is consistent with, and dependant on, monsoonal flooding events. Following floods, the waterholes self-seal and retain water for extended periods, with sulfate-δ34S and δ18O isotopes suggesting bacterial reduction processes within the hyporheic zone, and limited interaction between the surface water and groundwater during no-flow conditions. The area occupied by the freshwater lenses (TDS<5000mg/L) is locally asymmetrical with respect to the channel flow direction, extending down gradient along distances of ∼300m. [ABSTRACT FROM AUTHOR]

• Agro-ecohydrological processes were simulated in an arid region with shallow groundwater. • An enhanced agro-hydrological model (SWAT-AG) were employed. • Regional characteristics of agro-ecohydrological processes were presented and analyzed. • Water use efficiency was low with considering water applied during non-growing period. • A large proportion of non-beneficial water use was found during growing period. High water consumption in large irrigation districts has resulted in large river water diversion and serious water use conflicts in the Yellow River basin (YRB). The excessive water applied is also a cause of the dual problems of soil salinity and water logging in the upper YRB, which further negatively impacts crop yields. This study is aimed at modeling and analyzing the agro-ecohydrological processes and water use performance in large irrigation districts of the YRB. The Hetao Irrigation District (Hetao), a super-large irrigation district located in the upper YRB, was selected as a typical research area. A watershed hydrological model (SWAT-AG) with enhanced capacity for simulating regional agro-ecohydrological processes was adopted in this study. We simulated the agro-ecohydrological processes in Hetao and analyzed the regional characteristics of depth to groundwater table, soil moisture and salt content, evapotranspiration and crop yield. Water accounting was further carried out to interpret water cycle processes and evaluate water use performance in Hetao. Results indicated that Hetao was characterized by shallow depth to groundwater table, high soil moisture and high evapotranspiration. For farmland, there was a decreasing trend for both soil moisture and evapotranspiration from the upper to lower regions in Hetao, but this decreasing trend was not found for natural land. Further, results showed soil salt content has strong spatial variations with land use type, crops and shallow groundwater environments. Correspondingly, crop yields presented a decreasing trend from the upper to lower regions of Hetao, with large spatial variabilities closely related to the spatial distribution of soil water and salt conditions across Hetao. It was worth noting that the regional water use efficiency was relatively low in Hetao, especially with large proportions of non-beneficial water consumption to total water consumption. Overall, this study provided a thorough understanding of agro-ecohydrological processes and their regional characteristics, and the current situation of irrigation water use issues in the whole Hetao district, and further provided valuable insights in improving water use and management in large irrigation districts of the upper YRB. [ABSTRACT FROM AUTHOR]

Summary: The objective of this work is to identify hydrological processes controlling nitrate export and base flow concentration at the year scale in agricultural headwater catchment streams fed by an unconfined aquifer. The study is based on the hydrological and hydrochemical monitoring of the stream and shallow groundwater of three headwater catchments (0.1–5km2) in Western France over three to five water years. Results show that at the year scale nitrate export from the three catchments is a transport-limited process. The stream nitrate flux depends on how much water flows in the stream and not on the distribution of the flow over the year. Seasonal variations of concentration over the water year were complex. Variations were different for a given year from one catchment to another, and also different for a given catchment from one year to another. We show that the seasonal variations are controlled by water table depth dynamics along hillslope associated with spatially distributed nitrate concentration in the groundwater. The groundwater displays high nitrate concentrations, from 6 to 22mg, in upland and in the deeper zones of bottom lands. Persistence of such high concentrations results from the geochemical and mineralogical properties of the aquifers that consist of old, very oxidised and strongly weathered material, such that any primary electron donors have been leached. In riparian zones, concentrations are close to zero due to denitrification with oxidation of organic matter. In winter, stream nitrate concentration is controlled by the nitrate rich groundwater flow from upland. In summer, groundwater flow from upland decreases and stream concentration is controlled by bottom land hydrological and biogeochemical processes. The shift between winter and summer control depends on the water table dynamics along hillslopes. As long as the water table remains deep in upland, summer controls prevail. As soon as water table rises in upland, hydraulic gradient and groundwater flow from this zone increase, leading to an increase in the stream nitrate concentration. The shift from summer to winter control can be considered as the result of a connection between the upland nitrate rich groundwater and the stream, connection triggered by upland water table rise. [Copyright &y& Elsevier]

The three-dimensional groundwater flow and the hydrogeochemical regime have been determined in the Bere Stream valley, North Dorset Downs, southern England. The dual porosity characteristics of the Portsdown Chalk have been established using geophysical and hydrochemical borehole logging. Chemical properties have been established using major and trace element analyses of depth samples and groundwaters. The study site is located at the unconfined-confined boundary of the Chalk aquifer, where it is overflowing in the observation boreholes. The Chalk dips locally at about 5 m/km to the south–east under Palaeogene confining beds and three distinctive flow horizons may be recognised. The Chalk groundwater is of Ca–HCO3 type and three separate geochemical groundwater zones were also determined with depth, having different oxygen levels and trace element characteristics. (1) A shallow O2-rich zone with around 80% dissolved O2 and low trace element concentrations. (2) A mixing and transition zone with significant concentrations of trace elements and high trace metal concentrations at its base: manganese 29 μg/l, nickel 55 μg/l, cadmium 146 μg/l, and zinc 214 μg/l. (3) A deeper zone with depleted oxygen (5–20% dissolved O2) and with longer water residence times shown by higher Mg/Ca and K/Na ratios as well as higher Sr and F. The groundwater geochemistry in the Chalk aquifer is dominated by incongruent reactions with the fine-grained carbonate sediments, which release trace element impurities to the water. Some of the metals are co-precipitated with Mn- and Fe-oxide phases on fissure surfaces, whilst producing a purer calcite. During subsequent recrystallisation to purer iron- and manganese-oxides on fissure surfaces under specific geochemical and hydrodynamic conditions, trace metals are released into the fissure water. The results demonstrate the need to monitor quality stratification and the changes in the groundwater baseline chemistry in areas close to the redox boundary which, in the dual porosity Chalk is likely to be a diffuse zone with exchange between oxygen poor matrix waters and more oxic water flowing through the fissures. [Copyright &y& Elsevier]

• Groundwater discharge may hydrologically be as important as artificial drainage. • Nitrate concentrations in artificial drainage typically increased with flow. • The increase in nitrate concentrations varied between sites and seasons. • Nitrate, the main form of N exported, was not discharged through reduced groundwater. • Organic N and ammonium N exported via groundwater pathway can be important. When considering hydrological export pathways from drained pastoral fields, the shallow groundwater underlying the artificial drainage system may fulfil an as important role as the artificial drainage system itself. However, the important split between artificial drainage and groundwater flows at a field site is usually unknown. Consequently, the effects that the groundwater system's redox status may have on the forms and total amounts of nitrogen (N) exported from the site cannot be confidently assessed. We addressed these deficiencies by investigating the export of various forms of N in the artificial drainage and the shallow groundwater beneath two dairy farming sites (Tatuanui, Waharoa) within the Piako River catchment in New Zealand. Due to the very low hydraulic conductivity of the degraded peat in the saturated zone perched on a clay aquiclude, no significant water or contaminants were exported at Tatuanui via the unconfined shallow groundwater. Accordingly, artificial drainage discharge into the receiving surface collector drain was the only export pathway for N at this site. The sealed nature of the groundwater zone resulted in the accumulation of organic-N and ammonium-N in the peaty shallow saturated zone underlying the mineral soil. In contrast, the Waharoa site featured a more mobile shallow groundwater system, which on average conveyed approximately equal volumes of water offsite as the artificial drainage system. Nitrate (NO 3 -N) was at both sites the predominant form of N (76%) leached from the aerobic soil profile and discharged through the artificial drainage. As the shallow groundwater was at both sites in a reduced redox state, any NO 3 -N leached from the rootzone into the saturated zone was consumed. Accordingly, even the mobile shallow groundwater at Waharoa did not convey significant NO 3 -N. The NO 3 -N consumption in the reduced groundwater presumably occurred due to dissimilatory NO 3 -N reduction (concomitant increases in ammonium-N concentrations, with no organic material available for mineralisation in the saturated zone) and microbial denitrification (producing gaseous forms of N). The NO 3 -N reductions in the groundwater resulted in the average total N exports via the shallow groundwater pathway at the Waharoa site being substantially lower (31% of the exported N) than in the artificial drainage (69%). This is despite both pathways exporting similar water volumes. Because of the NO 3 -N consumption in the reduced shallow groundwater at Waharoa, over 98% of the N exported through this pathway was either as ammonium-N or organic-N. [ABSTRACT FROM AUTHOR]

• A dual-medium model was established to understand fracturing fluids migration. • Overpressure in shale and high permeability in overburden rocks are mutually exclusive. • Gas production greatly limits the upward driving force of fracturing fluids. • Shallow aquifers are hardly contaminated even considering preferential flow paths. One of the main environmental issues associated with hydraulic fracturing (HF) is the upward migration of the HF fluids from a shale reservoir to reach the shallow drinking water aquifers through preferential flow paths, such as fractures or faults. Oversimplified model structures or arbitrary boundary conditions were used in the previous studies, resulting in inaccurate results. To better describe the flow and transport in fractures/faults and matrix pores, a dual-medium model including fracture (fault) and matrix was established in this study to understand the evolution that occurs in reservoir pressure during HF and the long-term migration of HF fluids in deep geological formations, considering Sichuan Basin, China, as a case study. Both HF fractures and pre-existing natural faults were characterized as highly permeable preferential flow channels. Simulation results show that the spatial distribution of pressure in shallow aquifers is mainly controlled by the topography while it is controlled by overpressure in the deep strata. HF fluids were not found to contaminate the shallow groundwater throughout the simulation period, even in the worst scenario. A sensitivity analysis is carried out to quantify and understand the influence of a broad range of possibilities of parameters, such as HF fracture, reservoir properties and HF operation. The results suggest that the reservoir properties and the duration over which the injection of HF fluid take place are the most important factors influencing the spatiotemporal distribution of HF. Moreover, HF fluids are removed from the formation by the production of gas, meaning that these fluids are unlikely to contaminate the shallow aquifers of the Sichuan Basin. [ABSTRACT FROM AUTHOR]

• A new irrigation-drainage collaborative optimization model is developed. • It explores the more applicable irrigation and drainage water allocation solutions. • The developed model will lead to greater benefits than conventional optimal model. Agricultural production in arid and semi-arid area faces a global problem of water resources shortage and land salinization. Irrigation and drainage are the important measure to enhance crop yield and control soil salinity. Generally, at field scale only irrigation is optimized to pursue the higher water use efficiency and crop yield, while drainage is difficult to optimize owing to controlled by dynamic groundwater levels. Here, we develop a new collaborative optimization model of irrigation and drainage to improve irrigation water use efficiency and to control soil salinity. The model is formulated by integrating simulation of physical processes of field water - salt balance and a genetic algorithm-based optimization model. The new model is to search optimized irrigation and drainage strategic decision for enhancing field economic benefit with the condition controlling salinity with limited water resources. Then, a case study on optimally allocating irrigation and drainage water to different growth stages of maize field in the Hetao Irrigation District, arid area of northwest China shows enhanced applicability of the developed model. Five groundwater depth levels (1 m, 1.5 m, 2 m, 2.5 m and 3 m) and five groundwater salinity levels (2 g/L, 2.25 g/L, 2.5 g/L, 2.75 g/L and 3 g/L) are provided to show and compare the solutions of the optimal irrigation and drainage water allocation. Results indicate the developed model can supply reasonable field monthly irrigation and drainage decision with considering field hydrology, especially contribution of groundwater to crop water demand and groundwater role to soil salt accumulation. The contrary relationship between system benefit and irrigation water use efficiency was described successfully by the developed model. And compared with traditional single irrigation optimization model, the developed irrigation-drainage collaborative optimization model can enhance drainage function to keep the optimal groundwater levels and improve the system benefit by 0–8%. Overall, the developed model can provide more applicable irrigation water and drainage strategies to the sustainable development of irrigation agriculture. [ABSTRACT FROM AUTHOR]