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With the growing irrigation water scarcity, the researchers and policymakers are more concerned to improve the irrigation water use efficiency at farmers’ field level. The water-saving technologies provide greater control over water delivery to the crop root zone and reduce the non-beneficial evaporation from the crop field. Water productivity is an important concept for measuring and comparing water use efficiency. The present study tried to estimate the irrigation water use and physical water productivity of cotton under alternate furrow and drip irrigation methods in the Bhavnagar district of Gujarat. Results suggest that crop yield and physical water productivity were higher for cotton irrigated by drip method than alternate furrow method during normal rainfall and drought year. The irrigation water use under the drip method of irrigation was lower as compared to the alternate furrow method. In the case of total water (effective rainfall + irrigation water) use, per hectare crop yield and physical water productivity were higher for the drip method of irrigation than the alternate furrow method of irrigating cotton crop during normal rainfall and drought year. In the case of total water use (effective rainfall + irrigation water), it was lower for drip irrigation than the alternate furrow method of irrigating cotton crop during normal rainfall year and drought year. While estimating total water (effective rainfall + irrigation water) use, it was assumed that there is no return flow of water from the cotton field in the study area under both irrigation methods.

Groundwater sources are drastically changing in their quantity and quality depending on local and regional level natural and anthropogenic factors, influencing their suitability for drinking and irrigation purposes. The objective of this study is to characterize the hydrochemistry and assess the groundwater quality in the fluvial deltaic plains of Cauvery river basin, Tamil Nadu, India. A total of 50 georeferenced groundwater samples were collected across Needamangalam block of Thiruvarur district and analyzed for major ions and hydrochemical processes. The results showed an ionic sequence of Cl− > Na+ > HCO3− > Mg2+ > Ca2+ > CO32− > SO42− > K+ based on their relative proportions. The scatter diagram indicated that groundwater chemistry was mostly influenced by weathering dominance followed by evaporation and silicate weathering. The dominant hydro-chemical facies were Na+-Cl−-HCO3− type, Na+-Mg2+-Cl−-HCO3− type, Na+-Cl−-HCO3−-CO32− type and Na+-Mg2+-Cl−-HCO3−-CO32− type influenced by the ion-exchange reaction. Most of the groundwater samples are suitable for drinking and irrigation except few with higher Na+ and Cl− content caused by the mixing of salt from fluvio-marine sources or agriculture return flow. The high sodium content in irrigation water may affect the soil hydraulic and nutrient properties in the long run.

The North Shandong Coastal Current (NSCC) is an important transporting route for sediment, drifting algae, and spilled oil from the Bohai Sea to the Yellow Sea. This study investigated the features and formation mechanism of the NSCC using observational data of current velocity and a numerical coastal ocean model. Our results confirmed the existence of the NSCC in the sense of climatological current in winter when northerly wind prevails in the Bohai Sea and Yellow Sea. The magnitude of the NSCC in the monthly time scale ranged 0.07–0.12 m/s and the current direction was parallel to the coastline. The detided residual current on the pathway of the NSCC was unstable, but variable with the local wind speed. The residual current was well correlated with northerly wind speed and tended to be parallel to coastline with the increase of wind speed. Strong wind plays key roles in the formation of eastward mean flow on the pathway of the NSCC in winter. We found that strong wind can generate a stronger eastward current in the southern side of the northern Yellow Sea, but a smaller westward return flow during the strong wind relaxation period. The asymmetry of wind-related residual current during and after strong wind events accounts for the formation of the eastward NSCC. A momentum analysis was performed using the numerical model results during strong wind events. We found that the barotropic pressure gradient was the dominant driving force of the residual current both during and after a strong wind event.

Awareness concerning the degradation of groundwater quality and their exacerbating adverse effects due to salinization processes is gaining traction, raising for adequate understanding of the distribution, sources, genesis, and environmental concerns of salinity in groundwater. Saline groundwater is widely distributed all over the world, with an area of 24 million km2 (16% of the total land area on earth) and 1.1 billion people living in the affected areas, especially the arid/semi-arid areas in developing countries. These large-scale groundwater salinization problems are sourced from two major ways: natural and anthropogenic. The natural sources are diversified from connate saline groundwater, seawater intrusion, evaporation, dissolution of soluble salts, membrane filtration process to geothermal origin. The anthropogenic sources include irrigation return flow, road deicing salts, industrial and agricultural wastewater, and gas and oil production activities. The integrated approach of geochemical tracers and multiple isotopes (δ18OH2O, δ2HH2O, δ11B, δ36Cl, δ34Ssulfate, 87Sr/86Sr, and δ7Li) is proved to be useful in the constraints of the origin and transport of solutes in groundwater. Groundwater salinization is often associated with high levels of some toxic elements like arsenic, fluoride, selenium, and boron. Four "triggers" lead to this association: salt effect, competing adsorption, microbial processes, and cation exchange.

International audience; Large proportions of rain water and snowmelt infiltrate into the subsurface before contributing to stream flow and stream water quality. Subsurface flow dynamics steer the transport and transformation of contaminants, carbon, weathering products and other biogeochemistry. The distribution of groundwater ages with depth is a key feature of these flow dynamics. Predicting these ages are a strong test of hypotheses about subsurface structures and time varying processes. CFC‐based groundwater ages revealed an unexpected groundwater age stratification in a 0.47 km2 forested catchment called Svartberget in northern Sweden. An overall groundwater age stratification, representative for the Svartberget site, was derived by measuring CFCs from 9 different wells with depths of 2 m to 18 m close to the stream network. Immediately below the water table, CFC‐based groundwater ages of already 30 years that increased with depth were found. Using complementary groundwater flow models, we could reproduce the observed groundwater age stratification and show that the 30 year lag in rejuvenation comes from return flow of groundwater at a subsurface discharge zone that evolves along the interface between two soil types. By comparing the observed groundwater age stratification with a simple analytical approximation, we show that the observed lag in rejuvenation can be a powerful indicator of the extent and structure of the subsurface discharge zone, while the vertical gradient of the age‐depth relationship can still be used as a proxy of the overall aquifer recharge even when sampled in the discharge zone. The single age stratification profile measured in the discharge zone, close to the aquifer outlet, can reveal the main structure of the groundwater flow pattern from recharge to discharge. This groundwater flow pattern provides information on the participation of groundwater in the hydrological cycle and indicates the lower boundary of hydrological connectivity.

Evapotranspiration (ET) from riparian vegetation can be difficult to estimate due to relatively abundant water supply, spatial vegetation heterogeneity, and interactions with anthropogenic influences such as shallower groundwater tables, increased salinity, and nonpoint source pollution induced by irrigation. In semiarid south‐eastern Colorado, reliable ET estimates are scarce for the riparian corridor that borders the Arkansas River. This work investigates relationships between the riparian ecosystem along the Arkansas River and an underlying alluvial aquifer using ET estimates from remotely sensed data and modelled water table depths. Results from a calibrated, finite‐difference groundwater model are used to estimate weekly water table fluctuations in the riparian ecosystem from 1999 to 2009, and estimates of ET are calculated using the Operational Simplified Surface Energy Balance (SSEBop) model with over 200 Landsat scenes covering over 30 km² of riparian ecosystem along a 70‐km stretch of the river. Comparison of calculated monthly SSEBop ET to estimated alfalfa reference ET from local micrometeorological station data indicated statistically significant high linear correspondence (R² = .87). Daily calculated SSEBop ET showed statistically significant moderate linear correspondence with data from a local weighing lysimeter (R² = .59). Simulated monthly SSEBop ET values were larger in drier years compared with wetter years, and ET variability was also larger in drier years. Peak ET most commonly occurred during the month of June for all 11 years of analysis. Relationships between ET and water table depth showed that peak monthly ET was highest when groundwater depths were less than about 3 m, and ET values were significantly lower for groundwater depths greater than 3 m. Negative sample Spearman correlation highlighted riparian corridor locations where ET increased as a result of decreased groundwater depths across years with different hydroclimatic conditions. This study shows how a combination of remotely sensed riparian ET estimates and a regional groundwater model can improve our understanding of linkages between riparian consumptive use and near‐river groundwater conditions influenced by irrigation return flow and different climatic drivers.

Identifying the key factors controlling groundwater chemical evolution in mountain‐plain transitional areas is crucial for the security of groundwater resources in both headwater basins and downstream plains. In this study, multivariate statistical techniques and geochemical modelling were used to analyse the groundwater chemical data from a typical headwater basin of the North China Plain. Groundwater samples were divided into three groups, which evolved from Group A with low mineralized Ca‐HCO₃ water, through Group B with moderate mineralized Ca‐SO₄‐HCO₃ water, to Group C with highly saline Ca‐SO₄ and Ca‐Cl water. Water‐rock interaction and nitrate contamination were mainly responsible for the variation in groundwater chemistry. Groundwater chemical compositions in Group A were mainly influenced by dissolution of carbonates and cation exchange, and suffered less nitrate contamination, closely relating to their locations in woodland and grassland with less pronounced human interference. Chemical evolution of groundwater in Groups B and C was gradually predominated by the dissolution of evaporites, reverse ion exchange, and anthropogenic factors. Additionally, the results of the inverse geochemical model showed that dedolomitization caused by gypsum dissolution, played a key role in the geochemical evolution from Group A to Group B. Heavy nitrate enrichment in most groundwater samples of Groups B and C was closely associated with the land‐use patterns of farmland and residential areas. Apart from the high loads of chemical fertilizers in irrigation return flow as the main source for nitrate contamination, the stagnant zones, flood irrigation pattern, mine drainage, and groundwater‐exploitation reduction program were also important contributors for such high mineralization and heavy NO₃⁻ contents in Group C. The important findings of this work not only provide the conceptual framework for the headwater basin but also have important implications for sustainable management of groundwater resources in other headwater basins of the North China Plain.

Nitrogen (N) and phosphorus (P) runoff loads of paddy fields in situ under conventional water and fertilizer management measures caused severe water pollution in the Taihu Lake basin. The “water balance method” was implemented to calculate the surface runoff and the N and P runoff loads of the paddy field for 2 years. The total inflow difference in the paddy field between the 2 years was 22.5% with more irrigation and less precipitation in 2013 than in 2014. The soil water depletion of the paddy field was in the range of 47.0–83.3 mm, accounting for an average of 6.4% of the total inflow. The surface runoff was mainly caused by precipitation and artificial drainage, but irrigation return flow should not be neglected in paddy fields in situ. The rainfall runoff volume accounted for an average of 42.2% of the total precipitation. The artificial drainages occurred mainly during the tillering stage of drying, during maturity stage of drying and when the water level was too high in the paddy field. More than half of the irrigation cases would occur as irrigation return flow, which accounted for an average of 11.8% of the irrigation. The variation in N and P concentrations in the surface runoff of the paddy field was predominantly governed by the times of fertilization. Compared to the irrigation volume control, the runoff pollution loads were predominantly governed by the fertilization rate. The mean reduction potential of N and P fertilization was 38.9 kg ha−1 and 7.5 kg ha−1, respectively. Reducing fertilizer rate is the effective way to improve the runoff pollution in paddy fields in the Taihu Lake basin.

HighlightsPrivate and public irrigation development was important for expanding agricultural production in the western U.S.The Twin Falls Canal Company is an excellent example of a successful Carey Act project.Cooperative efforts during the last 30 years have dramatically improved the water quality of irrigation return flow.Electricity generated by six hydroelectric facilities improves the sustainability of the irrigation project.Abstract. Private and public irrigation development projects were fundamental to bringing irrigation to arid regions of the western U.S. The Twin Falls Canal Company in southern Idaho provides a case study of private and public irrigation development because the project was developed by private investors under the Carey Act and receives a portion of its irrigation water supply from U.S. Bureau of Reclamation reservoirs. The project survived initial financial struggles and waterlogged soil to focus on sustaining crop production by reducing chronic furrow irrigation erosion and nutrient losses in irrigation return flow. Average sediment loss from the project was 460 kg ha-1 in 1970. A cooperative effort by the canal company, state and federal agencies, and farmers improved water quality by installing sediment ponds on fields, applying polyacrylamide with furrow irrigation, converting from furrow to sprinkler irrigation, and constructing water quality ponds on irrigation return flow streams. From 2006 to 2018, more sediment and total phosphorus flowed into the watershed than returned to the Snake River, and the project removed 13,000 Mg of sediment and 30 Mg of total phosphorus from the Snake River each year. However, nitrate-N from subsurface drainage was lost at 10 kg ha-1 each year, or 800 Mg year-1, for the entire watershed. While sediment and phosphorus concentrations in irrigation return flow have decreased, these concentrations were still greater than the irrigation water, indicating that more can be done to reduce the project’s influence on water quality in the Snake River. Keywords: Nitrogen, Phosphorus, Sediment, Soluble salts, Water quality.

Agricultural water is of considerable interest to water managers and policymakers as irrigation—particularly flood irrigation—accounts for the largest portion of freshwater use. However, characterization of how and when flood applied water contributes to storage and adjacent surface water bodies via return flow remains limited, particularly relative to the volume of studies addressing surface processes. This study focuses on improving our understanding of return flow paths and explicitly quantifying subsurface return flow contributions using hydro-geophysical observations coupled with a vadose zone transport flow model. Our approach, which simulates return flow in agricultural fields in a small, heavily instrumented field site in northern Wyoming, leverages daily measured values of evapotranspiration obtained from a large aperture scintillometer and atmospheric conditions, and uses seismic data to characterize heterogeneous structure of the soil profiles at field scale. We use these high-resolution, site specific datasets to parameterize a process-based vadose zone flow model that quantifies hydrologic response to flood irrigation along the stream reach, allowing us to identify return flow fluxes out of the fields. Using our approach, we found that the majority of applied irrigation water (60%) left the fields through subsurface flow paths, while only 13% was consumed by vegetation. Subsurface runoff over less-permeable soil layers, governed by the subsurface structural characteristics, was identified as the dominant subsurface flow process contributing to fluxes out of the fields. Importantly, our findings about the importance of subsurface return flow paths nuance previous research which has emphasized surface flow paths. By improving our understanding of return flow processes through a high- resolution vadose zone model that leverages climatological, geophysical, and hydrological observations, this study advances our understanding of return flow processes. Understanding how subsurface characteristics influence return flow generation in semi-arid agriculturally productive regions will help managers and policy makers balance desires for improved irrigation efficiency against concerns about irrigation-dependent ecosystem services.

Improving water availability and distribution is key to combatting water stress in semi-arid regions. This study presents an approach that integrates hydrogeological surveys and economic analysis at a pilot site in the eastern Lower Jordan Valley to assess the potential for managed aquifer recharge (MAR). Based on exploratory drillings, hydraulic tests, water-quality analyses and previous studies, an appropriate storage space in the alluvial fans along the eastern margin of the valley was found. Average measured infiltration rates of 9.8 × 10−5 m/s in a gravel pit and average hydraulic conductivity values of 6.8 × 10−5 m/s in the aquifer indicate that up to 1 Mm3/year can be infiltrated and stored underground. It is assumed that water is trapped towards the center of the valley where the alluvial sediments inter-finger with fine-grained layers. An increase in electrical conductivity values in the same direction probably indicates irrigation return flow and the presence of evaporitic layers and suggests a water recovery before reaching these saline zones. Depending on whether the water provided by MAR is used for irrigation or domestic water purpose, an average incremental profit of 0.81 and 2.44 US$/m3 can be achieved, respectively. These calculations result in a net present value of the MAR plant of 10.6 or 31.9 million US$, respectively, over a lifetime of 30 years. The presented study serves as a basis for further investigations and planning of a MAR plant, and can be transferred and upscaled to other sites in the Jordan Valley.

The Atlantic Meridional Overturning Circulation (AMOC) plays a central role in the decadal variability of global and regional climate through changing poleward transport of heat. However, realistic simulation of the AMOC, i.e., its strength and spatial structure, remains a challenge for ocean general circulation models (OGCMs) and coupled climate models. Here, we investigate how the simulated AMOC could be affected by improved accuracy of the seawater equation of state (EOS) with an OGCM. Two EOSs used in this study: the UNESCO EOS80, and the “stiffened” EOS derived from the compressibility of sea water and the UNESCO EOS80. Compared to the model using the UNESCO EOS80, the model using the “stiffened” EOS yields stronger deep convection in the Labrador Sea, the Irminger-Iceland-Scotland Basin, and the Greenland-Iceland-Norwegian (GIN) seas, which leads to an improvement in the simulation of the AMOC: Along 26.5°N, the maximum transport is increased from 14.9 to 17.4 Sv and the interface between the upper clockwise cell and lower counterclockwise cell is deepened from 2.8 to 3.3 km, both matching the observations better. Taken the Labrador Sea as an example, the processes, including both direct and indirect causes, that in part responsible for the improved AMOC are as follows. The use of “stiffened” EOS increases the density throughout the water column and weakens the stability of sea water. Moreover, the enhanced cabbeling and thermobaric effect strengthen the vertical advection, intensifying the deep convection and increasing formation of deep water, which eventually improves the simulation of the AMOC. The intensified AMOC, in turn, speeds up the surface return flow, transporting more warm and saline water to the high latitudes in the North Atlantic, which contributes to the densification of surface water. Similar analyses can be applied to the Iceland-Scotland Basin and GIN seas. Thus, the enhanced deep convection and formation of deep water in the Labrador Sea, as well as in the Iceland–Scotland Basin and GIN seas, improve the simulated AMOC.

Early kick and loss detection is a major concern in any drilling operation. A real-time return flow rate measurement would be an added advantage for the early kick and loss detection for conventional drilling systems. In this paper, the real-time estimation of the return flow rate during drilling is studied, using a Venturi channel. A reduced order, 1D, mathematical equation for the shallow water flow/open flow in the Venturi channel is investigated with various non-Newtonian friction models. The volumetric fluid flow rate is estimated online using an unscented Kalman filter. In order to improve the fluid flow rate estimation, the model is adapted to some of the rheological parameters in real time. A simple and effective fit–for–purpose friction model is developed and compared with the other friction models from literature. The fit–for–purpose model has the least estimation error (RMSE 1.465%) and requires less information as inputs. The proposed method exhibits promising potential to be used as an online return flow rate measurement for early kick and loss detection in conventional drilling.

A thorough study on understanding of groundwater recharge sources and mechanisms was attempted by integrating the hydrogeological, geochemical and isotopic information along with groundwater dating and end-member mixing analysis (EMMA). This study was necessitated due to prolonged dryness and unavailability of freshwater in semi arid Deccan trap regions of Central India. In addition, groundwater resources are not characterized well in terms of their geochemical nature and recharge sources. The hydrogeochemical inferences suggest that aquifer I consists of recently recharged water dominated by Ca–Mg–HCO3 facies, while groundwater in aquifer II shows water–rock interaction and ion exchange processes. Presence of agricultural contaminant, nitrate, in both aquifers infers limited hydraulic interconnection, which is supported by unconfined to semi-confined nature of aquifers. Groundwater in both aquifers is unsaturated with respect to carbonate and sulfate minerals indicating lesser water–rock interaction and shorter residence time. This inference is corroborated by tritium age of groundwater (aquifer I: 0.7–2 years old and aquifer II: 2–4.2 years old). Stable water isotopes (δ2H, δ18O) suggest that groundwater is a mixture of rainwater and evaporated water (surface water and irrigation return flow). EMMA analysis indicates three groundwater recharge sources with irrigation return flow being the dominant source compared to others (rainwater and surface waters). A conceptual model depicting groundwater chemistry, recharge and dynamics is prepared based on the inferences.

When a ship model test is performed in a tank, particularly when the tank is small and the ship model is relatively large, the blockage effect will inevitably occur. With increased ship model scale and speed, the blockage effect becomes more obvious and must be corrected. In this study, the KRISO 3600 TEU Container Ship (KCS) is taken as a model and computational fluid dynamics techniques and ship resistance tests are applied to explore the mechanism and correction method of the blockage effect. By considering the degrees of freedom of the sinkage and trim, the resistance of the ship model is calculated in the infinite domain and for blockage ratios of 1.5%, 1.8%, 2.2%, and 3.0%. Through analysis of the free surface, pressure distribution, and flow field around the ship model, the action law of the blockage effect is studied. The Scott formula and mean flow correction formula based on the average cross sectional area are recommended as the main correction methods, and these formulas are improved using a factor for the return flow velocity correction based on comparison of the modified results given by different formulas. This modification method is verified by resistance test data obtained from three ship models with different scale ratios.

In electromembrane desalination processes such as electrodialysis (ED) and ion concentration polarization (ICP) desalination, ion-depleted boundary layers constitute the desalted, product stream, yet also cause high resistivity and voltage drop. Directly manipulating fluid flow streams is a new method to break this fundamental trade-off for electromembrane desalination. In this work, we are introducing a novel electromembrane desalination architecture that allows a feed stream to return to the feed inlet side of the membrane (hereby named as return-flow (RF) architecture) to improve the energy efficiency by re-distributing and controlling the depleted boundary layer, even at high current values. The technical feasibility of this idea was examined in ICP desalination process (RF-ICP) with a wide range of feed salinity from 10 to 70 g/L. For a partial desalination, RF-ICP (∼75 cm2 of membrane area) has achieved similar power consumption compared to batch-ED with 3 times bigger membrane area (200 cm2) with a higher area efficiency for salt removal, which translates into lower optimal desalination cost. The techno-economic analysis of RF-ICP have been performed for the treatment of 70 g/L brine waste. For partial desalination of 70 g/L brine down to 35 g/L, RF-ICP desalination achieved overall water cost as low as $2.57/m3 ($0.41/barrel). This could translate into reduction in total water cost up to 31% for zero brine release scenarios, depending on the concentrated brine treatment cost. These results show that return-flow architecture can improve the performance of electromembrane desalination, enabling more flexible water treatment for many real-world applications.

Estimation of groundwater recharge is more essential for managing aquifers, particularly in arid regions with limited access to water resources. Low potentiality of aquifer recharge, deep groundwater levels and impacts from other important sources like irrigation return flow add more complexities to estimating recharge in these areas. In this research, physical and chemical (tracer) techniques including water-table fluctuation (WTF), chloride mass balance (CMB), stable isotopes (2H and 18O) and water-budget equation (WBE) were applied to estimate recharge into the Birjand alluvial aquifer in arid areas of SE Iran with a mean annual precipitation of about 147 mm. Recharge rates, in ascending order, were calculated about 5.0, 9.2, 33.2 and 41.7 mm year−1 using isotopes, CMB, WTF and WBE methods, respectively. Isotopes and chloride tracers assessed the rainfall-related aquifer recharge, while the other methods represented total recharge including rainfall infiltration plus deep percolation of irrigation water (irrigation return flow). Total recharge rate of the Birjand aquifer was estimated in average at 37.5 mm year−1, partitioned between the rainfall recharge (7.0 mm year−1) and irrigation return flow (30.5 mm year−1). The values represent an average rainfall recharge coefficient of about 4.8% of the annual precipitation and return flow coefficient of 15% of the total annual applied water for irrigation. Recharge estimates are recommended as initial guesses for calculating water-balance in order to manage valuable groundwater resources in other arid-land aquifers. The study highlighted priorities of the stable isotope and WTF methods in estimating rainfall and total (rainfall + return flows) recharge of the groundwater in arid regions, respectively.

The 1960 M9.5 Valdivia and 1964 M9.2 Alaska earthquakes caused a decimeters high secondary zone of uplift a few hundred kilometers landward of the trench. We analyze GPS data from the 2010 M8.8 Maule and 2011 M9.0 Tohoku-Oki earthquakes to reveal the persistent existence of a secondary zone of uplift due to great earthquakes at the megathrust interface. This uplift varies in magnitude and location, but consistently occurs at a few hundred kilometers landward from the trench and is likely mainly coseismic in nature. This secondary zone of uplift is systematically predicted by our 2D visco-elasto-plastic seismo-thermo-mechanical numerical simulations, which model both geodynamic and seismic cycle timescales. Through testing hypotheses in both simple and realistic setups, we propose that a superposition of two physical mechanisms could be responsible for this phenomenon. First, a wavelength is introduced through elastic buckling of a visco-elastically layered fore-arc that is horizontally compressed in the interseismic period. The consequent secondary zone of interseismic subsidence is elastically rebound during the earthquake into a secondary zone of relative uplift. Second, absolute and broader uplift is ensured through a mass conservation-driven return flow following accelerated slab penetration due to the megathrust earthquake. The dip and width of the seismogenic zone and resulting (deep) coseismic slip seem to have the largest affect on location and amplitude of the secondary zone of uplift. These results imply that subduction and mantle flow do not occur at constant rates, but are rather modulated by earthquakes. This suggests a link between deep mantle and shallow surface displacements even at time scales of minutes.

Recent studies have highlighted the agronomic and environmental importance of phosphorus (P) movement through the soil profile. Thus, faced with challenges such as high‐profile cases of P enrichment of surface water, better understanding of nutrient movement through soil is needed to better manage agricultural fertilizers and manures and their contribution to water quality degradation. In particular, field‐scale research is especially needed in soils with preferential flow transport pathways. Thus, we collected nitrogen (N) and P transport data in run‐off and seepage (lateral subsurface return flow) from 13 field‐ and farm‐scale watersheds on Vertisols in Central Texas for a 14‐year period. For 2004–2017, seepage accounted for ~20% of the total surface flow, and nutrient concentrations were generally similar in run‐off and seepage. As surface run‐off contributed ~80% of the flow, it follows that median annual N and P loads in run‐off were significantly greater than in seepage for every watershed. N loads in both run‐off and seepage flow from cultivated land were an order of magnitude greater than in native prairie and improved pasture, and the highest run‐off and seepage P loads both occurred on cultivated land with organic fertilizer sources. Increasing watershed scale (size) did not to produce consistent patterns in N or P loss in run‐off or seepage. Land use and watershed scale produced significant differences in seepage volume but did not affect run‐off volumes or total surface flow/rainfall. Although less significant in terms of total offsite flux, nutrient movement in vadose zones has important agronomic and environmental implications as considerable N and P are transported through and within the root zone and eventually offsite. And in terms of P, this contradicts the traditionally held scientific viewpoint that P movement through the vadose zone is unimportant agronomically and environmentally.

We develop an idealised one dimensional (cross-shore) morphodynamic model that couples wave, tide and sediment dynamics to study the effect and evolution of a shoreface nourishment. Sediment fluxes driven by wave skewness, wave asymmetry (both onshore) and return flow (offshore) are considered. With the aid of new analytical expressions for the skewness and standard deviation of wave velocity and acceleration, sediment fluxes are calculated. Nourishment is viewed as a perturbation to the system in equilibrium that is subject to the divergence of the perturbed sediment flux and a gravity driven diffusion term. Depending on the location, a nourishment may provide a feeder or lee effect. In moderate and mild wave conditions, the evolution of a nourishment primarily depends on the relative location of nourishment and break point. Placed well offshore of the break point, the nourishment induces an overall positive perturbation in sediment flux, resulting in onshore migration (feeder effect). Located closer to the break point, the nourishment induces an earlier wave breaking, which dissipates part of the wave energy (lee effect), leading to a negative sediment flux perturbation around this break point and a positive sediment flux perturbation around the break point of the un-nourished beach. Depending on the intensity of the earlier breaking, the nourishment either migrates onshore (weak break) or splits into onshore and offshore moving parts (strong break). The relative importance of the diffusion term and the divergence of perturbed sediment flux may lead to a primarily migrating or decaying evolution of nourishment. In storm wave conditions, the nourishment tends to move offshore due to the predominance of return flow driven sediment flux. The sensitivity to wave period and tide are also studied. Model results are consistent with observations, as well as prevailing theory on cross-shore sediment transport.

We report on experiments of turbulent particle-laden plumes descending through a stratified environment. We show that provided the characteristic plume speed $(B_{0}N)^{1/4}$ exceeds the particle fall speed, where the plume buoyancy flux is $B_{0}$ and the Brunt–Väisälä frequency is $N$, then the plume is arrested by the stratification and initially intrudes at the neutral height associated with a single-phase plume of the same buoyancy flux. If the original fluid phase in the plume has density equal to that of the ambient fluid at the source, then as the particles sediment from the intruding fluid, the fluid finds itself buoyant and rises, ultimately intruding at a height of about $0.58\pm 0.03$ of the original plume height, consistent with new predictions we present based on classical plume theory. We generalise this result, and show that if the buoyancy flux at the source is composed of a fraction $F_{s}$ associated with the buoyancy of the source fluid, and a fraction $1-F_{s}$ from the particles, then following the sedimentation of the particles, the plume fluid intrudes at a height $(0.58+0.22F_{s}\pm 0.03)H_{t}$, where $H_{t}$ is the maximum plume height. This is key for predictions of the environmental impact of any material dissolved in the plume water which may originate from the particle load. We also show that the particles sediment at their fall speed through the fluid below the maximum depth of the plume as a cylindrical column whose area scales as the ratio of the particle flux at the source to the fall speed and concentration of particles in the plume at the maximum depth of the plume before it is arrested by the stratification. We demonstrate that there is negligible vertical transport of fluid in this cylindrical column, but a series of layers of high and low particle concentration develop in the column with a vertical spacing which is given by the ratio of the buoyancy of the particle load and the background buoyancy gradient. Small fluid intrusions develop at the side of the column associated with these layers, as dense parcels of particle-laden fluid convect downwards and then outward once the particles have sedimented from the fluid, with a lateral return flow drawing in ambient fluid. As a result, the pattern of particle-rich and particle-poor layers in the column gradually migrates upwards owing to the convective transport of particles between the particle-rich layers superposed on the background sedimentation. We consider the implications of the results for mixing by bubble plumes, for submarine blowouts of oil and gas and for the fate of plumes of waste particles discharged at the ocean surface during deep-sea mining.

The results of estimated calculations of fluxes of particles of the working fluid in peripheral regions from the axial direction of the jet of a magnetoplasmodynamic (MPD) thruster with an external magnetic field with a power of 100 kW as well as the results of measurements in laboratory conditions of reverse fluxes of Li particles from the thruster are presented. Return flow measurements were conducted using the quartz microbalance method. Calculations and measurements of particle fluxes indicate that their values do not exceed ~10–8–10–7 g cm–2 s–1.

Coarse temporal (i.e., annual) and spatial (i.e., watershed) scales camouflage water stress associated with withdrawals from surface water and groundwater sources. To address this "curse of scale", we developed a framework to characterize water stress at different time scales and at fine spatial scales that have not been explored before. Our framework incorporates surface water-groundwater interactions by accounting for spatially cumulative consumptive and nonconsumptive use impacts and associated changes in flow due to depletion and return flow along stream networks. We apply the framework using a rich data set of water withdrawals from more than 6800 principal facilities (i.e., withdrawal capacity >380 000 L/day) across the U.S. Great Lakes Basin. Results underscore the importance of spatiotemporal scale and return flows when characterizing water stress. Although the majority of catchments in this water-rich region do not experience large stress, a number of small headwater catchments with sensitive streams are vulnerable to flow depletion caused by surface water and shallow groundwater withdrawals, especially in a high-withdrawal, low-flow month (e.g., August). The return flow from deep groundwater withdrawals compensates for the streamflow depletion to the extent that excess flow is likely in many catchments. The improved ability to pinpoint the imbalance between natural water supply and withdrawals based on stream-specific ecological water stress thresholds facilitates protecting fragile aquatic ecosystems in vulnerable catchments.

Documenting the sources and residence times of water that contributes to streamflow is important for understanding processes in river catchments. The residence times of bank storage and return flow and its influence on river water chemistry in the upper Barwon River of southeast Australia were investigated using stable (18O, 2H, and 13C) and radioactive (3H and 36Cl) isotopes, major ion geochemistry, river discharge data, and electrical conductivity (EC)-discharge hysteresis. Elevated 3H activities following high winter flows indicate that bank storage and return flow contributes to river discharge for at least several months. However, EC-discharge hysteresis patterns suggest that individual storm events make additional contributions to bank storage and return flow throughout the year over periods of a few weeks. 3H activities in the upper Barwon River are >1.75 TU throughout the year, suggesting that the contribution of older regional groundwater, which has 3H activities

Impact of irrigation return flow induced mineral weathering and ion exchange reactions in the groundwater was studied in Limpopo region, South Africa . Based on EC (>500) and HCO 3 (>150 mg/l), groundwater samples are classified into two groups due to its heterogeneous nature. Water types in group 1 and 2 wells are Ca-Mg-Cl-SO4 and Ca-Mg-HCO3, respectively. Gibbs plots, bivariate plots and ionic ratios (Mg/Na vs Ca/Na; HCO3/Na vs Ca/Na; mCa + Mg vs mHCO3; mNa + K vs mHCO3; Ca + Mg-HCO3-SO4 vs Na + K-Cl; mCa + Mg/HCO3 vs Cl; Ca + Mg/SO4 +HCO3 >1; Na/Cl ratio; Cl + SO4 vs HCO3 ), chloro alkaline indices (CAI, CA2) and Pearson correlation analysis imply that groundwater chemistry is influenced by mineral weathering (carbonate > silicate minerals), reverse ion exchange and irrigation return flow. In group 1 wells, Cl + SO 4 and HCO3 have strong positive correlation and both increases together, suggesting induced mineral weathering caused by nitrification and wastewater infiltration from the surface. Group 2 wells are generally deep with low groundwater level fluctuation and high concentration of major ions. Overall, water chemistry in the group 1 wells is highly affected by the contamination sources than group 2 wells.

Coastal aquifers are highly vulnerable to salinization, especially by seawater intrusion. This problem is exacerbated by increasing demands for freshwater in coastal zones due to urbanization and industrialization. This paper investigates seawater intrusion in coastal aquifers of Kachchh district in Gujarat, an economic hub of the country in western India. A total of 26 groundwater samples were collected from various villages of the district from Kandla to Mandavi. In this study hydro-chemical, stable isotope (δ18O and δD) and GIS analysis were used to infer salinization processes. The origin of salinity and mixing of various water masses has been studied through the integration of major ions chemistry with stable isotopes δ18O and δD. Most of the samples were showing Na-Cl type water facies. It was observed that besides natural processes (such as water–rock interaction, ion exchange, dissolution/precipitation dynamics and evaporation) which are governing the groundwater quality, current land use practices have augmented the salinization in this poorly drained semi-arid area. Various ionic ratios (such as Mg2+/Ca2+, Na+/Cl−, SO42−/Cl−, K+/Cl−) and isotopic composition (δ18O and δD) of groundwater suggest that while in coastal areas seawater intrusion and formation of saline plumes is taking place, due to upconing of underlying natural saline water which is enhanced by overexploitation. In inland areas groundwater quality is deteriorating due to infiltration of wastewater and irrigation return flow. The continued stress on land and water resources has shifted the natural balance and accelerated the salinization process in this water scarce region. For securing freshwater in future, the integrated water resources management is required which should also include calculation of groundwater budget in the coastal aquifer.

This study investigates the occurrence, distribution, and potential ecological risk of trace elements (TEs) along with the hydrogeochemical characteristics of coastal surface water collected from the southern Caspian coasts in the Mazandaran province of Iran. Eighteen coastal water sites were sampled and analyzed using inductively coupled plasma–mass spectrometry and ion chromatography to determine concentrations of TEs and major ions, respectively. Mean concentrations (µg/l) of TEs in the water followed the order: Al (154.3) > Fe (73.6) > Zn (67.8) > Mn (29.9) > Cu (5.7) > Mo (3.7) > Cd (2.8) > Se (2.3) > V (1.9) > Co = As (1.8) > Sb (1.2) > Pb (0.6). TEs displayed high variations within samples, reflecting many sources that control their concentrations in the coastal water. Most TEs displayed elevated concentrations in the east and west of the study area. The Na-Cl water type in the majority of investigated sites indicates excess alkaline elements and strong acid anions relative to alkaline earth cations and weak acid anions. Considering the saturation states of minerals, carbonate and evaporite minerals are oversaturated and unsaturated in surface water, respectively. Compositional interrelations between aqueous species showed that reverse cation exchange may have occurred. The excess SO42− content, derived from irrigation return flow and domestic greywater, probably plays a crucial role in the mobilization and transport of Zn and Pb by binding to sulfate ligands and forming aqueous complexes. Ecological risk assessment of TEs revealed that water in the majority of sites is safe in terms of As, Se, Pb, and Cd content, and unsuitable with respect to Zn and Cu. Acute and chronic toxicities of Cu and Zn are reported in several sites, thus coastal water cannot be used for fishery and protecting “nature reserve” purposes. However, industrial activity and tourism are safe to be conducted in most coastal water sites.

International audience; Climate and human pressures can influence the evolution of estuarine sediment dynamics concurrently, but the understanding and quantification of their cause–effect relationships are still challenging due to the occurrence of complex hydro-morpho-sedimentary feedbacks. The Garonne Tidal River (GTR, upper Gironde Estuary, France) is a clear example of a system stressed by both anthropogenic and climate change, as it has been subject to decreasing river discharges, natural morphological changes, and gravel extraction. To understand the relative effect of each hydrological and geomorphological pressure on the turbidity maximum zone (TMZ), the sediment dynamics in the GTR over the last six decades was evaluated using the width-averaged idealized iFlow model. Model results show a gradual increase in tidal amplitude and currents over the decades that has led to the upstream shift of the landward sediment-transport capacity components (external M4 tide, spatial settling lag, and tidal return flow). The upstream displacement of the TMZ between the 1950s and the 2010s was estimated to be at least 19 km, of which about three fourth was induced by geomorphological changes and one fourth by hydrological changes. Concerning the geomorphological changes, the natural evolution of the lower Gironde morphology was the main pressure inducing the displacement of the TMZ in the GTR. Anthropogenic and natural changes in morphology and bed roughness in the GTR itself also contributed to this evolution. The natural geomorphological changes were, in turn, probably promoted by the evolution of sediment dynamics, so this study reveals the closed circle that governs the intensification of the TMZ.

In 2019, Taiwan completed its first thorough heavy metal investigation of irrigation canal sediments by this study with the support of Taiwan Environmental Protection Administration. Box-and-whisker plots were used to analyze the sediment distribution and to define metal concentrations. Possible metal pollution sources, the polluted agricultural land, irrigation area, and water sources were also evaluated using spatial analysis to understand the possible causes of sediment pollution. Results showed that the main heavy metal in agricultural land was Cu, found in 77% of contaminated agricultural land sites. Most sites with Cu pollution in sediments were in Taoyuan City and Changhua County. The heavy metals present in the sediment pollution sites in Taoyuan were consistent with those of possible pollution sources upstream, namely, Cr, Cu, and Pb. The main heavy metals in sediment pollution sites in Changhua were Cr, Cu, Ni, Pb, and Zn, whereas those for the polluted agricultural land sites were Cr, Cu, Ni, and Zn, without Pb. The main irrigation water sources in Changhua include drainages and rivers, with some receiving most wastewater pollution mass of release of Changhua, and functions as an irrigation water source with a high release mass in Cr, Cu, Ni and Zn. These findings indicate that the sites of sediment pollution, sites of polluted agricultural land, and the sources of pollution share corresponding heavy metal characteristics. Therefore, in Changhua, the sediments were polluted mainly because (1) the irrigation canals received the highest masses of pollutant releases into drainage wastewater of the county, and (2) the return flow from irrigation and the illegal discharge of wastewater. The preliminary assessment results for sediment pollution in Taoyuan also suggest that the main causes may be irrigation by polluted rivers or drainages and return flows.

Depiction of terrestrial water flux constituents in respect of geospatial distribution and consecutive disparity are important for water resources assessment. Due to the excessive and indiscriminate withdrawal of groundwater (GW), the water table (WT) in Dhaka area is continuously depleting during the last few decades. The consequences of increasing water scarcity being a grave concern for the policymakers of the country become a serious issue to save Dhaka areas from its possible failure in terms of human survivability and sustainability. These challenges set the urgency and great importance for holistic assessment of GW resources in the present context. GW recharge is estimated using combined Water Balance approach, Groundwater Estimation Committee methodology, and empirical technique. The monsoon and non-monsoon rainfall recharges (RRM and RRNM) are found as 63% and 37% in Dhaka City; and 55% and 45% in Dhaka district areas. The contributions for overall GW recharge in Dhaka City and Dhaka district from rainfall (RRAnl), water bodies (RSW), return flow (RRFD), and unaccounted recharge (RUAc) are 40.2% and 59.1%; 8.6% and 23.26%; 39.1% and 17.64%; and 12.1% and 0% in 1996, respectively. These have been altered as 22.1% and 55.9%; 2.8% and 16.2%; 37.6% and 27.9%; and 37.5% and 0% in 2015, respectively. It is observed that the total GW recharge in Dhaka City and Dhaka district has altered from 590 and 440 mm in 1996 to 730 mm and 422 mm in 2015, respectively. But the water draft due to domestic, private and industrial use in Dhaka City has increased from 726 and 373 mm in 1996 to 1092 and 582 mm in 2015, respectively. The water draft due to irrigation, and private & industrial use in Dhaka district is found as 256 and 110 mm in 1996 to 1369 and 166 mm in 2015, respectively. The seasonal WT fluctuation varies from 1.1 to 2.3 m in falling trend except Dhaka City where no significant variations are observed. The WT in Dhaka City has depleted from 17 m bgl with a rate of 1.5 m/year in 1996 to 63 m bgl in with a rate of 3.2 m/year in 2015. The WT in Dhaka district has been uplifted from 6.4 m bgl with a rate of 0.82 m/year in 1996 and then falling downward at a rate of 0.8 m/year in 2003 and reached to 14.45 m bgl at a rate of 1.52 m/year in 2015. The stage of GW development (SGD) and categorization in Dhaka City fall within 172–208% as ‘vulnerable’ during 1996 which have further aggravated to 229–272% as ‘highly vulnerable’ during 2015. The SGD and categorization in peri-urban area are estimated as 108–112% as ‘overexploited’ during 1996 which has further risen to 150–158% as ‘vulnerable’ during 2015, respectively. But, the condition is better in rural areas where, SGD and categorization are analysed as 61–70% as ‘safe’ in 1996 which has further deteriorated to 108–117% as ‘overexploited’ in 2015.

Yinchuan Plain is a typically intensive cultivated region in the northwest of China. The irrigation return infiltration from Yellow River is the main source of groundwater recharge. Deep soil layers, sandy vadose zones, and dense irrigation canals make the groundwater susceptible to the return flow which contains pollutants originating mainly from agriculture applications, particularly from the extensive use of nitrogen fertilizer and manure. The pollution levels of phreatic water and confined water in NWS areas (non-water source areas) and WS areas (water source areas) of Yinchuan Plain in 2004 and 2014 were evaluated by the single-factor evaluation method, fuzzy comprehensive evaluation method, and average benchmark coefficient method, respectively. Piper trilinear diagram and scatter plots of major ions were used to classify water types and chemical facies, and further analyze the causes of groundwater pollution and the variation tendency of agricultural pollution. The results show that in 2014, about 50% of the groundwater samples were heavily polluted in Yinchuan Plain, the pollution level of phreatic water and confined water in NWS areas was up to level 5. And the groundwater within the standard in 2004 was heavily polluted in 2014 in WS areas, three-nitrogen pollution was the most serious pollutant, and the organic pollution level was grade IV. From the scatter plots of ions, it can be seen that the increase in concentrations of major ions was affected by evaporation-condensation and cation exchange reaction, but the complex ion contents indicated that groundwater was affected by human activities. The intensive agricultural activities, such as over fertilization, artificial irrigation, have led to concentrations increase of some chemical composition in groundwater.

The two-parameter recursive digital filter method (Eckhardt) and the conductivity mass balance (CMB) method are two widely used baseflow separation methods favored by hydrologists. Some divergences in the application of these two methods have emerged in recent years. Some scholars believe that deviation of baseflow separation results of the two methods is due to uncertainty of the parameters of the Eckhardt method and that the Eckhardt method should be corrected by reference to the CMB method. However, other scholars attribute the deviation to the fact that they contain different transient water components. This study aimed to resolve this disagreement by analyzing the effectiveness of the CMB method for correcting the Eckhardt method through application of the methods to 26 basins in the United States by comparison of the biases between the generated daily baseflow series. The results showed that the approach of calibrating the Eckhardt method against the CMB method provides a “false” calibration of total baseflow by offsetting the inherent biases in the baseflow sequences generated by the two methods. The baseflow sequence generated by the Eckhardt method usually includes slow interflow and bank storage return flow, whereas that of the CMB method usually includes high-conductivity water flushed from swamps and depressions by rainfall, but not low-conductivity interflow and bank storage return flow. This difference results in obvious peak misalignment and periodic deviation between the baseflow sequences obtained by the two methods, thereby preventing calibration. However, multi-component separation of streamflow can be achieved through comparison. Future research should recognize the deviations between the separation results obtained by the different methods, identify the reasons for these differences, and explore the hydrological information contained therein.

The main groundwater aquifer in El Fayoum region is a Quaternary aquifer. It shows increasing signs of groundwater quality deterioration. Therefore, a comprehensive assessment of groundwater quality has been performed for determining the geochemical processes affecting the groundwater quality evolution and assessing groundwater quality for drinking and irrigation uses. Physicochemical parameters, geochemical model (NETPATH), and water quality indices (WQIs) are applied to improve water quality assessment and controlling mechanisms with help of statistical analysis and GIS techniques. Thirty-eight groundwater samples from the Quaternary aquifer at depths less than 60 m and 12 irrigation drainage samples were collected. Thirteen physicochemical parameters including temperature (°C), TDS, pH, EC, K+, Na+, Ca+2, Mg2+, HCO3−, Cl−, SO42−, CO32−, and NO3− were measured by standard analytical techniques. The analytical findings reveal that the groundwater facies are Na-Cl, mixed Ca-Mg-Cl, and Na-Ca-HCO3 water types. In addition, the minerals saturation indices (SI) estimated by a geochemical model revealed that the groundwater samples are supersaturated by kaolinite, Ca-montmorillonite, dolomite, gibbsite, calcite, quartz, and aragonite and are unsaturated by gypsum, anhydrite, and partial pressure of CO2. The drinking water quality index (DWQI) results revealed that 58% of the groundwater samples are very poor quality, while 42% of samples are unsuitable for drinking uses. The irrigation water quality index (IWQI) results revealed that all groundwater samples range from 26.1 to 36.1, indicating a medium class of quality for irrigation purposes. These results reflect the influences of the high mineralization processes, cumulative effect of cation exchange, leaching processes from aquifer materials, contamination by irrigation return flow, and seepage from drains on the groundwater quality. In conclusion, utilization of physicochemical parameters, geochemical model, and WQIs can be considered as a promising and applicable approach to determine the controlling geochemical mechanisms and for assessing of groundwater quality in El Fayoum Depression, Egypt.

Idealized simulations show that the approximate colocation between the ITCZ and the energy flux equator (EFE), which holds on the annual and zonal average, breaks down on subseasonal timescales, as the Hadley cell develops a shallow return flow and negative gross moist stability (GMS). Here, we explore if similar mechanisms are seen in reanalysis data. In the zonal mean, a temporal offset exists between the ITCZ and the EFE as the ITCZ is retreating from the Northern to Southern Hemisphere and the Hadley cell transports energy northward across the equator despite a northward‐shifted ITCZ. At these times, the southern cell has a bottom‐heavy structure, with a distorted cell boundary and northward energy transport. In the Eastern Pacific, while bottom‐heavy structures exist throughout the year, the bottom heaviness is stronger in boreal fall, when GMS is negative, and SSTs are weak while their Laplacian is large and negative below the ITCZ.

During the last decades, the endorheic Lake Urmia basin in northwestern Iran has suffered from declining groundwater tables and a very strong recent reduction in the volume of Lake Urmia. For the case of Lake Urmia basin, this study explores the value of different locally and globally available observation data for adjusting a global hydrological model such that it can be used for distinguishing the impacts of human water use and climate variations. The WaterGAP Global Hydrology Model (WGHM) was for the first time calibrated against multiple in situ and spaceborne data to analyze the decreasing lake water volume, lake river inflow, loss of groundwater, and total water storage in the entire basin during 2003–2013. The calibration process was done using an automated approach including a genetic algorithm (GA) and non-dominated sorting genetic algorithm II (NSGA-II). Then the best-performing calibrated models were run with and without considering water use to quantify the impact of human water use. Observations encompass remote-sensing-based time series of annual irrigated areas in the basin from MODIS, monthly total water storage anomaly (TWSA) from GRACE satellites, and monthly lake volume anomalies. In situ observations include time series of annual inflow into the lake and basin averages of groundwater level variations based on 284 wells. In addition, local estimates of sectoral water withdrawals in 2009 and return flow fractions were utilized. Calibration against MODIS and GRACE data alone improved simulated inflow into Lake Urmia but inflow and lake volume loss were still overestimated, while groundwater loss was underestimated and seasonality of groundwater storage was shifted as compared to observations. Lake and groundwater dynamics could only be simulated well if calibration against groundwater levels led to an adjustment of the fractions of human water use from groundwater and surface water. Thus, in some basins, globally available satellite-derived observations may not suffice for improving the simulation of human water use. According to WGHM simulations with 18 optimal parameter sets, human water use was the reason for 52 %–57 % of the total basin water loss of about 10 km3 during 2003–2013, for 39 %–43 % of the Lake Urmia water loss of about 8 km3, and for up to 87 %–90 % of the groundwater loss. Lake inflow was 39 %–45 % less than it would have been without human water use. The study shows that even without human water use Lake Urmia would not have recovered from the significant loss of lake water volume caused by the drought year 2008. These findings can support water management in the basin and more specifically Lake Urmia restoration plans.

Accurate forecasting of passenger flow (i.e., ridership) is critical to the operation of urban metro systems. Previous studies mainly model passenger flow as time series by aggregating individual trips and then perform forecasting based on the values in the past several steps. However, this approach essentially overlooks the fact that passenger flow consists of trips from each individual traveler. For example, a traveler’s work trip in the morning can help predict his/her home trip in the evening, while this causal structure cannot be explicitly encoded in standard time series models. In this paper, we propose a new forecasting framework for boarding flow by incorporating the generative mechanism into standard time series models and leveraging the strong regularity rooted in travel behavior. In doing so, we introduce returning flow from previous alighting trips as a new covariate, which captures the causal structure and long-range dependencies in passenger flow data based on travel behavior. We develop the return probability parallelogram (RPP) to summarize the causal relationships and estimate the return flow. The proposed framework is evaluated using real-world passenger flow data, and the results confirm that the returning flow—a single covariate—can substantially and consistently improve various forecasting tasks, including one-step ahead forecasting, multi-step ahead forecasting, and forecasting under special events. And the proposed method is more effective for business-type stations with most passengers come and return within the same day. This study can be extended to other modes of transport, and it also sheds new light on general demand time series forecasting problems, in which causal structure and long-range dependencies are generated by the user behavior.

Irrigation return flow (RF) is a critical component of the water cycle in an agricultural watershed, influencing the flow regime of downstream river. As such, it should be accurately quantified when developing water resources management plans and practices. Although many studies have proposed ways to quantify RF, uncertainty in RF estimates has not been determined to improve reliability and credibility. This study examines how conceptual (CON) and physically-based (PHY) parameterization approaches affect RF uncertainty. Results showed that PHY had a smaller amount of RF uncertainty compared to CON, as parameters of the PHY approach could be regulated based on their physical meanings. This study also found that the application of constraints created based on the relationship between the conceptual parameter and physical characteristics of irrigated plots could effectively reduce RF uncertainty made using the CON approach. This study demonstrates the benefits of the physically-based parameterization approach and the application of constraints on conceptual parameters to RF estimation.

Nitrate concentrations in groundwater have been historically high (N≥11.3 mg L−1) in an area surrounding Tinwald, Ashburton, since at least the mid-1980s. The local community is interested in methods to remediate the high nitrate in groundwater. To do this, they need to know where the nitrate is coming from. Tinwald groundwater exhibits two features stemming from irrigation with local groundwater (i.e. irrigation return flow). The first feature is increased concentrations of nitrate (and other chemicals and stable isotopes) in a “hotspot” around Tinwald. The chemical concentrations of the groundwater are increased by recirculation of water already relatively high in chemicals. The irrigation return flow coefficient C (irrigation return flow divided by irrigation flow) is found to be consistent with the chemical enrichments. The stable isotopes of the groundwater show a similar pattern of enrichment by irrigation return flow of up to 40 % and are also enriched by evaporation (causing a loss of about 5 % of the original water mass). Management implications are that irrigation return flow needs to be taken into account in modelling of nitrate transport through soil–groundwater systems and in avoiding overuse of nitrate fertiliser leading to greater leaching of nitrate to the groundwater and unnecessary economic cost. The second feature is the presence of “denitrification imprints” (shown by enrichment of the δ15N and δ18ONO3 values of nitrate) in even relatively oxic groundwaters. The denitrification imprints can be clearly seen because (apart from denitrification) the nitrate has a blended isotopic composition due to irrigation return flow and N being retained in the soil–plant system as organic N. The nitrate concentration and isotopic compositions of nitrate are found to be correlated with the dissolved oxygen (DO) concentration. This denitrification imprint is attributed to localised denitrification in fine pores or small-scale physical heterogeneity where conditions are reducing. The implication is that denitrification could be occurring where it is not expected because groundwater DO concentrations are not low.

Environmental tracers were used to characterize the origin and determine the age of the groundwater in the Motril-Salobrena aquifer (south-eastern Spain). The stable isotope concentrations (delta O-18/delta H-2), compared to the results obtained in previous studies, indicate that most of the recharge during the sampling period was from irrigation return flow and the carbonate Escalate aquifer. The combined dating of H-3, He-3, He-4, Kr-85, and Ar-39 allowed establishing the presence of modern water throughout the aquifer, although with different mixing percentages. Thus, there is a large zone characterized by a fluvial domain with 100% young waters ( 170 years), and the percentage of young water is reduced (22.5%). This is explained by the greater distance that groundwater travels (aquifer thicknesses is over 250 m) and the lower permeability of the aquifer in the deeper sectors.

Motivated by the observation of sediments being carried by meltwater plumes originating at the base of a marine terminating glacier, laboratory experiments are performed to examine the transport and deposition of particles settling out from a buoyant line-plume rising along a sloping upper boundary. If the plume source has relatively high momentum and is located near the bottom of the domain, then a strong recirculating region develops near the source. Emanating from this region is a particle bearing buoyant plume that moves at near constant speed along the slope. Particles are observed to settle within the plume itself and then descend from the plume toward the tank bottom being drawn back in the direction of the source through a return flow driven by the plume's entrainment of the underlying ambient fluid. A light attenuation technique is employed to measure nonintrusively the depth of the sediment bed after the source is turned off and all the particles settled out. Sediments are found to accumulate near the source over the extent of the recirculating region and then decrease approximately linearly with distance from the source. Conceptual theoretical models suggest that the linearly sloping bed results from a combination of vertical mixing in the plume near the recirculation region and the return flow acting to detrain from the plume particles more effectively near the plume source where the shear between the plume and the return flow is largest. In many aspects the experiments are not representative of a steady glacial meltwater plume due to restrictions of the experimental setup, notably the relatively low Reynolds number of the flow. Nonetheless the experiments are suggestive of the complicated dynamics and sediment deposition patterns that may occur near the base of a marine terminating glacier, with a crude estimate of mean clay deposition at a rate of 6 cm per year over a distance of 2 km from the source.

The concentration of solids in secondary sludge before anaerobic digestion in a wastewater treatment plant, bring about the production of a return flow, which contains high concentrations of all the common pollutant parameters. This return flow could unfavourably affect the performance of the processes and effluent quality of the waterline. Here, we report the utilisation of three similar microbial electrolysis cells reactors that performs simultaneous carbon and nitrogen removal to reduce the impact of the return flow in the plant. The result of the batch-fed (72 h) experiment showed COD and total nitrogen removal efficiencies that reached 90% and 80%, respectively, supporting the premise that return flows are suitable substrates for a bioelectrochemical treatment. The three reactors followed similar trends, showing good replicability and confirming the potential of MECs as a feasible technology for return flow treatment. Furthermore, when cathodic conversion efficiency was higher than 80%, the pure hydrogen production allows to recover the electric energy consumption, indicating that the system could be theoretically energy neutral.

Water managers around the world face the increasingly challenging task to evaluate the impacts of technological measures and policy mechanisms from the local to the river basin scale. A toolset providing quantitative, actionable information on dependencies and trade-offs between upstream and downstream water users is currently lacking. Yet, any intervention needs to be assessed in terms of consequences for downstream water users. This study evaluates the potential of a tracer-like approach, implemented in the water allocation software WEAP, to quantitatively track return flows and their downstream reuse in the river basin context. The WEAP-Virtual Tracer (WEAP-VT) approach was successfully applied to one of Europe&rsquo, s driest river basins, the Segura River Basin in Spain. For each water demand site, the different original sources of water supply, dependency on upstream return flows, and downstream reuse of its return flow were assessed. Based on these results, agricultural, urban, and environmental water users were evaluated in terms of their suitability for water saving measures and their vulnerability to the reduction in upstream return flows. A scenario analysis simulating the improvement of local efficiency improvements shows that specific irrigation schemes and ecosystems become deprived of water. Hence, efficiency improvement in water-scarce basins should be considered with caution. The demonstrated ability to quantify key water reuse indicators for individual water users and at different aggregation levels makes WEAP-VT a valuable tool to support water resource management decisions.

A portion of water not consumed by crops during flood irrigation can flow back across the surface or through the subsurface to adjacent surface water bodies and streams as return flow. Few studies have directly addressed subsurface processes governing return flow and the importance of structural complexity on hydrologic process representation. It is challenging to measure and model these subsurface flow paths using traditional hydrologic observations. In this study, we assess the impact of subsurface structural complexity on vadose zone flow representation in a two‐dimensional transport model by varying structural complexity derived from background geophysical data. We assessed four model structures each with three soil types of homogeneous hydrologic properties, two of which were evaluated with and without an anisotropy factor. Wetting front arrival times, derived from time‐lapse electrical resistivity measurements during flood irrigation field experiments, were used to evaluate the different representations of soil profile structures. These data indicated both vertical and lateral preferential flow in the subsurface during flood irrigation. Inclusion of anisotropy in the saturated hydraulic conductivity field improved the ability to model subsurface hydrologic behavior when flow processes shifted from uniform to heterogeneous flow, as occurs with lateral subsurface return flow under flood irrigation driven by a large pressure gradient. This reduced the need for detailed spatial discretization to represent these observed subsurface flow processes. The resulting simple three‐layer model structure was better able to model both the vertical and lateral flow processes than a more complex geospatial structure, suggesting that overinterpretation of smoothed inverted profiles could lead to misrepresentation of the subsurface structure.

We present the results of a detailed structural study in the Cycladic Blueschist Unit at Fabrika on Syros Island, Greece, and discuss their significance for tectonic processes at the subduction interface. Some samples record top‐to‐the‐west shear reflecting prograde (burial), peak high‐pressure (HP) and initial decompression (exhumation) conditions. Other nearby samples record top‐to‐the‐east shear during HP metamorphism and exhumation. Some rocks re‐equilibrated at greenschist‐facies conditions and record top‐to‐the‐west shear. Greenschist‐facies top‐to‐the‐west shear is also found at the base of non‐HP upper units above the Fabrika HP sequence. We interpret the HP structures to reflect forced return flow and incipient formation of an extrusion wedge in the subduction channel. The HP top‐to‐the‐west structures resulted from thrusting along the base of the wedge and started to form during burial before the rocks reached their deepest point. The HP top‐to‐the‐east structures reflect deformation near the top of the developing extrusion wedge. After considerable exhumation during ongoing subduction, out‐of‐sequence, top‐to‐the‐west thrusts emplaced the non‐HP upper units above the exhuming extrusion wedge ~10 Myr after the wedge initially formed. Our work suggests that the HP rocks were considerably exhumed during sustained lithospheric shortening in the subduction channel by forced return flow. Because return flow is controlled by the velocity of the subducting slab, it may explain why HP rocks can be exhumed at subduction rates. On the regional scale we find that four distinct HP belts were sequentially accreted and exhumed between ~50 and 20 Ma suggesting continuous subduction‐channel return flow in the Hellenic subduction zone.

A calibrated fate and reactive transport model is applied to evaluate alternative water and land best management practices (BMPs) in Colorado’s intensively irrigated Arkansas River Valley to attenuate nonpoint source pollution and more closely meet regulatory standards for selenium (Se) and nitrogen (N) in groundwater and streams. Reduced irrigation (RI), lease fallowing (LF), canal sealing to reduce seepage (CS), reduced fertilizer application (RF), and enhanced riparian buffers (ERB) are explored as stand-alone BMPs, and in combination, at basic to more aggressive levels of implementation. The distributed-parameter model, which couples MODFLOW-SFR with RT3D-OTIS, predicts impacts that vary significantly over a region encompassing about 500 km2 and across time. Results suggest that, over the course of several decades, average Se and nitrate-nitrogen (NO3-N) groundwater concentrations within the region could be lowered by as much as 23% and 40%, respectively, using combined BMPs. Average Se concentration in the river could be decreased by up to 56% with combined BMPs, and NO3-N concentrations by up to 32% by using ERB. The CS-RF-ERB combination type may be the most promising for simultaneously lowering both Se and NO3-N concentrations. To insure compliance with Colorado water rights and the Arkansas River Compact with Kansas, measures must be taken to compensate for altered return flow patterns that will be a consequence of BMP implementation. Results also point to the need to consider the targeting of BMPs to specific locations within the region to maximize their effectiveness and efficiency.

The Panama Basin serves as a laboratory to investigate abyssal water upwelling. The basin has only a single abyssal water inflow pathway through the narrow Ecuador Trench. The estimated critical inflow through the Trench reaches 0.34 ± 0.07 m s−1, resulting in an abyssal water volume inflow of 0.29 ± 0.07 Sv. The same trench carries the return flow of basin waters that starts just 200 m above the bottom and is approximately 400 m deeper than the depth of the next possible deep water exchange pathway at the Carnegie Ridge Saddle. The curvature of temperature‐salinity diagrams is used to differentiate the effect of geothermal heating on the deep Panama Basin waters that was found to reach as high as 2200 m depth, which is about 500 m above the upper boundary of the abyssal water layer.

Hydrochemical investigations, including geochemical analyses, multivariate statistics and geostatistics, were conducted to assess the factors that influence groundwater geochemistry and pollution potentiality in Luxor area, Upper Egypt. A total of thirty-one groundwater and surface water samples from the Quaternary aquifer and the River Nile were analyzed for fourteen physical and chemical variables for each sample. Spatial variations in total dissolved solids and nitrate concentration were mapped. Piper and Durov diagrams indicate that the hydrochemistry of groundwater is influenced by the secondary processes; mixing with fresh water from the River Nile and El Kalabia Canal, irrigation return flow, and sewage leakage, and reverse ion-exchange process. The hydrochemical modeling of mineral phase saturation indices shows that nearly all of the groundwater points are undersaturated with reference to calcite, aragonite, dolomite, anhydrite, gypsum, and halite. Correlation coefficients of the different variables are consistent with the saturation indices. Cluster analysis was used to identify four significant, distinct groundwater zones where the original groundwater was influenced differently by mixing processes. Factor analysis showed four mutually interfering factors reveal the chemical characteristics of the groundwater; these factors are caused by rock-water interactions, mixing of waters of different origins, and anthropogenic effects. Integration of hydrochemical and statistical analyses approach can be applied for the better management of water resources at a regional scale and in areas with comparable conditions.

High level of arsenic-containing groundwater has been used for irrigation purposes for several decades in Asia, leading to fluctuating water table and redox conditions in the unsaturated zone, thereby potentially affecting the mobilization of arsenic in the unsaturated and saturated zones. A field plot experiment was conducted in the arsenic – affected area of the Datong Basin, China to determine the effects of irrigation return flow on the hydrogeochemical behavior of arsenic and iron in the unsaturated zone. High-arsenic groundwater was extracted from a shallow aquifer far from the irrigation site and used as irrigation water. Soil water/shallow groundwater and soil/sediment samples were collected at different depths from the field site during the experiment. Water samples were analyzed to determine hydrochemical properties and arsenic concentrations, and soil/sediment samples were analyzed to determine total iron and arsenic. Infiltration of irrigation water into the unsaturated zone led to fluctuations in the water table, varying redox conditions, and the redistribution of arsenic and iron in the near-surface soil/sediment. Soil/sediment bulk geochemical analysis results indicate obvious increases of arsenic and iron in topsoil after irrigation. Infiltration of irrigation water carrying organic matter, sulfate, and nitrate into the subsurface affects the (bio)geochemistry of the unsaturated and saturated zones, and ultimately the behavior of arsenic and iron. Geochemical modeling results suggest desorption and leaching processes are responsible for the temporal changes of arsenic concentrations in both pore water, groundwater and sediment during irrigation. The results of this study indicate that flood irrigation using arsenic-contaminated groundwater should be controlled and gradually replaced by drip irrigation, sprinkler irrigation or irrigation using non-contaminated water resources to mitigate arsenic accumulation in the unsaturated zone and shallow groundwater.

During the last 25 years, rapid and unplanned land reclamation activity has been carried out in the areas located in both south and east of Wadi El - Natrun Depression of Egypt. Accordingly, negative effects on groundwater levels and vulnerability are frequently caused by localized high levels of abstraction and the return-flow of polluted irrigation water respectively. A groundwater model is a computational method that presents an approximation of an underground water system. In this study the groundwater system is simulated both in quantity and quality by using Mass Balance Transfer Model (NETPATH), Groundwater Modeling System (GMS) and DRASTIC Model to investigate the water - rock interactions, groundwater levels drawdown and vulnerability respectively. Three main geochemical processes namely dedolomitisation, dissolution of halite and silicate weathering were estimated during the flow path. The present over-abstraction of groundwater (105.84 million m3/year) has induced a general head drawdown from 3 to 40 m in years 2015 and 2050 respectively. Best estimate using a 3D GMS hydraulic model was (157000 m3/day) a strategy proposed for the management of groundwater without critical depletion (second scenario). The results document the extent to which a high drawdown can greatly reach 4 m by the end of simulation year 2050. The vulnerability maps of groundwater were constructed using the DRASTIC index method. The results indicated that, the southeastern and central portions of the study area are having high vulnerability rate (> 110). Modified DRASTIC map showed many more dominant high risk areas in the eastern parts of the study area that were low risk, which may be attributed to return flow of polluted irrigation water.

Irrigation efficiency plays an important role in agricultural productivity; it affects farm-scale water demand, and the partitioning of irrigation losses into evaporative and non-evaporative components. This partitioning determines return flow generation and thus affects water availability. Over the last two decades, hydrologic and agricultural research communities have significantly improved our understanding of the impacts of climate change on water availability and food productivity. However, the impacts of climate change on the efficiency of irrigation systems, particularly on the partitioning between evaporative and non-evaporative losses, have received little attention. In this study, we incorporated a process-based irrigation module into a coupled hydrologic/agricultural modeling framework (VIC-CropSyst). To understand how climate change may impact irrigation losses, we applied VIC-CropSyst over the Yakima River basin, an important agricultural region in Washington State, U.S. We compared the historical period of 1980–2010 to an ensemble of ten projections of climate for two future periods: 2030–2060 and 2060–2090. Results averaged over the watershed showed that a 9% increase in evaporative losses will be compensated by a reduction of non-evaporative losses. Therefore, overall changes in future efficiency are negligible (−0.4%) while the Evaporative Loss Ratio (ELR) (defined as the ratio of evaporative to non-evaporative irrigation losses) is enhanced by 10%. This higher ELR is associated with a reduction in return flows, thus negatively impacting downstream water availability. Results also indicate that the impact of climate change on irrigation losses depend on irrigation type and climate scenarios.