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Fresh submarine groundwater discharge (FSGD) can deliver significant fluxes of water and solutes from land to sea. In the Arctic, which accounts for ∼34% of coastlines globally, direct observations and knowledge of FSGD are scarce. Through integration of observations and process‐based models, we found that regardless of ice‐bonded permafrost depth at the shore, summer SGD flow dynamics along portions of the Beaufort Sea coast of Alaska are similar to those in lower latitudes. Calculated summer FSGD fluxes in the Arctic are generally higher relative to low latitudes. The FSGD organic carbon and nitrogen fluxes are likely larger than summer riverine input. The FSGD also has very high CO2 making it a potentially significant source of inorganic carbon. Thus, the biogeochemistry of Arctic coastal waters is potentially influenced by groundwater inputs during summer. These water and solute fluxes will likely increase as coastal permafrost across the Arctic thaws. Plain Language Summary Groundwater flows from land to sea, transporting freshwater, organic matter, nutrients, and other solutes that impact coastal ecosystems. However, along coasts of the rapidly‐warming Arctic, there is limited knowledge regarding how much fresh groundwater enters the ocean. Using field observations and numerical models, we show that groundwater flowing from tundra in northern coastal Alaska carries large amounts of freshwater, organic matter, and carbon dioxide to the Arctic lagoons during summer. These inputs are likely significant to coastal biogeochemical cycling and marine food webs. Groundwater discharge and the associated transport of dissolved materials are expected to increase due to longer periods of above‐zero temperatures that thaw frozen soils below the tundra. Key Points Summer fresh submarine groundwater discharge (FSGD) to the Alaskan Beaufort Sea is only 3%–7% of rivers but carries as much organic matter Summer FSGD delivers a median of 116 (interquartile range: 35–405) and 6 (2–21) kg/d per km dissolved organic carbon and nitrogen Fresh groundwater at the beach of Simpson Lagoon (SL) has a median PCO2 of ∼33,000 μatm implying substantial CO2 flux

Hyporheic flow can be extremely variable in space and time, and our understanding of complicated flow systems, such as exchange around small dams, has generally been limited to reach‐averaged parameters or discrete point measurements. Emerging techniques are starting to fill the void between these disparate scales, increasing the utility of hyporheic research. When ambient diurnal temperature patterns are collected at high spatial resolution across vertical profiles in the streambed, the data can be applied to one‐dimensional conduction‐advection‐dispersion models to quantitatively describe the vertical component of hyporheic flux at the same high spatial resolution. We have built on recent work by constructing custom fiber‐optic distributed temperature sensors with 0.014 m spatial resolution that are robust enough to be installed by hand into the streambed, maintain high signal strength, and permit several sensors to be run in series off a single distributed temperature sensing unit. Data were collected continuously for 1 month above two beaver dams in a Wyoming stream to determine the spatial and temporal nature of vertical flux induced by the dams. Flux was organized by streambed morphology with strong, variable gradients with depth indicating a transition to horizontal flow across a spectrum of hyporheic flow paths. Several profiles showed contrasting temporal trends as discharge decreased by 45%. The high‐resolution thermal sensors, combined with powerful analytical techniques, allowed a distributed quantitative description of the morphology‐driven hyporheic system not previously possible. Key Points Shallow hyporheic flux hot spots organize by streambed morphologic unit Flux may both increase and decrease through time within the same system High resolution fiber‐optic heat tracing is a valuable tool to the community

Current models on solute transport often fail to reproduce discharge‐dependent behavior of solute transport in stream reaches because they rely on the assumption of well‐mixed conditions and fail to account for the complex coupling between in‐stream and subsurface flow. StorAge Selection (SAS) functions describe outflow as a mixture of waters of different ages, providing a framework to overcome the well‐mixed assumption in “traditional” transport models. In this study, we applied SAS functions to model solute transport from 13 slug tracer experiments conducted under varying hydrological conditions in a headwater stream reach. Using SAS function parameters (expressed in units of volume) together with measurements of groundwater (GW) levels and streambed microtopography, we partitioned the total water storage within the study reach into distinct components: advective storage, in‐stream transient storage, tracer‐derived hyporheic storage, and GW level‐derived hyporheic storage. This partitioning assumes that transport processes and subsurface water flow in stream reaches are associated with different storage volumes. We found positive linear relationships between discharge and age‐ranked, advective, and tracer‐derived hyporheic storage. In‐stream transient storage increased with discharge up to 17 L s−1, corresponding to the discharge threshold above which streambed sediments became completely submerged, and declined at higher flows. This pattern likely reflects the contribution of eddies at lower discharge levels and highlights the importance of in‐stream transient storage for solute transport. Our results demonstrate that partitioning the total water storage in a reach–enabled only through applying SAS functions–is essential for understanding and modeling solute transport under varying hydrological conditions.

Groundwater return flow to streams is important for maintaining aquatic habitat and providing water to downstream users, particularly in irrigated watersheds experiencing water scarcity. However, in many agricultural regions, increased irrigation efficiency has reduced return flows and their subsequent in‐stream benefits. Agricultural managed aquifer recharge (Ag‐MAR)—where artificial recharge is conducted via irrigation canals and agricultural fields—may be a tool to recover these return flows, but implementation is challenged by water supply and water management. Using climate‐driven streamflow simulations, an integrated operations‐hydrology model, and a regional groundwater model, we investigated the potential for Ag‐MAR to recover return flows in the Henrys Fork Snake River, Idaho (USA). We simulated potential Ag‐MAR operations for water years 2023–2052, accounting for both future water supply conditions and local water management rules. We determined that Ag‐MAR operations reduced springtime peak flow at the watershed outlet by 10%–14% after accounting for return flows. Recharge contribution to streamflow peaked in July and November, increasing July–August streamflow by 6%–14% and November–March streamflow by 9%–14%. Furthermore, sites where Ag‐MAR was conducted incidental to flood irrigation had more water available for recharge, compared to sites requiring recharge rights, which are junior in priority to agricultural rights. Mean annual recharge volume for the incidental recharge sites averaged 12% of annual natural streamflow, ranged from 269 to 335 Mm3, and was largely available in April and October. We demonstrate Ag‐MAR can effectively recover groundwater return flows when applied as flood irrigation on agricultural land with senior‐priority water rights. Plain Language Summary Some water that seeps into the ground follows underground pathways and flows into rivers. This water is called groundwater return flow and is important in water‐scarce regions. Groundwater return flow increases streamflow and provides more water to farms and fish downstream but is decreasing in many agricultural regions. Agricultural managed aquifer recharge (Ag‐MAR) may be a tool to save these return flows. Ag‐MAR uses canals and agricultural fields to artificially increase the amount of water that seeps into the ground, but conducting Ag‐MAR can be difficult during water shortages and/or due to water management rules. We investigated the possibility of using Ag‐MAR to recover return flows in the Henrys Fork Snake River, Idaho (USA), accounting for both future water supply conditions and rules that limit access to water supply. We simulated 30 years of future conditions and found that water for Ag‐MAR is largely available at sites with senior‐priority water rights. We also found that conducting Ag‐MAR reduced peak springtime streamflows by 10%–14% but increased summer streamflow by 6%–14% and winter streamflow by 9%–14%. Thus, Ag‐MAR can effectively recover groundwater return flows in our study area. Key Points Water was reliably available to conduct agricultural managed aquifer recharge (Ag‐MAR) via flood irrigation with senior‐priority water rights, even in dry years Water was available in <30% of years to conduct Ag‐MAR at designated recharge basins with junior‐priority water rights Ag‐MAR generated return flows that increased July–August streamflow by 6%–14% and November–March streamflow by 9%–14%

Groundwater is a crucial resource to support surface water bodies via groundwater discharge. In this study, we applied two methods of estimating global environmentally critical groundwater discharge, defined as the flux of groundwater to streamflow necessary to maintain a healthy environment, from 1960 to 2010: the Presumptive Standard stipulates that a standard proportion of groundwater discharge should be maintained at all timesteps, while the Q* is a low‐flow index that focuses on critical periods. We calculated these critical flow thresholds using simulated natural groundwater discharge, and estimated violations of the thresholds when human‐impacted groundwater discharge dropped too low. Our global assessment of the frequency and severity of violations over all timesteps in our study period showed that the Presumptive Standard estimated more frequent and severe violations than the Q*, but that the spatial patterns were similar for both methods. During low‐flow periods, when the relative importance of groundwater to support streamflow is greatest, both methods estimated similar magnitudes of violation frequency and severity. We further compared our results to a method of estimating environmentally critical streamflow, Variable Monthly Flow, which does not explicitly consider groundwater. From the differences in violation frequency between these groundwater‐centric and surface water‐centric methods, we evaluated the influence of including groundwater contributions to streamflow in environmental flow assessments. Our results show that including groundwater in such assessments is particularly important for regions with high groundwater demands in the drier climates of the world, while it is less important for regions with low groundwater demands and more humid climates. Plain Language Summary We used two global methods of estimating the necessary flow of groundwater to surface water to protect environmental health. One method (Presumptive Standard) is designed to maintain environmental flows over the whole year, while the other method (Q*) focuses on low‐flow periods when groundwater plays a larger role in supporting surface water. We estimated historic violations of these environmentally critical flow thresholds and found that they estimated similar spatial patterns (although at different magnitudes). We then compared the violations to a method of estimating environmentally critical streamflow (Variable Monthly Flow), which does not directly consider groundwater. Here, we found that considering groundwater contributions to surface water affects the estimated environmental impacts of water use, particularly in river basins that are dry, have high amounts of agriculture, and are densely populated. From this study, we conclude that including groundwater in environmental flow assessments is important in regions with significant groundwater use, and that the choice of method should depend on the period of focus. Key Points First global comparison of methods to calculate the environmentally critical contributions of groundwater to streamflow The methods identified similar hotspots of historic violations of environmentally critical groundwater discharge The utility of the methods depends on whether an environmental flow assessment is important for all flow seasons or only low‐flow periods

Analytical solutions to the 1‐D heat transport equation can be used to quantify surface‐groundwater interactions directly from temperature time series data in streams and their streambeds. The solutions rely on three assumptions: purely vertical flow, no thermal gradient with depth in the streambed and sinusoidal temperature signals. Here numerical models of heat transport in streambeds are used to generate synthetic time series data under conditions that violate the aforementioned assumptions. These synthetic records are used to evaluate the impact of violations of model assumptions on vertical water velocity estimates. Analytical methods using the amplitude ratio to derive flux are less prone to error than methods that use lag time under nonideal field conditions. The greatest source of error is nonvertical flow in the streambed. Errors from analytical solutions for flux are comparable to or smaller than errors inherent to Darcy‐based flux estimates and therefore the use of temperature data to quantify flux across the streambed is a promising alternative to more commonly used approaches, such as Darcy flux calculations.

The exchange between surface water (SW) and groundwater (GW) influences water availability and ecosystems in stream networks. Assessing GW‐SW interactions can be based on various methods at different scales, such as point scale (e.g., local head differences, temperature profiles), reach scale (e.g., environmental tracers, water mass balance), and catchment scale (topographical‐driven groundwater flow), which all have distinct advantages and limitations. In this study, we combined the analysis of local hydraulic head differences with regional topographical‐driven groundwater flow to robustly reveal gaining and losing stream patterns in two study regions in Central Germany (Bode catchment and Free State of Thuringia). To evaluate local hydraulic gradients, we developed a method for estimating surface water levels across stream networks by modifying surface elevations from a coarse digital elevation model (25 m) and compared these to measured groundwater levels. Our results reveal prevalent occurrences of losing streams. Numerous stream locations are characterized by mismatching classifications from the two methods providing additional insights for understanding water cycles. The most notable discrepancy is the classification as losing based on head differences and gaining from topographic analyses accounting for 37% and 47% of the stream locations in Thuringia and in Bode catchment. This mismatch indicates anthropogenically lowered groundwater levels, typically occurring in urban and mining areas in the study areas. Our approach, combining local hydraulic head analysis and topographical‐driven groundwater flow enhances the understanding of gaining and losing stream patterns at catchment scale, revealing widespread occurrences of losing streams and highlighting the significance of anthropogenic influences on water cycles. Key Points A method was developed to estimate surface water levels along the stream network by correcting a digital elevation model with 25 m resolution Combining head gradients and topographic analysis reveals catchment‐scale patterns and uncertainties in stream gains and losses About 42% of streams indicate locally losing conditions despite being in a topographical discharge zone

Hydrologic exchange processes are critical for ecosystem services along river corridors. Meandering contributes to this exchange by driving channel water, solutes, and energy through the surrounding alluvium, a process called sinuosity‐driven hyporheic exchange. This exchange is embedded within and modulated by the regional groundwater flow (RGF), which compresses the hyporheic zone and potentially diminishes its overall impact. Quantifying the role of sinuosity‐driven hyporheic exchange at the reach‐to‐watershed scale requires a mechanistic understanding of the interplay between drivers (meander planform) and modulators (RGF) and its implications for biogeochemical transformations. Here, we use a 2D, vertically integrated numerical model for flow, transport, and reaction to analyze sinuosity‐driven hyporheic exchange systematically. Using this model, we propose a dimensionless framework to explore the role of meander planform and RGF in hydrodynamics and how they constrain nitrogen cycling. Our results highlight the importance of meander topology for water flow and age. We demonstrate how the meander neck induces a shielding effect that protects the hyporheic zone against RGF, imposing a physical constraint on biogeochemical transformations. Furthermore, we explore the conditions when a meander acts as a net nitrogen source or sink. This transition in the net biogeochemical potential is described by a handful of dimensionless physical and biogeochemical parameters that can be measured or constrained from literature and remote sensing. This work provides a new physically based model that quantifies sinuosity‐driven hyporheic exchange and biogeochemical reactions, a critical step toward their representation in water quality models and the design and assessment of river restoration strategies. Plain Language Summary Meandering causes pressure gradients that induce water flow from the channel to the alluvial aquifer and back to the channel. This circulation process is known as sinuosity‐driven hyporheic exchange, and it has traditionally been associated with ubiquitous and favorable impacts on ecosystem services. However, its presence and biogeochemical implications can vary across river networks and even result in detrimental conditions. Here, we conducted a systematic modeling study to understand the hydrodynamics of sinuosity‐driven hyporheic exchange and its implications for nitrogen transformations. Our results show that the compressing effect of RGF can significantly reduce or vanish the hyporheic zone. Yet, narrow meander necks, characteristic of high‐sinuosity channels, shield the hyporheic zone even under extreme regional gradients. This shielding effect has been previously ignored and highlights the persistent nature of the exchange and its resilience against external modulators. We also use our model to propose and evaluate a framework based on measurable physical and biogeochemical parameters to identify the conditions leading to a meander acting as a net source or sink of nitrogen. These mechanistic insights can guide the design and evaluation of river restoration strategies and provide a critical foundation for its representation in water quality models. Key Points We assess the role of hydrodynamic drivers and modulators in the hyporheic exchange and the biogeochemical potential of meandering rivers The meander neck in high‐sinuosity channels shields the effect of regional groundwater fluxes, resulting in persistent hyporheic zones Hyporheic denitrification potential decreases with increasing sinuosity, and dissolved and particulate organic carbon availability limits it

Hyporheic flow in streams has typically been studied separately from geomorphic processes. We investigated interactions between bed mobility and dynamic hyporheic storage of solutes and fine particles in a sand‐bed stream before, during, and after a flood. A conservatively transported solute tracer (bromide) and a fine particles tracer (5 μm latex particles), a surrogate for fine particulate organic matter, were co‐injected during base flow. The tracers were differentially stored, with fine particles penetrating more shallowly in hyporheic flow and retained more efficiently due to the high rate of particle filtration in bed sediment compared to solute. Tracer injections lasted 3.5 h after which we released a small flood from an upstream dam one hour later. Due to shallower storage in the bed, fine particles were rapidly entrained during the rising limb of the flood hydrograph. Rather than being flushed by the flood, we observed that solutes were stored longer due to expansion of hyporheic flow paths beneath the temporarily enlarged bedforms. Three important timescales determined the fate of solutes and fine particles: (1) flood duration, (2) relaxation time of flood‐enlarged bedforms back to base flow dimensions, and (3) resulting adjustments and lag times of hyporheic flow. Recurrent transitions between these timescales explain why we observed a peak accumulation of natural particulate organic matter between 2 and 4 cm deep in the bed, i.e., below the scour layer of mobile bedforms but above the maximum depth of particle filtration in hyporheic flow paths. Thus, physical interactions between bed mobility and hyporheic transport influence how organic matter is stored in the bed and how long it is retained, which affects decomposition rate and metabolism of this southeastern Coastal Plain stream. In summary we found that dynamic interactions between hyporheic flow, bed mobility, and flow variation had strong but differential influences on base flow retention and flood mobilization of solutes and fine particulates. These hydrogeomorphic relationships have implications for microbial respiration of organic matter, carbon and nutrient cycling, and fate of contaminants in streams. Key Points Hyporheic exchange of solutes and fine particles was linked to bedform dynamics Solutes and fines stored in hyporheic were differentially mobilized by flood Flood duration, bedform relaxation, and hyporheic residence time were controls

Phosphorus (P) leaching from agriculture is a major driver of water eutrophication in downstream rivers and lakes. In drained lowland areas with intensive agriculture, a reduction in the fertilizer applications may be insufficient to improve the water quality in the short term as the P accumulated in the soil during decades of high fertilization may continue leaching for many years. A complementary approach to reduce P exports from agriculture is to implement edge-of-field mitigation measures at the farm scale. The selection of effective measures requires a detailed insight into the chemical and hydrological transport mechanisms. Here, we determined the main P sources, processes, and transport routes at the farm scale to support the selection of appropriate mitigation measures. We quantified the legacy P, the different P pools stored in the upper soil, and related it to the yearly P export downstream. To do this, we combined high-resolution monitoring data from the soil, groundwater, surface water, and ditch sediments. The legacy P in the topsoil was high, about 2500 kg ha−1. The predominant subsurface flow and the subsoils' P sorption capacity retained the P mobilized from the topsoil and explained the relative moderate flux of P to surface waters (0.04 kg ha−1 during the 2018-2019 drainage season). The dissolved P entering the drainage ditch via groundwater discharge was bound to iron-containing particles formed due to the oxidation of dissolved ferrous iron. Once leached from the soil to the drainage ditch, resuspension of P-rich sediment particles during flow peaks were the most important P transport mechanism (78%). Therefore, we expect that hydraulic constructions that reduce flow velocities and promote sedimentation of P-containing particles could reduce the export of P further downstream.