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The mechanisms driving catchment nitrogen storage and release operate at multiple spatiotemporal scales. Consequently, analyses grounded in different observational timescales can yield discrepant interpretations of underlying mechanisms. To assess the consistency of nitrate export patterns between event‐ and inter‐annual scales, we evaluated multiple years of high‐frequency observations of nitrate concentrations (C) and discharge (Q) including 3,480 discrete discharge events from 28 dominantly agricultural catchments. We observed consistent and often drastic divergence between long‐term and median event‐specific C‐Q patterns. Most catchments showed long‐term enrichment patterns (positive C‐Q slope), but events were, on average, more chemostatic (close‐to‐zero C‐Q slopes). C‐Q slope variability was high for small events but decreased with event magnitude, approaching chemostatic patterns during the largest storms, yielding compelling evidence against nitrate source limitation. We conclude that nitrate export patterns observed at different temporal scales and event magnitudes are controlled by different processes, therefore embedding complementary information. Plain Language Summary: Storage and mobilization of nitrogen in the landscape ultimately shape nitrate concentration dynamics in streams and their transport to downstream receiving waters. However, the temporal resolution of data analyses (e.g., long‐term or high‐flow events) can affect the interpretation of nitrate export from catchments and underlying mechanisms. To test whether nitrate export patterns differ between event and long‐term time scales, we evaluated several years of daily observations of nitrate concentration and discharge, including 3,480 high‐flow events from 28 dominantly agricultural catchments. We observed consistent and often drastic divergence between long‐term and average event‐specific export patterns. The variability in export patterns between events decreased with event magnitude toward patterns that indicate no nitrogen source limitation. Nitrate export patterns are regulated by different controls depending on the temporal scale of analysis, which must be considered to avoid incomplete and misleading interpretations about the underlying mechanisms. Key Points: Long‐term and event‐scale nitrate export patterns diverge, pointing at complementary drivers of catchment functioning across time scalesEvent‐scale C‐Q slopes decreased in variability with event magnitude and showed no signs of source limitation at high‐magnitude eventsMulti‐catchment high‐frequency data revealed the crucial role of time scale and event magnitude for interpreting nitrate export patterns [ABSTRACT FROM AUTHOR]

Per‐ and polyfluoroalkyl substances (PFAS) are surface‐active contaminants experiencing strong retention in vadose zones due to adsorption at air–water and solid–water interfaces. Leaching of PFAS through vadose zones poses great risks of groundwater contamination. Prior PFAS transport studies have focused on homogenous or layered vadose zones that significantly underrepresented the impact of preferential flow caused by soil heterogeneities—a primary factor known to dominantly control the subsurface transport of many contaminants. We conduct numerical simulations to investigate the impact of preferential flow on PFAS leaching in stochastically generated heterogeneous vadose zones. The simulations show that while shorter‐chain PFAS experience accelerated leaching similar to non‐surfactant solutes, the accelerated leaching of more surface‐active longer‐chain PFAS is uniquely amplified by 1.1–4.5 times due to reduced accessible air–water interfacial areas along preferential flow pathways. Our study highlights the criticality of characterizing soil heterogeneities for accurately predicting the leaching of long‐chain PFAS in vadose zones. Plain Language Summary: Per‐ and polyfluoroalkyl substances (PFAS) are a group of emerging contaminants used in many industrial and domestic applications. A growing body of field investigations have shown that PFAS are widespread in the environment. Notably, soils above the groundwater table at many contamination sites serve as significant PFAS reservoirs. PFAS are active at interfaces and tend to accumulate at air–water and solid–water interfaces. This surface activity has led to the strong retention of PFAS in shallow soils (especially the more surface‐active PFAS with longer carbon chains). However, field observations have shown that a small fraction of longer‐chain PFAS has migrated to deep soils at many contamination sites—in contrast to prior model predictions that underrepresented subsurface heterogeneities. We conduct systematic numerical experiments to test the hypothesis that subsurface heterogeneities accelerate the migration of PFAS in soils. We demonstrate that the heterogeneity‐generated preferential flow reduces the air–water interfacial area accessible by PFAS. The accelerated migration is unique to PFAS and is much stronger for longer‐chain than that for shorter‐chain PFAS. Overall, the present study suggests that characterizing and representing soil heterogeneities are critical for determining the accessible air–water interfacial area and accurately quantifying PFAS contamination risks of groundwater. Key Points: The presence of heterogeneity significantly reduces the accessible air–water interface area in vadose zonesReduction of accessible air–water interfacial area amplifies the preferential‐flow‐induced acceleration of Per‐ and polyfluoroalkyl substances (PFAS) leachingThe amplified acceleration is unique for PFAS and is greater for longer chains, drier climates, and more heterogeneous vadose zones [ABSTRACT FROM AUTHOR]

Long‐term nitrogen (N) transport and retention dynamics across catchments are not well understood. Using a process‐based model for 89 German catchments, results across study catchments reveal that most N surplus (during 1950–2014) was removed by denitrification (mean ± standard deviation: 58 ± 15%) while the remaining fraction was mostly stored in the soil (14% ± 11%). The mean groundwater transit times in these catchments varied from 3.2 to 20.3 years. These results indicate that past N inputs could continue to affect surface and groundwater quality in the coming years. We identified four catchment groups with distinct archetypal N transport and retention dynamics, which are linked to the catchments' climate, topographic, and geological conditions. Overall, our results shed light on long‐term N dynamics in German catchments and how they are linked to catchment characteristics, emphasizing the role of long‐term N accumulation and transport for water quality management and evaluation programs. Plain Language Summary: High nitrate concentrations in German water bodies are quite common. It is unclear to what degree current nitrogen levels in water bodies are due to current or past nitrogen (N) application on agricultural fields. What happened to excess N (N was not taken up by plants) in catchments has not been fully understood. In this study, we modeled the fate of excess N in 89 German catchments during the period 1950–2014. Our results suggest that most of the excess N was removed from the catchment in gaseous form (denitrification), and a substantial portion of the remaining excess N was stored in the soil zone. This soil N can leach into the deeper zone (groundwater) where it may travel for 3–20 years to reach the catchment outlet. We also identified four distinct groups of catchments with different behavior in terms of N transport and storage, which further exhibited different climatic, topographic, and subsurface conditions, suggesting that these factors could play a role in catchment N transport and retention. Overall, our results show that there could be a substantial delay between implemented management practices and resulting changes in surface or groundwater quality, which should be considered in water quality management. Key Points: We provided insights into the long‐term (1950–2014) nitrogen (N) transport and retention across various German catchmentsLarge‐sample assessment shows that 57% of N surplus was removed by denitrification and 15% of N surplus was accumulated in the soil zoneFour catchment clusters with distinct nitrogen transport and retention dynamics were linked to climatic, topographic, and geological factors [ABSTRACT FROM AUTHOR]

The mechanisms underlying observed global patterns of partitioning precipitation (P $P$) to evapotranspiration (E $E$) and runoff (Q $Q$) are controversially debated. We test the hypothesis that asynchrony between climatic water supply and demand is sufficient to explain spatio‐temporal variability of water availability. We developed a simple analytical model for Q $Q$ that is determined by four dimensionless characteristics of intra‐annual water supply and demand asynchrony. The analytical model, populated with gridded climate data, accurately predicted global runoff patterns within 2%–4% of independent estimates from global climate models, with spatial patterns closely correlated to observations (R2=0.93 ${R}^{2}=0.93$). The supply‐demand asynchrony hypothesis provides a physically based explanation for variability of water availability using easily measurable characteristics of climate. The model revealed widespread responsiveness of water budgets to changes in climate asynchrony in almost every global region. Furthermore, the analytical model using global averages independently reproduced the Budyko curve (R2=0.99 ${R}^{2}=0.99$) providing theoretical foundation for this widely used empirical relationship. Plain Language Summary: Local water availability is ultimately controlled by the amount of precipitation that is evaporated from a landscape. Empirical measurements have long shown a coherent global pattern in precipitation partitioning, but a consensus theoretical explanation is surprisingly lacking. We test the hypothesis that intra‐annual asynchrony between climatic water supply and demand drives the observed global patterns. We developed a simple analytical framework to predict runoff (available water) based on deficits between sinusoidal representations of precipitation (supply) and potential evapotranspiration (demand). Our framework accurately predicted global runoff patterns and reproduced a widely applied empirical precipitation partitioning relationship. Thus, supply and demand asynchrony provides both a mechanistic explanation for observed variation in global water budgets and the capability to predict water availability under future climates. Key Points: We developed a simple analytical runoff model based on four dimensionless characteristics of intra‐annual water supply and demand asynchronyThe climate asynchrony framework accurately predicted global runoff patterns and explains the widely applied empirical Budyko curveThe model revealed local responsiveness of water budgets to changes in climate asynchrony in almost every global region [ABSTRACT FROM AUTHOR]

This paper exploits triggered lightning as a point source for the basin‐scale electromagnetic tomographic survey to image 3‐D subsurface electrical properties in basins. This paper further develops a new temporal moment approach, overcoming the difficulties in forward and inverse modeling of 3‐D Maxwell's equations with heterogeneous parameter fields. Using this approach, we find that the influence of a single triggered lightning strike covers a radius of 20–70 km with detectable signals. The cross‐correlation analysis between the moment difference of the electric and electric/magnetic property field indicates that the approach is suitable for mapping subsurface electric conductivity (σ $\sigma $) heterogeneity. A numerical experiment with 3‐D spatially random parameter fields demonstrates that the method captures the spatial distribution of electric conductivity over large areas with a sparse monitoring network. It reveals the potential of using triggered lightning as a basin‐scale electric/magnetic tomography survey. Plain Language Summary: Triggered lightning experiments traditionally aim at adverse impacts of lightning phenomena on near‐surface structures (such as buildings, power, communication, and transportation networks). Magnetotellurics surveys have exploited electromagnetic (EM) waves from thunderstorm activities and the interaction of solar wind with the Earth's magnetosphere to map the subsurface structure, assuming that electromagnetic waves are planar and propagate vertically into the Earth. This paper, in contrast, explores the EM waves generated by flashes of lightning triggered by a lightning rocket at designated locations as EM point sources and their measurements at different depths and distances in the subsurface. Such experiments are tantamount to an EM tomographic survey, viewing the subsurface from different perspectives. This paper further develops a new stochastic methodology to analyze the propagation of EM waves in heterogeneous geologic media over hundreds of kilometers. These accomplishments permit harvesting the lightning signals to image the subsurface over greater depths and areas and address the image's uncertainty. Numerical experiments confirm the robustness of this proposed concept, which could be a new technology to explore subsurface processes and natural resources in basins and mountain terrains. Key Points: An efficient 3‐D forward and inverse model based on the temporal moment is developed to solve Maxwell's equations in random parameter fieldsTriggered lightning tomography yields an excellent subsurface electric conductivity spatial distribution in a basin of several kilometersDeep lightning rods in the subsurface improve vertical resolution and depth of investigation [ABSTRACT FROM AUTHOR]

Diffuse Nitrogen pollution from agriculture maintains high pressures on groundwater and aquatic ecosystems. Further mitigation requires targeted measures that reconcile agricultural interests in environmental protection. However, the agriculture‐related processes of catchment N modeling remain poorly defined due to discipline‐specific data and knowledge gaps. Using field‐experimental data, crop N uptake responses to fertilizer management were parsimoniously conceptualized and integrated into a catchment diffuse‐N model. The improved catchment modeling further facilitated integration with agricultural budget‐based assessments. The integrated analysis in a mesoscale catchment disentangled contrasting agri‐environment functional mechanisms in typically flashy chemodynamic and transport‐limited chemostatic export regimes. Moreover, the former was actively responsive to interannual climatic variability and agricultural practices; the latter exhibited drought‐induced enhancement of N enrichment, which could likely be mitigated through reduced fertilization. This interdisciplinary integration of data and methods provided an insightful evidence base for multi‐sector targeted measures, especially under cumulative impacts of changing climate and fertilizer‐use intensities. Plain Language Summary: Due to intensive nitrogen fertilizer use in agriculture over recent decades, excessive nitrogen has largely accumulated in soil and groundwater systems and is gradually being released to surface waters, polluting aquatic environments. Considering the changing climate and growing food demand, future environmental mitigation should pursue actions that specifically target high‐risk areas and periods in order to better reconcile agricultural demand and environmental protection needs. This study integrated long‐term agricultural data sets into catchment nonpoint N pollution modeling, in ways that are methodologically complementary and provide interdisciplinary benefits. The integrated analysis advances scientific understanding of important catchment agri‐environmental processes that are coupled both spatially and temporally. The enhanced knowledge base can further support decision‐making for sustainable solutions between the needs of agriculture and the water environment. Key Points: Parsimonious crop responses to Nitrogen fertilizer are derived from agricultural experimental data and upscaled by catchment N modelingIntegrating agricultural budgets and catchment modeling reveals spatio‐temporally different agri‐environmental behaviors across scalesThe integrated analysis provides implications for targeted mitigation measures under varying climatic conditions and fertilizer inputs [ABSTRACT FROM AUTHOR]

We examined how climate variability affects the mobilization of material from six watersheds. We analyzed one to seven years of high‐frequency sensor data from a temperate ecosystem and a tropical rainforest. We applied a windowed analysis to correlate concentration‐discharge (C‐Q) behavior with climate anomalies, providing insight into how hydrological and biogeochemical processes change in response to climate variability. Positive precipitation anomalies homogenized the C‐Q responses for dissolved organic matter, nitrate, specific conductance and turbidity, indicating that hydrological processes dominate the C‐Q signal and watersheds act as "conveyor belts" of material. In contrast, drier and warmer conditions led to C‐Q behavior associated with variation in solute concentration, suggesting that biogeochemical processes are a primary control on solute export and their response to flow. Results indicate that climate variability can move watersheds along a continuum from transporter‐to‐transformer of biologically active solutes and responses can potentially vary by biome. Plain Language Summary: Watersheds transport and transform material as water moves through the landscape to downstream waterbodies. We evaluated how variability in precipitation and temperature affect the movement of solutes and sediment from watersheds to river networks in temperate forests and tropical rainforests by examining the relationships between concentration and flow across six watersheds. We found that during wetter than typical conditions, hydrology dominates the relationship between concentration and discharge and watersheds act as a conveyor belt of material to the stream network. During drier and warmer than typical conditions, we observed high variability in concentration of solutes relative to discharge suggesting that watershed and in‐stream biogeochemical processes are the primary control on solute export. Under these conditions, the stream and watershed acts as a transformer of material. These results indicate that variability in climate can move watersheds and river networks along a continuum from transporter‐to‐transformer of biologically active solutes. Key Points: Watersheds transition along a transporter‐transformer continuum across a range of precipitation anomalies in tropical and temperate biomesSynergistic effects of dry conditions and warming temperatures may affect the export of biologically reactive solutes in temperate systemsHigher nitrate flux occurred during negative precipitation anomalies than during the largest positive precipitation anomalies [ABSTRACT FROM AUTHOR]

Understanding how thermokarst lakes on arctic river deltas will respond to rapid warming is critical for projecting how carbon storage and fluxes will change in those vulnerable environments. Yet, this understanding is currently limited partly due to the complexity of disentangling significant interannual variability from the longer‐term surface water signatures on the landscape, using the short summertime window of optical spaceborne observations. Here, we rigorously separate perennial lakes from ephemeral wetlands on 12 arctic deltas and report distinct size distributions and climate trends for the two waterbodies. Namely, we find a lognormal distribution for lakes and a power‐law distribution for wetlands, consistent with a simple proportionate growth model and inundated topography, respectively. Furthermore, while no trend with temperature is found for wetlands, a statistically significant decreasing trend of mean lake size with warmer temperatures is found, attributed to colder deltas having deeper and thicker permafrost preserving larger lakes. Plain Language Summary: Arctic river deltas are landscapes facing significant risk from climate change, in part due to their unique permafrost features. In particular, thermokarst lakes in ice‐rich permafrost are expected to both expand and drain under warming‐induced permafrost thaw, reconfiguring deltaic hydrology and impacting the arctic carbon cycle. A limitation in understanding how thermokarst lake cover might be changing is the significant interannual variability in water cover in flat regions such as deltas, which makes it difficult to distinguish between perennially inundated, thermally relevant waterbodies, and ephemerally inundated waterbodies. Here, we present a pan‐Arctic study of 12 arctic deltas wherein we classify observed waterbodies into perennial lakes and ephemeral wetlands capitalizing on the historical record of remote sensing data. We provide evidence that thermokarst lake sizes are universally lognormally distributed and that historical temperature trends are encoded in lake sizes, while wetland sizes are power law distributed and have no temperature trend. These findings pave the way for quantitative insight into lake cover changes on arctic deltas and associated carbon and hydrologic cycle impacts under future climate change. Key Points: Lake areas in arctic deltas exhibit a lognormal distribution associated with a simple mechanistic growth processWetland areas exhibit a power law distribution consistent with inundated topographyColder arctic deltas have larger average lake sizes, likely due to thicker permafrost restricting sub‐lake hydrologic connectivity [ABSTRACT FROM AUTHOR]

Catchments are complex systems containing channel networks and hillslopes. Channel networks interact with hillslopes and are pathways for transporting water, sediment, and nutrients. Understanding the branching and flux transport patterns of channel networks is critical for predicting the response of catchments to external forcing such as climate and tectonics. However, factors creating complexities in catchments are not fully understood. Here, we propose a new framework based on multiscale entropy approach to evaluate complexity of catchments using two different representations of channel networks. First, we investigate the structural complexity using the width‐function, which characterizes the spatial arrangement of channels. Second, we utilize the incremental area‐function along the main channel to study the functional complexity that captures the patterns of transport of fluxes. Our analysis reveals stronger controls of topological connectivity on the functional complexity than on structural complexity, indicating unchannelized surface (hillslope) contribution to the increase of heterogeneity in transport processes. Plain Language Summary: Catchments are complex natural landscape systems that contain hillslopes and channel networks. Understanding and quantifying features and processes that result in catchment complexity is important for predicting their response to changing human and climatic conditions. In this paper, we use an entropy‐based approach to explore the role of channel network and hillslope toward the contribution to catchment's complexity. Based on 40 natural catchments with minimum human impact across the United States in different climatic and geologic conditions, our results show that hillslopes add significant complexity to the catchments and suggest the amount of hillslope information one needs to account for in accurate predictive modeling of hydrologic processes at the catchment scale. Key Points: Topology controls structural and functional complexityFunctional complexity is higher than structural complexity due to contribution from hillslope processesScale of peak difference between the impact of topology on functional and structural complexity indicates the average hillslope length scale [ABSTRACT FROM AUTHOR]

Lowland rivers regularly flood and create complex inundation patterns where energy and matter are exchanged between landscape patches over a dynamic network of surface‐water connections. Scale‐freeness of networks for phenomena in many disciplines have been studied with mixed results. Here we present the first documented example of a (roughly) scale‐free network of surface‐water connections within a river‐floodplain landscape. We accomplish this by simulating 23 inundation maps across the historical range of flows for the Mission River in Texas. We then analyze the topology of the surface‐water connections between the river and two habitat patch types. Results show that surface‐water connectivity is scale‐free for ≥64% of simulated flows (≥70% for flows with floodplain inundation). Moreover, the dynamic surface‐water connections meet five of the six conceptual criteria of scale‐free networks. Our findings indicate that river‐floodplain landscapes are self‐organizing toward scale‐free surface‐water connections among patches that optimizes energy and matter exchange. Plain Language Summary: Rivers that flow over relatively flat landscapes that are formed by their own sediment deposits flood in a manner where river water escapes the confines of the river channel and spills onto the adjacent floodplain. This flooding connects the river to a variety of habitats on the floodplain with the number of connections changing with the size of the river flow. The scale‐free network conceptual model has been used to represent connections and interactions among phenomena within numerous disciplines that include the geophysical, physical, biological, mathematical, engineering, and social sciences. Here we present the first documented example of a pattern of connections between a river and surrounding habitats induced by flooding that resembles the scale‐free network model. We accomplish this by modeling various river flows for Mission River in Texas using specialized computer programs. We then analyze the pattern of connections induced by flooding using specialized statistical methods and computer codes. Our results indicate that most of our modeled river flows create connections that meet five of the six conceptual criteria for scale‐free networks. This indicates that the parts of the river landscape are organizing themselves in a manner that attempts to optimize exchange of energy and matter. Key Points: Backwater inundation is a major component in the structure and function of river‐floodplain interactionsSurface‐water connectivity creates a dynamic network with a size, configuration, and function indicative of a scale‐free networkChannel and channel‐like features in the river landscape act as hubs that hold together the structure of surface‐water connectivity [ABSTRACT FROM AUTHOR]

The Upper Mississippi River Basin is the largest source of reactive nitrogen (N) to the Gulf of Mexico. Concentration‐discharge (C‐Q) relationships offer a means to understand both the terrestrial sources that generate this reactive N and the in‐stream processes that transform it. Progress has been made on identifying land use controls on C‐Q dynamics. However, the impact of basin size and river network structure on C‐Q relationships is not well characterized. Here, we show, using high‐resolution nitrate concentration data, that tile drainage is a dominant control on C‐Q dynamics, with increasing drainage density contributing to more chemostatic C‐Q behavior. We further find that concentration variability increases, relative to discharge variability, with increasing basin size across six orders of magnitude, and this pattern is attributed to different spatial correlation structures for C and Q. Our results show how land use and river network structure jointly control riverine N export. Plain Language Summary: Nitrate is a major agricultural pollutant that can harm freshwater and marine ecosystems. Understanding the relationships between the concentration of nitrate in a river and the river's flow rate (discharge) can allow us to infer drivers of nitrate release from land into waterways and to better manage nutrient pollution. Here, we found that artificial (i.e. tile) drainage increases the stability of nitrate concentrations in the rivers of the Upper Mississippi basin across a wide range of flow rates. We also found that the variability of nitrate concentrations, relative to discharge, increased as rivers grew larger and had more tributaries contributing to flow. This shows that both on‐land and in‐river processes control the concentration‐discharge relationships of nitrate. Key Points: Nitrate concentrations in UMRB show a threshold response, chemodynamic at low flows but chemostatic at high flowsTile drainage increases the degree of nitrate chemostasis in the UMRBNitrate concentrations for rivers within Upper Mississippi River Basin (UMRB) are more chemodynamic with increasing basin size [ABSTRACT FROM AUTHOR]

Plant response to elevated CO2 concentration is known to increase leaf‐level water‐use efficiency through a reduction in stomatal opening. Recent studies have emphasized that increased plant water‐use efficiency can ameliorate the impact of drought due to climate change. However, there is a potentially counterbalancing impact due to the increased leaf area. We investigate long‐term trends (1951 to 2015) of observed streamflow in the Southeastern United States (SE US) and quantify the contribution of major drivers of streamflow changes using single factor climate modeling experiments from Community Land Model Version 5 (CLM5). The SE US streamflow observations do not exhibit a trend, which is in agreement with the CLM5 control experiment. Using the factorial set of CLM5 experiments, we find that increased leaf area under elevated CO2 leads to decreased runoff and completely counteracts increased runoff due to water‐use efficiency gains under elevated CO2 and land‐use change. Plain Language Summary: Understanding drivers of hydrological changes are societally important. Many studies have investigated the effects of climate change and land‐use change on the regional water cycle. However, the effect of plant response under elevated CO2 concentration is less known. Plant response can play two primary roles: (1) It can increase water availability due to increased water‐use efficiency, and (2) it can decrease water availability due to increased plant growth. The observations show no significant changes in water availability from 1951 to 2015 in the Southeastern United States (SE US). Climate modeling experiments show that plant growth under elevated CO2 concentration has negated the effect of increased water‐use efficiency on water availability in the SE US. Hence, we conclude that the plant growth, this seemingly less known and slowly moving driver, can play an important role in the water cycle changes at the regional scale. Key Points: Long‐term streamflow observations (1951 to 2015) do not exhibit a significant trend in the Southeastern United StatesIncreased leaf area or plant growth under elevated CO2 decreases runoff or streamflowPlant growth completely counteracts increased runoff due to water‐use efficiency gains under elevated CO2 [ABSTRACT FROM AUTHOR]

Agricultural runoff from the Mississippi‐Atchafalaya River Basin delivers nitrogen (N) and phosphorus (P) to the Gulf of Mexico, causing hypoxia, and climate drives interannual variation in nutrient loads. Climate phenomena such as El Niño–Southern Oscillation may influence nutrient export through effects on river flow, nutrient uptake, or biogeochemical transformation, but landscape variation at smaller spatial scales can mask climate signals in load or discharge time series within large river networks. We used multivariate autoregressive state‐space modeling to investigate climate signals in the long‐term record (1979–2014) of discharge, N, P, and SiO2 loads at three nested spatial scales within the Mississippi‐Atchafalaya River Basin. We detected significant signals of El Niño–Southern Oscillation and land‐surface temperature anomalies in N loads but not discharge, SiO2, or P, suggesting that large‐scale climate phenomena contribute to interannual variation in nutrient loads through biogeochemical mechanisms beyond simple discharge‐load relationships. Plain Language Summary: Runoff of excess nutrients from crop fertilizers applied throughout the Mississippi‐Atchafalaya River Basin, particularly nitrogen (N) and phosphorus (P), pollute freshwater and coastal ecosystems such as the Gulf of Mexico. Though agriculture is the main source, year‐to‐year variation in the size of nutrient loads is largely controlled by precipitation and river flow, which mobilize nutrients from the landscape. Additional climate variables, such as temperature, influence nutrient loads by controlling rates of nutrient uptake or transformation by plants, algae, and microbes, but these processes may be difficult to detect in a nearly continental‐scale river network with heterogeneous subbasins. We identified signals of multiple large‐scale climate phenomena in the long‐term record (1979–2014) of nutrient loads from the Mississippi River and its major tributaries. Climate effects on nutrient loads, particularly N, were different and often stronger than on river flow, indicating that long‐term patterns in nutrient loads were influenced by processes beyond simple precipitation‐driven runoff. Variable effects of climate on nutrient export present challenges for reducing nutrient loads to the Mississippi River and Gulf of Mexico. Adjustments to targeted reductions may be needed as global and regional climates change. Key Points: Significant signals of large‐scale climate phenomena appear in N loads but not discharge, SiO2, or P loads in the Mississippi River Basin at multiple spatial scalesEffects of climate variables differ among nutrients (N, P, SiO2) and nutrient forms (nitrate, ammonium)Climate‐driven processes independent of river flow contribute to temporal variation in nutrient loads within the Mississippi River Basin [ABSTRACT FROM AUTHOR]

Abstract: Because Earth's soil contains more carbon than the atmosphere and all terrestrial vegetation combined, forecasting and managing the global carbon cycle in the face of natural and anthropogenic change requires accurate representations of this carbon. Here from regional geomorphic and soil databases, we characterize the mass, distribution, and cycling of previously unaccounted for soil carbon across the southeastern U.S. Coastal Plain, referred to as “deep‐podzolized carbon.” We show that geomorphologic‐hydrologic interactions stabilize approximately 1.1 × 10−9 t of deep‐podzolized carbon (equivalent to roughly 18% of the soil organic carbon stored across the entire region from 0–30 cm), and that this potentially ancient carbon is predictably distributed coincident with Pleistocene marine transgressions. We not only redefine soil carbon storage in the region but we also introduce the Earth Sciences to a massive organic carbon pool that interacts with landscape evolution and hydrology, has essentially never been studied, and is ripe for interdisciplinary research. [ABSTRACT FROM AUTHOR]

Abstract: Natural wetlands are ecological, biogeochemical, and hydrological hot spots yet continue to disappear under human pressure. Their shapes and sizes control their hydroecological functions. We propose that elevation data can be used to delineate potential wetlands and that the (statistical) distributions of potential wetlands should be identical to the distributions of actual wetlands. We compare the shape and size distributions of wetlands reported in the National Wetland Inventory with those of potential wetlands identified using a topographic depression identification model. We estimated area and perimeter distributions as well as shoreline fractal dimension in six contrasting locations in the United States. Pareto distributions described the tails of these distributions, with similar slopes for both model and data. The shape of shorelines was also similar, and their fractal dimension clustered around D = 4/3, a pervasive value in nature. We also analyzed the entire wetland inventory data set for the conterminous United States (~20 million wetlands) for reference and found the statistics to be invariant across scales. Our results demonstrate that a simple topographic model can identify most reported wetlands as well as potential wetlands missing from the inventory. These findings could inform strategic surveys and the conservation of wetlandscapes. [ABSTRACT FROM AUTHOR]

Fluid transit time is understood to be an important control on the shape of concentration‐discharge (C‐q) relationships, yet empirical evidence supporting this linkage is limited. We investigated C‐q relationships for weathering‐derived solutes across seven Antarctic glacial meltwater streams. We hypothesized that (H1) solute fluxes in McMurdo Dry Valley streams are reaction limited so that C‐q relationships are characterized by dilution and that (H2) transit time explains between‐stream variability in the degree of C‐q dilution. Results show that C‐q relationships are chemostatic because solute equilibrium times are shorter than stream corridor fluid transit times. Between‐stream variability in the efficiency of solute production is positively correlated with transit time, suggesting that transit time is an important control on the solute export regime. These results provide empirical evidence for the controls on weathering‐derived C‐q relationships and have important implications for Antarctic ecosystems and solute export regimes of watersheds worldwide. Plain Language Summary: Relationships between solute concentration and stream discharge (i.e., streamflow) rates in rivers contain important information regarding how water moves through a watershed. In this research, we use a combination of observations and mathematical modeling to show that the shape of concentration‐discharge relationships is related to hydrologic transit time, the duration of time a parcel of water spends moving through the watershed. Because the time scales of chemical reactions are shorter than hydrologic transit times, solute concentration is invariant with discharge. Results of this work empirically corroborate previous theoretical work regarding the physical controls on concentration‐discharge relationships, and have important implications for understanding chemical concentration patterns in rivers worldwide. Key Points: Antarctic streams exhibit chemostatic concentration‐discharge relationships for weathering‐derived solutesHydrologic transit times are short (less than four months), yet solute export is predominantly transport limited because equilibrium time scales are also short (<13 days)Hydrologic transit time explains between‐stream variability in the shape of concentration‐discharge relationships [ABSTRACT FROM AUTHOR]

Catchments act as a sequence of hierarchical filters, damping input signals and generating less variable streamflow and stream solute concentration output signals. Spectral analysis across a wide array of solutes and catchments has revealed ~1/f scaling behavior (i.e., spectral power inversely proportional to frequency) spanning periods of hours to decades, attributed to hillslope filtering. We hypothesize that additional filtering by stream channel processes should occur. However, most catchments in which solute scaling behavior has been resolved have channel travel times too short for this to be evident from the relatively low sampling rates utilized. We use high‐frequency (1–4 hr−1) sampling in larger catchments (up to 3 × 106 km2) to show that solute signals are indeed multifractal, steepening from ~1/f spectral at low frequencies to ~1/f2 at higher frequencies. Across conservative, reactive, and gaseous solutes we demonstrate that departure from 1/f scaling occurs at frequencies that correspond with metrics of catchment size. Plain Language Summary: Concentrations of elements in stream water are less variable than concentrations in rainfall or other sources. This damping occurs due to mixing during transport through the subsurface and in the stream channel. Subsurface travel times are typically much longer than channel travel times. So while subsurface filtering has been well recognized, subsequent channel filtering only becomes apparent when frequent measurements are taken, or in large catchments. Here we demonstrate that this channel filtering also occurs and produces concentrations in large rivers, which are proportionally less variable than those in small streams over similar time scales. The results have important implications for how we measure and interpret water quality measurements. Key Points: Catchments filter stream solute signalsChannel filtering only apparent in large catchments or with high frequency samplingSolute signals exhibit multifractal scaling [ABSTRACT FROM AUTHOR]

Although the flux of dissolved organic carbon (DOC) through freshwaters is nearly equivalent to the net carbon uptake of all terrestrial ecosystems, uncertainty remains about how source processes (carbon production and location) and transport processes (hydrologic connectivity and routing) interact to determine DOC flux across flow conditions and ecoregions. This limits our ability to predict the fluvial carbon flux responses to changes in climate and land use. We used DOC concentration and discharge patterns with ensemble modeling techniques to quantify DOC flux behavior for 1,006 U.S. watersheds spanning diverse climate and land cover conditions. We found that DOC flux was transport‐limited (concentration increased with discharge) in 80% of watersheds and that this flux behavior spanned ecoregions and watershed sizes. The generality of transport limitation demonstrates how coupling discharge models with widely available watershed properties could allow DOC flux to be efficiently integrated into landscape and Earth system models. Plain Language Summary: When water flows through ecosystems, it picks up dissolved organic carbon (DOC) from plants and soils, sometimes determining whether the ecosystem is a net carbon source or sink. DOC is also an important water quality parameter and understanding how it is produced and transported affects society's ability to provide water for industrial, agricultural, and domestic uses. Because DOC flux through rivers varies widely with flow and in different regions, DOC flux remains a major source of uncertainty in the global carbon cycle. Based on one of the largest and most geographically diverse analyses of river DOC dynamics to date, we found surprising similarities in DOC flux behavior. From southwestern deserts to northeastern forests, hydrologic flow, not DOC sources, determined DOC flux behavior in 80% of watersheds in the conterminous United States. In other words, DOC concentration systematically increased with river flow, even during large flow events, indicating that organic matter stocks provide ample DOC to maintain delivery to rivers. Additional analysis of this large data set identified several landscape and climate conditions that predict DOC flux behavior in watersheds. Together, these findings demonstrate that watershed DOC flux can be simulated across spatial scales using river flow and widely available watershed properties. Key Points: Across ecoregions of the United States, 80% of watersheds express transport limitation in DOC flux behaviorWetland abundance in watersheds is nonlinearly related to river DOC flux behaviorThe generality of transport limitation in DOC flux presents an opportunity for improving ecosystem carbon balance models [ABSTRACT FROM AUTHOR]