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Freshwater mussels are an imperiled group of organisms that are vital to aquatic ecosystems. Services performed by freshwater mussels, coupled with their use for biomonitoring, make them an invaluable asset. Neogobius melanostomus (Round Goby), a recently introduced invasive species to the French Creek watershed, was once restricted to the watershed of Lake Erie in Pennsylvania. The Round Goby’s propensity to consume Dreissena polymorpha (Zebra Mussel) and Dreissena bugensis (Quagga Mussel) in their native habitat raises concerns about this recent introduction into Pennsylvania’s Allegheny River watershed. Since the discovery of their introduction within the watershed, we have followed the range expansion and dispersal rate of Round Gobies, which makes this study unique. The objectives of this study were to quantify baseline data on the contemporary diversity and abundance of unionid mussels in the upper French Creek watershed, and to explore potential habitat factors that influence or limit the size of the mussel populations. We gathered baseline data on freshwater mussel diversity and abundances across eight sites in the French Creek watershed and examined substrate particle size and host availability as potential limiting factors of the freshwater mussel distribution. Freshwater mussel surveys were conducted during the summer months (July–September) of 2017 using area-constrained surveys. Results showed a significant relationship between mussel diversity and substrate particle size (p &lt, 0.05). From the data collected, we were able to calculate population estimates for the species found across the sample sites. Our results regarding the locations of native mussel populations and characteristics of their habitat provide the needed insight for establishing priority areas for the conservation of freshwater mussels, facilitating planning for protection, mitigation, and adaptation as the invasive Round Goby continues its spread.

This study evaluates the capacity of three gridded precipitation products (MSWEP V2.2, TRMM-3B42 V7, and GPM-IMERG V6) to detect precipitation in the Min Jiang watershed, a data-scarce and mountainous region in western China. A set of statistical and contingency indices is calculated for the precipitation products and compared with rain gauge observations at 23 ground stations from July 2000 to May 2016. Consistency between gridded and ground precipitation datasets is examined at different temporal (i.e., daily, monthly, seasonally, and annually) and spatial (i.e., site level, sub-regional level, and watershed level) resolutions. We identify possible reasons for discrepancies among precipitation datasets. Our results indicate that: (1) the MSWEP product is best suited for the study of long-term mesoscale rainfall, rather than short-term light or extreme rainfall; (2) the IMERG product represents stable performance for the simulation of rainfall spatial variability and detection capability; and (3) Composition of the datasets, climatic systems, and regional topography are key factors influencing the consistency between gridded and ground precipitation datasets. Therefore, we suggest using MSWEP V2.2 and GPM-IMERG V6 as potential precipitation data sources for hydrometeorological studies over the Min Jiang watershed. The findings of this study inform future hydrometeorological and climate applications in data-scarce regions with complex terrain.

The Special Issue (SI) of Hydrological Processes features invited contributions led by women scientists at an advanced career stage who have made sustained contributions to the study of hydrological processes, advancing the field. This preface article briefly introduce the contributors and their papers.

Access to clean water is a critical challenge and opportunity for community-level collaboration. People rely on local water sources, but awareness of water quality and participation in water management is often limited. Lack of community engagement can increase risks of water catastrophes, such as those in Flint, Michigan, and Cape Town, South Africa. We investigated water quality practices in a watershed system serving c.100 000 people in the United States. We identified a range of entities including government and nonprofit citizen groups that gather water quality data. Many of these data are accessible in principle to citizens. However, the data are scattered and diverse; information infrastructures are primitive and not integrated. Water quality data and data practices are hidden in plain sight. Based on fieldwork, we consider sociotechnical courses of action, drawing on best practices in human–computer interaction and community informatics, data and environmental systems management.

Nutrient enrichment is currently a leading cause of impairment to streams in Pennsylvania. Evaluating the water quality condition and eutrophic status of streams and rivers is a challenge without established thresholds for nutrient concentrations, which can vary depending on climate and landscape characteristics. The US Environmental Protection Agency (USEPA) has published nutrient criteria for nutrient ecoregions nationwide that are used as regional baseline values, and has encouraged states to develop more refined values if better data are available. In this study, we quantified long-term nutrient concentrations observed in streams and rivers across Pennsylvania using a robust water quality dataset compiled from monitoring data collected over the past two decades (2000&ndash, 2019) by multiple agencies. We estimated nutrient criteria concentration thresholds for each ecoregion using USEPA&rsquo, s percentile approach. The 25th percentile median concentrations observed in streams and rivers ranged from 0.27 to 2.30 mg/L for total nitrogen (TN), and from 0.010 to 0.053 mg/L for total phosphorus (TP). The percent of sites with available data that exceeded the 25th percentile was 53% for TN and 60% for TP, reflecting longstanding problems with nutrient pollution of rivers and streams in Pennsylvania. The 25th percentile may overestimate background condition levels, as nutrient conditions vary substantially within and among ecoregions. We compared our contemporary concentrations at the threshold values to other published recommended criteria for the region and explored the influence of landscape heterogeneity and seasonality on nutrient concentrations. Our results provide environmental managers with new insights regarding the status of nutrient conditions in streams and rivers across Pennsylvania ecoregions toward further developing numeric nutrient criteria.

Concentration&ndash, discharge (C-Q) relationships are a convenient and increasingly popular tool for interpreting the episodic hydrochemical response to the varying discharge in small basins, providing insights into solute transport and streamflow generation. While most studies are focused on total runoff, this study quantified C-Q relationships in four runoff components during precipitation events at the Hydrohill experimental catchment in Chuzhou, China. This unique artificial catchment is carefully engineered, allowing observations of the interacting runoff components that collectively determine total flow issuing from the catchment. The four runoff components, or flow paths, include surface runoff (SR), shallow interflow at 0&ndash, 30 cm depth (SSR30), deeper interflow at 30&ndash, 60 cm depth (SSR60), and groundwater flow at 60&ndash, 100 cm depth (SSR100). Water samples were collected during three consecutive precipitation events to study how the concentrations of primary solutes vary with flow. Analysis of C-Q relationships reveals that concentrations of Na+, Ca2+, Mg2+, SO42&minus, and HCO3&minus, in the four runoff components had a negative relationship with discharge, while the concentration of K+ and Cl&minus, were negatively correlated with discharge in SR and SSR30 but positively correlated in SSR60 and SSR100. Further insights were gained from principal component analysis. Three eigenvectors explained 92% of the variability in hydrochemistry in surface runoff, while two eigenvectors explained most of the variability in the hydrochemistry of subsurface flows observed at various depths in the soil profile (73% for SSR30, 79% for SSR60, and 76% for SSR100). PC1 (the first Principal Component) can be interpreted as a salinity factor, deriving from carbonate minerals such as dolomites and limestone minerals. Results indicated that leaching and dilution processes, water&ndash, soil interaction, and macropore flows in soils are the primary factors controlling the C-Q relationships. Our work sheds light on the coupled processes and streamflow generation mechanisms that control water quality at the catchment scale.

Irrigated agriculture contributes 40% of total global food production. In the US High Plains, which produces more than 50 million tons per year of grain, as much as 90% of irrigation originates from groundwater resources, including the Ogallala aquifer. In parts of the High Plains, groundwater resources are being depleted so rapidly that they are considered nonrenewable, compromising food security. When groundwater becomes scarce, groundwater withdrawals peak, causing a subsequent peak in crop production. Previous descriptions of finite natural resource depletion have utilized the Hubbert curve. By coupling the dynamics of groundwater pumping, recharge, and crop production, Hubbert-like curves emerge, responding to the linked variations in groundwater pumping and grain production. On a state level, this approach predicted when groundwater withdrawal and grain production peaked and the lag between them. The lags increased with the adoption of efficient irrigation practices and higher recharge rates. Results indicate that, in Texas, withdrawals peaked in 1966, followed by a peak in grain production 9 y later. After better irrigation technologies were adopted, the lag increased to 15 y from 1997 to 2012. In Kansas, where these technologies were employed concurrently with the rise of irrigated grain production, this lag was predicted to be 24 y starting in 1994. In Nebraska, grain production is projected to continue rising through 2050 because of high recharge rates. While Texas and Nebraska had equal irrigated output in 1975, by 2050, it is projected that Nebraska will have almost 10 times the groundwater-based production of Texas., This paper stems from discussions during the Ettersburg Ecohydrology Workshop in Germany (October 2018), with the corresponding manuscript preparation ensuing in subsequent months. The workshop was funded by the UNIDEL Foundation, Inc. and the University of Delaware. Accordingly, partial support for this paper derived from funding for the workshop. A.M. was supported by the US NSF (Grants NSF-AGS-1644382 and NSF-IOS-175489).

Land-use and land-cover changes are accelerating. Such changes can homogenize the water cycle and undermine planetary resilience. Policymakers and practitioners must consider water–vegetation interactions in their land-management decisions., This paper stems from the Ettersburg Ecohydrology Workshop (October 2018). Funding was provided by the UNIDEL Foundation, Inc. and the University of Delaware.

Retention, processing, and transport of solutes and particulates in stream corridors are influenced by the travel time of streamflow through stream channels, which varies dynamically with discharge. The effects of streamflow variability across sites and over time cannot be addressed by time-averaged models if parameters are based solely on the characteristics of mean streamflow. We develop methods to that account for the effects of streamflow variability on travel time and compare our estimates with to flow-weighted (“effective”) travel time at 100 streams in the southeastern United States. Velocity time series were generated for each stream from multiple-year (median 15.5 years), high-frequency (15-minute interval) records of instantaneous streamflow and field measurements of velocity and inverted to produce time series of specific travel time [T/L]. The effective travel times for streams are 60 to 90% of the specific travel time of mean streamflow because a large fraction of the total streamflow volume is discharged during higher flows with higher velocities. We find that adjusting the specific travel time of mean streamflow at a site by a factor of 0.81 generally accounts for the effect of a skewed streamflow distribution, but at-site estimates of the coefficient of variation of streamflow are necessary to resolve differences in streamflow variability between streams or changes in variability over time. For example, the effective travel time of urban streams is less than the effective travel of forested streams in the southeastern United States as a result of increased streamflow variability in urban streams. Effective travel time accounts for both the variation in velocity with streamflow and the large fraction of streamflow discharged during high flows in most streams and provides time-averaged models with limited capability to account for effects of streamflow variability that otherwise they lack. This capability is needed for continental-scale modeling where streamflow variability is not uniform because heterogeneous surficial geology, hydro-climatology, and vegetation and applications where streamflow variability is not stationary as a response to climate change or hydrologic alteration.

Author(s): Tague, CL; Papuga, SA; Gerlein-Safdi, C; Dymond, S; Morrison, RR; Boyer, EW; Riveros-Iregui, D; Agee, E; Arora, B; Dialynas, YG; Hansen, A; Krause, S; Kuppel, S; Loheide, SP; Schymanski, SJ; Zipper, SC

Forested watersheds provide multiple ecosystem benefits to society, playing a key role in the supply of fresh water and the regulation of climate. In this chapter we review two classic paired watershed studies carried out in temperate forests in contrasting world regions; Tatsunokuchi-Yama Experimental Watershed in Japan and at the Leading Ridge Experimental Watershed in the United States. Long-term measurements of the water balance have been made at both experimental watersheds, with observations at the Tatsunokuchi-Yama Experimental Watershed beginning in 1937 and at the Leading Ridge Experimental Watershed in 1957. The catchments were observed over a range of changing climatic conditions. The forests changed over time due to a number of factors, such as changing tree species, gypsy moth defoliation, pine wilt disease, forest fire, clear-cutting, herbicide application, selective logging, and forest management. We summarize results from monitoring of these watersheds from 1937 to 2002 in Japan and from 1957 to 2007 in the United States. In the Tatsunokuchi-Yama watersheds annual precipitation ranged from about 600 to 1730 mm averaging 1220 mm year−1, with about 33% of the average annual incident precipitation delivered as streamflow in the watersheds. In the Leading Ridge watersheds annual precipitation ranged from about 470 to 1470 mm averaging 1060 mm year−1, with about 45% of the average annual precipitation delivered to streamflow. Results from these studies show that increases in water yield occur on small watersheds in response to removal of vegetation and that increases in streamflow diminish as vegetation is replanted or naturally recovers. Long-term watershed monitoring data are becoming increasingly useful to link the impacts of changing climate, vegetation, soil, and water as the data are synthesized and shared publicly, allowing application to new research questions and hypotheses.

The direction and magnitude of responses of evapotranspiration (ET) to climate change are important to understand, as ET represents a major water and energy flux from terrestrial ecosystems, with consequences that feed back to the climate system. We inferred multidecadal trends in water balance in 11 river basins (1940–2012) and eight smaller watersheds (with records ranging from 18 to 61 years in length) in the Northeastern United States. Trends in river basin actual ET (AET) varied across the region, with an apparent latitudinal pattern: AET increased in the cooler northern part of the region (Maine) but decreased in some warmer regions to the southwest (Pennsylvania–Ohio). Of the four small watersheds with records longer than 45 years, two fit this geographic pattern in AET trends. The differential effects of the warming climate on AET across the region may indicate different mechanisms of change in more‐ vs. less‐energy‐limited watersheds, even though annual precipitation greatly exceeds potential ET across the entire region. Correlations between AET and time series of temperature and precipitation also indicate differences in limiting factors for AET across the Northeastern U.S. climate gradient. At many sites across the climate gradient, water‐year AET correlated with summer precipitation, implying that water limitation is at least transiently important in some years, whereas correlations with temperature indices were more prominent in northern than southern sites within the region.

Planned beneficial re-use of water has become an increasingly common conservation practice worldwide, sparking questions about the degree of water treatment needed to mitigate negative environmental impacts. Since the early 1980s, as an alternative to surface discharge, the Pennsylvania State University has spray-irrigated all of its treated wastewater effluent via land application onto an environmental setting known as the “Living Filter” site (∼245 ha). The impacts of spray irrigation on nearby ephemeral wetlands, known as vernal pools, were explored. The pools gain water from both natural rainfall and spray-irrigation of the University’s treated wastewater. The occurrence and persistence of estrogens in three vernal pools were quantified by analyzing >137 water samples collected from the pools over an eight-week period coincident with the development period of native amphibian larvae. Additionally, dissolved oxygen, oxidation-reduction potential, water level, water temperature, electrical conductivity, pH, and rainfall data were measured continuously throughout the study period within each pool. Further, the treated wastewater effluent was sampled during each weekly spray-irrigation event. Estrone was detected in nearly 100% of the vernal pool samples, with concentrations up to 6.2 ng L−1. Additionally, 17α-estradiol was not detected in the wastewater effluent, but was present in 52% of the vernal pool samples. 17β-estradiol, estriol, and 17α-ethinylestradiol were detected in fewer than 10% of the vernal pool samples. The findings of this research have important implications for management practices that can help protect these critical habitats.

Carbon cycling in the coastal zone affects global carbon budgets and is critical for understanding the urgent issues of hypoxia, acidification, and tidal wetland loss. However, there are no regional carbon budgets spanning the three main ecosystems in coastal waters: tidal wetlands, estuaries, and shelf waters. Here, we construct such a budget for Eastern North America using historical data, empirical models, remote-sensing algorithms, and process-based models. Considering the net fluxes of total carbon at the domain boundaries, 59 ± 12% (± 2 standard errors) of the carbon entering is from rivers and 41 ± 12% is from the atmosphere, while 80 ± 9% of the carbon leaving is exported to the open ocean and 20 ± 9% is buried. Net lateral carbon transfers between the three main ecosystem types are comparable to fluxes at the domain boundaries. Each ecosystem type contributes substantially to exchange with the atmosphere, with CO2 uptake split evenly between tidal wetlands and shelf waters, and estuarine CO2 outgassing offsetting half of the uptake. Similarly, burial is about equal in tidal wetlands and shelf waters, while estuaries play a smaller but still substantial role. The importance of tidal wetlands and estuaries in the overall budget is remarkable given that they respectively make up only 2.4 and 8.9% of the study domain area. This study shows that coastal carbon budgets should explicitly include tidal wetlands, estuaries, shelf waters and the linkages between them; ignoring any of them may produce a biased picture of coastal carbon cycling.

Flood drainage rights (FDR) refer to the rights of all regions in a watershed to drain floodwaters into river reaches under the permission of law. FDR is an important measure used to manage the flooding in various regions of China. This paper explores the problem of flood drainage conflicts caused by regional competition for FDR during flood disasters. We designed an FDR allocation indicator system based on the principles of fairness, efficiency, sustainable development, respect for historical context, and flood resilience strategy. We used the interval multi-objective allocation model based on the projection method to create an FDR pre-allocation plan, and iteratively improved it until it exceeded the standards of a harmonious diagnosis method, thereby obtaining a harmonious allocation plan. The Jiangsu section of the Sunan Canal in the Yangtze River Delta region of China was taken as an example, using data from 2009 to 2018, to study the suitability of our approach and compare it with other indicator systems. Our results showed that: (1) in the FDR pre-allocation, the proportions of FDR obtained by Suzhou, Zhenjiang, Wuxi, and Changzhou were 32.81%, 19.83%, 24.68%, and 22.69%, respectively. However, the FDR pre-allocation scheme did not pass the harmonious diagnosis and would have been objectionable to some regions. (2) After two rounds of harmonious adjustment, the model yielded a final scheme that would promote the harmonious development of the region and improve the acceptability of the FDR allocation plan. In the harmonious distribution of FDR, the proportions of FDR obtained by Suzhou, Zhenjiang, Wuxi, and Changzhou were changed to 29.18%, 20.03%, 28.10%, and 22.69%, respectively. (3) The allocation results based on our five-principle approach were the most harmonious of the tested plans. Our method is geared toward solving the problem of flood drainage conflicts between regions to ensure an orderly flood drainage situation, reduce flood losses, and promote the harmonious development of the entire watershed. As such, it provides a reference for governments and agencies involved in FDR management.

Mercury methylation, where inorganic mercury (Hg) is converted to methylmercury (MeHg), can increase in soils when flooded. While effects of the initial flooding of soils on MeHg production have been well studied, less is known about impacts of re-flooding on MeHg production. Lake Perez, an impounded recreational reservoir in the Appalachian Highlands, was completely drained then re-filled 7 years later. We use a combination of chemical, soil physical, and microbial data to quantify changes in MeHg before and after re-flooding of the lakebed. Portions that were transiently saturated due to pluvial flooding had the highest pre-flooded MeHg concentrations. When the lake was re-flooded, concentrations of MeHg in subaqueous soils increased by a factor of 2.74 (+174%) on average. Substantial variability was observed among the sampling sites, with smaller increases in MeHg at sites subjected to seasonal flooding during periods when the reservoir was drained. The increase of soil MeHg after re-flooding was lower in this study compared to studies that evaluated soil MeHg after initial flooding, indicating that re-flooding of a former lake bed caused a smaller response in MeHg production compared to initial flooding of terrestrial land. This study advances understanding of the environmental impact of impounded reservoirs.

The dramatic increase of bioreactive nitrogen entering the Earth's ecosystems continues to attract growing attention. Increasingly large quantities of inorganic nitrogen are flushed from land to wa ...

Atmospheric mercury (Hg) deposition to forests is important because half of the land cover in the eastern USA is forest. Mercury was measured in autumn litterfall and weekly precipitation samples at a total of 27 National Atmospheric Deposition Program (NADP) monitoring sites in deciduous and mixed deciduous-coniferous forests in 16 states in the eastern USA during 2007–2014. These simultaneous, uniform, repeated, annual measurements of forest Hg include the broadest area and longest time frame to date. The autumn litterfall-Hg concentrations and litterfall mass at the study sites each year were combined with annual precipitation-Hg data. Rates of litterfall-Hg deposition were higher than or equal to precipitation-Hg deposition rates in 70% of the annual data, which indicates a substantial contribution from litterfall to total atmospheric-Hg deposition. Annual litterfall-Hg deposition in this study had a median of 11.7 μg per square meter per year (μg/m 2 /yr) and ranged from 2.2 to 23.4 μg/m 2 /yr. It closely matched modeled dry-Hg deposition, based on land cover at selected NADP Hg-monitoring sites. Mean annual atmospheric-Hg deposition at forest study sites exhibited a spatial pattern partly explained by statistical differences among five forest-cover types and related to the mapped density of Hg emissions. Forest canopies apparently recorded changes in atmospheric-Hg concentrations over time because litterfall-Hg concentrations decreased year to year and litterfall-Hg concentrations were significantly higher in 2007–2009 than in 2012–2014. These findings reinforce reported decreases in Hg emissions and atmospheric elemental-Hg concentrations during this same time period. Methylmercury (MeHg) was detected in all litterfall samples at all sites, compared with MeHg detections in less than half the precipitation samples at selected sites during the study. These results indicate MeHg in litterfall is a pathway into the terrestrial food web where it can accumulate in the prey of songbirds, bats, and raptors.

Organic compounds are removed from the atmosphere and deposited to the earth's surface via precipitation. In this study, we quantified variations of dissolved organic carbon (DOC) in precipitation during storm events at the Shale Hills Critical Zone Observatory, a forested watershed in central Pennsylvania (USA). Precipitation samples were collected consecutively throughout the storm during 13 events, which spanned a range of seasons and synoptic meteorological conditions, including a hurricane. Further, we explored factors that affect the temporal variability by considering relationships of DOC in precipitation with atmospheric and storm characteristics. Concentrations and chemical composition of DOC changed considerably during storms, with the magnitude of change within individual events being comparable or higher than the range of variation in average event composition among events. While some previous studies observed that concentrations of other elements in precipitation typically decrease over the course of individual storm events, results of this study show that DOC concentrations in precipitation are highly variable. During most storm events concentrations decreased over time, possibly as a result of washing out of the below-cloud atmosphere. However, increasing concentrations that were observed in the later stages of some storm events highlight that DOC removal with precipitation is not merely a dilution response. Increases in DOC during events could result from advection of air masses, local emissions during breaks in precipitation, or chemical transformations in the atmosphere that enhance solubility of organic carbon compounds. This work advances understanding of processes occurring during storms that are relevant to studies of atmospheric chemistry, carbon cycling, and ecosystem responses.

We measured wet atmospheric deposition of dissolved organic carbon (DOC) over 6 years at a network of 12 monitoring sites across Pennsylvania, quantified rates of wet DOC deposition, and developed the first statewide estimates of inputs of DOC to watersheds via wet deposition. Average annual volume-weighted concentration of DOC was 0.71 mg C L−1. Annual wet deposition fluxes of DOC varied between sites and years, ranging from 3 to 13 kg C ha−1 yr−1, with an average value of 8 kg C ha−1 yr−1 across all sites and years and are of the same order of magnitude as literature values for riverine organic carbon fluxes in the northeastern United States. The rates of wet DOC deposition showed a pronounced seasonality and spatial distribution, with highest deposition rates observed in the summer, especially at the sites located in western Pennsylvania. Significant links between DOC and inorganic constituents in precipitation, such as sulfate and inorganic nitrogen forms, point to the similarity of sources and atmospheric processing and suggest that DOC may potentially affect their atmospheric transport and ecological fate. Observational data resulting from this study underscore the potential significance of atmospheric deposition as an external input of reactive carbon species to watersheds and may be useful for constraining atmospheric carbon models and evaluating atmospheric influences on ecosystems.

Downstream flow in rivers is repeatedly delayed by hydrologic exchange with off-channel storage zones where biogeochemical processing occurs. We present a dimensionless metric that quantifies river connectivity as the balance between downstream flow and the exchange of water with the bed, banks, and floodplains. The degree of connectivity directly influences downstream water quality - too little connectivity limits the amount of river water exchanged and leads to biogeochemically inactive water storage, while too much connectivity limits the contact time with sediments for reactions to proceed. Using a metric of reaction significance based on river connectivity, we provide evidence that intermediate levels of connectivity, rather than the highest or lowest levels, are the most efficient in removing nitrogen from Northeastern United States' rivers. Intermediate connectivity balances the frequency, residence time, and contact volume with reactive sediments, which can maximize the reactive processing of dissolved contaminants and the protection of downstream water quality. Our simulations suggest denitrification dominantly occurs in riverbed hyporheic zones of streams and small rivers, whereas vertical turbulent mixing in contact with sediments dominates in mid-size to large rivers. The metrics of connectivity and reaction significance presented here can facilitate scientifically based prioritizations of river management strategies to protect the values and functions of river corridors. U.S. Geological SurveyUnited States Geological Survey; National Science Foundation Hydrologic Sciences ProgramNational Science Foundation (NSF)NSF - Directorate for Geosciences (GEO); USGS National Water Quality Program; DOE Office of Biological and Environmental Research (BER) in the Subsurface Biogeochemistry Program (SBR) as part of SBR's Scientific Focus Area at the Pacific Northwest National Laboratory (PNNL) The work is a product of the John Wesley Powell Center River Corridor Synthesis Group, supported by U.S. Geological Survey and National Science Foundation Hydrologic Sciences Program. USGS authors received additional support from the USGS National Water Quality Program. Gomez-Velez received additional support from the DOE Office of Biological and Environmental Research (BER) in the Subsurface Biogeochemistry Program (SBR) as part of SBR's Scientific Focus Area at the Pacific Northwest National Laboratory (PNNL). The synthesis is based entirely on analysis of published information and publicly available data sources. Any use of trade, firm, or product is for descriptive purposes only and does not imply endorsement by the U.S. Government. Public domain – authored by a U.S. government employee

Atmospheric deposition is the major pathway for removal of organic carbon (OC) from the atmosphere, affecting both atmospheric and landscape processes. Transfers of OC from the atmosphere to land occur as wet deposition (via precipitation) and as dry deposition (via surface settling of particles and gases). Despite current understanding of the significance of organic carbon inputs with precipitation to carbon budgets, transfers of organic matter between the atmosphere and land are not explicitly included in most carbon cycle models due to limited data, highlighting the need for further information. Studies regarding the abundance of OC in precipitation are relatively sparse, in part due to the fact that concentrations of organics in precipitation and their associated rates of atmospheric deposition are not routinely measured as a part of major deposition monitoring networks. Here, we provide a new data synthesis from 83 contemporary studies published in the peer reviewed literature where organic matter in precipitation was measured around the world. We compiled data regarding the concentrations of organic carbon in precipitation and associated rates of atmospheric deposition of organic carbon. We calculated summary statistics in a common set of units, providing insights into the magnitude and regional variability of OC in precipitation. A land to ocean gradient is evident in OC concentrations, with marine sites generally showing lower values than continental sites. Our synthesis highlights gaps in the data and challenges for data intercomparison. There is a need to concentrate sampling efforts in areas where anthropogenic OC emissions are on the rise (Asia, South America), as well as in remote sites suggesting background conditions, especially in Southern Hemisphere. It is also important to acquire more data for marine rainwater at various distances from the coast in order to assess a magnitude of carbon transfer between the land and the ocean. Our integration of the recent published information on OC in precipitation provides a unique data set (shared here as supplemental information) and a regional perspective that will be useful in carbon budgets, environmental modeling, and ecosystem studies. This can be used for comparison with past conditions and as a baseline toward exploring future changes, since changes in emissions, land use, and climatic variability are reflected in the amount and quality of OC deposited to ecosystems.

The objectives of this study were to determine the quantity and chemical composition of precipitation inputs of dissolved organic carbon (DOC) to a forested watershed; and to characterize the associated temporal variability. We sampled most precipitation that occurred from May 2012 through August 2013 at the Susquehanna Shale Hills Critical Zone Observatory (Pennsylvania, USA). Sub-event precipitation samples (159) were collected sequentially during 90 events; covering various types of synoptic meteorological conditions in all climatic seasons. Precipitation DOC concentrations and rates of wet atmospheric DOC deposition were highly variable from storm to storm, ranging from 0.3 to 5.6 mg C L−1 and from 0.5 to 32.8 mg C m−2 h−1, respectively. Seasonally, storms in spring and summer had higher concentrations of DOC and more optically active organic matter than in winter. Higher DOC concentrations resulted from weather types that favor air advection, where cold frontal systems, on average, delivered more than warm/stationary fronts and northeasters. A mixed modeling statistical approach revealed that factors related to storm properties, emission sources, and to the chemical composition of the atmosphere could explain more than 60% of the storm to storm variability in DOC concentrations. This study provided observations on changes in dissolved organic matter that can be useful in modeling of atmospheric oxidative chemistry, exploring relationships between organics and other elements of precipitation chemistry, and in considering temporal changes in ecosystem nutrient balances and microbial activity.

The quantity and chemical composition of dissolved organic matter (DOM) in surface waters influence ecosystem processes and anthropogenic use of freshwater. However, despite the importance of understanding spatial and temporal patterns in DOM, measures of DOM quality are not routinely included as part of large-scale ecosystem monitoring programs and variations in analytical procedures can introduce artifacts. In this study, we used consistent sampling and analytical methods to meet the objective of defining variability in DOM quantity and quality and other measures of water quality in streamflow issuing from small forested watersheds located within five Critical Zone Observatory sites representing contrasting environmental conditions. Results show distinct separations among sites as a function of water quality constituents. Relationships among rates of atmospheric deposition, water quality conditions, and stream DOM quantity and quality are consistent with the notion that areas with relatively high rates of atmospheric nitrogen and sulfur deposition and high concentrations of divalent cations result in selective transport of DOM derived from microbial sources, including in-stream microbial phototrophs. We suggest that the critical zone as a whole strongly influences the origin, composition, and fate of DOM in streams. This study highlights the value of consistent DOM characterization methods included as part of long-term monitoring programs for improving our understanding of interactions among ecosystem processes as controls on DOM biogeochemistry.

Nutrient pollution of surface waters is a widespread problem, calling for regional assessments of water quality conditions. In this study, we quantified long-term median nutrient concentrations of total nitrogen (TN) and total phosphorus (TP) in streams and rivers of Pennsylvania and explored relationships between stream nutrient concentrations and the land use of their watersheds. Our analysis is based on a synthesis of monitoring data from multiple agencies that included records of nutrient concentrations observed between 2000–2019. Across the state, stream nutrient concentrations observed in predominantly undeveloped areas (e.g., forests, shrubs, and grasslands) have median concentration values of 0.42 mg l−1 for TN and 0.011 mg l−1 for TP, reflecting background concentrations for minimally impacted watersheds. Median stream concentrations of TN in agricultural areas are about eleven times higher than in undeveloped areas; and are about five times higher in developed areas than in undeveloped areas. Median stream concentrations of TP in developed areas have about eight times higher concentrations than undeveloped areas; and are about four times higher in agricultural areas than in undeveloped areas. Concentrations of TN and TP increased substantially as the combined percentage of agricultural and developed land use increased. Fragmented data storage practices (e.g. incomplete metadata, ambiguous site names, and missing coordinates) and inconsistencies in monitoring protocols (e.g., differences in constituents measured, parameter names, and measurement methods) made leveraging the secondary use of multiple sources of data challenging. Nonetheless, our integrated dataset is robust, represents the best data available, and provides a new window into the nutrient status of Pennsylvania’s surface waters. The long-term median nutrient concentrations reveal the magnitude of variability in TN and TP concentrations across the state’s diverse environmental settings of land use, physiography, and geology. This information is useful for interpreting additional monitoring data, informing evaluation of water quality conditions, and guiding watershed management.

River corridors store, convey, and process nutrients from terrestrial and upstream sources, regulating delivery from headwaters to estuaries. A consequence of chronic excess nitrogen loading, as supported by theory and field studies in specific watersheds, is saturation of the biogeochemically-mediated nitrogen removal processes that weakens the capacity of the river corridor to remove nitrogen. Regional nitrogen models typically assume that removal capacity exhibits first-order behavior, scaling positively and linearly with increasing concentration, which may bias the estimation of where and at what rate nitrogen is removed by river corridors. Here we estimate the nitrogen concentration saturation threshold and its effects on annual nitrogen export from the Northeastern United States, revealing an average 42% concentration-induced reduction in headwater removal capacity. The weakened capacity caused an average 10% increase in the predicted delivery of riverine nitrogen from urban and agricultural watersheds compared to estimates using first-order process assumptions. Our results suggest that nitrogen removal may fall below a first-order rate process as riverine concentration increases above a threshold of 0.5 mg N l(-1). Threshold behavior indicates that even modest mitigation of nitrogen concentration in river corridors above the threshold can cause a self-reinforcing boost to nitrogen removal. USGSUnited States Geological Survey; National Science Foundation Hydrologic Sciences ProgramNational Science Foundation (NSF)NSF - Directorate for Geosciences (GEO); USGS Water Resources Availability Program; USGS Mendenhall Postdoctoral FellowshipUnited States Geological Survey; US Department of Agriculture National Institute of Food and AgricultureUnited States Department of Agriculture (USDA); US Department of Energy Subsurface Biogeochemistry Research Program, Pacific Northwest National Laboratory's scientific focus area; Virginia Agricultural Experimental Station The US Geological Survey (USGS) River Corridor Working Group developed the project idea in their meetings at the JohnWesley Powell Center with support from the USGS and the National Science Foundation Hydrologic Sciences Program. NMS, JWH, RBA, GES, and CPK also received support from the USGS Water Resources Availability Program. NMS was additionally supported by a USGS Mendenhall Postdoctoral Fellowship. EWB and DS received support from the US Department of Agriculture National Institute of Food and Agriculture. JDGV received support from the US Department of Energy Subsurface Biogeochemistry Research Program as part of Pacific Northwest National Laboratory's scientific focus area. DS also received support from the Virginia Agricultural Experimental Station. The SPARROW source code is also publicly available (https://water.usgs.gov/nawqa/sparrow/). Expressions and equations in the Methods section can be used to reproduce the results. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government. The authors declare no competing interests.

Lakes, reservoirs, and other ponded waters are ubiquitous features of the aquatic landscape, yet their cumulative role in nitrogen removal in large river basins is often unclear. Here we use predictive modeling, together with comprehensive river water quality, land use, and hydrography datasets, to examine and explain the influences of more than 18,000 ponded waters on nitrogen removal through river networks of the Northeastern United States. Thresholds in pond density where ponded waters become important features to regional nitrogen removal are identified and shown to vary according to a ponded waters’ relative size, network position, and degree of connectivity to the river network, which suggests worldwide importance of these new metrics. Consideration of the interacting physical and biological factors, along with thresholds in connectivity, reveal where, why, and how much ponded waters function differently than streams in removing nitrogen, what regional water quality outcomes may result, and in what capacity management strategies could most effectively achieve desired nitrogen loading reduction., Lakes, reservoirs, and other ponded waters are common in large river basins yet their influence on nitrogen budgets is often indistinct. Here, the authors show how a ponded waters’ relative size, shape, and degree of connectivity to the river network control nitrogen removal.

Net ecosystem production (NEP) and the overall organic carbon budget for the estuaries along the East Coast of the United States are estimated. We focus on the open estuarine waters, excluding the fringing wetlands. We developed empirical models relating NEP to loading ratios of dissolved inorganic nitrogen to total organic carbon, and carbon burial in the sediment to estuarine water residence time and total nitrogen input across the landward boundary. Output from a data-constrained water quality model was used to estimate inputs of total nitrogen and organic carbon to the estuaries across the landward boundary, including fluvial and tidal-wetland sources. Organic carbon export from the estuaries to the continental shelf was computed by difference, assuming steady state. Uncertainties in the budget were estimated by allowing uncertainties in the supporting model relations. Collectively, U.S. East Coast estuaries are net heterotrophic, with the area-integrated NEP of −1.5 (−2.8, −1.0) Tg C yr−1 (best estimate and 95% confidence interval) and area-normalized NEP of −3.2 (−6.1, −2.3) mol C m−2 yr−1. East Coast estuaries serve as a source of organic carbon to the shelf, exporting 3.4 (2.0, 4.3) Tg C yr−1 or 7.6 (4.4, 9.5) mol C m−2 yr−1. Organic carbon inputs from fluvial and tidal-wetland sources for the region are estimated at 5.4 (4.6, 6.5) Tg C yr−1 or 12 (10, 14) mol C m−2 yr−1 and carbon burial in the open estuarine waters at 0.50 (0.33, 0.78) Tg C yr−1 or 1.1 (0.73, 1.7) mol C m−2 yr−1. Our results highlight the importance of estuarine systems in the overall coastal budget of organic carbon, suggesting that in the aggregate, U.S. East Coast estuaries assimilate (via respiration and burial) ~40% of organic carbon inputs from fluvial and tidal-wetland sources and allow ~60% to be exported to the shelf.

summary Shallow groundwater dynamics play a critical role in determining the chemistry and movement of nitrogen (N) in the riparian zone. In this study, we characterized N concentration variability and hydrologic transport pathways in shallow groundwater draining areas of a riparian area with and without emergent groundwater seeps. The study was conducted in FD36, an agricultural headwater catchment in the Ridge and Valley physiographic region of central Pennsylvania, USA. Three seep and adjacent non-seep areas were each instrumented with a field of 40 piezometers installed in a grid pattern (1.5-m spacing) at both 20- and 60-cm depths. Piezometers were monitored seasonally for approximately two years (October 2010–May 2012). Results showed that hydraulic head within seep areas was variable and some regions exhibited upward vertical hydraulic gradients of 0.18–0.27. Non-seep areas were characterized by uniform hydraulic head levels and were relatively hydrostatic. Nitrate-N (NO3-N) concentrations in seep areas were significantly greater than those in the non-seep areas at two of the three study sites. A two-component mixing model using chloride as a conservative tracer indicated that shallow groundwater in seep areas was primarily (53–75%) comprised of water from a shallow fractured aquifer, which had elevated NO3-N concentrations (5.7 mg L � 1 ). Shallow groundwater in non-seep areas, however, was comprised (58–82%) of perched water on top of the fragipan that was likely recharged locally in the riparian zone and had low NO3-N concentrations (0.6 mg L � 1 ). Higher NO3-N concentrations, variable hydraulic head, and groundwater emergence onto the land surface in seep areas provided evidence for preferential flow paths as an important conduit for water and N movement in these areas of the riparian zone. We conclude that the potential for N delivery to the stream in FD36 was much greater from seep areas compared to non-seep areas. Targeted management of seeps should be a priority in efforts to reduce NO3-N levels in riparian zones within headwater agricultural catchments. Published by Elsevier B.V.

Autumn is a season of dynamic change in forest streams of the northeastern United States due to effects of leaf fall on both hydrology and biogeochemistry. Few studies have explored how interactions of biogeochemical transformations, various nitrogen sources, and catchment flow paths affect stream nitrogen variation during autumn. To provide more information on this critical period, we studied (1) the timing, duration, and magnitude of changes to stream nitrate, dissolved organic nitrogen (DON), and ammonium concentrations; (2) changes in nitrate sources and cycling; and (3) source areas of the landscape that most influence stream nitrogen. We collected samples at higher temporal resolution for a longer duration than typical studies of stream nitrogen during autumn. This sampling scheme encompassed the patterns and extremes that occurred during base flow and stormflow events of autumn. Base flow nitrate concentrations decreased by an order of magnitude from 5.4 to 0.7 µmol L-1 during the week when most leaves fell from deciduous trees. Changes to rates of biogeochemical transformations during autumn base flow explained the low nitrate concentrations; in-stream transformations retained up to 72% of the nitrate that entered a stream reach. A decrease of in-stream nitrification coupled with heterotrophic nitrate cycling were primary factors in the seasonal nitrate decline. The period of low nitrate concentrations ended with a storm event in which stream nitrate concentrations increased by 25-fold. In the ensuing weeks, peak stormflow nitrate concentrations progressively decreased over closely spaced, yet similarly sized events. Most stormflow nitrate originated from nitrification in near-stream areas with occasional, large inputs of unprocessed atmospheric nitrate, which has rarely been reported for nonsnowmelt events. A maximum input of 33% unprocessed atmospheric nitrate to the stream occurred during one event. Large inputs of unprocessed atmospheric nitrate show direct and rapid effects on forest streams that may be widespread, although undocumented, throughout nitrogen-polluted temperate forests. In contrast to a week-long nitrate decline during peak autumn litterfall, base flow DON concentrations increased after leaf fall and remained high for 2 months. Dissolved organic nitrogen was hydrologically flushed to the stream from riparian soils during stormflow. In contrast to distinct seasonal changes in base flow nitrate and DON concentrations, ammonium concentrations were typically at or below the detection limit, similar to the rest of the year. Our findings reveal couplings among catchment flow paths, nutrient sources, and transformations that control seasonal extremes of stream nitrogen in forested landscapes.

The availability of light detection and ranging data (LiDAR) has resulted in a new era of landscape analysis. For example, improvements in LiDAR data resolution may make it possible to accurately model microtopography over a large geographic area; however, data resolution and processing costs versus resulting accuracy may be too costly. We examined two LiDAR datasets of differing resolutions, a low point density (0.714 points/m2spacing) 1 m DEM available statewide in Pennsylvania and a high point density (10.28 points/m2spacing) 1 m DEM research-grade DEM, and compared the calculated roughness between both resulting DEMs using standard deviation of slope, standard deviation of curvature, a pit fill index, and the difference between a smoothed splined surface and the original DEM. These results were then compared to field-surveyed plots and transects of microterrain. Using both datasets, patterns of roughness were identified, which were associated with different landforms derived from hydrogeomorphic features such as stream channels, gullies, and depressions. Lowland areas tended to have the highest roughness values for all methods, with other areas showing distinctive patterns of roughness values across metrics. However, our results suggest that the high-resolution research-grade LiDAR did not improve roughness modeling in comparison to the coarser statewide LiDAR. We conclude that resolution and initial point density may not be as important as the algorithm and methodology used to generate a LiDAR-derived DEM for roughness modeling purposes.