Water-quality trends in US rivers: Exploring effects from streamflow trends and changes in watershed management

Dynamic disparities in inorganic nitrogen and phosphorus fluxes into estuarine systems under different flow regimes and streamflow droughts

Elongated periods of low flow conditions, which can be termed as streamflow droughts, influence the nutrient (e.g., nitrogen and phosphorus) balance in estuarine systems. Analyzing temporal trends of nutrient fluxes into such systems under different streamflow regimes can complement the understanding about the dynamic evolution of streamflow droughts and their impacts on nutrient levels. The objective of this paper was to evaluate how dynamic evolution of streamflow droughts (from low flow conditions) affects the inorganic nutrient flux in a tropical estuarine system. We analyzed a 20-year time series of streamflow data together with the concentrations of two nutrient parameters-dissolved inorganic phosphorus (DIP) and dissolved inorganic nitrogen (DIN)-in the Lower Apalachicola River that drains into Apalachicola Bay in northeastern Gulf of Mexico, Florida. Our findings revealed that droughts affect the seasonal patterns and fluxes of both DIP and DIN. We also observed post-drought flushing patterns in DIP and contrasting changes in DIP and DIN fluxes in the long-term (20 years here) under different streamflow conditions. Dynamically changing correlations between the streamflow and the fluxes were found throughout different phases of droughts. In the long-term (from 2003 to 2021), the DIP flux in high flows increased by 35.3%, while the flux decreased by 15.7% in low flows. Conversely, DIN flux in high flows showed a decrease of <1.2%, but an increase of <23.7% in low flows after droughts end. The insights from this study highlighted the need for effective regulation plans such as proper nutrient management against streamflow droughts to mitigate negative ecological consequences in estuarine systems such as harmful algal blooms.

Effect of varying hydrologic regime on seasonal total maximum daily loads (TDML) in an agricultural watershed

Rising hypoxia due to the eutrophication of riverine ecosystems is primarily caused by the transport of nutrients. The majority of existing TMDL models cannot be efficienty applied to represent nutrient concentrations in riverine ecosystems having varying flow regimes due to seasonal differences. Accurate TMDL assessment requires nutrient loads and suspended matter estimation under varying flow regimes with minimal uncertainty. Though a large database can enhance accuracy, it can be resource intensive. This study presents the design of an innovative modeling strategy to optimize the use of existing datasets to effectively represent streamflow-load dynamics while minimizing uncertainty. The study developed an approach to assess TMDLs using six different flux models and kriging techniques (i) to enhance the accuracy of nutrient load estimation under different hydrologic regimes (flow stratifications) and (ii) to derive an optimal modeling strategy and sampling scheme for minimizing uncertainty. The flux models account for uncertainty in load prediction across varying flow strata, and the deployment of multiple load calculation procedures. Further, the proposed flux approach allows the determination of load exceedance under different TMDL scenarios aimed at minimizing uncertainty to achieve reliable load predictions. The study employed a 10-year dataset (2009-2018) consisting of daily flow data (m3/sec) and weekly data (mg/L) for nitrogen (N), phosphorus (P) and total suspended solids (TSS) concentrations in three distinct agricultural sites in+ the Minnesota River Watershed. The outcomes were analyzed geospatially in a Geographic Information System (GIS) environment using the kriging interpolation technique. The study recommends (i) triple stratification of flows to obtain accurate load estimates, and (ii) an optimal sampling scheme for nitrogen and phosphorous with 30.6 % and 49.8 % datapoints from high flow strata. The study outcomes are expected to contribute to the planning of economically and technically sound combinations of best management practices (BMPs) required for achieving total maximum daily loads (TMDL) in a watershed.

Recent, widespread nitrate decreases may be linked to persistent dissolved organic carbon increases in headwater streams recovering from past acidic deposition

Long-term monitoring of water quality responses to natural and anthropogenic perturbation of watersheds informs policies for managing natural resources. Dissolved organic carbon (DOC) and nitrate (NO3-) in streams draining forested landscapes provide valuable information on ecosystem function due to their biogeochemical reactivity and solubility in water. Here we evaluate a 20-year record (2001-2021) of biweekly stream-water samples (n > 3000) and continuous discharge in three forested catchments in the Adirondack region of New York to investigate and interpret long-term trends in DOC and NO3- concentrations. Results from the intensively monitored catchments were compared with data from synoptic surveys of streams throughout the Adirondack region. A weighted regressions on time, discharge, and season (WRTDS) model, used to estimate daily flow-normalized concentrations, determined that DOC increased by ~30 to 50 % while NO3- decreased by ~50 to 70 % over the study period. The large amount of data from catchments with different soil properties permitted us to assess the relative effects of hydrology, season, and land cover factors on temporal trends in DOC and NO3- concentrations. We found weak evidence of climatic forcing of long-term increases in DOC, and instead contend that declining ionic strength in precipitation linked to declining anthropogenic acid deposition is driving DOC trends in stream waters. Nitrate concentrations were more variable but clearly decreased in recent years possibly related to declining N deposition. The recent increase in DOC:NO3- in all catchments indicates a major shift in stream stoichiometry that reflects changes in ecosystem functioning that may have important biogeochemical implications for terrestrial as well as aquatic ecosystems.

Effects of point and nonpoint source controls on total phosphorus load trends across the Chesapeake Bay watershed, USA

Reduction of total phosphorus (TP) loads has long been a management focus of Chesapeake Bay restoration, but riverine monitoring stations have shown mixed temporal trends. To better understand the regional patterns and drivers of TP trends across the Bay watershed, we compiled and analyzed TP load data from 90 non-tidal network stations using clustering and random forest (RF) approaches. These stations were categorized into two distinct clusters of short-term (2013-2020) TP load trends, i.e. monotonic increase (n = 35) and monotonic decline (n = 55). RF models were developed to identify likely regional drivers of TP trend clusters. Reductions in point sources and agricultural nonpoint sources (i.e. fertilizer) both contributed to water-quality improvement in our period of analysis, thereby demonstrating the effectiveness of nutrient management and the importance of continuing such efforts. In addition, declining TP trends have a larger chance to occur in carbonate areas but a smaller chance in Coastal Plain areas, with the latter likely reflecting the effect of legacy P. To provide spatially explicit information, TP trend clusters were predicted for the entire watershed at the scale of river segments, which are more directly relevant to watershed planning. Among the 975 river segments, 544 (56%) and 431 (44%) were classified as 'monotonic increase' and 'monotonic decrease', respectively. Furthermore, these predicted TP trend clusters were paired with our previously published total nitrogen (TN) trend clusters, showing that TP and TN both declined in 185 segments (19%) and neither declined in 337 segments (35%). Broadly speaking, large-scale nutrient reduction efforts are underway in many regions to curb eutrophication. Water-quality responses and drivers may differ among systems, but our work provides important new evidence on the effectiveness of management efforts toward controlling point and nonpoint sources.

Turbidity dynamics in Indian peninsular river mouths derived from Kd490 reveals key anthropogenic drivers

Large rivers, which act as natural integrators of surface processes, contribute massive volume of terrestrial materials to the coastal oceans. However, the accelerated climate warming and increasing anthropogenic activities recorded in recent years have been severely affecting the hydrologic and physical regimes of river systems. These changes have a direct impact on river discharge and runoff, some of which are occurred rapidly in the past two decades. Here, we present a quantitative analysis on the effects of changes in surface turbidity at coastal river mouths using diffuse attenuation coefficient at 490 nm (Kd490) as a proxy of turbidity for six major Indian peninsular rivers. The time series (2000-2022) trends of Kd490 obtained from Moderate Resolution Imaging Spectrometer (MODIS) images shows a significant decreasing trend in Kd values (p < 0.001) at the mouths of the Narmada, Tapti, Cauvery, Krishna, Godavari, and Mahanadi rivers. This is despite an increased rainfall trend observed for the six studied river basins which can likely intensifies the surface runoff and deliver more sediments, suggesting that other factors such as land use changes and increased number of dam constructions are primarily responsible for the decreased sediment load from rivers to coastal mouths.

Four decades of regional wet deposition, local bulk deposition, and stream-water chemistry show the influence of nearby land use on forested streams in Central Appalachia

Hydrologic monitoring began on two headwater streams (<1 km²) on the University of Kentucky's Robinson Forest in 1971. We evaluated stream-water (1974-2013) and bulk-deposition (wet + dust) (1984-2013) chemistry in the context of regional wet-deposition patterns that showed decreases in both sulfate and nitrate concentrations as well as proximal surface-mine expansion. Decadal time steps (1974-83, 1984-93, 1994-2003, 2004-2013) were used to quantify change. Comparison of the first two decades showed similarly decreased sulfate (minimum flow-adjusted annual-mean concentration of ≈13.5 mg/L in 1982 to 8.8 mg/L in 1992) and increased pH (6.6-6.8) in both streams, reflecting contemporaneous changes in both bulk and wet deposition. In contrast, concentrations of nitrate (0.14 to >0.25 mg/L) and base cations increased between these two decades, coinciding with expansion of surface mining between 1985 and 1995. In 2004, stream-water pH (6.7 in 2004), sulfate (9.2 mg/L), and nitrate (>0.11 mg/L) were similar to 1982, despite wet-deposition concentrations being lower. Base-cation concentrations were higher in the stream adjacent to ongoing surface mining relative to the stream situated near the middle of the experimental forest. However, pH decreased to approximately 5.7 by 2013 for both streams, which, combined with a shift in dominant cations from calcium to magnesium and potassium, indicates that the soil-buffering capacity of this landscape has been exceeded. Ratios of bulk deposition and stream-water concentrations indicate enrichment of sulfate (1.7-25.2) and cations (0.5-64.8), but not nitrogen (0.1-5.6), indicating that the Forest is not nitrogen saturated and that ongoing changes in water-quality are sulfate driven. When concentrations were adjusted to account for changes in streamflow (climate) over the 4 decades, external influences (land management/regulation) explained most change. The amount and direction of change differed among constituents, both between consecutive decades and between the first and last decades, reflecting the influence of localized surface mining even as regional wet deposition continued to improve due to the Clean Air Act. The implication is that localized stressors have the potential to out-pace the benefits of national environmental policies for communities that depend on local water-resources in similar environments.

Lake morphological characteristics and climatic factors affect long-term trends of phytoplankton community in the Rotorua Te Arawa lakes, New Zealand during 23 years observation

Monitoring the long-term dynamics of lake phytoplankton can help understand their natural temporal variability, as well as assess potential impacts of interventions aimed at improving lake ecological condition. However, investigating long-term changes in lake ecosystems has received scant attention. In the present study, we analyzed a long-term dataset of phytoplankton communities collected from 1990 to 2013 from eleven of the 12 Rotorua Te Arawa lakes in New Zealand, to explore their responses to changing abiotic conditions. We used a sequential algorithm to examine the likelihood of regime shifts in abiotic and biotic factors during the study period that could be attributable to lake interventions. Our analysis suggests that lake interventions have improved the abiotic factors, whereas the response of biotic factors was less clear. Total phosphorus levels were implicated in the decline in lake condition, including in two lakes subject to lake interventions, and in four control lakes. Both abiotic and biotic factors showed diverse trends (e.g., increase, decrease or no change), and abiotic factors had more regime shifts than biotic factors. Shifts in biotic indices also displayed time lags to shifts in abiotic factors. Long-term responses of abiotic and biotic factors were also influenced by lake morphological characteristics and climatic variables. This latter finding underscores the importance of considering lake morphological characteristics and climate changes when planning management practices. A sound understanding of resilience and threshold of phytoplankton shifts to environmental changes are needed to assess the effectiveness of previous management strategies and prioritize the future conservation efforts toward water quality goals.

Spatial patterns and seasonal timing of increasing riverine specific conductance from 1998 to 2018 suggest legacy contamination in the Delaware River Basin

Increasing salinization of freshwater threatens water supplies that support a range of human and ecological uses. The latest assessments of Delaware River Basin (DRB) surface-water-quality changes indicate widespread salinization has occurred in recent decades, which may lead to meaningful degradation in water quality. To better understand how and when salinity transport occurs and implications for DRB streams, this study: 1) explores the variability of specific conductance (SC) trends spatially and seasonally from 1998 to 2018, and 2) investigates how trends relate to streamflow, land disturbance, and impervious surface area to better understand regional salinization drivers. We find widespread increases in SC across the DRB, with several sites in the lower basin exceeding thresholds for aquatic life and experiencing increasing frequencies of exceedance over time. In general, the greatest basin wide increases in SC occurred during low flow conditions, indicating that a legacy component resulting from subsurface retention and transport processes has driven observed changes in riverine SC. For a subset of sites in the lower basin, where impervious area and cumulative land disturbance are higher, the greatest SC increases occurred during high flow conditions in winter months. Given the patterns of SC and watershed changes across the basin, as well as strong relationships between SC trends and sodium and chloride trends, deicing salt appears to be a likely driver of observed SC change. Even if deicing salt application plateaus or declines in coming years, the continued release and transport of the legacy subsurface component may still contribute to elevated DRB riverine SC.

Regional social-ecological system coupling process from a water flow perspective

The social-ecological system is receiving more and more attention, and water resources have been a focal point for linking social systems and ecosystems, but how to clarify the regional social-ecological system coupling process through the water flow perspective and how to make ecosystem services management decisions still needs further research. This study integrates water quantity and quality and proposes a water-related ecosystem services flow framework. This study applied the framework to the Wuding River watershed and simulated water quantity and quality by SWAT model. The results showed that: (1) there is significant spatial heterogeneity in ecosystem service provisioning and meaningful improvement in water quality under the function of human-made capital in the green phase of the ecosystem services flow; (2) in the red phase, beneficiaries use the water supply for their production and life and discharge >7400 tons pollution loads into the ecosystem; (3) in this process, human-made capital reduces about 35 % of the ammonia pollution, and meanwhile, the ecosystem relies on its environment to further clean up about 44 % of the load. The research framework is suitable for watershed social-ecological systems with simplistic interactions, guiding ecological compensation schemes and related management policies. Furthermore, providing a scientific basis for the sustainable use of regional water resources.

Impact of artisanal small-scale (gold and diamond) mining activities on the Offin, Oda and Pra rivers in Southern Ghana, West Africa: A scientific response to public concern

The surface water systems of Ghana serve as a major source of drinking water, besides other multi-purpose benefit of hydro-electrical power generation and transportation. Thus, the dependence and benefits from such resources are of national interest. For instance, the Pra river of the South-Western surface water system of Ghana was a major consideration for a projected 5 billion m³ water demand in the year 2020 and "African Water Vision 2025". In recent times, the colour state of the Pra river and similar surface water bodies of the Offin and Oda rivers has attracted intense public discussion. The prime issue relates to incessant illegal artisanal gold/diamond mining on or along these rivers. In order to assess the state of these rivers, water samples were taken, and analysed at the Council of Scientific and Industrial Research Laboratory (CSIR, Accra-Ghana) to investigate their physico-chemical quality. The research objective was to assess the extent of their water pollution by measuring physico-chemical parameters of turbidity, colour, pH and content of selected metals. A total of 18 preserved bottled samples [(5 from Offin river and 2 boreholes), 5 from Oda river and 5 from Pra river and 1 borehole)] were analysed, and results compared with portable water standards as defined by the WHO and CSIR (GS-175-1) of Ghana. Results on turbidity, colour, mercury and iron from the river and water samples generally exceed WHO or GS-175-1 limit. The Pra river recorded the most alarming result; range for turbidity (2,010 to 2,745 NTU), colour (3,000 to 4,500 Hz), total suspended solutes (2,240 to 2,570 mg/L) and total dissolved solutes (97.80-99.60 mg/L, excluding 319.00 to 25,440 mg/L). The Oda river shows lowest parameter values among the three rivers, as the areas have been dormant from illegal gold mining for 5 years. Current data suggests polluted river bodies and boreholes, and that none of these water resources meets the portable water consumption criteria unless treated prior to usage. As the current state of the water bodies may incur higher cost of water treatment or purification, an integrated water governance under Ghana's Ministry of Water Resource, Work and Housing, and the Minerals Commission and Environmental Protection Agency are recommended for the management of these valuable water resources.

Long-term impacts of impervious surface cover change and roadway deicing agent application on chloride concentrations in exurban and suburban watersheds

Roadway deicing agents, including rock salt and brine containing NaCl, have had a profound impact on the water quality and aquatic health of rivers and streams in urbanized areas with temperate climates. Yet, few studies evaluate impacts to watersheds characterized by relatively low impervious surface cover (ISC; < 15 %). Here, we use long-term (1997-2019), monthly streamwater quality data combined with daily streamflow for six exurban and suburban watersheds in southeastern Pennsylvania to examine the relations among chloride (Cl^-) concentrations and ISC. Both flow-normalized Cl^- concentrations and ISC increased over time in each of the six watersheds, consistent with changes in watershed management (e.g., ISC, road salt application, etc.). The watersheds that experienced the greatest changes in percent ISC (e.g., agriculture replaced by residential and commercial development) experienced the greatest changes in flow-normalized Cl^- concentrations. We also utilized a comprehensive mass-balance model (2011-2018) that indicated Cl^- inputs exceeded the outputs for the study watersheds. Road salt applied to state roads, non-state roads, and other impervious surfaces accounted for the majority of Cl^- inputs to the six watersheds. Furthermore, increasing Cl^- concentrations during baseflow conditions confirm impacts to shallow groundwater. Although flow-normalized Cl^- concentrations are below the U.S. Environmental Protection Agency's chronic threshold value for impacts to aquatic organisms, year-round exceedances may result before the end of this century based on current trends. Though reduced Cl^- loading to streams may be achieved by limiting the expansion of impervious surfaces in exurban and suburban watersheds, changes in baseflow concentrations are likely to be gradual because of the accumulated Cl^- in groundwater.

A systematic increase in the slope of the concentration discharge relation for dissolved organic carbon in a forested catchment in Vermont, USA

The production, mobilization and fluvial transport of dissolved organic carbon (DOC) in temperate forests are important components of the carbon cycle that are influenced by ongoing changes in climate. Numerous studies have reported temporal trends in stream water DOC concentrations and have attributed changes in concentrations to climatic and hydrologic variables. Fewer studies have reported trends in concentration-discharge (C-Q) relations for DOC. The goal of this study was to detect and quantify changes in DOC concentration and slope of the C-Q relation from 1991 to 2018 in an intensively sampled forested research watershed in northern Vermont. Stream water DOC concentration and slope of the C-Q relation increased over time as did precipitation, stream discharge, and air temperature. The increases in DOC concentration and slope of the C-Q were substantially greater in the summer and fall (autumn) than in winter and spring. The largest increases in the magnitude of C-Q slopes occurred in the December, October and September. The increases in slope of the C-Q relation in summer and fall were larger for baseflow than for storm flow. The increases in DOC concentration and slope of the C-Q relation over time may be related to increasing temperature, longer growing seasons, and associated increases in production and microbial decomposition of soil organic matter that supplies DOC for mobilization to streams. The results suggest that in a changing climate, C-Q relations may not necessarily be stationary and therefore analyses that attempt to estimate future DOC concentrations and loads should consider potentially changing C-Q relations over time.

Natural and anthropogenic forces on suspended sediment dynamics in Asian estuaries

Climate change and anthropogenic activities are affecting the hydrological conditions of rivers and may have altered nutrient and suspended sediments released into coastal seas. However, testing this hypothesis is difficult, confounded by the lack of observational data and the unavailability of globally accepted suspended sediment concentration (SSC) algorithms. Here, we analyzed the trends in SSC (2000-2020) at the mouths of 10 major Asian rivers using 10 available satellite-SSC algorithms. We identified spatially distinct trends, with SSC decreasing at the mouths of the Yellow, Pearl, and Indus rivers, and increasing trends at the mouths of the Narmada and Ganges-Brahmaputra rivers, while there were no significant trends at the mouths of the remaining rivers. River discharge, dams, and land use changes in basins individually did not suffice, but reproduced the observed SSC trends when used together. Our results imply that anthropogenic activities threaten the marine ecosystem more than climate forcing on Asian coasts.

Regional patterns and drivers of total nitrogen trends in the Chesapeake Bay watershed: Insights from machine learning approaches and management implications

Anthropogenic nutrient inputs have led to nutrient enrichment in many waterbodies worldwide, including Chesapeake Bay (USA). River water quality integrates the spatial and temporal changes of watersheds and forms the foundation for disentangling the effects of anthropogenic inputs. We demonstrate with the Chesapeake Bay Non-Tidal Monitoring Network that machine learning approaches - i.e., hierarchical clustering and random forest (RF) classification - can be combined to better understand the regional patterns and drivers of total nitrogen (TN) trends in large monitoring networks, resulting in information useful for watershed management. Cluster analysis revealed regional patterns of short-term TN trends (2007-2018) and categorized the stations into three distinct trend clusters, namely, V-shape (n = 23), monotonic decline (n = 35), and monotonic increase (n = 26). RF models identified regional drivers of TN trend clusters by quantifying the effects of watershed characteristics (land use, geology, physiography) and major N sources on the trend clusters. Results provide encouraging evidence that improved agricultural nutrient management has resulted in declines in agricultural nonpoint sources, which in turn contributed to water-quality improvement in our period of analysis. Moreover, water-quality improvements are more likely in watersheds underlain by carbonate rocks, reflecting the relatively quick groundwater transport of this terrain. By contrast, water-quality improvements are less likely in Coastal Plain watersheds, reflecting the effect of legacy N in groundwater. Notably, results show degrading trends in forested watersheds, suggesting new and/or remobilized sources that may compromise management efforts. Finally, the developed RF models were used to predict TN trend clusters for the entire Chesapeake Bay watershed at the fine scale of river segments (n = 979), providing fine spatial information that can facilitate targeted watershed management, including unmonitored areas. More broadly, this combined use of clustering and classification approaches can be applied to other regional monitoring networks to address similar water-quality questions.

Evaluation of sampling frequency impact on the accuracy of water quality status as determined considering different water quality monitoring objectives

Water quality sampling is a key element in tracking water quality monitoring objectives. However, frequencies adapted by different agencies might not be sufficient to provide an accurate indication of water quality status. In this study, data from low- and high-resolution water quality datasets were analyzed to determine the extent to which monitoring objectives could be achieved with different sampling frequencies, with a view to providing recommendations and best practices for water quality monitoring frequency in places with limited resources with which to implement a high-frequency monitoring plan. Water quality data from two watersheds (Maumee River and Raisin River) located in the Western Lake Erie Basin (WLEB) were used since these watersheds have consistent records over substantial periods of time, and the water quality data available have a high resolution (at least daily). The water quality constituents analyzed included suspended solids (SS), total phosphorus (TP), soluble reactive phosphorus (SRP), and nitrate + nitrite (NO₂₊₃). Sources of pollutants for watersheds located in the WLEB include contributions from point sources like discharges from sewage treatment plants and non-point sources such as agricultural and urban storm runoff. Weekly, bi-weekly, monthly, and seasonal datasets were created from the original datasets, following different sampling rules based on the day of the week, week of the month, and month of the year. The resulting datasets were then compared to the original dataset to determine how the sampling frequency would affect the results obtained in a water quality assessment when different monitoring objectives are considered. Results indicated that constituents easily transported by water (such as sediments and nutrients) require more than 50 samples/year to provide a small error (< 10%) with a confidence interval of 95%. Monthly and seasonal sampling were found appropriate to report a stream's prevailing water quality status and statistical properties. However, these resolutions might not be sufficient to capture long-term trends, in which case bi-weekly samples would be preferable. Limitations of low-resolution sampling frequency could be overcome by including rainfall events and random sampling during specific time windows as part of the monitoring plan.

Climate extremes as drivers of surface-water-quality trends in the United States

Surface-water quality can change in response to climate perturbations, such as changes in the frequency of heavy precipitation or droughts, through direct effects, such as dilution and concentration, and through physical processes, such as bank scour. Water quality might also change through indirect mechanisms, such as changing water demand or changes in runoff interaction with organic matter on the landscape. Many studies predict future changes in water-quality related to climate changes; however, fewer studies specifically document changes in water quality related to changes in climate, and they are usually limited in geographic scope. Recently, the U.S. Geological Survey's National Water-Quality Program reported nearly 12,000 trends in concentration and load for numerous water-quality constituents, including nutrients, sediment, major ions, and carbon. The results provide an unprecedented opportunity to examine sites across the conterminous United States for changes in water quality related to climate changes. We used published water-quality trends, modeled using the method of Weighted Regressions on Time, Season and Discharge, and calculated trends in climate extremes indices, using a modified Mann-Kendall trend method. The water-quality and the climate extremes trends were combined to identify areas in the conterminous United States where changes in climate extremes may have changed water quality. We investigated the water-quality trends in these areas to determine whether the trends related to changes in climate. We found that it was important to go beyond spatial correlation and examine trends on a watershed scale to investigate key drivers of trends. We found successful management practices in Iowa to reduce chloride concentrations, despite increases in icing days. For sediment, it appeared that management practices were having a larger effect than climate changes. For nutrients, complex forces affecting water quality make it difficult to unequivocally attribute water-quality change to climate change.

Vulnerable Waters are Essential to Watershed Resilience

Watershed resilience is the ability of a watershed to maintain its characteristic system state while concurrently resisting, adapting to, and reorganizing after hydrological (for example, drought, flooding) or biogeochemical (for example, excessive nutrient) disturbances. Vulnerable waters include non-floodplain wetlands and headwater streams, abundant watershed components representing the most distal extent of the freshwater aquatic network. Vulnerable waters are hydrologically dynamic and biogeochemically reactive aquatic systems, storing, processing, and releasing water and entrained (that is, dissolved and particulate) materials along expanding and contracting aquatic networks. The hydrological and biogeochemical functions emerging from these processes affect the magnitude, frequency, timing, duration, storage, and rate of change of material and energy fluxes among watershed components and to downstream waters, thereby maintaining watershed states and imparting watershed resilience. We present here a conceptual framework for understanding how vulnerable waters confer watershed resilience. We demonstrate how individual and cumulative vulnerable-water modifications (for example, reduced extent, altered connectivity) affect watershed-scale hydrological and biogeochemical disturbance response and recovery, which decreases watershed resilience and can trigger transitions across thresholds to alternative watershed states (for example, states conducive to increased flood frequency or nutrient concentrations). We subsequently describe how resilient watersheds require spatial heterogeneity and temporal variability in hydrological and biogeochemical interactions between terrestrial systems and down-gradient waters, which necessitates attention to the conservation and restoration of vulnerable waters and their downstream connectivity gradients. To conclude, we provide actionable principles for resilient watersheds and articulate research needs to further watershed resilience science and vulnerable-water management.

Effects of change in streamflow patterns on water quality

Streamflow patterns are closely linked with the quality of stream water, but they are often dealt separately. Due to this, the effects of change in streamflow patterns resulting from river regulation and flow diversion on stream water quality remain under-investigated. This study models change in water quality indicators including pollutants (total suspended solids and turbidity), nutrients (total nitrogen and phosphorus), dissolved oxygen, nitrogen (kjeldahl), pH, and salinity caused by the change in streamflow patterns under different scenarios of river regulation, flow diversion, and rainfall. The generalized additive model was used and the Goulburn-Broken catchment, Australia was chosen as the case study. It was found that concentrations of pollutants and nutrients increased by 38% while dissolved oxygen and nitrogen (kjeldahl) decreased by 35% during the period 1990-2018. These changes were associated with an average increase of 20% in low and medium flows, an average decline of 22% in high and overbank flows and a 15% decline in rainfall. Under the scenario of climate change, river regulation and flow diversion, the overbank flow patterns would mimic the effects of low and medium flows on the water quality indicators that would raise the concentration of pollutants, nutrients, and salinity by 19%. Restoration of high flows would decrease these concentrations by 28% relative to current concentrations, however, it would also reduce dissolved oxygen, nitrogen (kjeldahl), and pH. Effects of streamflow patterns on water quality have implications for environmental flow management, thus, this study recommends critical adjustments in low, medium, and high flows for improving water quality.

Predicting suspended sediment load in Peninsular Malaysia using support vector machine and deep learning algorithms

High loads of suspended sediments in rivers are known to cause detrimental effects to potable water sources, river water quality, irrigation activities, and dam or reservoir operations. For this reason, the study of suspended sediment load (SSL) prediction is important for monitoring and damage mitigation purposes. The present study tests and develops machine learning (ML) models, based on the support vector machine (SVM), artificial neural network (ANN) and long short-term memory (LSTM) algorithms, to predict SSL based on 11 different river data sets comprising of streamflow (SF) and SSL data obtained from the Malaysian Department of Irrigation and Drainage. The main objective of the present study is to propose a single model that is capable of accurately predicting SSLs for any river data set within Peninsular Malaysia. The ANN3 model, based on the ANN algorithm and input scenario 3 (inputs consisting of current-day SF, previous-day SF, and previous-day SSL), is determined as the best model in the present study as it produced the best predictive performance for 5 out of 11 of the tested data sets and obtained the highest average RM with a score of 2.64 when compared to the other tested models, indicating that it has the highest reliability to produce relatively high-accuracy SSL predictions for different data sets. Therefore, the ANN3 model is proposed as a universal model for the prediction of SSL within Peninsular Malaysia.

Sedimentary organic carbon and nutrient distributions in an endorheic lake in semiarid area of the Mongolian Plateau

Due to the lack of outlets, inflowing pollutants are often deposited in an endorheic lake, posing potential pressure on the environment. With climate change, extreme weather is expected to be more frequent and will contribute to the release of carbon and nutrients buried in the lakebeds. However, the distribution of sedimentary organic carbon and nutrients and the mechanisms that control the distribution are not fully understood, despite their significance to environmental development in endorheic lakes being widely recognized. In this study, the mechanisms controlling the sedimentary organic carbon and nutrient distributions in endorheic lakes were examined based on the analysis of an endorheic lake in the semiarid area of the Mongolian Plateau. The field survey results indicate that the concentrations of sedimentary organic carbon (TOC) and nutrients (NH₃-N and TP) on the lakebed have significant correlations and present spatial heterogeneities. To further study the distribution mechanisms, numerical models were established to calculate the age of the water discharged from the rivers around the lake, and satellite remote sensing data were applied to examine the external source of organic carbon and nutrients and the factors influencing their movements to the lake. Based on the distribution of the water age, the water flow and mass transport trends in Lake Hulun were determined, and the time scales of the environmental processes were compared with those of water circulation. Further analysis indicates that the water circulation in the lake favors the accumulation of sedimentary organic carbon and nutrients in the northwestern part of the lake, and the organic carbon produced in the lake is transported to this region within an ice-free period. Satellite remote sensing data indicate that the region on the northwest bank of the lake experiences a larger terrestrial slope and better vegetation coverage than that on the southeast bank, which corresponds to a higher concentration of sedimentary organic carbon and nutrients in the northwest of the lake. This suggests that the sediment quality is closely related to the environment around the endorheic lake, and the larger slope and better vegetation coverage are significant factors for the high concentration of sedimentary organic carbon and nutrients on the lakebed under the conditions of scarce precipitation and low temperature. This study provides a theoretical basis and direction for further protection and management of the ecological environment of endorheic lakes.

Irrigation suitability of White River in Indiana, Midwestern USA

Climate change models consistently project future precipitation reduction and temperature increase during the crop growing season in the US Midwest, which may exacerbate surface water scarcity issues confronting regional agriculture. To maintain consistent crop yields under the risk of increased droughts, farmers may shift from rain-fed agriculture to irrigation agriculture, particularly during drought periods. There is an urgent need to understand whether surface water in the Midwest is suitable for irrigation. In this study, irrigation water quality was comprehensively analyzed for commonly used parameters regarding salt content including sodium adsorption ratio (SAR), adjusted sodium adsorption ratio (SAR_adj), soluble sodium percentage (SSP), electrical conductivity (EC), total dissolved solids (TDS), residual sodium bicarbonate (RSBC), magnesium adsorption ratio (MAR), permeability index (PI), Kelley's ratio (KR), synthetic harmful coefficient (SHC), and salinity. Results indicate that water in the White River at Muncie was rated mostly in excellent to good condition with regard to irrigation quality. However, the irrigation suitability level exhibited two distinct patterns between May-July and August-October. Specifically, an average of 7.8% of the samples from May to July were unsuitable for irrigation, and an average of 24.5% of samples from August to October were unsuitable for irrigation considering all parameters. Flow rate change over time and the release of pollutants from wastewater treatment plants and combine sewage outflows to the White River impacted on the irrigation water quality variations of the river. This study showed that there are higher risks during the fall season for farmers to use surface water as an irrigation source, and this risk might be greater if extended or more frequent drought events occur in the future. To our best knowledge, this is the first peer-reviewed study on irrigation water quality assessment in the Midwest and provides useful information for farmers and decision makers to consider while formulating applications for irrigation.

An increase in the slope of the concentration-discharge relation for total organic carbon in major rivers in New England, 1973 to 2019

The mobilization and transport of organic carbon (OC) in rivers and delivery to the near-coastal ocean are important processes in the carbon cycle that are affected by both climate and anthropogenic activities. Riverine OC transport can affect carbon sequestration, contaminant transport, ocean acidification, the formation of toxic disinfection by-products, ocean temperature and phytoplankton productivity. There have been many studies reporting temporal trends in OC concentrations in comparatively small streams with minimal anthropogenic influences but there have been fewer studies on larger rivers and fewer still that have investigated changes in OC concentration-discharge (C-Q) relations. This study examined changes in C-Q relations for total organic carbon (TOC) from 1973 to 2019 in 8 rivers in New England, USA. TOC concentrations declined in all rivers, and in most rivers, and in most seasons, the slope of the C-Q relation increased between 1973 to 1995 and 1996 to 2019. The increase in C-Q slope between periods may be related to changes in the magnitude of TOC sources. The most likely sources to have changed are wastewater inputs, urban runoff, production through photosynthesis in aquatic systems, and runoff from agricultural and forestry practices. Changes in wetland abundance and changes in sulfate concentrations can be ruled out as drivers of the observed changes in C-Q.

Increased nitrogen use efficiency in crop production can provide economic and environmental benefits

Potential economic and environmental benefits of increasing nitrogen-use efficiency (NUE) are widely recognized but scarcely quantified. This study quantifies the effects of increased NUE on 1) the national agricultural economy using a simulation model of US agriculture and 2) regional water quality effects using a biogeochemical model for the Arkansas-White-Red river basin. National economic effects are reported for NUE improvement scenarios of 10%, 20%, 50%, and 100%, whereas regional water quality effects are estimated for a 20% NUE improvement scenario in the Arkansas-White-Red river basin. Simulating a 20% increase in NUE in row crops is shown to reduce N requirements by 1.4 million tonnes y^-1 and increase farmer net profits by 1.6% ($743 million) per year by 2026 over the baseline simulation for the same period. For each 10% increase in NUE, annual farm revenues for commodity crops increased over the baseline by approximately $350 million per year by 2026. Changes in crop prices and land-use relative to the baseline were less than 2%. This suggests a net benefit even though fertilizer cost savings can result in increased cultivation of land, i.e., 'Jevon's paradox'. Results from the biogeochemical model of the Arkansas-White-Red river basin suggest that a 20% increase in NUE corresponds to a 5.72% reduction in nitrate loadings to freshwaters, with higher reductions in agricultural watersheds. The value of these reductions was estimated as $43 ha^-1, for a total of $15.3 to 136.7 million yr^-1 in avoided water treatment costs. After estimating the social value of increased NUE, we conclude with a discussion of potential strategies to increase efficiency and the research needed to achieve this goal. These include perennialization of the agricultural landscape, genetic crop improvement, targeted fertilizer application, and manipulation of the plant-root microbiome.

An approach for decomposing river water-quality trends into different flow classes

A number of statistical approaches have been developed to quantify the overall trend in river water quality, but most approaches are not intended for reporting separate trends for different flow conditions. We propose an approach called FN_2Q, which is an extension of the flow-normalization (FN) procedure of the well-established WRTDS ("Weighted Regressions on Time, Discharge, and Season") method. The FN_2Q approach provides a daily time series of low-flow and high-flow FN flux estimates that represent the lower and upper half of daily riverflow observations that occurred on each calendar day across the period of record. These daily estimates can be summarized into any time period of interest (e.g., monthly, seasonal, or annual) for quantifying trends. The proposed approach is illustrated with an application to a record of total nitrogen concentration (632 samples) collected between 1985 and 2018 from the South Fork Shenandoah River at Front Royal, Virginia (USA). Results show that the overall FN flux of total nitrogen has declined in the period of 1985-2018, which is mainly attributable to FN flux decline in the low-flow class. Furthermore, the decline in the low-flow class was highly correlated with wastewater effluent loads, indicating that the upgrades of treatment technology at wastewater treatment facilities have likely led to water-quality improvement under low-flow conditions. The high-flow FN flux showed a spike around 2007, which was likely caused by increased delivery of particulate nitrogen associated with sediment transport. The case study demonstrates the utility of the FN_2Q approach toward not only characterizing the changes in river water quality but also guiding the direction of additional analysis for capturing the underlying drivers. The FN_2Q approach (and the published code) can easily be applied to widely available river monitoring records to quantify water-quality trends under different flow conditions to enhance understanding of river water-quality dynamics.

Multidecadal water quality deterioration in the largest freshwater lake in China (Poyang Lake): Implications on eutrophication management

Poyang Lake is the largest freshwater lake in China and a globally important wetland with various functions. Exploring the multidecadal trend of water quality and hydroclimatic conditions is important for understanding the adaption of the lake system under the pressure from multiple anthropogenic and meteorological stressors. The present study applied the Mann-Kendall trend analysis and Pettitt test to detect the trend and breakpoints of hydroclimatic, and water quality parameters (from the 1980s to 2018) and the trend of monthly-seasonal ammonia (NH₄-N) and total phosphorus (TP)concentrations (from 2002 to 2018) in Poyang Lake. Results showed that Poyang Lake had undergone a highly significant warming trend from 1980 to 2018, with a warming rate of 0.44 °C/decade in terms of annual daily mean air temperature. The wind speed and water level of the lake presented a highly significant decreasing trend, whereas no notable trend was detected for precipitation variations. The annual mean total nitrogen (TN), NH₄-N, TP, and permanganate index (COD_Mn) concentrations showed significant upward trends from the 1980s to 2018. Remarkable abrupt shifts were detected for TN, NH₄-N, and COD_Mn in around 2003. They were in accordance with the water level breakpoint of the lake, thus implying the important role of hydrological conditions in water quality variations in floodplain lakes. A significant increasing trend has been detected for Chl-a variations during wet season from 2008 to 2018, which could be attributed to the increasing trend of nutrient concentration during the nutrient-limited phase of Poyang Lake. These hydroclimatic and water quality trends suggest a high risk of increasing phytoplankton growth in Poyang Lake. This study thus emphasizes the need for adaptive lake eutrophication management for floodplain lakes, particularly the consideration of the strong trade-off and synergies between hydroclimatic conditions and water quality variations.

Estimation of nonlinear water-quality trends in high-frequency monitoring data

Recent advances in high-frequency water-quality sensors have enabled direct measurements of physical and chemical attributes in rivers and streams nearly continuously. Water-quality trends can be used to identify important watershed-scale changes driven by natural and anthropogenic influences. Statistical methods to estimate trends using high-frequency data are lacking. To address this gap, an evaluation of the generalized additive model (GAM) approach to test for trends in high-frequency data was conducted. Our proposed framework includes methods for handling serial correlation, trend estimation and slope-change detection, and trend interpretation at arithmetic scale for log-transformed variables. Water-temperature and turbidity data, representing two analytes with different temporal patterns, collected from the James River at Cartersville, Virginia, USA, were chosen for this analysis. Results indicated that the model, including flow, season, time covariates, and interaction between flow and season performed well for both analytes. The same model structure was applied to specific conductance data, collected from a small highly urbanized watershed, with satisfactory model performance. The water temperature GAM results indicated that the significant decreasing-then-increasing patterns after 2012 were mainly driven by air temperature changes. The turbidity trend was not significant over time. The specific conductance results showed a consistently upward trend over the last decade due to ever-increasing urbanization in the small watershed. This study suggests that the GAM method has great potential as a useful tool for trend analysis on high-frequency data, and for informing watershed managers of hydro-climatic and human influences on water quality by detecting crucial signal variation over time.

Recent Changes in Nitrogen Sources and Load Components to Estuaries of the Contiguous United States

Regional Spatially Referenced Regressions on Watershed models were used to update 2002 delivered nitrogen (N) loads to estuaries of the contiguous US for 2011, supplemented by direct estuarine atmospheric deposition from the Community Multiscale Air Quality Model. Median 2011 watershed N yields were greatest for the Puget Trough, Virginian. and Oregon-Washington-Vancouver Coast marine ecoregions (MEs; 13. 7, 11.0, and 9.9 kg N/ha watershed/year, respectively); intermediate for the Floridian, Southern California Bight, and Northern California MEs (4.4-6.3 kg N/ha watershed/year); and lowest for the Northern Gulf of Mexico, Carolinian, and Gulf of Maine MEs (2.4-3.2 kg N/ha watershed/year). Dominant sources varied across marine ecoregions, with direct atmospheric deposition as the dominant source only in the far northern Gulf of Maine ME. Delivered N loads from atmospheric deposition have significantly decreased (p < 0.05) for most estuaries on the Atlantic and Gulf coasts for 2002-2012. Estimated point source delivered N loads for 2002-2012 increased for most estuaries with upstream treatment plants, with estimated loads to only seven estuaries decreasing by more than 50%. Urban runoff increased for most estuaries in the Puget Trough and Carolinian MEs and either increased or had no significant trend for the remaining marine ecoregions. The magnitude of change in total N delivered loads is uncertain due to incomplete monitoring for most minor dischargers. In areas with increased population growth and decreases in agricultural land, decreasing agricultural fertilizer inputs have been insufficient to offset increases in urban runoff.

Long-Term Changes in Precipitation and Temperature Have Already Impacted Nitrogen Loading

Increases in nitrogen loading over the past several decades have led to widespread water quality impairments across the U.S. Elevated awareness of the influence of climate variability on nitrogen loading has led to several studies investigating future climate change impacts on water quality. However,  it remains unclear whether long-term climate impacts can already be observed in the historical record. Here, we quantify long-term trends in total nitrogen loading over the period 1987-2012 across the contiguous U.S. and attribute these trends to long-term changes in nitrogen inputs and climatic variables. We find that annual precipitation, extreme springtime precipitation, and springtime temperature are key drivers of trends in historical loading in most regions. These decadal climate trends have either  amplified or offset loading trends expected from nitrogen inputs alone. We also find that rising temperatures have been insufficient to offset precipitation-induced loading increases, suggesting that future increases in temperature under climate change may have limited potential to counteract loading increases expected as a result of anticipated changes in precipitation. This work demonstrates the important role of decadal climate variability in long-term nitrogen loading, emphasizing the need to consider climate change risks when designing and monitoring nutrient reduction programs.

Hydrologic Trends in the Upper Nueces River Basin of Texas—Implications for Water Resource Management and Ecological Health

Reliable water sources are central to human and environmental health. In south Texas, USA, the Nueces River Basin (NRB) directly or indirectly plays that important role for many counties. Several NRB stream segments are designated as ecologically significant because they serve crucial hydrologic, ecologic, and biologic functions. The hydrologically significant streams recharge the Edwards Aquifer, an essential water source for the region’s agricultural, industrial, and residential activities. Unfortunately, the semiarid to arid south Texas climate leads to large inter-annual precipitation variability which impacts streamflow, and as a consequence, the aquifer’s recharge. In this study, we used a suite of hydrologic metrics to evaluate the NRB’s hydroclimatic trends and assess their potential impacts on the watershed’s ecologically significant stream segments using precipitation and streamflow data from the National Climatic Data Center (NCDC) and Hydroclimatic Data Network (HCDN) respectively from 1970 to 2014. The results consistently showed statistically significant decreasing streamflow for certain low-flow indicators over various temporal scales, likely due to water rights diversions and minimal land use changes. This research could help decision-makers develop the necessary tools to manage water resources in south Texas, given the NRB’s significance as a source of water for domestic consumption and ecological health.