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We have quantified inputs and fate of nutrients in European fresh and marine waters from 1990 to 2018. We have used the conceptual model GREEN to assess the impact of efforts on curbing nutrient pollution in European regions. In the first two decades, i.e. in the 1990s and through the start of the new millennium, nutrient inputs to waters decreased significantly. Nutrient pollution in freshwaters and to the sea largely reduced in all regions, although at different pace. However, around 2008-2010 trends in nutrient inputs changed, marking an increase in the last decade, particularly from agricultural diffuse sources. In some regions, current nutrient inputs to waters are close to those estimated at the beginning of the 1990s. At the end of the study period, nutrient concentrations in freshwaters remain above thresholds congruent with good ecological status of water bodies in most downstream reaches. European policies tackling point sources are close to reach their maximum impact. In the face of this approaching ceiling, sustainable nutrient management on agricultural land becomes pivotal for effective nutrient control in river basins. The regional approach highlighted differences across Europe that may provide tailored opportunities to plan effective strategies for achieving environmental targets., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)

ESPRES (Efficient Strategies for anthropogenic Pressure Reduction in European waterSheds) is a web-based Decision Support System (DSS) designed to explore management options for achieving environmental targets in European freshwaters. The tool integrates multi-objective optimization (MOO) algorithms for selecting the best management options in a river basin and models assessing the consequent changes in the water quantity (water flow) and quality (nutrient concentration). The MOO engine identifies Pareto front strategies that are trade-offs between environmental objectives for water bodies and the effort required for reducing the pressures. The web interface provides tools to set the effort perceived by different river basin stakeholders considering technical feasibility, political difficulty, and social acceptability of the alternative options. The environmental impact of management options (scenarios) is assessed with models developed at the European scale. ESPRES enables comparison of management solutions and allows quantifying environmental and socio-economic trade-offs inherent to the decision making process., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Authors. Published by Elsevier B.V. All rights reserved.)

Invasive alien species (IAS) induce changes to community structure and functions which lead to a decline of endemic species and major irreversible changes to the local physical habitat. The distribution and the impacts of multiple freshwater IAS are not well known, and they have not been investigated simultaneously at catchment and at European scales. This study provides an assessment of the distribution and cumulative impact of freshwater IAS over European catchments. IAS occurrences were retrieved from the European Alien Species Information Network geospatial dataset and updated with the most recent records from the literature. The Cumulative Impact Index of Invasive Alien Species (CIMPAL) was derived by aggregating the impacts of species and their occupied area at catchment level by following three steps: i) IAS were scored by both the magnitude of impacts on freshwater ecosystems and the strength of evidence in the literature, ii) scores were mapped over the catchment area, and iii) scores were summed across IAS over the catchment. The distribution of CIMPAL in the river ecological classes of the Water Framework Directive was examined and increasing/decreasing patterns identified across ecological statuses. Results showed strong spatial variation in the documented distribution and impacts of IAS in Europe. Catchments with CIMPAL scores >40 (range 0-55) clustered in Western European countries (e.g. Belgium and France) were characterised by plant, invertebrate and vertebrate IAS that had both a large impact in magnitude and colonisation at local (catchment level) and large scale (across catchments). CIMPAL showed statistically significant and increasing values from high to bad ecological classes in eight countries only (Belgium, the Czech Republic, Germany, France, Hungary, Italy, Luxemburg, Poland). This study provides comprehensive evidence of the distribution and impact of IAS within freshwater environments that could be used to improve understanding of the ecological pressures at catchment scale., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Authors. Published by Elsevier B.V. All rights reserved.)

Biochemical Oxygen Demand (BOD) is an indicator of organic pollution in freshwater bodies correlated to microbiological contamination. High BOD concentrations reduce oxygen availability, degrade aquatic habitats and biodiversity, and impair water use. High BOD loadings to freshwater systems are mainly coming from anthropogenic sources, comprising domestic and livestock waste, industrial emissions, and combined sewer overflows. We developed a conceptual model (GREEN + BOD ) to assess mean annual current organic pollution (BOD fluxes) across Europe. The model was informed with the latest available European datasets of domestic and industrial emissions, population and livestock densities. Model parameters were calibrated using 2008-2012 mean annual BOD concentrations measured in 2157 European monitoring stations, and validated with other 1134 stations. The most sensitive model parameters were abatement of BOD by secondary treatment and the BOD decay exponent of travel time. The mean BOD concentrations measured in monitored stations was 2.10 mg O 2 /L and predicted concentrations were 2.54 mg O 2 /L; the 90th percentile of monitored BOD concentration was 3.51 mg O 2 /L while the predicted one was 4.76 mg O 2 /L. The model could correctly classify reaches for BOD concentrations classes, from high to poor quality, in 69% of cases. High overestimations (incorrect classification by 2 or more classes) were 2% and large underestimations were 5% of cases. Across Europe about 12% of freshwater network was estimated to be failing good quality due to excessive BOD concentrations (>5 mg O 2 /L). Dominant sources of BOD to freshwaters and seas were point sources and emissions from intensive livestock systems. Comparison with previous assessments confirms a decline of BOD pollution since the introduction of EU legislation regulating water pollution., (Copyright © 2019 The Authors. Published by Elsevier B.V. All rights reserved.)

Water scarcity and droughts are a major concern in most Mediterranean countries. Agriculture is a major user of water in the region and releases significant amounts of surface and ground waters, endangering the sustainable use of the available resources. Best Management Practices (BMPs) can mitigate the agriculture impacts on quantity of surface waters in agricultural catchments. However, identification of efficient BMPs strategies is a complex task, because BMPs costs and effectiveness can vary significantly within a basin. In this study, sustainable agricultural practices were studied based on optimal allocation of irrigation water use for dominant irrigated crops in the island of Crete, Greece. A decision support tool that integrates the Soil and Water Assessment Tool (SWAT) watershed model, an economic model, and multi-objective optimization routines, was used to identify and locate optimal irrigation strategies by considering crop water requirements, impact of irrigation changes on crop productivity, management strategies costs, and crop market prices. Three spatial scales (crop type, fields, and administrative regions) were considered to point out different approaches of efficient management. According to the analysis, depending on the spatial scale and complexity of spatial optimization, water irrigation volumes could be reduced by 32%-70% while preserving current agricultural benefit. Specific management strategies also looked at ways to relocate water between administrative regions (4 prefectures in the case of Crete) to optimize crop benefit while reducing global water use. It was estimated that an optimal reallocation of water could reduce irrigation water volumes by 52% (148 Mm 3 /y) at the cost of a 7% (48 M€) loss of agricultural income, but maintaining the current agricultural benefit (626.9 M€). The study showed how the identification of optimal, cost-effective irrigation management strategies can potentially address the water scarcity issue that is becoming crucial for the viability of agriculture in the Mediterranean region., (Copyright © 2018 The Authors. Published by Elsevier B.V. All rights reserved.)

Multiple abiotic stressors affect the ecological status of water bodies. The status of waterbodies in the Ebro catchment (NE Spain) is evaluated using the biological quality elements (BQEs) of diatoms, invertebrates and macrophytes. The multi-stressor influence on the three BQEs was evaluated using the monitoring dataset available from the catchment water authority. Nutrient concentrations, especially total phosphorus (TP), affected most of the analyzed BQEs, while changes in mean discharge, water temperature, or river morphology did not show significant influences. Linear statistical models were used to evaluate the change of water bodies' ecological status under different combinations of future socioeconomic and climate scenarios. Changes in land use, rainfall, water temperature, mean discharge, TP and nitrate concentrations were modeled according to the future scenarios. These revealed an evolution of the abiotic stressors that could lead to a general decrease in the ecosystem quality of water bodies within the Ebro catchment. This deterioration was especially evidenced on the diatoms and invertebrate biological indices, mainly because of the foreseen increase in TP concentrations. Water bodies located in the headwaters were seen as the most sensitive to future changes., (Copyright © 2018 The Author(s). Published by Elsevier B.V. All rights reserved.)

Sustainable water basin management requires characterization of flow regime in river networks impacted by anthropogenic pressures. Flow regime in ungauged catchments under current, future, or natural conditions can be assessed with hydrological models. Developing hydrological models is, however, resource demanding such that decision makers might revert to models that have been developed for other purposes and are made available to them ('off-the-shelf' models). In this study, the impact of epistemic uncertainty of flow regime indicators on flow-ecological assessment was assessed at selected stations with drainage areas ranging from about 400 to almost 90,000km 2 in four South European basins (Adige, Ebro, Evrotas and Sava). For each basin, at least two models were employed. Models differed in structure, data input, spatio-temporal resolution, and calibration strategy, reflecting the variety of conditions and purposes for which they were initially developed. The uncertainty of modelled flow regime was assessed by comparing the modelled hydrologic indicators of magnitude, timing, duration, frequency and rate of change to those obtained from observed flow. The results showed that modelled flow magnitude indicators at medium and high flows were generally reliable, whereas indicators for flow timing, duration, and rate of change were affected by large uncertainties, with correlation coefficients mostly below 0.50. These findings mirror uncertainty in flow regime indicators assessed with other methods, including from measured streamflow. The large indicator uncertainty may significantly affect assessment of ecological status in freshwater systems, particularly in ungauged catchments. Finally, flow-ecological assessments proved very sensitive to reference flow regime (i.e., without anthropogenic pressures). Model simulations could not adequately capture flow regime in the reference sites comprised in this study. The lack of reliable reference conditions may seriously hamper flow-ecological assessments. This study shows the pressing need for improving assessment of natural flow regime at pan-European scale., (Copyright © 2017 The Authors. Published by Elsevier B.V. All rights reserved.)

This study provides an innovative process-based modelling approach using the SWAT model and shows its application to support the implementation of the European environmental policies in large river basins. The approach involves several pioneering modelling aspects: the inclusion of current management practices; an innovative calibration and validation methodology of streamflow and water quality; a sequential calibration starting from crop yields, followed by streamflow and nutrients; and the use of concentrations instead of loads in the calibration. The approach was applied in the Danube River Basin (800,000km 2 ), the second largest river basin in Europe, that is under great nutrients pressure. The model was successfully calibrated and validated at multiple gauged stations for the period 1995-2009. About 70% and 61% of monthly streamflow stations reached satisfactory performances in the calibration and validation datasets respectively. N-NO 3 monthly concentrations were in good agreement with the observations, albeit SWAT could not represent accurately the spatial variability of the denitrification process. TN and TP concentrations were also well captured. Yet, local discrepancies were detected across the Basin. Baseflow and surface runoff were the main pathways of water pollution. The main sinks of TN and TP diffuse emissions were plant uptake which captured 58% of TN and 92% of TP sources, then soil retention (35% of TN and 2% of TP), riparian filter strips (2% both for TN and TP) and river retention (2% of TN and 4% of TP). Nitrates in the aquifer were estimated to be around 3% of TN sources. New reliable "state-of-the-art" knowledge of water and nutrients fluxes in the Danube Basin were thus provided to be used for assessing the impact of best management practices and for providing support to the implementation of the European Environmental Directives., (Copyright © 2017 The Authors. Published by Elsevier B.V. All rights reserved.)

Sediment management is of prior concern in the Danube Basin for provision of economic and environmental services. This study aimed at assessing current (1995-2009) sediment fluxes of the Danube Basin with SWAT model and identifying sediment budget knowledge gaps. After hydrologic calibration, hillslope gross erosion and sediment yields were broadly calibrated using ancillary data (measurements in plots and small catchments, and national and European erosion maps). Mean annual sediment concentrations (SSC) from 269 gauging stations (2968 station-year entries; median 19mg/L, interquartile range IQR 10-36mg/L) were used for calibrating in-stream sediments. SSC residuals (simulations-observations) median was 2mg/L (IQR -14; +22mg/L). In the validation dataset (172 gauging stations; 1457 data-entries, median 17mg/L, IQR 10-28), median residual was 9mg/L (IQR -9; +39mg/L). Percent bias in an independent dataset of annual sediment yields (SSY; 689 data-entries in 95 stations; median 52t/km 2 /y, IQR 20-151t/km 2 /y) was -21.5%. Overall, basin-wide model performance was considered satisfactory. Sediment fluxes appeared overestimated in some regions (Sava and Velika Morava), and underestimated in others (Siret-Prut and Romanian Danube), but unbiased elsewhere. According to the model, most sediments were generated by hillslope erosion. Streambank degradation contributed about 5% of sediments, and appeared important in high stream power Alpine reaches. Sediment trapping in reservoirs and floodplain deposition was probably underestimated and counterbalanced by high stream deposition. Factor analysis showed that model underestimations were correlated to Alpine and karst areas, whereas underestimations occurred in high seismicity areas of the Lower Danube. Contemporary sediment fluxes were about one third of values reported for the 1980s for several tributaries of the Middle and Lower Danube. Knowledge gaps affecting the sediment budget were identified in the contributions of some erosion processes (glacier erosion, gully erosion and mass movements), and in-stream sediment dynamics., (Copyright © 2017 The Authors. Published by Elsevier B.V. All rights reserved.)

The Soil and Water Assessment Tool (SWAT) is used worldwide for water quality assessment and planning. This paper aimed to assess and adapt SWAT hillslope sediment yield model (Modified Universal Soil Loss Equation, MUSLE) for applications in large basins, i.e. when spatial data is coarse and model units are large; and to develop a robust sediment calibration method for large regions. The Upper Danube Basin (132,000km(2)) was used as case study representative of large European Basins. The MUSLE was modified to reduce sensitivity of sediment yields to the Hydrologic Response Unit (HRU) size, and to identify appropriate algorithms for estimating hillslope length (L) and slope-length factor (LS). HRUs gross erosion was broadly calibrated against plot data and soil erosion map estimates. Next, mean annual SWAT suspended sediment concentrations (SSC, mg/L) were calibrated and validated against SSC data at 55 gauging stations (622 station-years). SWAT annual specific sediment yields in subbasin reaches (RSSY, t/km(2)/year) were compared to yields measured at 33 gauging stations (87station-years). The best SWAT configuration combined a MUSLE equation modified by the introduction of a threshold area of 0.01km(2) where L and LS were estimated with flow accumulation algorithms. For this configuration, the SSC residual interquartile was less than +/-15mg/L both for the calibration (1995-2004) and the validation (2005-2009) periods. The mean SSC percent bias for 1995-2009 was 24%. RSSY residual interquartile was within +/-10t/km(2)/year, with a mean RSSY percent bias of 12%. Residuals showed no bias with respect to drainage area, slope, or spatial distribution. The use of multiple data types at multiple sites enabled robust simulation of sediment concentrations and yields of the region. The MUSLE modifications are recommended for use in large basins. Based on SWAT simulations, we present a sediment budget for the Upper Danube Basin., (Copyright © 2015. Published by Elsevier B.V.)