Displays

Studying the response of streamwater chemistry to changes in discharge can improve our understanding of how catchments store and release water and solutes. Previous studies have determined concentration-discharge (cQ) relationships from long-term, low-frequency data for many different solutes. These analyses, however, provide little insight into the coupling of solute concentrations and flow during individual hydrologic events. Although intra-event cQ relationships have been determined for selected solutes and storm events, they have rarely been investigated across a wide range of solutes and over extended periods of time. Thus, little is known about how intra-event and longer-term cQ relationships may differ, potentially providing different perspectives on processes regulating transport through the landscape.

We present cQ relationships of 14 different solutes, ranging from major ions to trace metals, as well as electrical conductivity, in the Swiss Erlenbach catchment (0.7 km²). From a 2-year time series of sub-hourly solute concentration measurements, we determined long-term cQ relationships for each solute. We compared these to cQ relationships spanning the hydrograph recessions of 30 individual events. Solutes sharing the same dominant water sources exhibited similar behavior. Groundwater-sourced solutes exhibited dilution patterns, and their long-term cQ behavior was representative of their cQ behavior during hydrologic events. Other solutes, however, exhibited highly variable cQ behavior from one event to the next, and very different cQ patterns at intra-event and longer-term time scales. This was particularly true for trace metals as well as atmospherically derived and/or biologically active solutes. Most of the observed event-to-event variability in cQ behavior could be explained by factors such as catchment wetness, season, event size, input concentrations, and event-water contributions. These relationships help to clarify how the release of solutes depends on their catchment sources and pathways. Our analysis thus provides insight into controls on solute variations at the hydrologic event scale.

How to cite: Knapp, J., von Freyberg, J., Studer, B., Kiewiet, L., and Kirchner, J.: Concentration-discharge relationships vary among hydrological events, reflecting differences in event characteristics , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5981, https://doi.org/10.5194/egusphere-egu2020-5981, 2020.

Since the 1980s, an increase in dissolved organic carbon (DOC) concentrations in waters of the Northern hemisphere has been observed. However, no general explanation has been found so far. Our study focuses on investigating the mechanisms influencing DOC mobilization and export in the streams of a forested headwater catchment in the Bavarian Forest National Park, Germany, as controlled by topography and hydrological conditions. Our goal is to identify differences in DOC mobilization processes between steep hillslopes and riparian zones and between different precipitation events. We hypothesize that different hydrological conditions and the topographical position (steep hillslopes vs. riparian zones) influence sources of DOC and mobilization processes. 

Three continuous sampling sites were established in different topographical positions within the catchment of the Große Ohe in the Bavarian Forest National Park along one of the streams: at a steep hillslope (880 m.a.s.l.), in a transition zone where the steep hillsides level off (805 m.a.s.l.) and in a flat and wide riparian zone (770 m.a.s.l). At these three locations, DOC concentrations in stream water have been measured continuously using UV-Vis spectrometry since early summer 2018, in combination with continuous discharge measurements. In addition, we regularly conducted a longitudinal sampling in order to analyze stream water chemistry parameters at 16 sampling points along the investigated stretch of about 3 km.

We analyzed discharge and DOC dynamics and DOC-Q hysteresis patterns, derived from the high-resolution data, to investigate if DOC mobilization differed between the topographical positions. We focus on two large events with different antecedent hydrological conditions in October 2018 (P_tot: 30 mm, API₁₄: 1.9 mm) and May 2019 (P_tot: 28.9 mm, API₁₄: 46.8 mm). At all topographical positions, maximal DOC concentrations were higher during the event in October 2018 (up to 15 mg/l) than during the event in May 2019 (up to 10 mg/l). These maximal concentrations also persisted much longer on the falling limb of the hydrograph during the October event, following dry conditions, than during the May event, following wet conditions.  This behavior results in wider hysteresis loops at all topographical positions during the event in October 2018 than in May 2019. However, peak concentrations dropped more quickly at the site of the steep hillslope than at the site of the transition zone and the riparian zone, resulting in more compressed hysteresis loops during both events. We use these differences in the DOC-Q hysteresis patterns to identify key processes for DOC mobilization and to create a perceptual model for DOC export from small, forested catchments.

How to cite: Blaurock, K., Gilfedder, B., Fleckenstein, J., Peiffer, S., and Hopp, L.: High-resolution DOC measurements indicate differences in DOC mobilization processes depending on topography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1522, https://doi.org/10.5194/egusphere-egu2020-1522, 2020.

There is a need to better understand the driving factors of and interactions between hydro-biogeochemical processes at different temporal and spatial scales to inform water management. Especially for tropical montane ecosystems, which are recognized as important freshwater sources, the required data and knowledge are missing. To address this knowledge gap, a long-term hydrological monitoring network was set up in the Mau Forest Complex, Kenya. The network covers three sub-catchments (27–36 km²) dominated by either tropical montane forest, smallholder agriculture or commercial tea plantations, within a 1021 km² mixed land use catchment. A 5-year dataset of nitrate and dissolved organic carbon concentrations measured at 10-minute interval with in situ UV-Vis sensors was analysed for short-term changes in solute concentrations. The analysis revealed small diurnal patterns (amplitude <0.25 mg N or C L⁻¹, decreasing with increasing discharge) in solute concentrations in all streams. In addition, the timing of the minima and maxima differed between catchments and seasons, suggesting and influence of land use and seasonality on the occurrence of in-stream biogeochemical processes. However, unusual and abrupt changes in the diurnal patterns were observed after a change in sensor position or exchange of sensors. We therefore developed an experiment to test the validity of the observed diurnal patterns. A second, mobile sensor was installed at each site for a period of more than three weeks. After measuring in parallel position to the fixed sensor for two weeks, the position (orientation, depth) was changed or the measurement gap was shaded. In parallel position, the patterns in solute concentrations recorded by the mobile sensor agreed better with those measured by the fixed sensor for dissolved organic carbon (r>0.98) than for nitrate (r=0.43–0.81). However, shading the sensor or a position change resulted in inconsistent changes in the recorded patterns. Larger changes in solute concentrations, e.g. as a result of rainfall events, were reproduced well by the mobile sensor. The results of our study suggest that diurnal changes in solute concentrations with an amplitude close sensor accuracy measured with in situ UV-Vis sensors should be interpreted with caution. The experiment was not conclusive as to what caused the differences in observed patterns. Further experimental work is required to understand the causes and to develop recommendations for the use of UV-Vis sensors in hydro-biogeochemical research.

How to cite: Jacobs, S., Weeser, B., Rufino, M., and Breuer, L.: In situ UV-Vis sensors record spatially and temporally different diurnal patterns in solute concentrations in tropical montane headwater streams: reality or artefact?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12537, https://doi.org/10.5194/egusphere-egu2020-12537, 2020.

Changes in seasonal climate regimes, and related changes in seasonal nutrient dynamics, are occurring across a range of climates and land use types. Although it is known that seasonal patterns in nutrient availability are key drivers of both stream metabolism and eutrophication, there has been little success in developing a comprehensive understanding of seasonal variations in nutrient export across watersheds or of the relationship between nutrient seasonality and watershed characteristics. In the present study, we have used concentration and discharge data from more than 200 stations across US and Canadian watersheds to identify (1) archetypal seasonal concentration regimes for nitrate, soluble reactive phosphorus, and total phosphorus, and (2) dominant watershed controls on these regimes across a gradient of climate, land use, and topography. Our analysis shows that less impacted watersheds, with more forested and wetland area, most commonly exhibit concentration regimes that are in phase with discharge, with concentration lows occurring during summer low-flow periods. Agricultural watersheds also commonly exhibit in-phase behavior, though the seasonality is usually muted compared to that seen in less impacted areas. With increasing urban area, however, nutrient concentrations frequently become essentially aseasonal or even exhibit clearly out-of-phase behavior. In addition, our data indicate that seasonal SRP concentration patterns may be strongly influenced by proximal controls such as the presence of dams and reservoirs. In all, these results suggest that human activity is significantly altering nutrient concentration regimes, with large potential consequences for both in-stream metabolism and eutrophication risk in downstream water bodies.

How to cite: Van Meter, K., Basu, N., and Byrnes, D.: Biogeochemical Asynchrony: Ecosystem Drivers of Seasonal Concentration Regimes across the Great Lakes Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20970, https://doi.org/10.5194/egusphere-egu2020-20970, 2020.

Large-scale efforts to reduce cultural eutrophication of freshwater systems have had varied success because internal feedbacks can stabilize the high nutrient, high productivity, and turbid conditions associated with eutrophic systems. We examined these feedbacks using a unique 40-year water quality data set from the middle Loire River, France, where phosphorus and phytoplankton concentrations have decreased by an order of magnitude from 1980–2018. We focused on ecosystem metabolism as an integrative measure to elucidate cause-effect relationships of both bottom-up (e.g., nutrient concentrations) and top-down (e.g., consumer populations) effects on river trophic state.

The dataset combined both long-term (30 years), high-frequency (hourly) measurements of dissolved oxygen (DO) and long-term (40 years), low-frequency (monthly) measures of nutrients, plus several supporting biological surveys of primary producer and consumer densities. Using hourly measurements of DO, we estimated gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production (NEP = GPP – ER), and from the resulting long time series of metabolic fluxes, we tested the hypothesis that GPP and ER responded to changes in water column concentrations of algal pigments (chlorophyll a) and phosphorus. We further tested the hypothesis that change points in the patterns of ecological behavior were contemporaneous with notable changes in river management.

Despite well-established links between phosphorus, chlorophyll-a and primary production, GPP was resilient to the drastic reductions in both P concentrations and phytoplankton. Indeed, GPP has only recently decreased (~25%), despite chlorophyll-a concentrations reaching a new minima 10 years earlier in response to colonization of the invasive Corbicula sp. clam in the year 2000. Declines in ER are only half (~12%) the decline in GPP, shifting the river from an autotrophic state (i.e., positive NEP) to a heterotrophic state (i.e., negative NEP). Moreover, Granger causality analysis suggested that daily primary production and respiration have decoupled over this period. With earlier phytoplankton dominance, daily ER was strongly linked to recent autochthonous GPP, but more recently daily GPP has far less influence on subsequent ER. We interpret this partially as a reduction in carbon and nutrient turnover rates resulting from the community shift from algae to macrophytes, and attendant changes in nutrient sources (now primarily from sediment) and carbon stocks (now principally in the sediment). This study illustrates the benefit of long-term high-frequency data collection for understanding pattern and process in aquatic ecosystems, and illustrates a compelling example of process resilience contrasted with an ecosystem tipping point in the context of global change.

How to cite: Diamond, J., Moatar, F., Cohen, M., Poirel, A., Martinet, C., Maire, A., and Pinay, G.: Thirty years of hourly dissolved oxygen in a large shallow river illustrates discrepancy between a primary producer tipping point and river metabolism, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9667, https://doi.org/10.5194/egusphere-egu2020-9667, 2020.

Selection of non-point nutrient pollution management and mitigation options in catchments requires in-depth understanding of both spatial and temporal controlling aspects on nutrient dynamics for covering a diversity of factors controlling nutrient transfer to surface waters. Such an understanding can be obtained by analyzing the hysteresis behaviors and export regime in concentration-discharge (c-Q) relationships from the monitoring stations in smaller streams. 
A classification scheme developed by Pohle et al. (2019), including nine different c-Q relationships classes were defined as a combination of export behavior (dilution, neutral, enrichment) and rotational pattern of the hysteresis (clock-wise, no rotation, anti-clockwise). To perform this, the export behavior was assessed based on the theoretical c-Q relationships by checking whether concentrations decrease, increase or do not change with discharge (Mann-Kendall test). The rotational pattern was also determined by comparing concentrations at the rising and the falling limb of the hydrograph (Kruskal-Wallis test).
The classification has been applied to a 8 years record (2010-2017) of daily discharge and discrete nutrient concentration  data from 88 small streams including forms such as - Nitrate, Organic N, Dissolved Reactive Phosphorus and Particulate P  from Denmark, Sweden and Finland. The streams drains catchments with a size ranging from 0.1 km² – 65 km². Additionally similarity in types of c-Q relationships were investigated by multivariate analysis for N and P forms  considering effects of land use, climate, soil type and the size of the catchments  . 
The dilution behavior of the catchments might dominantly be related to arable catchments with low groundwater inputs and with a good direct contact from root zone to the stream (e.g. through tile drains for N) and macropore or surface runoff for P. The constant behavior of the catchments might dominantly be related to dominance of groundwater fed streams in arable or natural catchments and the enrichment behavior might dominantly be found in catchments influenced by point source discharges of nutrients.
This kind of catchments classification can be used for planning of optimal sampling frequencies in monitoring programs, cost-optimal dosing of mitigation options in catchments and inform about expected inertias in catchment responses to management. 

How to cite: Hashemi, F., Pohle, I., Tornbjerg, H., Kyllmar, K., Marttila, H., Lepistö, A., and Kronvang, B.: Classification of Mini-catchment typologies for analyzing dominant controls of nutrient dynamic in three Nordic countries, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13219, https://doi.org/10.5194/egusphere-egu2020-13219, 2020.

Empirical dynamic modelling (EDM) is a relatively novel method to assess causality from time series in coupled dynamic systems. From the literature, EDM appeared to be useful in a number of applications including analysis of water quality data. It was therefore hypothesized that this technique has a potential to revisit existing long term data of solutes from catchment streams. More specifically, we proposed that causal links between concentration time series could be revealed, which were previously overlooked when only standard linear regression and correlation methods were used. We applied EDM to long term concentrations of dissolved organic carbon (DOC), total Fe and pH from the Lehstenbach stream in Germany, time series that were formerly evaluated using various other techniques in the context of DOC mobilisation from peat catchments. To assess causal links between solute time series, three steps of analysis were conducted. Firstly, the embedding dimension for each time series (that is the number of time lags required for the best possible one-time-step-ahead forecast for a particular time series) was computed. In a second step, using the computed embedding dimension, non-linearity of the system was assessed using a technique called s-mapping to explore if EDM is expected to be a beneficial tool for time series analysis. Finally, convergent cross mapping was applied to test different combinations of variables for causal links. The basic idea of convergent cross mapping is that - if X causes Y - the response variable Y contains information on its driver X but not vice versa (because X is independent of Y) and hence X can be predicted from time series of Y. So X is predicted from Y by mapping nearest neighbours in state space (so called cross mapping). With increasing length of Y, predictions shall improve to a saturation level, that is cross mapping skill converges, which is used as a criterion of causality. We applied EDM to weekly, discharge-corrected, temporal changes of concentration time series. Discharge corrections was conducted to eliminate discharge as a common driver of concentrations. Our data analysis implied a causal interaction between Fe and DOC that was proposed in earlier work. But surprisingly, DOC seems to drive pH but not the other way around, a result that needs to be investigated in more detail, but a likely explanation would be that pH of streamwater is mainly decreasing with acidic inputs of DOC from riparian wetlands. We concluded that using EDM additional insights on the links between catchment time series can be obtained.

How to cite: Selle, B. and Knorr, K.-H.: Empirical dynamic modelling - a promising tool to assess causal links between water quality time series, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5760, https://doi.org/10.5194/egusphere-egu2020-5760, 2020.

Strom event-scale analysis provides insights into nitrate transport dynamics at catchment scale. Investigating different hysteretic relationships between nitrate and discharge can disentangle catchment nitrate functioning both spatially and temporally. In this study, we explored seasonality and landscape gradiemt effects on nitrate concentration-discharge (C-Q) hysteresis patterns based on six-year (2012-2017), high-frequency (15 min) data in the Selke catchment (central Germany). The Selke catchment exhibits heterogeneous combinations of meteorological, hydrogeological, and anthropogenic conditions. Three nested gauging stations were built along the main Selke River, capturing discharge and nitrate concentration from the dominant uppermost mixed forest and arable land, middle catchment pure steep forest and lowland arable and urban land areas, respectively. Amongst the 227 storm events that have been detected, anticlockwise and accretion of C-Q relationships accounted for 76.6% and 75.3%, respectively, while the proportions decreased with the increasing areal share of arable land during summer season. Accretion pattern predominated forest areas (e.g., the middle catchment) throughout the whole year suggesting higher nitrate concentration in dominating interflow than baseflow. In contrast, dilution pattern was almost exclusively observed in lowland areas (dominated by arable and urban areas) in dry periods, indicating lower nitrate concentration in quick runoff components like surface runoff. We further investigated the consistency and variability of hysteresis patterns from upstream to downstream based on shared events. Results indicated hysteresis patterns seemed to be consistent at the three stations when discharge was high enough. Moreover, we found that nitrate load contributions from the upper and lower areas changed seasonally, albeit the dominant share of runoff volume from the upper area throughout the whole year. Such a comprehensive analysis (i.e., clockwise vs. anticlockwise, accretion vs. dilution) enables in-deep discussion of the plausible mechanisms of nitrate dynamics under different landscape conditions. We are also aware of limitations of such statistical data analysis, which can likely be tackled by mechanistic modelling at higher temporal resolutions.

How to cite: Zhang, X., Yang, X., Jomaa, S., and Rode, M.: Analyzing impacts of seasonality and landscape gradient on event scale nitrate-discharge dynamics based on nested high-frequency monitoring , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3188, https://doi.org/10.5194/egusphere-egu2020-3188, 2020.

Using historical data to identify future water quality trends

1. Lintern
2. Kho
3. Guo
4. Liu
5. Duvert

Climate change is expected to have a severe impact on water resources management in Australia. This is expected to lead to increasing frequency in extreme hydrological events such as droughts and floods, which will in turn contribute to higher risks of bushfires, fish kills, and water shortage for both humans and the environment. The potential impacts of these climate-change-induced extreme events on the quantity of water available to humans and the environment are relatively well understood. However, we have little understanding of the effect on the water quality of Australian rivers. This project aims to start filling this gap in our understanding.

Our key objectives are:

(1) to identify how extreme hydrological events such as droughts and floods have affected river water quality over the last two decades, and explore how spatially variable these impacts have been across the Australian continent.

(2) to use these past observations as a basis to predict how river water quality will be affected by climate change across the continent, and identify the locations within Australia that will be most vulnerable to water quality deterioration in the near future.

There is a wealth of historical water quality data for each state in Australia, but these datasets have not yet been investigated systematically to develop a nation-wide understanding of water quality patterns. We believe that only a continental-scale understanding of the response of river water quality to extreme hydrological events will allow for the development of robust predictive models of climate change impacts on water quality. Knowing the potential hotspots for future water quality deterioration will be a key step towards identifying priorities for catchment planning and management.

In this poster, we will present the preliminary findings of this project by detailing the spatial variability in the impact of hydrological events on water quality across the state of Victoria in South-East Australia.

How to cite: Lintern, A., Kho, N., Guo, D., Liu, S., and Duvert, C.: Using historical data to identify future water quality trends at a regional scale, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4308, https://doi.org/10.5194/egusphere-egu2020-4308, 2020.

This study aimed at reconstructing the dynamics of radiocesium (¹³⁴Cs, ¹³⁷Cs) fluxes exported at the outlet of catchments impacted by the Fukushima Dai-ichi Nuclear Power Plant accident. 

River flowrate, load and dissolved/particulate radiocesium concentrations were simultaneously monitored during the period 2012-2016 in 4 coastal forested catchments of Uda (0.3, 97 km²), Mano (20 km²) and Ohta rivers (43 km²). Precipitation time series were derived from raingauges (JMA) and different satellite datasets (AMeDAS, GSmap, Aphrodite). Contamination maps were derived from MEXT airborne monitoring surveys (MEXT/JAEA).
 A class of stochastic models adapted from Mass Response Functions (originally introduced by Rinaldo & Marani, 1987), was implemented to reconstruct radiocesium fluxes. This theory, describing the coupled transport of water and contaminant particles with transit/holding time distributions, was proposed non only because it encompasses the classical assessment models, but also because it allows some improvements with other hypotheses about the catchment response, notably: transit time distribution of effective rainfall, exchanges between mobile and immobile phases (mass transfer rate, equilibrium concentration) and the macroscopic mass balance at the basin scale.
Classic hypotheses about the catchment response (corresponding to variants of removal coefficients and transfer function models) were tested by Bayesian inference. Inferences were conducted with routines provided by the R environment (R Development Core Team, 2013) and the package BayesianTools (Hartig et al., 2018). The oversampling of extreme events in the monitoring design was counterbalanced by assigning weights to the observations corresponding to the likelihood of the carrier flow rate (liquid or solid flow rate).
For catchments of Mano (43.64 km²) and Ohta (20.28 km²) rivers, observations covered both low-flow and high-flow periods. The short-term fluctuations of the wash-off catchment response were strongly transport-limited: dissolved ¹³⁷Cs fluxes varied linearly with river flow rate (m³/s), whereas particulate ¹³⁷Cs fluxes varied linearly with solid flow rate (kg/s) or nonlinearly with river flow rate (m³/s). The longer-term decline of radionuclide availability to wash-off was credible for dissolved wash-off, but was not plausible for solid wash-off, certainly due to the short period of observations of the monitorings. 
For assessment purposes, the removal coefficient approach appeared as a good option for both dissolved and particulate ¹³⁷Cs wash-off. For Mano and Ohta catchments, median model predictions agreed with observations within a factor ranging from 1.47 to 1.66 for dissolved wash-off, and from 19.22 to 22.44 for particulate wash-off. The explicative power of the proposed models will need to be updated when more recent measurements are available and their predictive power needs to be confirmed on independent observations on other catchments.

How to cite: Garcia-Sanchez, L. and Hayashi, S.: Radiocesium wash-off from japanese rivers impacted by the Fukushima accident. Calibration and comparison of simple assessment models for exported dissolved and particulate fluxes by bayesian inference, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4548, https://doi.org/10.5194/egusphere-egu2020-4548, 2020.

Abstract
Hydrochemical assessment have been carried out for a tropical, east flowing Western Ghats river Cauvery, southern India, to understand the dissolved load sources, acquisition processes and their controlling factors. Silicate weathering rates (SWR) and associated CO2 consumption rates (CCR) are estimated along with quantification of source wise solute load contribution towards total solute load of the Cauvery River Basin (CRB). Atmospheric input, anthropogenic activities and water-mineral interaction processes are identified as the major solute sources. Using the chemical mass balance forward model, source wise solute load contributions are estimated to be 13%, 32%, 47% and 8% from atmospheric input, anthropogenic activities, silicates and carbonates weathering respectively. It’s found that, chemical weathering followed by anthropogenic activities are the controlling factors for the solute load of CRB with minor influence of atmospheric input. Weathering index calculated for CRB (Re > 3), suggest incomplete weathering of drainage rocks resulting in formation of secondary soil minerals along the river course. Further, detailed analysis of chemical weathering mechanisms is accomplished via end-member mixing analysis approach (EMMA) by using Ca/Na and Mg/Na ratios of different end-members including primary minerals form country rocks and secondary soil (weathered profile) minerals. End-member mixing diagram referred as modified Na-normalized Ca versus Mg, reveal that chemical weathering of secondary soil minerals is as significant as primary minerals and source wise solute load contribution to the river is almost equal from both sources primary and secondary. At outlet of the basin (Musiri), SWR and associated CCR values are estimated to be 12.9 t.km-2.y-1 and 3.3 × 105 mole.km-2.y-1 respectively. Results indicate that average SWR values estimated for the east flowing Cauvery river are several times (~ 4) lower than the average SWR values of west flowing Western Ghats rivers, even though the associated CO2 consummation rates are comparable for both river systems.
Keywords: Cauvery river, solute acquisition mechanisms, chemical weathering, anthropogenic sources, primary minerals, secondary soil minerals, silicate weathering, CO2 consumption rates.

How to cite: Upendra, B., Manohar, C., Aji, A., Vasudevan, V. D., and Krishnan, A. K.: Mechanisms controlling the chemical weathering flux and CO2 consumption rate of Cauvery river, South India: Role of secondary soil minerals (in weathered profiles) versus primary minerals and anthropogenic sources, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6573, https://doi.org/10.5194/egusphere-egu2020-6573, 2020.

Understanding the influence of solute inputs on their export dynamics at the catchment scale is an open challenge hampered by the general lack of distributed and time varying input data. The export dynamics is thus often investigated by analyzing concentration and discharge data at catchment outlets. These data together with knowledge of typical solute source location can provide insights about the export dynamics. We collected concentration and discharge data across 492 catchments in 9 countries and analyzed the solute concentration magnitude and dependence on discharge at the catchment outlet. The observations indicate that solutes with typically higher abundancy in the deeper subsurface have a quite different temporal dynamics and concentration-discharge (C-Q) relation than solutes produced near the surface. We further interpret results from observations by running synthetic experiments with a tracer-aided distributed model. The results clearly show that the depth at which the solute is produced is indeed the key-player in shaping the C-Q relation, especially on solutes exhibiting consistent diluting (Ca, Mg, K, Na, Cl) or weakly enriching (DOC) behavior. Such a generalization is not straightforward when moving to nutrients (NO₃ and PO₄), mostly injected sporadically through point or distributed sources. Their temporal variability is enhanced compared to the other solutes, and it adds uncertainties in the determination of the exponent of the C-Q relation.

How to cite: Burlando, P., Botter, M., Li, L., Hartmann, J., and Fatichi, S.: Vertical distribution of solute input shapes concentration-discharge relations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8930, https://doi.org/10.5194/egusphere-egu2020-8930, 2020.

Glaciers are retreating across the Canadian Cordillera. As this ice melts trace elements and other contaminants, accumulated from millennia of atmospheric deposition, are subject to release with uncertain consequences for downstream water quality. It is therefore imperative to constrain the rate and magnitude of contaminant input to river systems from glacierized watersheds. Meltwater chemistry was monitored and modelled at a high temporal frequency using a combination of grab-sampling and sondes for physical, chemical, and hydrological parameters at the outlet of proglacial Sunwapta Lake, Athabasca Glacier, Canada. Principal component analysis revealed that chemical parameters could be split into two groups with distinct seasonal trends. Group A encompasses solutes and endogenic bedrock weathering associated elements. Group B includes particulate, and exogenic dust-associated elements. Group A element concentrations were highest during low flow conditions and were correlated positively with conductivity. Group B element concentrations were highest during high flow conditions and had a moderate positive correlation with turbidity. Concentrations of potentially hazardous trace elements remained below Canadian Environmental Quality Guidelines throughout the hydrological season (THg < 2.7 ng/L; TPb < 1.7 µg/L; TAs < 0.34 µg/L; TCr < 1.9 µg/L). Trace element fluxes (kg/year) and yields (kg/year/watershed area) were modelled at a high temporal-resolution by pairing grab sampling results with corresponding strongly correlated high-frequency physical parameters: conductivity or turbidity. Annual fluxes and yields were comparable or lower than fluxes and yields from other glacial meltwater streams globally. Annual fluxes and yields were THg: 95 kg/yr  and 3.2 g/yr/km²; TPb: 34 kg/yr and 1.2 kg/yr/km²; TCr 39.5 kg/yr and 1.4 kg/yr/km²; TAs: 7.3 kg/yr and 0.25 kg/yr/km². Numerous studies have suggested that glaciers are a significant source of high concentrations, fluxes, and yields of contaminants, including: pesticides; PAHs; PCBs; and toxic trace elements. In contrast, we found low concentrations, fluxes, and yields of trace elements in meltwater from the rapidly retreating Athabasca Glacier. Grab-sampling complemented by high-frequency monitoring of physical and chemical water parameters allowed a high-resolution view of water chemistry variation in meltwater from the Athabasca Glacier.

How to cite: Staniszewska, K., Cooke, C., and Reyes, A.: Are melting alpine glaciers a source of legacy priority contaminants to downstream environments? A high-frequency analysis of water chemistry in the Canadian Rockies., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11516, https://doi.org/10.5194/egusphere-egu2020-11516, 2020.

The mean surface water concentration and the concentration variance of nutrients are major water quality characteristics of catchments that directly relates to exported nutrient loads and ecosystems functioning. The mean concentration reflects nutrient input, discharge (Q) and retention within different compartments of the catchment. The concentration variability defines the export regime of a certain solute and can be characterized by the ratio of CV_C and CV_Q and the slope b of the logC-logQ relationship. Recent explorative modelling studies argue that the export regime is shaped by spatial variance of the solute source in the catchment and by the subsurface reactivity (Musolff et al. 2017, Zhi et al. 2019). Here, we seek large scale evidence of this hypothesis by analyzing nitrate concentration and discharge (C-Q) time series in more than 1400 catchments across France and Germany. We found a consistent relationship between mean nitrate concentrations and the fraction of cultivated area within the catchments pointing to agriculture as the dominant nitrate source. The upper boundary of this relationship follows an exponential function with catchments showing mean nitrate concentrations around this envelope function being characterized by chemostatic export regimes with low concentration variance and slope b near zero. In contrast, catchments deviating from this relationship i.e. with lower than expected mean nitrate concentrations are characterized by higher concentration variance and steep, positive logC-logQ slopes. We argue, that subsurface retention is the major control of this behavior: i.e., effective denitrification decreases groundwater nitrate concentration. This was mainly observed in catchments with sedimentary aquifers and low topographic slopes. Here, old water components in the catchment storage that dominate discharge under low flow conditions are low in nitrate. Under high flow conditions, young water components high in nitrate concentrations are activated. Catchments without effective nitrate retention are characterized by a low concentration gradient between younger and older water components. The observed relationship between the fraction of cultivated areas, mean nitrate concentration and export regime was found to be surprisingly consistent across the wide range of hydroclimatic conditions, geology and topography. In consequence, steeply positive logC-logQ slopes can be used as indicators of effective subsurface reactivity. Future work will further elucidate the catchment characteristics that favor effective denitrification.

References

Musolff, A., Fleckenstein, J.H., Rao, P.S.C., Jawitz, J.W., 2017. Emergent archetype patterns of coupled hydrologic and biogeochemical responses in catchments. Geophys Res Lett, 44(9): 4143-4151. DOI:10.1002/2017GL072630

Zhi, W., Li, L. Dong, W.M., Brown, W., Kaye, J., Steefel, C., Williams, K.H., 2019. Distinct Source Water Chemistry Shapes Contrasting Concentration-Discharge Patterns. Water Resour Res, 55(5): 4233-4251. DOI:10.1029/2018wr024257

How to cite: Musolff, A., Ebeling, P., Fleckenstein, J. H., Kumar, R., and Dupas, R.: Subsurface reactivity dominates regional patterns of riverine nitrate concentration variability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13078, https://doi.org/10.5194/egusphere-egu2020-13078, 2020.

Increased anthropogenic inputs of nitrogen (N) to the biosphere during the last decades have resulted in increased groundwater and surface water concentrations of N (primarily as nitrate), posing a global problem. Although measures have been implemented to reduce N inputs, they have rarely led to decreasing riverine nitrate concentrations and loads. This limited response to the measures can either be caused by the accumulation of organic N in the soils (biogeochemical legacy) –or by long travel times (TTs) of inorganic N to the streams (hydrological legacy). Both legacy types determine the temporal dimension of catchment response on the one hand and the quantitative dimension on the other hand.

Here we analyze several decades of N input, water quality and discharge observations from 62 catchments in 8 federal states in Germany. The selection of catchments represents a wide range of land use, geology and soils, topography and hydroclimate. In an input-output assessment, N input from atmospheric deposition, waste water treatment and agriculture is compared with riverine N concentrations (nitrate-N) as N output. We assess jointly the N budget and the effective TTs of N through the soil and groundwater compartments. In combination with long-term trajectories of the C–Q relationships, we evaluate the potential for and the characteristics of an N legacy.

Our data-driven approach shows a mean legacy of 73 % (spanning 0 – 90 %), cumulating to a total missing mass of 4270 kg N/ha a. Log-normal distributed TTs have a mean of 6 years (0.8 – 34 years) with an R² of 89 % between the convolved N input and N output. Due to the chemostatic export regime (mean CV_C/CV_Q: 0.36 < 0.5) and relatively short TTs in most of the catchments, the biogeochemical legacy seems to dominate the catchment responses nowadays. Further analyses aim to investigate the controlling parameters determining the N time lags and legacies type. A correlation analyses hint to topographic parameters, mainly slope and topographic wetness index, as main controls of the legacy i.e. that flat catchments have the tendency to higher legacies or retention.

Legacies of almost ¾ of the N input pose a challenge to the limited denitrification potential of soils and aquifers or indicate a massive N accumulation in the catchment. Latter can cause elevated N concentrations for the next decades explaining at the same time a limited response to measures. The dominant biogeochemical legacy suggests that management needs to address both a longer-term reduction of N inputs and shorter-term mitigation of past high N loads by favoring denitrification.

How to cite: Ehrhardt, S., Musolff, A., Weber, M., Ebeling, P., and Kumar, R.: Joint analyses of nitrate transit time distributions and legacy effects in catchments with contrasting physical settings in Germany , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13971, https://doi.org/10.5194/egusphere-egu2020-13971, 2020.

In this study, time series over 14 years of climate, stream flow, land management, nitrate-nitrogen (NO₃^--N) concentrations and losses were analyzed to identify potential drivers for temporal trends at two tile-drained catchments under cropland use in northeastern Germany. Mean annual NO₃^--N concentrations were 9.7 (drainage plot) and 6.8 mg l^-1 (ditch catchment), while mean annual NO₃^--N losses amounted to 22 and 20 kg ha^-1, respectively. Results indicated decreasing trends for discharge, NO₃^--N concentrations and losses, and N surpluses at both the drainage plot and the ditch catchment scale. However, a significant downward trend was only detected for flow-weighted mean annual NO₃^--N concentrations of the ditch water. A significant positive relationship between annual discharge and mean annual NO₃^--N concentrations of the ditch underlines the importance of the hydrologic conditions on the NO₃^--N concentrations. No direct relationships were found between NO₃^--N concentrations and N surpluses. We conclude that the decreasing NO₃^--N concentrations could be primarily attributed to decreasing discharge rates. The possible impact of reduced N surpluses was overridden by the hydrologic conditions in the catchment. The statistically significant downward trend of flow-weighted mean annual NO₃^--N concentrations in the ditch water suggests, however, a positive effect of reduced N surpluses on stream water quality. Our analysis has further shown that effects of land management aiming at reducing N surpluses might only become visible with a delay of years or even decades.

How to cite: Bauwe, A., Kahle, P., Tiemeyer, B., and Lennartz, B.: Decreasing trends of nitrogen export to waters in tile-drained landscapes are linked to a shifting water balance, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18120, https://doi.org/10.5194/egusphere-egu2020-18120, 2020.

Solute and sediment catchment exports from drainage basins are difficult to quantify because they are exported during “hot moments”, generally during high water periods.  

We present here a simple model for predicting load flashiness (M₂, proportion of load exported during the highest 2% of flows) from flow flashiness (W₂, proportion of flows exported during the highest 2% of flows) and export pattern (b_50high, linear slope of the log concentration-log discharge relationship based on data above the median flow only). The model is parameterised based on an extensive monthly-sample data set from France, and validated using an independent daily-sample long-term data set from US including several rivers near Lake Erie (altogether over 1.5 Million discharge-concentration

Based on this model, we constructed a load-flashiness diagram to determine optimal monitoring frequency of dissolved or particulate constituents as a function of b_50high and W₂. Based on M₂, optimal temporal monitoring frequency of the studied constituents decreases in the following order: TSS, TP, DOC, NO₃, and TDS. Finally, we analyzed relationships between these metrics and catchments characteristics. Depending on the constituent, we explained between 30 to 40% of their M₂ variance with simple catchment characteristics, such as stream network density or percentage of intensive agriculture. Therefore, catchment characteristics can be used as a first approach to set up water quality monitoring design where no hydrological and/or water quality monitoring exist.

The load flashiness M₂ can also be used to optimize monitoring frequency to reach a certain level of annual load uncertainty (here 10%) for loads trend detection required for instance by international conventions such as OSPAR and HELCOM. Regulatory monitoring in Europe, recommended by the WFD, promotes the monthly sampling for any monitored constituents (dissolved and particulate) and for any basin size. Such standardized monitoring does not take into account the actual variability of the constituent concentration and loads, particularly for the small (100 – 1000 km²) and very small (< 100 km²) basins. For instance, our results show that a 30 days sampling frequency is not appropriate to calculate loads with a reasonable uncertainty (+/- 10%) in more than 90% of cases.

How to cite: Moatar, F., Renard, B., Floury, M., Gold, A., Meybeck, M., Minaudo, C., Ferréol, M., Chandesris, A., Addy, K., Piffady, J., and Pinay, G.: Catchment exports and monitoring, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18928, https://doi.org/10.5194/egusphere-egu2020-18928, 2020.

In many ecosystems on land and sea, the supply of nutrients is a key factor controlling the nature and diversity of plant life, the population dynamics of both grazing animals and their predators, and vital ecological processes such as plant productivity and the cycling of carbon and soil minerals. Over the last century, runoff from farms and cities, along with land cover and land use changes, have drastically altered the mass balance of nutrients in aquatic systems, affecting both their ecological functioning and the living communities they support. Here we present the results of a multi-year nutrient assessment of streams and lakes from the Mohican Watershed, in North-Central Ohio, which drains to the Ohio River and into the Gulf of Mexico. A total of 64 streams and 8 lakes/reservoirs have been sampled periodically since the summer of 2008. A GIS-based landscape model was used to examine the relationships between streams and their catchments. Land use data from NLCD was used to select representative reach-catchment areas in one of four categories: forested, developed, cropland, and pasture. Nutrient concentrations (NO₂; NO₃; NH₄; PO₄) were measured and used for calculation of nutrient fluxes within the watershed. Sampling was undertaken during both baseflow and stormflow conditions in order to evaluate the effects of precipitation on nutrient transport. In order to assess nutrient contribution from atmospheric deposition, rainwater samples were also analyzed. Our results show that nutrient fluxes are highly controlled by the land use of the reach-catchment and by precipitation events. In addition, there is a marked shift between local and external controls on biogeochemical processes under baseflow and stormflow conditions. During stormflow, nutrient input is primarily hydrologically controlled. During baseflow, biological processes dominate both the production and removal of nutrient ions from the stream. This short-term hydrological variability is compounded by the effects of long-term geomorphic and climatic changes, and an increase of over 15% in nutrient export was observed during wetter years.

How to cite: Costa Jr, O.: Sources, sinks, and transport of nutrients across a mixed-use headwater catchment in north-central Ohio, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20027, https://doi.org/10.5194/egusphere-egu2020-20027, 2020.

Besides agricultural land, settlement areas are among the primary sources for diffuse contamination of surface waters. Both organic and inorganic compounds originate from wash-off of road and roof surfaces, industrial areas as well as illegal wastewater discharge.

In an 18-month measurement campaign, flow triggered composite water samples were gathered using an automatic sampler, partly in small urban creeks draining settlement areas, partly from storm water channels in 7 mid-sized to large towns (30,000 to 1,800,000 inhabitants) in Hungary. Besides the automatic samples, characteristic runoff events were manually grab-sampled, leading to a time series of the contaminants. Both types of samples were analyzed for the total amount of nutrients (N and P), heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb, Sb, Zn) and 16 PAH forms.

In this contribution, the first results of the sample analyses are presented. The concentration of the measured contaminants is significantly higher during runoff events than in dry periods and can be linked to the amount of road and roof areas on the catchment. Flow triggered composite water samples are efficient in estimating total event load amounts, which were calculated for the pilot catchment areas.

How to cite: Kardos, M. K., Budai, P., Clement, A., and Knolmár, M.: Estimating load of emerging pollutants originating from urban surfaces, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20457, https://doi.org/10.5194/egusphere-egu2020-20457, 2020.

The mechanism of the water pollution process is becoming more complex due to changes in climate and river environment. There has so far been little effort to explore uncertainty considering these factors in water quality management. The water quality of rivers in Korea has become an issue and even led to a socio-political problem, especially after the environmental changes caused by the development project. We used a machine learning based classification apporoach to investigate the overall pattern of water quality changes over the past 16 years including the construction period. Water quality models are commonly based on a numerical-based deterministic model that has limitations representing stochastic behaviors properly. We employed a statistical Markov process approach to classifying the states of water quality within an unsupervised learning framework. Consequently, the spatio-temporal transition of water quality was accurately identified, and a discussion of the potential causes of the transition is offered.

KEYWORDS: Classification, Hidden Markov chain model, Water quality

Acknowledgement

This work is supported by the Korea Agency for Infrastructure Technology Advancement(KAIA) grant funded by the Ministry of Land, Infrastructure and Transport (Grant 19AWMP-B121100-04)

How to cite: Cho, H., Yu, J.-U., Uranchimeg, S., and Kwon, H.-H.: Analysis of Spatio-temporal Variation on Water Quality using a Statistical Markov Process with the Unobservable States, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21215, https://doi.org/10.5194/egusphere-egu2020-21215, 2020.

Nitrogen (N) loads and concentrations have been successfully reduced in most Danish streams during the last 30 years. Thereby also reducing the impact of the main driver of marine eutrophication in Danish coastal waters. However, the trend in N-loads and concentrations vary substantially among the monitored streams. The understanding of this variation are of great importance and interest for the evaluation of measures implemented to combat N eutrophication and for forecasting of effects of further measures.

River hydrographs can be split into base flow and quick flow components and the N concentrations in these two components can, thereafter, be calculated. The N concentration in the two components varies over time showing both longer term and seasonal variation. The quick flow component typically having a high variation reflecting present days leaching of N from fields and this strata has been significantly reduced during the last 3 decades due to a more sustainable farming practices.

During base flow conditions, stream water typically holds less nitrogen due to N removal in groundwater. Reductions in agricultural nitrogen leaching over the past three decades has reduced concentrations in the quick flow component and reduced the load to ground water aquifers. As groundwater aquifers are often large with a capacity of several years of recharge, the response in base flow N-concentrations is expected to be slow compared to the response in quick flow. The low response of the N-concentrations in base flow have implications on the rate of change of the river concentrations and consequently riverine N-loads to coastal waters. In some cases, the base flow N-concentration might still be influenced by the larger N-leaching of the past (1960-1990).

We have analyzed a national data set for developments in N-concentrations during base flow and quick flow. The data set covers the in country range in catchment size, land use and geology. The data set spans 29 years covering the period 1990 – 2018. In addition, measurements from a few streams monitored for a longer period have been included in the analyses

How to cite: Windolf, J., Thodsen, H., Tornbjerg, H., Kronvang, B., and B. Sørensen, P.: Nitrogen concentrations in flow separated river water, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22020, https://doi.org/10.5194/egusphere-egu2020-22020, 2020.