Relationships between in‐stream dissolved solute concentrations (C) and discharge (Q) are useful indicators of catchment‐scale processes. We combine a synthesis of observational records with a ...parsimonious stochastic modeling approach to test how C‐Q relationships arise from spatial heterogeneity in catchment solute sources coupled with different timescales of reactions. Our model indicates that the dominant driver of emergent archetypical dilution, enrichment, and constant C‐Q patterns was structured heterogeneity of solute sources implemented as correlation of source concentration to travel time. Regardless of the C‐Q pattern, with weak correlation between solute‐source concentration and travel time, we consistently find lower variability in C than in Q, such that the predominant solute export regime is chemostatic. Consequently, the variance in exported loads is determined primarily by variance of Q. Efforts to improve stream water quality and ecological integrity in intensely managed catchments should lead away from landscape homogenization by introducing structured source heterogeneity.
Key Points
A stochastic modeling approach reproduces observed archetypical dilution, enrichment, and constant concentration‐discharge (C‐Q) patterns
Even for dilution or enrichment C‐Q patterns, chemostatic export regimes with low concentration variability prevail
Weak correlation between solute‐source concentration and travel time leads to a prevalence of chemostatic export regimes
The analysis of concentration‐discharge (C‐Q) relationships from low‐frequency observations is commonly used to assess solute sources, mobilization, and reactive transport processes at the catchment ...scale. High‐frequency concentration measurements are increasingly available and offer additional insights into event‐scale export dynamics. However, only few studies have integrated inter‐annual and event‐scale C‐Q relationships. Here, we analyze high‐frequency measurements of specific conductance (EC), nitrate (NO3‐N) concentrations and spectral absorbance at 254 nm (SAC254, as a proxy for dissolved organic carbon) over a two year period for four neighboring catchments in Germany ranging from more pristine forested to agriculturally managed settings. We apply an integrated method that adds a hysteresis term to the established power law C‐Q model so that concentration intercept, C‐Q slope and hysteresis can be characterized simultaneously. We found that inter‐event variability in C‐Q hysteresis and slope were most pronounced for SAC254 in all catchments and for NO3‐N in forested catchments. SAC254 and NO3‐N event responses in the smallest forested catchment were closely coupled and explainable by antecedent conditions that hint to a common near‐stream source. In contrast, the event‐scale C‐Q patterns of EC in all catchments and of NO3‐N in the agricultural catchment without buffer zones around streams were less variable and similar to the inter‐annual C‐Q relationship indicating a homogeneity of mobilization processes over time. Event‐scale C‐Q analysis thus added key insights into catchment functioning whenever the inter‐annual C‐Q relationship contrasted with event‐scale responses. Analyzing long‐term and event‐scale behavior in one coherent framework helps to disentangle these scattered C‐Q patterns.
Key Points
We compare event‐scale and inter‐annual concentration‐discharge relationships in four adjoined catchments with contrasting land use
The variability of event‐scale C‐Q relationships was shaped by land use and antecedent conditions for biogeochemically reactive but not for geogenic solutes
For biogeochemically reactive solutes, event‐scale C‐Q patterns can contrast the inter‐annual pattern obtained from all observations
Increased anthropogenic inputs of nitrogen (N) to the
biosphere during the last few 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 not always 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).
Here, we compare atmospheric and agricultural N inputs with long-term
observations (1970–2016) of riverine nitrate concentrations and loads in a
central German mesoscale catchment with three nested subcatchments
of increasing agricultural land use. Based on a data-driven
approach, 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. We show that in the 40-year-long observation period, the catchment (270 km2) with 60 % agricultural area received an N input of
53 437 t, while it exported 6592 t, indicating an overall retention of
88 %. Removal of N by denitrification could not sufficiently explain this
imbalance. Log-normal travel time distributions (TTDs) that link the N input
history to the riverine export differed seasonally, with modes spanning
7–22 years and the mean TTs being systematically shorter during the high-flow season as compared to low-flow conditions. Systematic shifts in the
C–Q relationships were noticed over time that could be attributed to strong
changes in N inputs resulting from agricultural intensification before 1989,
the break-down of East German agriculture after 1989 and the
seasonal differences in TTs. A chemostatic export regime of nitrate was only
found after several years of stabilized N inputs. The changes in C–Q
relationships suggest a dominance of the hydrological N legacy over the
biogeochemical N fixation in the soils, as we expected to observe a stronger
and even increasing dampening of the riverine N concentrations after
sustained high N inputs. Our analyses reveal an imbalance between N input
and output, long time-lags and a lack of significant denitrification in the
catchment. All these suggest that catchment management needs to address
both a longer-term reduction of N inputs and shorter-term mitigation of
today's high N loads. The latter may be covered by interventions triggering
denitrification, such as hedgerows around agricultural fields, riparian
buffers zones or constructed wetlands. Further joint analyses of N budgets
and TTs covering a higher variety of catchments will provide a deeper insight into N trajectories and their controlling parameters.
Interest in groundwater (GW)-surface water (SW) interactions has grown steadily over the last two decades. New regulations such as the EU Water Framework Directive (WFD) now call for a sustainable ...management of coupled ground- and surface water resources and linked ecosystems. Embracing this mandate requires new interdisciplinary research on GW-SW systems that addresses the linkages between hydrology, biogeochemistry and ecology at nested scales and specifically accounts for small-scale spatial and temporal patterns of GW-SW exchange. Methods to assess these patterns such as the use of natural tracers (e.g. heat) and integrated surface-subsurface numerical models have been refined and enhanced significantly in recent years and have improved our understanding of processes and dynamics. Numerical models are increasingly used to explore hypotheses and to develop new conceptual models of GW-SW interactions. New technologies like distributed temperature sensing (DTS) allow an assessment of process dynamics at unprecedented spatial and temporal resolution. These developments are reflected in the contributions to this Special Issue on GW-SW interactions. However, challenges remain in transferring process understanding across scales.
►Rapidly growing interest in groundwater-surface water exchange processes. ►Research on groundwater-surface water interactions has become multidisciplinary. ►New focus on linkages between hydrology, biogeochemistry and ecology. ►Development of new methods and models to quantify spatial and temporal patterns. ►Challenges remain in transferring process understanding across scales.
Concentrations of dissolved organic carbon (DOC) in runoff from catchments are often subject to substantial short-term variations. The aim of this study was to identify the compartmental sources of ...DOC in a forested catchment and the causes for short-term variations in runoff. Furthermore, we investigated the implication of short-term variations for the calculation of annual runoff fluxes. High frequency measurements (30 min intervals) of DOC in runoff, of discharge and groundwater table were conducted for one year in the 4.2 km2 forested Lehstenbach catchment, Germany. Riparian wetland soils represent about 30% of the catchment area. The quality of DOC was investigated by three dimensional fluorescence excitation-emission matrices in samples taken from runoff, deep groundwater and shallow groundwater from the riparian wetland soils. The concentrations of DOC in runoff were highly variable at an hourly to daily time scale, ranging from 2.6 mg L−1 to 34 mg L−1 with an annual average of 9.2 mg L−1. The concentrations were positively related to discharge, with a counter clockwise hysteresis. Relations of DOC to discharge were steeper and the degree of hysteresis larger in the summer/fall than in the winter/spring period. Dynamics of groundwater table, discharge, DOC concentrations and DOC quality parameters indicated that DOC in runoff originated mainly from the riparian wetland soils, both under low and high flow conditions. The annual export of DOC from the catchment was 84 kg C ha−1 yr−1 when calculated from the high frequency measurements. If the annual export was calculated by simulated samplings of >2 days intervals substantial deviations resulted. Predicted changes in precipitation and discharge patterns as well as generally increasing temperatures likely will cause raising DOC exports from this catchment.
In humid upland catchments wetlands are often a prominent feature in the vicinity of streams and have potential implications for runoff generation and nutrient export. Wetland surfaces are often ...characterized by distinct micro-topography (hollows and hummocks). The effects of such micro-topography on surface–subsurface exchange and runoff generation for a 10 by 20
m synthetic section of a riparian wetland were investigated in a virtual modeling experiment. A reference model with a planar surface was run for comparison. The geostatistically simulated structure of the micro-topography replicates the topography of a peat-forming riparian wetland in a small mountainous catchment in South-East Germany (Lehstenbach). Flow was modeled with the fully-integrated surface–subsurface code HydroGeoSphere. Simulation results showed that the specific structure of the wetland surface resulted in distinct shifts between surface and subsurface flow dominance. Surface depressions filled and started to drain via connected channel networks in a threshold controlled process, when groundwater levels intersected the land surface. These networks expanded and shrunk in a spill and fill mechanism when the shallow water table fluctuated around the mean surface elevation under variable rainfall inputs. The micro-topography efficiently buffered rainfall inputs and produced a hydrograph that was characterized by subsurface flow during most of the year and only temporarily shifted to surface flow dominance (>
80% of total discharge) during intense rainstorms. In contrast the hydrograph in the planar reference model was much “flashier” and more controlled by surface runoff. A non-linear, hysteretic relationship between groundwater level and discharge observed at the study site was reproduced with the micro-topography model. Hysteresis was also observed in the relationship between surface water storage and discharge, but over a relatively narrow range of surface water storage values. Therefore it was concluded that surface water storage was a better predictor for the occurrence of surface runoff than groundwater levels.
Nitrate (NO3-) excess in rivers harms aquatic ecosystems and can induce detrimental algae growths in coastal areas. Riverine NO3- uptake is a crucial element of the catchment-scale nitrogen balance ...and can be measured at small spatiotemporal scales, while at the scale of entire river networks, uptake measurements are rarely available. Concurrent, low-frequency NO3- concentration and streamflow (Q) observations at a basin outlet, however, are commonly monitored and can be analyzed in terms of concentration discharge (C–Q) relationships. Previous studies suggest that steeper positive log (C)–log (Q) slopes under low flow conditions (than under high flows) are linked to biological NO3- uptake, creating a bent rather than linear log (C)–log (Q) relationship. Here we explore if network-scale NO3- uptake creates bent log (C)–log (Q)
relationships and when in turn uptake can be quantified from observed low-frequency C–Q data. To this end we apply a parsimonious mass-balance-based river network uptake model in 13 mesoscale German catchments (21–1450 km2) and explore the linkages between log (C)–log (Q) bending and different model parameter combinations. The modeling results show that uptake and transport in the river network can create bent log (C)–log (Q) relationships at the basin outlet from log–log linear C–Q relationships describing the NO3- land-to-stream transfer. We find that within the chosen parameter range the
bending is mainly shaped by geomorphological parameters that control the
channel reactive surface area rather than by the biological uptake velocity
itself. Further we show that in this exploratory modeling environment,
bending is positively correlated to percentage of NO3- load removed in the network (Lr.perc) but that network-wide flow velocities should be taken into account when interpreting log (C)–log (Q) bending. Classification trees, finally, can successfully predict classes of low (∼4 %), intermediate (∼32 %) and high (∼68 %) Lr.perc using information on water velocity and log (C)–log (Q) bending. These results can help to identify stream networks that efficiently attenuate NO3- loads based on low-frequency NO3- and Q observations and
generally show the importance of the channel geomorphology on the emerging
log (C)–log (Q) bending at network scales.
Hyporheic exchange transports solutes into the subsurface where they can undergo biogeochemical transformations, affecting fluvial water quality and ecology. A three‐dimensional numerical model of a ...natural in‐stream gravel bar (20 m × 6 m) is presented. Multiple steady state streamflow is simulated with a computational fluid dynamics code that is sequentially coupled to a reactive transport groundwater model via the hydraulic head distribution at the streambed. Ambient groundwater flow is considered by scenarios of neutral, gaining, and losing conditions. The transformation of oxygen, nitrate, and dissolved organic carbon by aerobic respiration and denitrification in the hyporheic zone are modeled, as is the denitrification of groundwater‐borne nitrate when mixed with stream‐sourced carbon. In contrast to fully submerged structures, hyporheic exchange flux decreases with increasing stream discharge, due to decreasing hydraulic head gradients across the partially submerged structure. Hyporheic residence time distributions are skewed in the log‐space with medians of up to 8 h and shift to symmetric distributions with increasing level of submergence. Solute turnover is mainly controlled by residence times and the extent of the hyporheic exchange flow, which defines the potential reaction area. Although streamflow is the primary driver of hyporheic exchange, its impact on hyporheic exchange flux, residence times, and solute turnover is small, as these quantities exponentially decrease under losing and gaining conditions. Hence, highest reaction potential exists under neutral conditions, when the capacity for denitrification in the partially submerged structure can be orders of magnitude higher than in fully submerged structures.
Key Points:
CFD model of in‐stream gravel bar coupled to reactive transport model
Losing and gaining conditions reduce aerobic respiration and denitrification
Submergence controls skewness of residence time distribution and reactions
Transit time distributions (TTDs) of streamflow are useful descriptors for understanding flow and solute transport in catchments. Catchment-scale TTDs can be modeled using tracer data (e.g. oxygen ...isotopes, such as δ18O) in inflow and outflows by employing StorAge Selection (SAS) functions.
However, tracer data are often sparse in space and time, so they need to be interpolated to increase their spatiotemporal resolution. Moreover, SAS functions can be parameterized with different forms, but there is no general agreement on which one should be used. Both of these aspects induce uncertainty in the simulated TTDs, and the individual uncertainty sources as well as their combined effect have not been fully investigated.
This study provides a comprehensive analysis of the TTD uncertainty resulting from 12 model setups obtained by combining different interpolation schemes for δ18O in precipitation and distinct SAS functions.
For each model setup, we found behavioral solutions with satisfactory model performance for in-stream δ18O (KGE > 0.55, where KGE refers to the Kling–Gupta efficiency). Differences in KGE values were statistically significant, thereby showing the relevance of the chosen setup for simulating TTDs.
We found a large uncertainty in the simulated TTDs, represented by a large range of variability in the 95 % confidence interval of the median transit time, varying at the most by between 259 and 1009 d across all tested setups. Uncertainty in TTDs was mainly associated with the temporal interpolation of δ18O in precipitation, the choice between time-variant and time-invariant SAS functions, flow conditions, and the use of nonspatially interpolated δ18O in precipitation.
We discuss the implications of these results for the SAS framework, uncertainty characterization in TTD-based models, and the influence of the uncertainty for water quality and quantity studies.
Subsurface contamination due to excessive nutrient surpluses is a persistent and widespread problem in agricultural areas across Europe. The vulnerability of a particular location to pollution from ...reactive solutes, such as nitrate, is determined by the interplay between hydrologic transport and biogeochemical transformations. Current studies on the controls of subsurface vulnerability do not consider the transient behaviour of transport dynamics in the root zone. Here, using state-of-the-art hydrologic simulations driven by observed hydroclimatic forcing, we demonstrate the strong spatiotemporal heterogeneity of hydrologic transport dynamics and reveal that these dynamics are primarily controlled by the hydroclimatic gradient of the aridity index across Europe. Contrasting the space-time dynamics of transport times with reactive timescales of denitrification in soil indicate that ~75% of the cultivated areas across Europe are potentially vulnerable to nitrate leaching for at least one-third of the year. We find that neglecting the transient nature of transport and reaction timescale results in a great underestimation of the extent of vulnerable regions by almost 50%. Therefore, future vulnerability and risk assessment studies must account for the transient behaviour of transport and biogeochemical transformation processes.
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