Riparian zones are known to modulate water quality in stream corridors. They can act as buffers for groundwater-borne solutes before they enter the stream at harmful, high concentrations or ...facilitate solute turnover and attenuation in zones where stream water (SW) and groundwater (GW) mix. This natural attenuation capacity is strongly controlled by the dynamic exchange of water and solutes between the stream and the adjoining aquifer, creating potential for mixing-dependent reactions to take place. Here, we couple a previously calibrated transient and fully integrated 3D surface–subsurface numerical flow model with a hydraulic mixing cell (HMC) method to map the source composition of water along a net losing reach (900 m) of the fourth-order Selke stream and track its spatiotemporal evolution. This allows us to define zones in the aquifer with more balanced fractions of the different water sources per aquifer volume (called mixing hot spots), which have a high potential to facilitate mixing-dependent reactions and, in turn, enhance solute turnover. We further evaluated the HMC results against hydrochemical monitoring data. Our results show that, on average, about 50 % of the water in the alluvial aquifer consists of infiltrating SW. Within about 200 m around the stream, the aquifer is almost entirely made up of infiltrated SW with practically no significant amounts of other water sources mixed in. On average, about 9 % of the model domain could be characterized as mixing hot spots, which were mainly located at the fringe of the geochemical hyporheic zone rather than below or in the immediate vicinity of the streambed. This percentage could rise to values nearly 1.5 times higher following large discharge events. Moreover, event intensity (magnitude of peak flow) was found to be more important for the increase in mixing than event duration. Our modeling results further suggest that discharge events more significantly increase mixing potential at greater distances from the stream. In contrast near and below the stream, the rapid increase in SW influx shifts the ratio between the water fractions to SW, reducing the potential for mixing and the associated reactions. With this easy-to-transfer framework, we seek to show the applicability of the HMC method as a complementary approach for the identification of mixing hot spots in stream corridors, while showing the spatiotemporal controls of the SW–GW mixing process and the implications for riparian biogeochemistry and mixing-dependent turnover processes.
Excessive amounts of nutrients and dissolved organic matter in freshwater bodies affect aquatic ecosystems. In this study, the spatial and temporal variability in nitrate (NO3−), dissolved organic ...carbon (DOC) and soluble reactive phosphorus (SRP) was analyzed in the Selke (Germany) river continuum from three headwaters draining 1–3 km2 catchments to two downstream reaches representing spatially integrated signals from 184–456 km2 catchments. Three headwater catchments were selected as archetypes of the main landscape units (land use × lithology) present in the Selke catchment. Export regimes in headwater catchments were interpreted in terms of NO3−, DOC and SRP land-to-stream transfer processes. Headwater signals were subtracted from downstream signals, with the differences interpreted in terms of in-stream processes and contributions from point sources. The seasonal dynamics for NO3− were opposite those of DOC and SRP in all three headwater catchments, and spatial differences also showed NO3− contrasting with DOC and SRP. These dynamics were interpreted as the result of the interplay of hydrological and biogeochemical processes, for which riparian zones were hypothesized to play a determining role. In the two downstream reaches, NO3− was transported almost conservatively, whereas DOC was consumed and produced in the upper and lower river sections, respectively. The natural export regime of SRP in the three headwater catchments mimicked a point-source signal (high SRP during summer low flow), which may lead to overestimation of domestic contributions in the downstream reaches. Monitoring the river continuum from headwaters to downstream reaches proved effective to jointly investigate land-to-stream and in-stream transport, and transformation processes.
Excess export of nitrate to streams affects ecosystem
structure and functions and has been an environmental issue attracting
worldwide attention. The dynamics of catchment-scale solute export from
...diffuse nitrogen sources can be explained by the changes of dominant flow
paths, as solute attenuation (including the degradation of nitrate) is
linked to the age composition of outflow. Previous data-driven studies
suggested that catchment topographic slope has strong impacts on the age
composition of streamflow and consequently on in-stream solute
concentrations. However, the impacts have not been systematically assessed
in terms of solute mass fluxes and solute concentration levels, particularly
in humid catchments with strong seasonality in meteorological forcing. To
fill this gap, we modeled the groundwater flow and nitrate transport for a
small agricultural catchment in Central Germany. We used the fully coupled
surface and subsurface numerical simulator HydroGeoSphere (HGS) to model
groundwater and overland flow and nitrate transport. We computed the
water ages using numerical tracer experiments. To represent various
topographic slopes, we additionally simulated 10 synthetic catchments
generated by modifying the topographic slope from the real-world scenario.
Results suggest a negative correlation between the young streamflow fraction
and the topographic slope. This correlation is more pronounced in flat
landscapes with slopes <1:60. Flatter landscapes tend to retain
more N mass in the soil (including mass degraded in soil) and export less N
mass to the stream, due to reduced leaching and increased degradation. The
mean in-stream nitrate concentration shows a decreasing trend in response to
a decreasing topographic slope, suggesting that a large young streamflow
fraction is not sufficient for high in-stream concentrations. Our results
improve the understanding of nitrate export in response to topographic slope
in a temperate humid climate, with important implications for the management
of stream water quality.
In 2018–2019, Central Europe experienced an unprecedented 2-year drought with severe impacts on society and ecosystems. In this study, we analyzed the impact of this drought on water quality by ...comparing long-term (1997–2017) nitrate export with 2018–2019 export in a heterogeneous mesoscale catchment. We combined data-driven analysis with process-based modeling to analyze nitrogen retention and the underlying mechanisms in the soils and during subsurface transport. We found a drought-induced shift in concentration–discharge relationships, reflecting exceptionally low riverine nitrate concentrations during dry periods and exceptionally high concentrations during subsequent wet periods. Nitrate loads were up to 73 % higher compared to the long-term load–discharge relationship. Model simulations confirmed that this increase was driven by decreased denitrification and plant uptake and subsequent flushing of accumulated nitrogen during rewetting. Fast transit times (<2 months) during wet periods in the upstream sub-catchments enabled a fast water quality response to drought. In contrast, longer transit times downstream (>20 years) inhibited a fast response but potentially contribute to a long-term drought legacy. Overall, our study reveals that severe droughts, which are predicted to become more frequent across Europe, can reduce the nitrogen retention capacity of catchments, thereby intensifying nitrate pollution and threatening water quality.
Essentially all hydrogeological processes are strongly influenced by the subsurface spatial heterogeneity and the temporal variation of environmental conditions, hydraulic properties, and solute ...concentrations. This spatial and temporal variability generally leads to effective behaviors and emerging phenomena that cannot be predicted from conventional approaches based on homogeneous assumptions and models. However, it is not always clear when, why, how, and at what scale the 4D (3D + time) nature of the subsurface needs to be considered in hydrogeological monitoring, modeling, and applications. In this paper, we discuss the interest and potential for the monitoring and characterization of spatial and temporal variability, including 4D imaging, in a series of hydrogeological processes: (1) groundwater fluxes, (2) solute transport and reaction, (3) vadose zone dynamics, and (4) surface–subsurface water interactions. We first identify the main challenges related to the coupling of spatial and temporal fluctuations for these processes. We then highlight recent innovations that have led to significant breakthroughs in high-resolution space–time imaging and modeling the characterization, monitoring, and modeling of these spatial and temporal fluctuations. We finally propose a classification of processes and applications at different scales according to their need and potential for high-resolution space–time imaging. We thus advocate a more systematic characterization of the dynamic and 3D nature of the subsurface for a series of critical processes and emerging applications. This calls for the validation of 4D imaging techniques at highly instrumented observatories and the harmonization of open databases to share hydrogeological data sets in their 4D components.
Transit time distributions (TTDs) integrate information
on timing, amount, storage, mixing and flow paths of water and thus
characterize hydrologic and hydrochemical catchment response unlike any
...other descriptor. Here, we simulate the shape of TTDs in an idealized
low-order catchment and investigate whether it changes systematically with
certain catchment and climate properties. To this end, we used a physically
based, spatially explicit 3-D model, injected tracer with a precipitation
event and recorded the resulting forward TTDs at the outlet of a small
(∼6000 m2) catchment for different scenarios. We found
that the TTDs can be subdivided into four parts: (1) early part – controlled
by soil hydraulic conductivity and antecedent soil moisture content, (2) middle part – a transition zone with no clear pattern or control, (3) later
part – influenced by soil hydraulic conductivity and subsequent
precipitation amount, and (4) very late tail of the breakthrough curve –
governed by bedrock hydraulic conductivity. The modeled TTD shapes can be
predicted using a dimensionless number: higher initial peaks are observed if
the inflow of water to a catchment is not equal to its capacity to discharge
water via subsurface flow paths, and lower initial peaks are connected to
increasing available storage. In most cases the modeled TTDs were humped
with nonzero initial values and varying weights of the tails. Therefore,
none of the best-fit theoretical probability functions could describe the
entire TTD shape exactly. Still, we found that generally gamma and
log-normal distributions work better for scenarios of low and high soil
hydraulic conductivity, respectively.
Increasing dissolved organic carbon (DOC) concentrations
and exports from headwater catchments impact the quality of downstream
waters and pose challenges to water supply. The importance of riparian ...zones
for DOC export from catchments in humid, temperate climates has generally
been acknowledged, but the hydrological controls and biogeochemical factors
that govern mobilization of DOC from riparian zones remain elusive. A
high-frequency dataset (15 min resolution for over 1 year) from a
headwater catchment in the Harz Mountains (Germany) was analyzed for
dominant patterns in DOC concentration (CDOC) and optical DOC quality
parameters SUVA254 and S275−295 (spectral slope between 275 and
295 nm) on event and seasonal scales. Quality parameters and CDOC
systematically changed with increasing fractions of high-frequency quick
flow (Qhf) and antecedent hydroclimatic conditions, defined by the
following metrics: aridity index (AI60) of the preceding 60 d and the
quotient of mean temperature (T30) and mean discharge (Q30) of the
preceding 30 d, which we refer to as discharge-normalized temperature
(DNT30). Selected statistical multiple linear regression models for the
complete time series (R2=0.72, 0.64 and 0.65 for
CDOC, SUVA254 and S275−295, resp.) captured DOC dynamics based on
event (Qhf and baseflow) and seasonal-scale predictors (AI60,
DNT30). The relative importance of seasonal-scale predictors allowed for
the separation of three hydroclimatic states (warm and dry, cold and wet,
and intermediate). The specific DOC quality for each state indicates a shift
in the activated source zones and highlights the importance of antecedent
conditions and their impact on DOC accumulation and mobilization in the
riparian zone. The warm and dry state results in high DOC concentrations
during events and low concentrations between events and thus can be seen as
mobilization limited, whereas the cold and wet state results in low
concentration between and during events due to limited DOC accumulation in
the riparian zone. The study demonstrates the considerable value of
continuous high-frequency measurements of DOC quality and quantity and its
(hydroclimatic) key controlling variables in quantitatively unraveling DOC
mobilization in the riparian zone. These variables can be linked to DOC
source activation by discharge events and the more seasonal control of DOC
production in riparian soils.
Export of dissolved organic carbon (DOC) from riparian zones (RZs) is
an important component of temperate catchment carbon budgets, but
export mechanisms are still poorly understood. Here we show ...that DOC
export is predominantly controlled by the microtopography of the RZ
(lateral variability) and by riparian groundwater level dynamics
(temporal variability). From February 2017 until July 2019 we studied
topography, DOC quality and water fluxes and pathways in the RZ
of a small forested catchment and the receiving stream in central
Germany. The chemical classification of the riparian groundwater and
surface water samples (n=66) by Fourier transform ion cyclotron
resonance mass spectrometry revealed a cluster of plant-derived,
aromatic and oxygen-rich DOC with high concentrations
(DOCI) and a cluster of microbially processed, saturated
and heteroatom-enriched DOC with lower concentrations
(DOCII). The two DOC clusters were connected to locations
with distinctly different values of the high-resolution topographic
wetness index (TWIHR; at 1 m resolution) within the study
area. Numerical water flow modeling using the integrated surface–subsurface model HydroGeoSphere revealed that surface runoff from high-TWIHR zones associated with the DOCI cluster
(DOCI source zones) dominated overall discharge generation
and therefore DOC export. Although corresponding to only 15 % of the
area in the studied RZ, the DOCI source zones contributed
1.5 times the DOC export of the remaining 85 % of the area
associated with DOCII source zones. Accordingly, DOC quality
in stream water sampled under five event flow conditions (n=73) was
closely reflecting the DOCI quality. Our results suggest
that DOC export by surface runoff along dynamically evolving surface
flow networks can play a dominant role for DOC exports from RZs with
overall low topographic relief and should consequently be considered
in catchment-scale DOC export models. We propose that proxies of
spatial heterogeneity such as the TWIHR can help to
delineate the most active source zones and provide a mechanistic basis
for improved model conceptualization of DOC exports.
Understanding the controls on event-driven dissolved organic carbon (DOC) export is crucial as DOC is an important link between the terrestrial and the aquatic carbon cycles. We hypothesized that ...topography is a key driver of DOC export in headwater catchments because it influences hydrological connectivity, which can inhibit or facilitate DOC mobilization. To test this hypothesis, we studied the mechanisms controlling DOC mobilization and export in the Große Ohe catchment, a forested headwater in a mid-elevation mountainous region in southeastern Germany. Discharge and stream DOC concentrations were measured at an interval of 15 min using in situ UV-Vis (ultraviolet–visible) spectrometry from June 2018 until October 2020 at two topographically contrasting subcatchments of the same stream. At the upper location (888 m above sea level, a.s.l.), the stream drains steep hillslopes, whereas, at the lower location (771 m a.s.l.), it drains a larger area, including a flat and wide riparian zone. We focus on four events with contrasting antecedent wetness conditions and event size. During the events, in-stream DOC concentrations increased up to 19 mg L−1 in comparison to 2–3 mg L−1 during baseflow. The concentration–discharge relationships exhibited pronounced but almost exclusively counterclockwise hysteresis loops which were generally wider in the lower catchment than in the upper catchment due to a delayed DOC mobilization in the flat riparian zone. The riparian zone released considerable amounts of DOC, which led to a DOC load up to 7.4 kg h−1. The DOC load increased with the total catchment wetness. We found a disproportionally high contribution to the total DOC export of the upper catchment during events following a long dry period. We attribute this to the low hydrological connectivity in the lower catchment during drought, which inhibited DOC mobilization, especially at the beginning of the events. Our data show that not only event size but also antecedent wetness conditions strongly influence the hydrological connectivity during events, leading to a varying contribution to DOC export of subcatchments, depending on topography. As the frequency of prolonged drought periods is predicted to increase, the relative contribution of different subcatchments to DOC export may change in the future when hydrological connectivity will be reduced more often.
PDF
