Changes In Flow Conditions Research Articles

Juvenile growth of a macrohabitat generalist tied to hydrological character of large‐river system

Summary Characteristics of river hydrology, including components of the natural flow regime, have been used to describe fish growth patterns. Contradictory growth responses among species, life‐stages within species and river systems suggest that eco‐hydrological relationships may not be scalable to all species and river systems; thus, increased knowledge regarding growth–flow relationships among a greater diversity of taxa and river systems may aid conservation and management of riverine fishes. We used channel catfish (Ictalurus punctatus: Ictaluridae), and a set of candidate mixed‐effect linear models to assess juvenile fish growth in relation to components of the flow regime within the Lower Platte River (LPR), Nebraska, U.S.A. The LPR is subjected to extremes in river flow conditions from both natural and anthropogenic sources, and provides a model system with which to assess growth–flow relationships. Juvenile channel catfish growth was influenced by the duration, magnitude and timing of in‐channel flows. Specifically, sustained high‐flow periods during the spring increased growth; whereas, reduced growth occurred during prolonged periods of low‐flow conditions within the LPR. Differences in juvenile channel catfish growth with respect to changing flow conditions may be due to changes in river productivity between low‐ and high‐flow conditions; thus, we discuss the applicability of alternative river production models with respect to juvenile fish growth in wide braided river systems such as the LPR. Reduced duration and changes to the timing of high‐flow periods may result in a myriad of deleterious effects including as our study shows, exacerbating slow growth of juvenile channel catfish and potentially other fishes with similar flow requirements. Conservation and management of large‐river species may benefit from water management policies that consider the complex and often contrasting growth responses of lotic fishes as well as the importance of using multiple aspects of the flow regime when assessing ecological responses.

Effect of unsteady stretching on the flame local dynamics

Numerical simulations employing detailed reaction kinetics and transport have been applied to calculate steady-state and harmonically excited laminar hydrogen–air flames with equivalence ratios ranging from fuel-lean to fuel-rich mixtures. The objective is to gain a basic understanding of the influence of unsteady flow stretching on the flame’s response. The flame displacement speed and consumption speed have been evaluated, together with stretch, curvature and strain. In the steady-state simulations, a quasi-linear correlation has been obtained for the flame speeds with stretch for a moderate range, which confirms the applicability of the asymptotic relation for small stretch. In the case of unsteady flow conditions, a phase shift is observed between the oscillations of the fluid flow and the flame surface position, indicating a delayed response of the flame to changing flow conditions. The corresponding delay time or relaxation time is found to be depending on the turnover time of the flow oscillations. The flame response in terms of displacement from its steady state location is shown to be dampened at higher excitation frequencies and for flames with large flame transit times τL0. Due to the delayed response, the local consumption speeds at different phase angles show different dependences on flame stretch. This discrepancy is large for flames in flows excited with lower frequencies as well as equivalence ratios with smaller flame transit times. Also, a larger variation of consumption speeds from different phase angles over flame stretch has been observed for regions with positive stretch, because the internal structure of the flame is altered, leading locally to smaller flame transit times. This explains the large scattering of flame propagation statistics in turbulent combustion, where the flow is characterized by a cascade of interacting vortices with different turnover frequencies. The unsteady effect is quantitatively analysed by comparing the characteristic times of the flame and the flow. In this way, an extension of the Markstein regression has been provided based on the simulation results, giving Ma as a function of the Damköhler number defined by Da=τflow/τL0=SL0/δL0/f. Evaluating the consumption speed for different phases during a turnover period of the oscillations and averaging over the period results in a correlation of the averaged Markstein number with the Damköhler number.

Computational Fluid Dynamics Analysis of the Fossil Crinoid Encrinus liliiformis (Echinodermata: Crinoidea).

Crinoids, members of the phylum Echinodermata, are passive suspension feeders and catch plankton without producing an active feeding current. Today, the stalked forms are known only from deep water habitats, where flow conditions are rather constant and feeding velocities relatively low. For feeding, they form a characteristic parabolic filtration fan with their arms recurved backwards into the current. The fossil record, in contrast, provides a large number of stalked crinoids that lived in shallow water settings, with more rapidly changing flow velocities and directions compared to the deep sea habitat of extant crinoids. In addition, some of the fossil representatives were possibly not as flexible as today’s crinoids and for those forms alternative feeding positions were assumed. One of these fossil crinoids is Encrinus liliiformis, which lived during the middle Triassic Muschelkalk in Central Europe. The presented project investigates different feeding postures using Computational Fluid Dynamics to analyze flow patterns forming around the crown of E. liliiformis, including experimental validation by Particle Image Velocimetry. The study comprises the analysis of different flow directions, velocities, as well as crown orientations. Results show that inflow from lateral and oral leads to direct transport of plankton particles into the crown and onto the oral surface. With current coming from the “rear” (aboral) side of the crinoid, the conical opening of the crown produces a backward oriented flow in its wake that transports particles into the crown. The results suggest that a conical feeding position may have been less dependent on stable flow conditions compared to the parabolic filtration fan. It is thus assumed that the conical feeding posture of E. liliiformis was suitable for feeding under dynamically changing flow conditions typical for the shallow marine setting of the Upper Muschelkalk.

Mechanistic Model for Nanoparticle Retention in Porous Media

With sizes larger than molecules but smaller than colloidal particles, nanoparticles exhibit unique transport properties in porous media. They can easily pass through typical pore throats in reservoir formations with micron diameters, but may get retained by physicochemical interaction with the pore walls. Based on detailed analysis of nanoparticle retention data from an extensive series of transport experiments, we examine the limitations of classical models of transport and interaction with a stationary phase. Some features of nanoparticle transport and retention are similar to those of adsorbing/desorbing solutes, while others are similar to those of depositing colloids. But neither solute sorption nor colloid filtration alone can explain all nanoparticle retention features, and of particular importance for subsurface applications, neither model can predict the effect of changing flow conditions on nanoparticle retention. The model that accounts for most observations is an independent two-site model which postulates physically independent sites of fixed capacity: one for reversible attachment and the other for irreversible attachment. We validate the model against five distinctly different groups of experimental data from the literature, through a rigorous approach of obtaining the model parameters from one experiment and blind testing against data from other experiments when experimental conditions vary.

The Response of a Heat Loss Flowmeter in a Water Pipe Under Changing Flow Conditions

A heat loss flowmeter for measuring the volume flow rate of water in real time ( $t)$ was formed using two thick film segmented thermistors, range constant voltage (RCV) power supply, acquisition card, PC, and software. The input water temperature $T_{w}(t)$ was measured by a cold thermistor, while the water flow rate $Q(t)$ was determined using the heat loss principle by measuring the self-heated thermistor current $I(t)$ . The flowmeter inertia, stability, and accuracy were measured and analyzed on a flowmeter prototype in real-time and real conditions on the water mains. The flowmeter response was measured for different durations of step input water flow-out functions and intervals between steps. An independent ultrasonic flowmeter was connected in series with the thermal flowmeter to measure the ordered input water flow functions. The realized intelligent functions in real time were: measuring the input water temperature $T_{w}(t)$ and self-heating temperature $T_{s}(t)$ , auto-selection of RCV supply voltage $U(T_{w})$ , determination and modeling of the water flow rate in real time $Q(t)$ , determination of the water volume $V(t)$ , and determination of the thermal gradient on the self-heating thermistor as a water flow indicator $\Delta R_{s}(t)$ .

Impact of gas flow rate on breakdown of filamentary dielectric barrier discharges

The influence of gas flow rate on breakdown properties and stability of pulsed dielectric barrier discharges (DBDs) in a single filament arrangement using a gas mixture of 0.1 vol. % O2 in N2 at atmospheric pressure was investigated by means of electrical and optical diagnostics, accompanied by fluid dynamics and electrostatics simulations. A higher flow rate perpendicular to the electrode symmetry axis resulted in an increased breakdown voltage and DBD current maximum, a higher discharge inception jitter, and a larger emission diameter of the discharge channel. In addition, a shift of the filament position for low gas flow rates with respect to the electrode symmetry axis was observed. These effects can be explained by the change of the residence time of charge carriers in the discharge region—i.e., the volume pre-ionization—for changed flow conditions due to the convective transport of particles out of the center of the gap.

Scour below a subsea pipeline in time varying flow conditions

This paper presents results of an experimental investigation into the time scale of local scour and backfill for a subsea pipeline in changing flow conditions, modelled as two consecutive but different flow conditions and as uniformly increasing flow velocities. Unidirectional current flow, waves and combined waves and current flow are all considered. Based on the results of the experiments, effective time scales of the scour and backfill are quantified and algorithms to predict the scour process in changing flow conditions are derived by accumulating the effects of the different flow conditions. It is found that these algorithms may be used to predict the scour observed in the experimental data reasonably well.

The role of mass-transport complexes in controlling channel avulsion and the subsequent sediment dispersal patterns on an active margin: The Magdalena Fan, offshore Colombia

Submarine channel avulsion is a fundamental process in the evolution of submarine fans that records abrupt changes in the sediment dispersal patterns, and the development of sand-rich splay deposits. On passive margins, changing flow conditions have been invoked to explain the location and timing of channel avulsion. On tectonically active basin margins, processes such as seismic activity, seabed faulting and folding and emplacement of mass-transport complexes (MTCs) are additional triggers. This study is based on the detailed mapping and interpretation of the first 1000 m below seabed of a 1900 km2 3D seismic volume in the southern Magdalena Fan, offshore Colombia, a tectonically-active margin. The emplacement of a large MTC deposit (>400 km2 in area and 200 m in thickness) is interpreted to have controlled the avulsion node of a major channel-levee complex-set and influenced the evolution of the subsequent avulsion lobe complex-set. The basal surface of the MTC is highly erosional resulting in net degradation of the seascape. Substrate entrainment by the MTC left behind a narrow erosional remnant ridge that formed a bathymetric anomaly upon which a channel-levee complex-set developed. The irregular levee geometries above the remnant ridge led to instability and levee collapse prior to channel avulsion. Map view geometries and seismic amplitude extractions suggest that the initial avulsion lobes were mud-prone and relatively erratically distributed, and evolved to form well-defined sand-prone lobes. The distribution, morphology and evolution of the avulsion lobe complexes were strongly influenced by the bathymetric anomalies on the MTC top surface which, are generally coincident with protruding megaclasts.The role of MTC emplacement in triggering submarine channel avulsion and the development of sand-prone deposits in proximal locations has important implications for hydrocarbon exploration and production. This study serves as a high-resolution, shallow-subsurface analogue for less well-imaged avulsion cycles on tectonically-active basin margins that are prone to MTCs.

The impact of inertial forces on morphing wind turbine blade in vertical axis configuration

A novel flexible blade concept with the ability to morph and geometrically adapt to changing flow conditions has been proposed to improve part-load performance of horizontal-axis wind turbines. The extension of these benefits to a vertical axis wind turbine would make wind technology a more competitive player in the energy market. Both flexible and rigid wind turbine rotor blades for vertical axis application were modeled, designed, manufactured and tested. Their performances were tested in a low speed wind tunnel. The predicted magnitude and direction of blade morph was validated using a high speed camera as well as finite element analysis. The comparative results of straight rigid and straight morphing blades show that the coefficient of performance greatly depends on the tip speed ratio. Overall, the morphing blade has better performance at low RPMs, but the rigid blade performed better at high RPMs. It was observed that the flexible blade self-started in the majority of the experiments. At high RPM, the centrifugal force overwhelmed the lift force, bending the flexible blade out of phase in an undesired direction increasing drag and therefore reducing the coefficient of performance.

Corrosion behavior of EN AC-AlSi10Mg in boiling coolant with various average flow temperatures

Even with an increasing share in electrical vehicles, the combustion engine can be assumed to be the main drive technology in automobiles over the next 20 years. Increasing demands to the engine, such as the reduction of CO2 emissions and minimization of fuel consumption while maintaining performance, require new concepts regarding the motor design. Known examples like “downsizing” and “thermal management” are already being developed. Even start-stop systems, which reduce fuel consumption while waiting at traffic lights or in traffic jams are increasingly used. These measures often result in an increasing engine temperature, which has to be balanced by the coolant. The increased thermal loading, associated with local boiling processes result in still unknown effects in terms of the formation of corrosion protective layers and coolant long-term stability. At this point, the reaction between materials and the respective coolant is of particular importance. In this report, the impact of increased coolant temperatures, along with stable and unstable boiling and changing flow conditions are discussed. The experimental results have been obtained with silicium, organic acid, and silicium/organic acid-based coolants in a special test device, mimicking the temperature and flow conditions in cooling cycles next to engine walls. An evaluation of the results is given on the basis of surface conditions (investigated by REM/EDX analyses and cross-sections), temperature gradients, and mass loss of the used specimens. Furthermore, the evaluation is also based on electrochemical measurements of the open circuit potential, which was recorded in the coolant flow. The obtained results provide information on damage relevant parameters and their interaction with coolant inducing corrosion on hot aluminum surfaces. In particular, the influence of boiling processes with complete coolant evaporation, as they can appear in common start-stop systems today, are investigated.

Wavelength selection and symmetry breaking in orbital wave ripples

AbstractSand ripples formed by waves have a uniform wavelength while at equilibrium and develop defects while adjusting to changes in the flow. These patterns arise from the interaction of the flow with the bed topography, but the specific mechanisms have not been fully explained. We use numerical flow models and laboratory wave tank experiments to explore the origins of these patterns. The wavelength of “orbital” wave ripples (λ) is directly proportional to the oscillating flow's orbital diameter (d), with many experimental and field studies finding λ/d ≈ 0.65. We demonstrate a coupling that selects this ratio: the maximum length of the flow separation zone downstream of a ripple crest equals λ when λ/d ≈ 0.65. We show that this condition maximizes the growth rate of ripples. Ripples adjusting to changed flow conditions develop defects that break the bed's symmetry. When d is shortened sufficiently, two new incipient crests appear in every trough, but only one grows into a full‐sized crest. Experiments have shown that the same side (right or left) wins in every trough. We find that this occurs because incipient secondary crests slow the flow and encourage the growth of crests on the next flank. Experiments have also shown that when d is lengthened, ripple crests become increasingly sinuous and eventually break up. We find that this occurs because crests migrate preferentially toward the nearest adjacent crest, amplifying any initial sinuosity. Our results reveal the mechanisms that form common wave ripple patterns and highlight interactions among unsteady flows, sediment transport, and bed topography.

The Impact of Low Flow on Riverine Food Webs in South-Central Newfoundland

Low-flow events can reduce food availability and decrease the feeding niche of consumers within rivers. Stable carbon (δ13C) and nitrogen (δ15N) isotope and stomach content analyses were employed to evaluate resource use and overlap between fish species in a natural and regulated river in normal and low-flow years, with the use of multiple methodological approaches providing the best means of understanding short-term and long-term observations on fish feeding and resource overlap under changing flow conditions. Diet analyses generally indicated significant inter-specific differences in the diets of key fish species within rivers and similarities in resource use between rivers. In comparison with fish from the natural river, fish from the regulated river had lower and less inter-annually variable δ13C values. In the natural river, there was a significant reduction and increase, respectively, in δ13C and δ15N variation in the low-flow year. Intra-annual or inter-annual differences in trophic niche area were not apparent in the regulated river, whereas within the natural river, intra-annual and inter-annual differences in trophic niche were found. Resource overlap between key fish species was also higher in the low-flow year and lower in the spring and higher in the summer as a result of differences in flow. Resource overlap was also higher between rivers in the low-flow year. High resource overlap between rivers during decreased summer flow indicates a strong effect of flow on river organisms, where both fish and their invertebrate prey resources are concerned. Copyright © 2014 John Wiley & Sons, Ltd.

Shifts in aquatic macroinvertebrate community structure in response to perenniality, southern Cape, South Africa

Stream flow and associated seasonal hydrologic variation is a critical driver affecting the structure of aquatic macroinvertebrate communities. Quarterly aquatic macroinvertebrate sampling was carried out for a single year along the longitudinal axes of two streams in the southern Cape of South Africa. Stones-in-current, marginal vegetation, gravel-sand-mud, and stones-out-of-current biotopes were sampled for aquatic macroinvertebrates. Associations between macroinvertebrate communities and hydrographs were analyzed to investigate whether a non-perennial stream with greater flow variability had a higher prevalence of common or opportunistic species than a perennial one with less variable flow. Macroinvertebrate species richness was greater in the perennial as opposed to the non-perennial stream, where, on a seasonal basis, species richness increased from winter to autumn. Temporal species turnover differed between sites and streams, where reduced flows transformed the more dominant aquatic biotopes from stones-in-current into standing pools. Findings are that aquatic macroinvertebrates respond to changing flow conditions and reduced mean daily flows resulted in the disappearance of flow-dependant taxa and the arrival of non-flow-dependant taxa.

A new approach to rapid, desktop-level, environmental flow assessments for rivers in South Africa

An approach is presented for desktop-level environmental flow requirement (EFR) determination that is aligned with the Habitat Flow–Stressor Response (HFSR) method which evolved in South Africa over recent years. The HFSR method integrates hydrological, hydraulic and ecological habitat data, involves ecological and hydraulic specialists and is data-intensive and time-consuming. The revised desktop method integrates hydrological information with estimates of channel hydraulic cross-sectional characteristics to generate habitat-type frequencies under changing flow conditions. This information is used with the expected natural habitat requirements to determine acceptable habitat availability under different levels of ecological protection, which is then used with the hydraulic data to define flow regime characteristics that meet the ecological objectives. The paper describes the model components, discusses the assumptions, data requirements and limitations and presents some example results. The revised desktop approach uses approaches that are aligned with the more complex methods and generates results that are similar.Editor D. Koutsoyiannis; Guest editor M. AcremanCitation Hughes, D.A., Desai, A.Y., Birkhead, A.L., and Louw, D., 2014. A new approach to rapid, desktop-level, environmental flow assessments for rivers in South Africa. Hydrological Sciences Journal, 59 (3–4), 673–687.

Numerical Modeling of Fluid‐Structure Interaction of Blood in a Vein by Simulating Conservation of Mass and Linear Momentum with the Finite Element Method in FEniCS

AbstractIn medicine a fundamental understanding for blood flow in human arteries is significantly important. In socalled hemodynamics engineers all over the world simulate blood flows in healthy and diseased vessels. Scientific findings in this area help to improve cardiovascular assisting devices and to plan surgical operations. The difficulty with such simulations is the interaction of the blood flow with the elastic vessel wall, which deforms due to changing flow conditions. We will present a two‐dimensional model for blood flowing through an arterial vessel and investigate the balance of mass and the balance of linear momentum. We will adjust these balance equations appropriately and define time dependent boundary conditions. The necessary partial differential equations for the fluid‐structure interaction will be solved by using the finite element method in FEniCS [1]. (© 2013 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Variations in the architecture of hydraulic‐jump bar complexes on non‐eroding beds

AbstractSediment accumulation downstream of hydraulic jumps can occur in many settings but the architectures of such deposits are poorly documented. Here, three flume runs were used to examine the influence of sediment grain size and transport rate on the characteristics of hydraulic‐jump unit bars. In one of these runs six hydraulic‐jump unit bars formed a hydraulic‐jump bar complex. In another, the same sediment was supplied more quickly and only two unit bars formed. In the third run with the same sediment supply rate, but different grain size, only one large unit bar formed. All unit bars developed in a similar way but their size and internal architecture differed; they all resulted from a reduction in sediment transport capacity at the transition from supercritical flow to subcritical flow in the hydraulic jump. After initial onset of sedimentation and unit bar formation, generation of subsequent unit bars may be: (i) related to small changes in sediment flux; and (ii) independent of changes in the hydraulic jump. Continued sedimentation caused changes from oscillating to weak hydraulic jumps and hydraulic‐jump unit bars formed in both circumstances. The flow of water and suspended sediment becomes shallower over the lee of the bar complex. This leads to flow acceleration and a return to supercritical flow conditions. In turn, a chain of such features can form and generate a chute and pool bed morphology. There is an inherent upper size limit to a hydraulic‐jump bar complex due to the changing flow conditions over the growing deposit as the water above it becomes shallower. There is also an amplitude minimum for the development of foresets and subsequent unit bar growth. Hydraulic‐jump unit bars have architectures that should be recognizable in the rock record and because their size is constrained by the flow conditions, their identification should be useful for interpreting palaeoenvironment.

Spatio‐temporal habitat selection shifts in brown trout populations under contrasting natural flow regimes

ABSTRACTUnderstanding the links between instream ecology and hydrology has become a critical task in contemporary river research and management. Habitat selection behaviour is a central dimension in applied ecology because it is a primary way that mobile organisms adapt to changing environmental conditions. Here, we analyzed brown trout habitat selection during two consecutive years in rivers presenting contrasting flow conditions to test the following hypotheses: (1) given that adaptation to flow regimes occurs as a response to the interaction between frequency, magnitude and predictability of mortality‐causing events, habitat selection would vary across populations subject to different disturbance regimes; (2) because adaptations are directed towards enduring both intra‐annual and interannual variations in flow, habitat selection would shift across years as a response to changing flow conditions and (3) such responses to yearly flow fluctuations would depend on the historical long‐term hydrologic regime. We found that trout from rivers with highly variable flow and more frequent, longer and stronger extreme flow events were more willing to occupy positions in high‐velocity habitats and showed stronger requirements for velocity refuges, whereas trout inhabiting more stable and benign flow environments selected visually‐covered habitats to minimize biotic interactions. Results also revealed that trout shifted habitat selection patterns across years differing in flow conditions irrespective of river typology, but this shift was markedly stronger in rivers with higher flow variability and extremity. Overall, observed ecological patterns have strong implications for predicting the consequences of flow alteration for species adapted to particular flow regimes. Copyright © 2013 John Wiley & Sons, Ltd.

Stage–discharge prediction in straight compound channels using 3D numerical models

An improved approach for applying three-dimensional (3D) computational fluid dynamics (CFD) models to estimate uniform flow stage–discharge relationships and velocity distributions in straight compound channels is presented. Commonly used modelling approaches tend to be over-specified. For a given flow and water level, desired results are obtained through calibration of resistance coefficients that can be artificially high and vary with changing flow conditions. Furthermore, the momentum interaction at the main channel–floodplain interface is sometimes ignored or is accounted for using a constant eddy viscosity. This potentially results in an overestimation of conveyance capacity in compound channels. The proposed approach represents an advance on these methods and uses a 3D CFD model with k–ε turbulence closure in a predictive capacity where a flow together with physically realistic resistance coefficients are specified. Downstream water levels are then iteratively adjusted until uniform flow conditions are established in the channel. The approach is validated against benchmark experimental data obtained from the large-scale UK Flood Channel Facility and is compared with predictions from divided channel methods.

Oxygen Is Nonbeneficial for Most Patients Who Are Near Death

ContextClinicians prescribe and administer oxygen in response to reports of dyspnea, in the face of dropping oxygen saturation, as a “routine” comfort intervention, or to support anxious family members. Oxygen may produce nasal irritation and increase the cost of care. ObjectivesTo determine the benefit of administering oxygen to patients who are near death. MethodsA double-blind, repeated-measure observation with the patient as his/her own control was conducted. The Respiratory Distress Observation Scale© measured presence and intensity of distress at baseline and at every gas or flow change. Medical air, oxygen, and no flow were randomly alternated every 10 minutes via nasal cannula with patients who were near death, at risk for respiratory distress, with no distress at the baseline of testing. Each patient had two encounters under each condition, yielding six encounters per patient. ResultsPatients were 66% female, 34% white, and 66% African American, and ages 56–97 years. Patients had heart failure (25%), chronic obstructive pulmonary disease (34%), pneumonia (41%), or lung cancer (9%). Most (91%) patients tolerated the protocol with no change in respiratory comfort. Three patients (9%) displayed distress and were restored to baseline oxygen; one patient died during the protocol while displaying no distress. Repeated-measure analysis of variance revealed no differences in the Respiratory Distress Observation Scale under changing gas and flow conditions. ConclusionThe routine application of oxygen to patients who are near death is not supported. The n-of-1 trial of oxygen in clinical practice is appropriate in the face of hypoxemic respiratory distress.

Modeling the impacts of climate change on nitrogen retention in a 4th order stream

Climate induced changes of temperature, discharge and nitrogen concentration may change natural denitrification processes in river systems. Until now seasonal variation of N-retention by denitrification under different climate scenarios and the impact of river morphology on denitrification have not been thoroughly investigated. In this study climate scenarios (dry, medium and wet) have been used to characterize changing climatic and flow conditions for the period 2050–2054 in the 4th order stream Weise Elster, Germany. Present and future periods of nitrogen turnover were simulated with the WASP5 river water quality model. Results revealed that, for a dry climate scenario, the mean denitrification rate was 71% higher in summer (low flow period between 2050 and 2054) and 51% higher in winter (high flow period) compared to the reference period. For the medium and wet climate scenarios, denitrification was slightly higher in summer (3% and 4%) and lower in winter (9% and 3% for medium and wet scenarios, respectively). Additionally, the variability of denitrification rates was higher in summer compared to winter conditions. For a natural river section, denitrification was a factor of 1.22 higher than for a canalized river reach. Besides, weirs along the river decrease the denitrification rate by 16% in July for dry scenario conditions. In the 42 km study reach, N-retention through denitrification amounted to 5.1% of the upper boundary N load during summer low flow conditions in the reference period. For the future dry climate scenario this value increased up to 10.2% and for the medium climate scenario up to 5.4%. In our case study the investigated climate scenarios showed that future discharge changes may have a larger impact on denitrification rates than future temperature changes. Overall results of the study revealed the significance of climate change in regulating the magnitude, seasonal pattern and variability of nitrogen retention. The results provide guidance for managing nitrogen related environmental problems for present and future climate conditions.