Exchange Velocity Research Articles

Inplane vibration analysis of rotating beams with elastic restraints

In this paper, the dynamic model of a rotating beam with elastic restraints is constructed by Euler–Bernoulli and Timoshenko beam theories. The study includes the Coriolis effect and the additional effect of dynamic centrifugal stiffness. A modified Fourier series method is used for expanding displacement field functions in the analysis. The standard eigenvalue equation is established and written in terms of the state space for dynamic analysis. Then, the natural frequencies of the rotating beam are calculated. The convergence analysis and the comparison of the results are carried out to verify the applicability of the method. The effects of the rotational velocity and boundary stiffness on the natural frequencies are analysed. The results show that the influence of the linear spring on the frequency is more evident than that of the torsional spring. The boundary can be considered to be elastic restraints when the boundary stiffness is within a certain interval. With the increase in the rotational velocity, the phenomenon of modal exchange appears. The elastic restraints cause the rotational velocity of the modal exchange to lag. Finally, the reason that the fundamental frequency goes to zero with increasing rotational velocity is explained. The Coriolis effect and elastic restraints reduce the critical velocity.

Impact of building façade geometrical details on pollutant dispersion in street canyons

The present study investigates the impact of building façade geometrical details on the pollutant transport mechanism in long street canyons. Large-eddy simulations (LES), extensively validated with experiments, are performed for four cases: (i) street canyon without façade balconies, (ii) street canyon with balconies at both windward and leeward façades, (ii) street canyon with balconies only at the windward façade and (iv) street canyon with balconies only at the leeward façade. The results show that the building balconies can strongly affect the wind flow field and pollutant dispersion in long street canyons. The most significant impact is observed for the two street canyon cases with balconies at the windward façade, which strongly obstruct the airflow from penetrating deep into the bottom of the canyon. The presence of balconies only at the windward façade and at both façades can increase the area-weighted mean pollutant concentration in the vertical center plane inside the canyon by 80% and 106%, respectively, and reduce the mean pollutant exchange velocity (Ue) by 46% and 54%, respectively. The analysis of the vertical mean convective and turbulent mass fluxes indicates that the presence of balconies mainly decreases the convective contribution to Ue, while the impact on the turbulent contribution is smaller.

Improving Predictions of Stream CO2 Concentrations and Fluxes Using a Stream Network Model: A Case Study in the East River Watershed, CO, USA

AbstractInland waters are an important component of the global carbon budget. However, our ability to predict carbon fluxes from stream systems remains uncertain, as pCO2 varies within streams at scales of 1–100 m. This makes direct monitoring of large‐scale CO2 fluxes impractical. We incorporate CO2 input and output fluxes into a stream network advection‐reaction model, representing the first process‐based representation of stream CO2 dynamics at watershed scales. This model includes groundwater (GW) CO2 inputs, water column (WC), benthic hyporheic zone (BHZ) respiration, downstream advection, and atmospheric exchange. We evaluate this model against existing statistical methods including upscaling and multiple linear regressions through comparisons to high‐resolution stream pCO2 data collected across the East River Watershed in the Colorado Rocky Mountains (USA). The stream network model accurately captures GW, evasion, and respiration‐driven pCO2 variability and significantly outperforms multiple linear regressions for predicting pCO2. Further, the model provides estimates of CO2 contributions from internal versus external sources suggesting that streams transition from GW‐ to BHZ‐dominated sources between 3rd and 4th Strahler orders, with GW, BHZ, and WC accounting for 49.3%, 50.6%, and 0.1% of CO2 fluxes from the watershed, respectively. Lastly, stream network model atmospheric CO2 fluxes are 4‐12x times smaller than upscaling technique predictions, largely due to relationships between stream pCO2 and gas exchange velocities. Taken together, this stream network model improves our ability to predict stream CO2 dynamics and efflux. Furthermore, future applications to regional and global scales may result in a significant downward revision of global flux estimates.

Cross‐Validating Hydromechanical Models and Tracer Test Assessments of Hyporheic Exchange Flow in Streams With Different Hydromorphological Characteristics

AbstractHyporheic exchange flow (HEF) can generally be quantified through two different approaches. The first approach, which is deductive, entails physically based models, supported with relevant observations. The second approach includes inductive assessments of stream tracer tests using solute transport models, which provide a useful mathematical framework that allows for upscaling of results, but included parameters often have a vague physical base, which limits the possibilities of generalizing results using independent hydromorphological observations. To better understand how the physical basis of HEF‐quantifying parameters relates to stream hydromorphology at different spatial scales, we cross‐validated the results from (a) tracer test assessments using a 1D solute transport model that accounts for HEF and (b) an independent hydromechanical model that represents HEF driven by multiscale pressure gradients along the streambed interface. To parameterize the models, topographical surveys, tracer tests, and streambed hydraulic conductivity measurements were performed in 10 stream reaches, differing in terms of geomorphology, slope, and discharge. The results show that the models were cross‐validated in terms of the average exchange velocity, providing a plausible physical explanation for this parameter in small alluvial streams with low discharges, shallow depth, and moderate slopes. However, the hydromechanical model generally resulted in wider residence time distributions and occasionally higher average residence times compared to the tracer test assessments. From the cross‐validated multiscale hydromechanical model, we learned that water surface profile variations were the main drivers of HEF in all investigated streams and that spatial scales between 20 cm and 5 m dominated the estimated HEF velocity.

Simulations of the Surf Zone Eddy Field and Cross‐Shore Exchange on a Nonidealized Bathymetry

AbstractX‐band radar observations from the 2017 Inner Shelf Dynamics Experiment (ISDE) in central California show multiple persistent and pulsatory rip currents on a relatively straight coastline with alongshore‐varying bathymetry. Although past studies have assessed the characteristics of transient rip currents on alongshore uniform beaches, the relative balance of transient versus steady rip current behavior on nonuniform beaches in realistic wave conditions remains poorly understood. Here, a phase‐resolving Boussinesq‐type wave model (funwaveC) is used to assess the role of alongshore‐varying bathymetry and incident conditions in controlling mean and transient surf zone vorticity and velocity fields and their effect on surf zone exchange. The model simulates wave conditions chosen from the ISDE observations and utilizes both an alongshore‐varying bathymetry estimated from the ISDE radar observations and a uniform bathymetry. Results show that the variable bathymetry significantly increases the alongshore‐ and time‐averaged kinetic energy but that this increase is primarily due to the increase in the standing component resulting from mean circulation patterns, with only small changes in the transient component. A variable bathymetry also increases the spectral energy of surf zone vorticity and time‐averaged vorticity forcing at large spatial scales (>100 m). Wave directional spreading has a large impact on the alongshore‐ and time‐averaged enstrophy and on the spectral energy of surf zone vorticity and vorticity forcing at smaller spatial scales (<100 m). In the presence of a directionally spread wavefield, an alongshore‐varying bathymetry slightly increases the total exchange velocity but has little effect on its transient component.

Spatial perspectives on niche empowerment: an agent-based model

ABSTRACT This paper aims at increasing our understanding of the role of space dimensions in niche evolution dynamics. To accomplish this goal, an agent-based model is developed, locating agents in geographical space. The model is then used to show how local niches built on geographical proximity perform differently from global niches built on relative proximity, in terms of the timing of niche creation, the velocity of niche maturation, average profit, environmental uncertainty and knowledge exchange. It concludes that the spatial dimension of niches should be considered in the development of policies to promote sustainable transitions.

Evaluating stream CO<sub>2</sub> outgassing via drifting and anchored flux chambers in a controlled flume experiment

Abstract. Carbon dioxide (CO2) emissions from running waters represent a key component of the global carbon cycle. However, quantifying CO2 fluxes across air–water boundaries remains challenging due to practical difficulties in the estimation of reach-scale standardized gas exchange velocities (k600) and water equilibrium concentrations. Whereas craft-made floating chambers supplied by internal CO2 sensors represent a promising technique to estimate CO2 fluxes from rivers, the existing literature lacks rigorous comparisons among differently designed chambers and deployment techniques. Moreover, as of now the uncertainty of k600 estimates from chamber data has not been evaluated. Here, these issues were addressed by analysing the results of a flume experiment carried out in the Summer of 2019 in the Lunzer:::Rinnen – Experimental Facility (Austria). During the experiment, 100 runs were performed using two different chamber designs (namely, a standard chamber and a flexible foil chamber with an external floating system and a flexible sealing) and two different deployment modes (drifting and anchored). The runs were performed using various combinations of discharge and channel slope, leading to variable turbulent kinetic energy dissipation rates (1.5×10-3<ε<1×10-1 m2 s−3). Estimates of gas exchange velocities were in line with the existing literature (4<k600<32 m2 s−3), with a general increase in k600 for larger turbulent kinetic energy dissipation rates. The flexible foil chamber gave consistent k600 patterns in response to changes in the slope and/or the flow rate. Moreover, acoustic Doppler velocimeter measurements indicated a limited increase in the turbulence induced by the flexible foil chamber on the flow field (22 % increase in ε, leading to a theoretical 5 % increase in k600). The uncertainty in the estimate of gas exchange velocities was then estimated using a generalized likelihood uncertainty estimation (GLUE) procedure. Overall, uncertainty in k600 was moderate to high, with enhanced uncertainty in high-energy set-ups. For the anchored mode, the standard deviations of k600 were between 1.6 and 8.2 m d−1, whereas significantly higher values were obtained in drifting mode. Interestingly, for the standard chamber the uncertainty was larger (+ 20 %) as compared to the flexible foil chamber. Our study suggests that a flexible foil design and the anchored deployment might be useful techniques to enhance the robustness and the accuracy of CO2 measurements in low-order streams. Furthermore, the study demonstrates the value of analytical and numerical tools in the identification of accurate estimations for gas exchange velocities. These findings have important implications for improving estimates of greenhouse gas emissions and reaeration rates in running waters.

Headwater stream ecosystem: an important source of greenhouse gases to the atmosphere

Although an increasing number of reports have revealed that rivers are important sources of greenhouse gases (GHGs), the magnitude and underlying mechanism of riverine GHG emissions are still poorly understood. The global extent of the headwater stream ecosystem may represent one of the important GHG emitters. A global database of GHG measurements from 595 rivers, indicated that the concentrations of riverine GHGs continually decrease as the stream order increases. Further analysis suggested that high GHG emissions from headwater streams (Strahler stream orders of 1 to 3) could be related to the low levels of dissolved oxygen, massive terrestrially derived carbon/nitrogen inputs and large gas exchange velocity. Through a combination of the predicted river surface areas and gas transfer velocities, we estimated that globally, the rivers emit approximately 6.6 (5.5–7.8) Pg CO2, 29.5 (19.6–37.3) Tg CH4, and 0.6 (0.2–0.9) Tg N2O per year, and totally emit 7.6 (6.1–9.1) CO2 equivalent into atmosphere per year. The headwater streams contribute 72.3%, 75.5%, and 77.2% of the global riverine CO2, CH4, and N2O emissions, respectively. This study presents a systematic estimation of GHG emissions from river ecosystems worldwide and highlights the dominant role played by headwater streams in GHG evasions from global rivers.

Analytical and numerical study on transient urban street air warming induced by anthropogenic heat emission

Anthropogenic heat emission is an important factor for street air warming. In this study, the transient street air warming process caused by anthropogenic heat is investigated by both the analytical model and Large-Eddy Simulation (LES). A new analytical model is developed based on the empirical exchange velocity modelling and heat exchange rate estimation. In the model, the buoyancy effect is included by introducing a buoyancy coefficient. With the new analytical model, both transient time and steady-state street air temperature increment (Θc) can be easily estimated in urban areas. This new model is calibrated and validated by cross-comparing with LES results. A good agreement is found between the results after including the buoyancy coefficient into the analytical model. Modelling results indicate that Θccould be limited because the buoyancy effect increases the exchange velocity between the urban canopy layer and overlying atmosphere. Furthermore, the buoyancy is also important to the transient air exchange process. A transformation of the urban canopy layer flow structure is found in the transient period, suggesting a large scale turbulence structure induced by anthropogenic heat. Lastly, the air temperature increment in the residential areas of Singapore is estimated with GIS (Geography Information System) tool to show the application of the analytical model in practice.

Floods increase carbon dioxide and methane fluxes in agricultural streams

Abstract Changes in climate are causing floods to occur more often and more intensely in many parts of the world, including agricultural landscapes of southern Wisconsin (U.S.A.). How flooding and greater flood frequency affect stream carbon dioxide (CO2) and methane (CH4) fluxes and concentrations is not obvious. Thus, we asked how diffusive fluxes of CO2 and CH4 varied over time, particularly in response to floods, in agricultural streams, and what were likely causes for observed flood responses. We measured concentrations and diffusive fluxes of CO2 and CH4 at 10 stream sites in mixed agricultural and suburban catchments in southern Wisconsin (U.S.A.) during the growing season (March–November) in a year that experienced multiple floods. Habitat, hydrologic, and water chemistry attributes were also quantified to determine likely drivers of changes in gas concentrations and fluxes. Habitat and water chemistry, as well as CO2 and CH4 concentrations and fluxes were temporally erratic and lacked any seasonality. Carbon dioxide and CH4 concentrations and fluxes were higher during floods along with increased water velocity, turbidity, and dissolved organic carbon and decreases in dissolved oxygen, soft sediment depth, and macrophyte cover. Increased gas concentrations and fluxes were probably due to flushing of gases from soils, respiration of organic matter in the channel, and increased gas exchange velocities during floods. Flooding alleviated both supply and transfer limits on CO2 and CH4 emissions in these agricultural streams, and frequent and prolonged flooding during the growing season led to sustained high emissions from these streams. We hypothesise that such persistent increases in emissions during floods may be a common response to high precipitation periods for many agricultural streams.

Surface Wetness as an Unexpected Control on Forest Exchange of Volatile Organic Acids

AbstractWe report bidirectional exchange of volatile acids, including isocyanic and alkanoic acids, over a pine forest across multiple seasons. The exchange velocity of these acids is well correlated with dew point depression, suggesting an equilibrium‐driven continuum of flux. Wetness on forest surfaces impacts the vertical exchange of gases, and we suggest that water films and droplets drive equilibrium partitioning, with acids being solvated in surface wetness and released through evaporation. Despite their volatility, these acids partition into neutral‐to‐alkaline aqueous films, consistent with reported dew pH. This relationship between exchange velocity and dew point depression holds for a wetter mixed forest, but not a very dry orchard. Dew point depression is an excellent indicator of acid fluxes so long as the canopy is occasionally wetted.

Mechanisms of Mid- to Outer-Shelf Transport of Shoreline-Released Tracers

AbstractTransport of shoreline-released tracer from the surfzone across the shelf can be affected by a variety of physical processes from wind-driven to submesoscale, with implications for shoreline contaminant dilution and larval dispersion. Here, a high-resolution wave–current coupled model that resolves the surfzone and receives realistic oceanic and atmospheric forcing is used to simulate dye representing shoreline-released untreated wastewater in the San Diego–Tijuana region. Surfzone and shelf alongshore dye transports are primarily driven by obliquely incident wave breaking and alongshore pressure gradients, respectively. At the midshelf to outer-shelf (MS–OS) boundary (25-m depth), defined as a mean streamline, along-boundary density gradients are persistent, dye is surface enhanced and time and alongshelf patchy. Using baroclinic and along-boundary perturbation dye transports, two cross-shore dye exchange velocities are estimated and related to physical processes. Barotropic and baroclinic tides cannot explain the modeled cross-shore transport. The baroclinic exchange velocity is consistent with the wind-driven Ekman transport. The perturbation exchange velocity is elevated for alongshore dye and cross-shore velocity length scales < 1 km (within the submesoscale) and stronger alongshore density gradient ∂ρ/∂y variability, indicating that alongfront geostrophic flows induce offshore transport. This elevated ∂ρ/∂y is linked to convergent northward surface along-shelf currents (likely due to regional bathymetry), suggesting deformation frontogenesis. Both surfzone and shelf processes influence offshore transport of shoreline-released tracers with key parameters of surfzone and shelf alongcoast currents and alongshelf winds.

Consensus in networks of uncertain euler-lagrange agents using adaptive gravity compensation

The leaderless and the leader-follower consensus are the most basic synchronization behaviors for multi-agent systems. For networks of Euler-Lagrange (EL) agents different controllers have been proposed to achieve consensus, requiring in all cases, either the cancellation or the estimation of the gravity forces. While, in the first case, it is shown that a simple Proportional plus damping (P+d) scheme with exact gravity cancellation can achieve consensus, in the latter case, it is necessary to estimate, not just the gravity forces, but the parameters of the whole dynamics. This requires the computation of a complicated regressor matrix, that grows in complexity as the degrees-of-freedom of the EL-agents increase. To simplify the controller implementation we propose in this paper a simple P+d scheme with only adaptive gravity compensation. In particular, two adaptive controllers that solve both consensus problems by only estimating the gravitational term of the agents and hence without requiring the complete regressor matrix are reported. The first controller is a simple P+d scheme that does not require to exchange velocity information between the agents but requires centralized information. The second controller is a Proportional-Derivative plus damping (PD+d) scheme that is fully decentralized but requires exchanges of speed information between the agents. Simulation results demonstrate the performance of the proposed controllers.

Tracer Exchange Across the Stratified Inner‐Shelf Driven by Transient Rip‐Currents and Diurnal Surface Heat Fluxes

AbstractExchange across the surf‐zone and inner‐shelf affects coastal water quality and larval recruitment. Surf‐zone generated transient rip‐currents (TRC) exchange shoreline released tracers onto and across a stratified inner‐shelf. Surface heat fluxes (SHF) modify inner‐shelf stratification and surf‐zone temperature, relative to the inner‐shelf, inducing nearshore thermally driven exchange. The coupled effect of TRC and diurnal SHF forcing on cross‐shore exchange is evaluated using idealized model surf‐zone tracer releases with TRC‐only, SHF‐only, and combined SHF+TRC forcing. For conditions representing Fall in Southern California, the TRC mechanism dominates cross‐shore exchange, relative to SHF, to 12LSZ offshore (LSZ = 100 m is the surf‐zone width). Tracer and velocity derived estimates of exchange velocity indicate that the TRC cross–inner‐shelf exchange mechanism is due to an alongshore mean baroclinic flow setup by TRC vertical mixing of inner‐shelf stratification.

Maximal lactate accumulation rate and post-exercise lactate kinetics in handcycling and cycling

The aim of this study was to assess lactate kinetics, maximal lactate accumulation rate (⩒Lamax) and peak power output (POmax) in a 15-s all-out exercise in handcycling (HC) and cycling (C) in terms of (1) reliability, (2) differences and (3) correlations between HC and C. Eighteen female and male competitive triathletes performed two trials (separated by one week) of a 15-s all-out sprint test in HC and C. Tests were performed in a recumbent racing handcycle and on the participants’ own road bike that were attached to an ergometer. Reliability was assessed using intraclass correlation coefficient (ICC). POmax and ⩒Lamax demonstrated high reliability in HC (ICC = 0.972, ICC = 0.828) and C (ICC = 0.937, ICC = 0.872). POmax (d = −2.54, P < 0.0005) and ⩒Lamax (d = −1.62, P < 0.0005) were lower in HC compared to C. POmax and ⩒Lamax correlated in HC (r = 0.729, P = 0.001) and C (r = 0.710, P = 0.001). There was no significant correlation between HC and C in POmax (r = 0.442, P = 0.066) and ⩒Lamax (r = 0.455, P = 0.058). Whereas the exchange velocity of lactate (k1) was similar in HC and C, the removal velocity (k2) was significantly higher in HC. ⩒Lamax and POmax during sprint exercise are highly reliable and demonstrate a correlation in both HC and C. However, since ⩒Lamax and POmax are significantly higher in C and not correlated between HC and C, ⩒Lamax and POmax seem to be extremity-specific.

Mitigating intensity of urban heat island by better understanding on urban morphology and anthropogenic heat dispersion

Anthropogenic heat is one of the key factors that causes intensive Urban Heat Island (UHI) due to its direct impact on ambient temperature in urban areas. Stagnated airflow due to closely packed tall buildings causes weak dilution and removal of anthropogenic heat. Consequently, research is critically needed to investigate the effect of urban morphology on anthropogenic heat dispersion and provide effective planning strategies to reduce UHI intensity, especially at the extreme scenarios, such as with very low wind speed and high heat emission. This study provides scientific understanding and develops a Geographic Information System (GIS)-based modelling tool to support decision-making in urban planning practice. We start from a computational parametric study at the neighbourhood scale to investigate the impact of urban morphology on heat dispersion. Site coverage ratio and frontal area density are two urban morphological parameters. Ten parametric cases with two heat emission scenarios are designed to study representative urban areas. Furthermore, based on the energy conservation within urban canopy layers, we develop a semi-empirical model to estimate spatially-averaged in-canopy air temperature increment, in which the exchange velocity between street canyons and overlaying atmosphere is estimated by the Bentham and Britter model. The performance of new model is validated by cross-comparing with Computational Fluid Dynamics (CFD) results from the parametric study. By applying this new model, the impact of anthropogenic heat on air temperature is mapped in residential areas of Singapore for both long-term annually averaged and short-term extreme low wind speed to improve urban climate sustainability and resilience.

Influence of Abyssal Mixing on the Multilayer Circulation in the South China Sea

AbstractBy parameterizing the abyssal mixing as the exchange velocity (entrainment/detrainment) between the middle and deep layers of the South China Sea (SCS), its effects on the multilayer circulation are examined. Results indicate that the cyclonic circulation in the deep SCS appears only when the mixing induces an entrainment of at least 0.72 Sv (1 Sv ≡ 106 m3 s−1) from the deep to the middle layer, which is equivalent to a diapycnal diffusivity of 0.65 × 10−3 m2 s−1 or a net input rate of gravitational potential energy (GPE) of 6.89 GW, respectively. It is also found that tidal mixing in the SCS is stronger than the threshold for the generation of the cyclonic abyssal circulation, but the pattern and evolution of the deep circulation and meridional overturning circulation also depend on the spatiotemporal variability of the mixing. Moreover, the abyssal mixing is able to intensify the anticyclonic circulation in the middle layer but weaken the cyclonic circulation in the upper layer. Vorticity analysis suggests that the upward net flux induced by the abyssal mixing leads to vortex stretching (squeezing) and modulates the pressure gradient by redistributing the layer thickness, hence affects the pattern and strength of the circulation in the middle (deep) layer of the SCS, respectively. The depth-integrated effect of the thickness variation can modulate the pressure gradient across all layers and hence influence the upper-layer circulation.

Carbon dioxide efflux and ecosystem metabolism of small forest lakes

Small lakes are numerous in the landscape and closely connected to the terrestrial environment, which strongly influences the system scale carbon cycling. However, despite their importance in large-scale carbon budgets, small lakes remain understudied compared to large lakes. We investigated oxygen and carbon dynamics in four small Danish forest lakes over a year. Continuous pH measurements were used to establish a high-frequency time series of CO2 partial pressure and 1169 direct measurements of air–water CO2 flux were performed using floating chambers. Net ecosystem oxygen production (NEP) was derived from free-water oxygen measurements in order to quantify the contribution of in-lake aerobic metabolism to the air–water CO2 flux. We found that the forest lakes, on average, were tenfold CO2 supersaturated. The two most intensively studied lakes had mean CO2 effluxes of 36.3 mol m−2 year−1. The CO2 effluxes exceeded NEP during all months implying that the CO2 generated by aerobic respiration alone could not account for the observed CO2 efflux. The observed discrepancy is likely promoted by a hydrologic CO2 input and/or anaerobic sediment processes generating CO2 without a concurrent consumption of O2. A broad-scale national analysis showed that the CO2 efflux increased as lake size decreased despite lower gas exchange velocity. Small lakes also showed higher excess CO2 efflux compared to O2 influx. Overall, small lakes and forest lakes in particular have high CO2 effluxes and high CO2 supersaturation. However, the contribution of hydrological inputs and anaerobic sediment processes to the net CO2 efflux remains elusive.

Gas exchange in streams and rivers

AbstractGas exchange across the air–water boundary of streams and rivers is a globally large biogeochemical flux. Gas exchange depends on the solubility of the gas of interest, the gas concentrations of the air and water, and the gas exchange velocity (k), usually normalized to a Schmidt number of 600, referred to as k600. Gas exchange velocity is of intense research interest because it is problematic to estimate, is highly spatially variable, and has high prediction error. Theory dictates that molecular diffusivity and turbulence drives variation in k600 in flowing waters. We measure k600 via several methods from direct measures, gas tracer experiments, to modeling of diel changes in dissolved gas concentrations. Many estimates of k600 show that surface turbulence explains variation in k600 leading to predictive models based upon geomorphic and hydraulic variables. These variables include stream channel slope and stream flow velocity, the product of which, is proportional to the energy dissipation rate in streams and rivers. These empirical models provide understanding of the controls on k600, yet high residual variation in k600 show that these simple models are insufficient for predicting individual locations. The most appropriate method to estimate gas exchange depends on the scientific question along with the characteristics of the study sites. We provide a decision tree for selecting the best method to estimate k600 for individual river reaches to scaling to river networks.This article is categorized under: Water and Life &gt; Nature of Freshwater Ecosystems Science of Water &gt; Water Quality Water and Life &gt; Methods

Listening to air–water gas exchange in running waters

AbstractAir–water gas exchange velocities (k) are critical components of many biogeochemical and ecological process studies in aquatic systems. However, their high spatiotemporal variability is difficult to capture with traditional methods, especially in turbulent flow. Here, we investigate the potential of sound spectral analysis to infer k in running waters, based on the rationale that both turbulence and entrained bubbles drive gas exchange and cause a characteristic sound. We explored the relationship between k and sound spectral properties using laboratory experiments and field observations under a wide range of turbulence and bubble conditions. We estimated k using flux chamber measurements of CO2 exchange and recorded sound above and below the water surface by microphones and hydrophones, respectively. We found a strong influence of turbulence and bubbles on sound pressure levels (SPLs) at octave bands of 31.5 Hz and 1000 Hz, respectively. The difference in SPLs at these bands and background noise bands showed a linear correlation with k both in the laboratory (R2 = 0.93–0.99) and in the field (median R2 = 0.42–0.90). Underwater sound indices outperformed aerial sound indices in general, and indices based on hydraulic parameters in particular, in turbulent and bubbly surface flow. The results highlight the unique potential of acoustic techniques to predict k, isolate mechanisms, and improve the spatiotemporal coverage of k estimates in bubbly flow.