Past Field Studies Research Articles

A synthesized field survey database of vegetation and active-layer properties for the Alaskan tundra (1972–2020)

Abstract. Studies in recent decades have shown strong evidence of physical and biological changes in the Arctic tundra, largely in response to rapid rates of warming. Given the important implications of these changes for ecosystem services, hydrology, surface energy balance, carbon budgets, and climate feedbacks, research on the trends and patterns of these changes is becoming increasingly important and can help better constrain estimates of local, regional, and global impacts as well as inform mitigation and adaptation strategies. Despite this great need, scientific understanding of tundra ecology and change remains limited, largely due to the inaccessibility of this region and less intensive studies compared to other terrestrial biomes. A synthesis of existing datasets from past field studies can make field data more accessible and open up possibilities for collaborative research as well as for investigating and informing future studies. Here, we synthesize field datasets of vegetation and active-layer properties from the Alaskan tundra, one of the most well-studied tundra regions. Given the potentially increasing intensive fire regimes in the tundra, fire history and severity attributes have been added to data points where available. The resulting database is a resource that future investigators can employ to analyze spatial and temporal patterns in soil, vegetation, and fire disturbance-related environmental variables across the Alaskan tundra. This database, titled the Synthesized Alaskan Tundra Field Database (SATFiD), can be accessed at the Oak Ridge National Laboratory Distributed Active Archive Center (ORNL DAAC) for Biogeochemical Dynamics (Chen et al., 2023: https://doi.org/10.3334/ORNLDAAC/2177).

Sampling data of macro-invertebrates collected in grasslands under restoration succession in a lowland stream-valley system.

Publication of data from past field studies on invertebrate populations is of high importance, as there is much added value for them to be used as baselines to study spatiotemporal population and community dynamics in these groups. Therefore, a dataset consisting of occurrence data on epigaeic invertebrates collected in 1996 was standardised into the Darwin core format and cross-checked in order to make it publicly available following FAIR data principles. With publication, it can contribute to the biodiversity assessment of terrestrial invertebrates, thereby improving the availability and accessibility of much-needed historical datasets on macro-invertebrates.Here, we present sampling event data on invertebrates from four grasslands taken out of agricultural production over the span of several decades, effectively displaying a chronosequence on the effects of agricultural extensification. The data were collected by means of a standardised sampling design using pyramid traps, pitfall traps and soil samples. The raw data presented in this data paper have not been published before. They consist of 20,000+ records of nearly 70,000 specimens from 121 taxonomic groups. The data were collected using a standardised field study set-up and specimens were identified by taxonomic specialists. Most groups were identified up to family level, with eight groups identified up to species level. The occurrence data are complemented by information on plant composition, meteorological data and soil physical characteristics. The dataset has been registered in the Global Biodiversity Information Facility (GBIF): http://doi.org/10.15468/7n499e.

Plant phenology drives seasonal changes in shear stress partitioning in a semi-arid rangeland

Accurate representation of surface roughness in predictive models of aeolian sediment transport and dust emission is required for model accuracy and for models to inform wind erosion management. While past wind tunnel and field studies have examined roughness effects on drag partitioning, the spatial and temporal variability of surface shear velocity and the shear stress ratio remain poorly described. Here, we use a four-month dataset of total shear velocity (u*) and soil surface shear velocity (us*) measurements to examine the spatiotemporal variability of the shear stress ratio (R) before, during, and after vegetation green-up at a honey mesquite (Prosopis glandulosa Torr.) shrub-invaded grassland in the Chihuahuan Desert, New Mexico, USA. Results show that vegetation green-up, the emergence of leaves, led to increased drag and surface aerodynamic sheltering and a reduction in us* and R magnitude and variability. We found that us* decreased from 20% to 5% of u* as the vegetation form drag and its sheltering effect increased. Similarly, the spatiotemporal variability of R was found to be linked directly to plant phenological phases. We conclude that drag partition schemes should incorporate seasonal vegetation change, via dynamic drag coefficients and/or R, to accurately predict the timing and magnitude of seasonal aeolian sediment fluxes. The drag partition response to mesquite phenological phases also provided insight to potential mesquite herbicide treatment effects which, if successful, could increase wind erosivity and the onsite and downwind impacts of wind erosion unless protection by herbaceous plants is maintained.

Energy recovery mechanism of air injection in higher methane cut reservoir

According to new research, methane emissions contribute 25 percent more to global warming than previously assumed. Methane is a crucial precursor gas of tropospheric ozone, a dangerous air pollutant. Methane emissions are responsible for almost half of the reported growth in tropospheric ozone levels on a global scale. But it plays a vital role as an energy source, therefore, the study of methane cut reservoirs is essential. The injection of air can extract additional energy in high methane-cut reservoirs. The analysis is made to compute the theoretical potential of low air flooding in high methane-cut reservoirs. Computational Fluid Dynamics (CFD) is used to study the impact of injecting air on methane. The presence of carbon dioxide and oxygen when injected into the methane cut reservoir produces high-pressure air injection that may cause significant safety damage such as the potential for corrosion or explosion. Past field studies and reported solutions indicate that there are no insurmountable problems in the execution of high-pressure air injection. The results are harmful and need to take useful safety precautions for the engineers who are interested in experiments. The users of CFD use mathematical laws and models that exactly represent the phenomenon they are dealing with. The pressure induced by the air injection is simulated with the help of the Finite Volume Method (FVM). The velocity field near the outlet arises as higher pressure of air from the inlet is observed, whereas, the pressure near the outlet declines. The findings of this study can help for better understanding of outflow of methane from methane reservoirs.

Bayesian inversion of laboratory experiments of transport through limestone fractures

In this study, a novel experimental setup is proposed for which a column filled with glass beads and parallelepiped-shaped limestone beams is used to reconstruct a multiple fracture limestone media. The proposed setup produces asymmetric breakthrough curves (BTCs) that are consistent with the shape expected from the past field and lab-scale studies. Three transport experiments have been conducted under fast, medium, and slow flow velocity conditions.The research focuses on parameter and state estimation using Bayesian inference via Markov Chain Monte Carlo (MCMC) sampler, investigating the degree to which three models of transport through fractured media can reproduce the experimental results under the three flow conditions. The first transport model, named ADE, is based on the equivalent porous medium (EPM) approach and corresponds to the linear advection dispersion equation (ADE). The second model, named FOMIM (first-order mobile immobile), is based on the mobile/immobile approach and uses the dual porosity model with a linear first-order transfer between mobile and immobile regions. The third model, named NLMIM (non-linear mobile-immobile), uses a nonlinear transfer function between these two regions.The results of the three models show that almost all the unknown model input parameters can be well-estimated with narrow confidence intervals using the MCMC method. With respect to state estimation, the ADE model fails to reproduce correctly the tail of the BTCs observed under slow and medium flow conditions. The FOMIM model improves the tailing of the BTCs, but significant discrepancies remain between simulated and measured concentrations. The NLMIM model with velocity-dependent parameters is the only model that captures BTCs under all three conditions of slow, medium, and fast flow velocities.

Effects of Gehl’s urban design guidelines on walkability: A virtual reality experiment in Singaporean public housing estates

Walkability has become an important theme of urban design research and practice. Evidence suggests that environmental attractiveness can have a significant impact on the amount of walking activities that take place, but relatively little research exists on which environmental features linked to attractiveness increase walkability. Using a virtual reality experiment, the present study examined the effects on walkability of three key features, as defined by Jan Gehl, an influential urban planning practitioner and theorist: liveliness, high-quality façades and low buildings. A virtual reality simulation allowed isolating the effects of these features, while avoiding confounding factors, such as the presence of shops, which has been difficult to do in past field studies. Our study confirmed that the combination of features recommended by Gehl promoted walking activity in the study’s context. Further exploratory analyses suggested that improved façade quality was positively linked to walking activity, and that building height and liveliness had negligible effects. Our findings contribute to the existing understanding of walkability, which may benefit urban planning practice and models of walkability. Further research is necessary to confirm our results regarding the effects of specific features on walking activity in different contexts.

Modeling Biomass Burning Organic Aerosol Atmospheric Evolution and Chemical Aging

The changes in the concentration and composition of biomass-burning organic aerosol (OA) downwind of a major wildfire are simulated using the one-dimensional Lagrangian chemical transport model PMCAMx-Trj. A base case scenario is developed based on realistic fire-plume conditions and a series of sensitivity tests are performed to quantify the effects of different conditions and processes. Temperature, oxidant concentration and dilution rate all affect the evolution of biomass burning OA after its emission. The most important process though is the multi-stage oxidation of both the originally emitted organic vapors (volatile and intermediate volatility organic compounds) and those resulting from the evaporation of the OA as it is getting diluted. The emission rates of the intermediate volatility organic compounds (IVOCs) and their chemical fate have a large impact on the formed secondary OA within the plume. The assumption that these IVOCs undergo only functionalization leads to an overestimation of the produced SOA suggesting that fragmentation is also occurring. Assuming a fragmentation probability of 0.2 resulted in predictions that are more consistent with available observations. Dilution leads to OA evaporation and therefore reduction of the OA levels downwind of the fire. However, the evaporated material can return to the particulate phase later on after it gets oxidized and recondenses. The sensitivity of the OA levels and total mass balance on the dilution rate depends on the modeling assumptions. The high variability of OA mass enhancement observed in past field studies downwind of fires may be partially due to the variability of the dilution rates of the plumes.

Simulation of the cooking organic aerosol concentration variability in an urban area

Cooking activities are a source of urban air pollution that has been often ignored. The corresponding particulate matter (PM) emissions are not included or are seriously underestimated in urban emission inventories. However, several field studies suggest that cooking organic aerosol (COA) can be an important component of the organic PM in urban areas. In this study we propose and evaluate a methodology for the simulation of the COA concentration and its variability in space and time in an urban area. The city of Patras, the third biggest in Greece, is used for this first application during a typical late summer period.The spatial distribution of COA emissions was based on the exact location of restaurants and grills, while the emission rates on the per person meat consumption in Greece. We estimated COA emissions of 0.6 g d−1 per person that corresponds to 150 kg d−1 for Patras. The temporal distribution of COA emissions was based on the known cooking times and the results of past field studies in the area. We estimated that half of the daily COA is emitted during dinner time (21:00–0:00 LT), while approximately 25% during lunch time (13:00–16:00 LT). The COA is simulated using the Volatility Basis Set approach and is treated as semivolatile and reactive. Its volatility distribution is based on laboratory measurements of COA from meat grilling.The chemical transport model PMCAMx predicts that the COA concentration reaches values up to 15 μg m−3 during the nights of the simulated summertime period in an area with high restaurant density. The average predicted COA concentration is around 1.2 μg m−3 in the center and 0.1–0.2 μg m−3 in the suburbs of the city. COA has a distinct daily profile that peaks during lunch (13:00–15:00 LT) and dinner (21:00–23:00 LT) periods. The local production of secondary COA is predicted to be slow and it represents just a few percent of the total COA. The model reproduces well the average PM2.5 concentrations at the outskirts of Patras and its overall performance for hourly PM2.5 values is rated as good. A 50% uncertainty of the reported COA emissions is estimated based on the results of the sensitivity tests and the observations in the city core. The emissions can be further constrained and the corresponding uncertainty can be further reduced with measurements focusing on COA and not on total PM2.5 as was the case in this study. These should take place in different areas of the city, selected based on the model predictions, to also improve our estimates of the spatial distribution of the emissions.

Soil and soil organic carbon effects on simulated Southern High Plains dryland Cotton production

The effect of increasing soil organic content (SOC) on the soil water retention and cotton yield productivity of two benchmark U.S. Southern High Plains (SHP) soils was estimated using pedotransfer functions and the CROPGRO-Cotton crop simulation model. Increasing plow layer (0−30 cm) SOC leads to increased wilting point (WP), field capacity (FC), and plant available water capacity (PAW = FC-WP) in both soils. The increase in a clay loam’s available water capacity is modest, with a 1% increase in SOC producing an additional 0.16 cm of PAW in the soil profile’s uppermost 30 cm. The fine sandy loam’s plow layer effect is twice that, with a 1% SOC increase producing a 0.32 cm PAW increase. These effect’s magnitudes were consistent with a recent meta-analysis of SOC on soil water retention, but considerably below those cited by national and regional extension services. As surface SOC levels in both soils were increased above baseline levels the fine sandy loam’s median simulated cotton lint yields were essentially unchanged, while clay loam yields decreased. The clay loam yield effect is attributed to increased soil evaporation rates. Conservation agriculture (CA) practices such as increased residue retention may compensate for these weak soil water retention effects, but cotton’s limited residue production would require winter cover crops or alternate crop rotations. As the success of terminated winter wheat – dryland cotton rotations is unclear in past SHP field studies, a CA sorghum-cotton rotation with periodic tillage is proposed as a SHP dryland production system.

Willows Used for Phytoremediation Increased Organic Contaminant Concentrations in Soil Surface

The Salix genus includes shrub species that are widely used in phytoremediation and various other phytotechnologies due to their advantageous characteristics, such as a high evapotranspiration (ET) rate, in particular when cultivated in short rotation intensive culture (SRIC). Observations made in past field studies suggest that ET and its impact on soil hydrology can also lead to increases in soil pollutant concentrations near shrubs. To investigate this, sections of a mature willow plantation (seven years old) were cut to eliminate transpiration (Cut treatment). Soil concentrations of polychlorinated biphenyls (PCBs), aliphatic compounds C10–C50, polycyclic aromatic hydrocarbons (PAHs) and five trace elements (Cd, Cr, Cu, Ni and Zn) were compared between the Cut and the uncut plots (Salix miyabeana ‘SX61’). Over 24 months, the results clearly show that removal of the willow shrubs limited the contaminants’ increase in the soil surface, as observed for C10–C50 and of 10 PAHs under the Salix treatment. This finding strongly reinforces a hypothesis that SRIC of willows may facilitate the migration of contaminants towards their roots, thus increasing their concentration in the surrounding soil. Such a “pumping effect” in a high-density willow crop is a prominent characteristic specific to field studies that can lead to counterintuitive results. Although apparent increases of contaminant concentrations contradict the purification benefits usually pursued in phytoremediation, the possibility of active phytoextraction and rhizodegradation is not excluded. Moreover, increases of pollutant concentrations under shrubs following migration suggest that decreases would consequently occur at the source points. Some reflections on interpreting field work results are provided.

Nutritional traits of riverine eucalypts across lowland catchments in southeastern Australia

Eucalyptus (Myrtaceae) trees are ubiquitous in riparian–floodplain zones of Australia’s south-eastern river catchments, where natural ecosystems continue to be affected. In the Murray–Darling Basin (MDB), provision of environmental flows to mitigate tree decline is informed by past field studies. However, broadscale empirical field data on tree nutrition and response to external changes remain scarce. This is the first study to gather soil and plant data across a large area of catchment lowlands to generate a low-resolution regional snapshot of tree nutrition and soil chemistry. Leaves and soils were sampled across and adjacent to the MDB; from and beneath mature trees of three key riverine eucalypts, Eucalyptus largiflorens, E. camaldulensis, and E. coolabah. Foliar sodium concentrations ranged from ∼500 mg kg−1 for E. coolabah up to ∼4500 mg kg−1 for E. largiflorens, with highest values at the River Murray sites. The results suggest E. largiflorens is highly salt tolerant by foliage accumulation given all trees sampled were in good condition. Further research into these species is needed to determine toxicity thresholds for elements such as sodium to aid early diagnosis of potential tree stress, which could provide an additional line of evidence for when environmental water is required to mitigate decline.

Integrative taxonomy of some Iranian peregrine earthworm species using morphology and barcoding (Annelida: Megadrili).

Despite the biological and economic importance of earthworms, the taxonomic status and evolutionary relationships of most lumbricid genera are still under debate. Further complications arise from the recognition that earthworms also show a high cryptic diversity. Past and current field studies of Iranian earthworm fauna have resulted in the identification of a total number of 28 earthworm species. However, many specimens do not fully fit into their original descriptions, making the species assignation very difficult. In this study, we evaluated the genetic diversity using mitochondrial markers as a tool to assess the species occurrence of some problematic species in Iran. Four species with high morphological variation were selected: Aporrectodea caliginosa (Savigny, 1826), Aporrectodea trapezoides (Dugès, 1828), Dendrobaena byblica (Rosa, 1893) and Dendrobaena veneta (Rosa, 1886). Morphological identification was contrasted with the molecular information generated through COI and 16S barcoding and the COI and 16S sequences stored in the Genbank. The results of this first integrative taxonomic analysis revealed that D. veneta consisted of two separated clades and that a number of species assigned to D. byblica showed very close relationships with those belonging to the genus Philomontanus. The lack of taxonomic expertise and identification characters providing a clear and unambiguous identification of earthworms highlights the urgent need of new tools to identify species unequivocally. Therefore, it is concluded that more taxonomical studies are needed to clarify the diagnostic characters and taxonomic status of the species belonging to two genera, Aporrectodea and Dendrobaena (Lumbricidae), in Iran.

Patterns in the spectral composition of sunlight and biologically meaningful spectral photon ratios as affected by atmospheric factors

Plants rely on spectral cues present in their surroundings, generated by the constantly changing light environment, to guide their growth and reproduction. Photoreceptors mediate the capture of information by plants from the light environment over a wide range of wavelengths, but despite extensive evidence that plants respond to various light cues, only fragmentary data have been published showing patterns of diurnal, seasonal and geographical variation in the spectral composition of daylight. To illustrate patterns in spectral photon ratios, we measured time series of irradiance spectra at two distinct geographical and climatological locations, Helsinki, Finland and Gual Pahari, India. We investigated the drivers behind variation of the spectral photon ratios measured at these two locations, based on the analysis of over 400 000 recorded spectra. Differences in spectral irradiance were explained by different atmospheric factors identified through multiple regression model analysis and comparison to spectral irradiance at ground level simulated with a radiative transfer model. Local seasonal and diurnal changes in spectral photon ratios were related to solar elevation angle, atmospheric water-vapour content and total ozone column thickness and deviated from their long-term averages to an extent likely to affect plant photobiology. We suggest that future studies should investigate possible effects of varying photon ratios on terrestrial plants. Solar elevation angle especially affects the patterns of B:G and B:R ratios. Water vapour has a large effect on the R:FR photon ratio and modelled climate scenarios predict that increasing global temperatures will result in increased atmospheric water vapour. The development of proxy models, utilising available data from weather and climate models, for relevant photon ratios as a function of solar elevation angle and atmospheric factors would facilitate the interpretation of results from past, present and future field studies of plants and vegetation.

Spatial Analysis of Livestock Grazing and Forest Service Management in the High Uintas Wilderness, Utah

This case study addresses the Forest Service reauthorization for grazing of domestic sheep in Utah’s High Uintas Wilderness, USA. It provides an approach using spatial analysis and aerial imagery to evaluate the lands capable of being grazed based on Forest Service criteria and field surveys. The resulting model and analysis demonstrated that the Forest Service has not applied its own criteria. This has led to the Forest Service overestimating the amount of land and numbers of sheep that can be supported in the study area. Past field studies show this has resulted in environmental damage by grazing sheep. Our analysis concludes that the numbers of domestic sheep should be greatly reduced to protect these lands and wilderness values. Limitations of the study include the lack of a suitably detailed soil survey to determine erosion susceptibility, a lack of ground cover data, a lack of Forest Service data for the level of grazing use, or utilization, and the lack of a Forest Service quantitative measurement of vegetation production in each plant community and soil type. In the end, our use of aerial imagery, GIS determinations of areas of steep slopes and dense forests, and our measurement of vegetation production in the dominant soil types showed most of the land is not capable for grazing domestic sheep even in the absence of this other data.

A spectral weighting function for abiotic photodegradation based on photochemical emission of CO2 from leaf litter in sunlight

Photodegradation can be a significant driver of leaf litter decomposition although the spectral effectiveness of sunlight in driving this process is not well characterized. We developed spectral weighting functions (WFs) for the photochemical emission of CO2 from three leaf litter types using 10 cutoff filters that provided contrasting polychromatic sunlight under clear skies in Tempe, AZ, USA. An iterative nonlinear least-squares regression fitting procedure was used to estimate how effective sunlight at a given wavelength was in eliciting CO2 emission. Although absolute CO2 emission rates varied appreciably among litter types, their WFs were very similar. Using the average WF of all litter types, the effectiveness of sunlight declined from 300 nm by one and two orders of magnitude at 399 and 498 nm, respectively. The slope of the WF was most similar to WFs for CO emission from terrestrial leaf litter and photobleaching of dissolved organic matter in lakes, and was much more gradual than WFs addressing UV damage to biotic processes. Peak effectiveness of clear-sky noon sunlight with our WF occurred at 330 nm, with UV-B (280–320 nm), UV-A (320–400 nm) and visible (400–550 nm) wavebands responsible for 9, 61 and 30% of CO2 emission, respectively. Results from past field studies suggest that solar UV is typically less effective in driving litter mass loss than our WF predicts; we discuss possible reasons for this discrepancy. The gradual slope of our WF implies that differences in UV-B irradiance associated with stratospheric ozone thickness or latitude are unlikely to significantly influence photochemical litter emission.

Differentiation and financial performance: a meta-analysis

PurposeThe purpose of this study is to meta-analytically compare the effect of differentiation strategy formulation on financial and non-financial performance to explore the agency problem.Design/methodology/approachThis study was conducted using a meta-analysis approach and CMA2 software. Hypotheses has been tested using cumulative effect sizes. Then, the cumulative effect sizes if some subsamples are also tested for robustness check by manipulation of circumstances.FindingsBased on the findings, differentiation affects performance dimensions. However, spite of shareholders’ wealth maximization, differentiation has a smaller effect on financial performance (as the proxy of shareholders’ wealth) compared with operational performance. Meanwhile, the robustness test showed that the results in all subcategories has also confirmed, and this supports the results.Research limitations/implicationsThe results catch attentions of shareholders to review the elements of management evaluation in the corporate governance system in a way that they focus more on stakeholders’ interests in developing differentiation strategy. Limitations of this study is firstly based on limitations of meta-analysis approach, which provide general results of relevant studies so that the local conditions of each firm should be considered in deploying implications.Originality/valueExploring the overall schema of firms (from past to present and in different research conditions) regarding the impact of differentiation strategy on performance has been the originality of this research because of potentials of meta-analysis against past field studies. This originality led to the discovery of the general confusion of firms in the development of differentiation strategy so that it emphasizes operational performance more than financial performance.

Molecular and isotopic composition of modern soils derived from kerogen-rich bedrock and implications for the global C cycle

Ancient sedimentary organic matter (kerogen) represents the largest terrestrial organic carbon (OC) reservoir on earth and is vulnerable to remineralization upon exposure to earth’s atmosphere during the oxidative weathering of sedimentary rocks on the continents. Due to the potential for large carbon-cycle implications, the mechanisms and rates involved in kerogen transformation by oxidative weathering are becoming increasingly well-constrained in contemporary models of the global carbon cycle. Past field studies have focused primarily on areas where high erosion rates deliver large amounts of kerogen to earth’s surface, making the relative importance of low-lying landscapes a key unknown in regional or global scale estimates of kerogen recycling. The weathered residuum of organic-rich sedimentary rocks serves as the parent material for many soils. Therefore, some aspects of the chemical structure and biogeochemical cycling of the soil organic matter are likely to be inherited from the bedrock. We used a combination of solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, and carbon isotope techniques to describe molecular and isotopic changes that occur throughout oxidative weathering of marine kerogens, and the subsequent formation of modern soils, in two outcropping Cretaceous mudstones of the Eagle Ford and Pepper Formations in central, Texas. Increasing production of oxygenated functional groups was coupled with reductions in characteristically abundant aliphatic components of marine kerogens along the weathering profiles. Organic matter structural parameters, derived from C–H dephasing NMR experiments, also provide the basis for a novel weathering index that accounts for the degree of post-sedimentary alteration of kerogen samples along the bedrock-soil continuum. An uncertain future marked by climatic shifts in temperature and/or precipitation and increased continental weathering and denudation rates highlights the potential for enhanced vulnerability of kerogen, and the need for molecular and isotopic tools for quantifying mechanisms and rates involved in kerogen weathering.

Meter-based synthesis of equipment schedules for improved models of electrical demand in multifamily buildings

The authors present a novel method for using electrical meter data to synthesize realistic equipment use schedules that can improve models of electrical demand in buildings. Other methods exist for creating schedules based on historical data from a limited set of past field studies, but the method presented here is unique because it allows a modeller to customize and calibrate equipment use schedules based on user-specified meter data and load profile metrics. A case study with water heaters demonstrates that this schedule synthesis method can outperform traditional modelling strategies by replicating key characteristics of a measured equipment load profile, including its magnitude, variability, and relationships between load and time. In particular, the new method was more effective at replicating the total water heater electrical load duration curve and ramp duration curve of a real-world reference building.

Three-dimensional flow structure, morphodynamics, suspended sediment, and thermal mixing at an asymmetrical river confluence of a straight tributary and curving main channel

Past field studies of confluence dynamics have focused mainly on experimental or small stream confluences and confluences of large rivers. Few studies have explored fluvial processes and forms at confluences of medium-size rivers, especially those where river planform varies from a straight alignment. This study examines changes in flow structure, bed morphology, sediment suspension, and mixing with changing incoming flow conditions at a river confluence where a straight tributary joins a curving main channel at the upstream end of a bend. Data include five sets of field measurements of three-dimensional velocity components, bed topography, acoustic backscatter intensity, and water-surface temperatures at the confluence of the curving Wabash River and its straight tributary, the Embarras River. Results show that, in contrast to large river confluences, channel-scale helical motion is a prominent feature of flow structure at this confluence. Dual counter-rotating helical cells, separated by a distinct shear layer, develop over a region of scour within the confluence and contribute to maintenance of scour by sweeping sediment away from this region. Helical motion develops through channel curvature and flow deflection within the confluence. The location and geometry of the scour hole are subject to change depending on the relative momentum fluxes of the two rivers with the zone of scour shifting outward and enlarging as the momentum flux of the tributary increases relative to the momentum flux of the main river. Patterns of backscatter intensity, used here as a surrogate for suspended sediment concentration, confirm that near-bed suspended sediment moves around the scour hole. The thermal mixing interface between the two rivers corresponds to the position of the shear layer, and thermal mixing at the surface within the immediate vicinity of the confluence is limited. The hydrodynamics, morphodynamics, and patterns of sediment suspension and thermal mixing at this river confluence reflect a combination of confluence and bend dynamics.

Linking functional response and bioenergetics to estimate juvenile salmon growth in a reservoir food web.

Juvenile salmon (Oncorhynchus spp.) use of reservoir food webs is understudied. We examined the feeding behavior of subyearling Chinook salmon (O. tshawytscha) and its relation to growth by estimating the functional response of juvenile salmon to changes in the density of Daphnia, an important component of reservoir food webs. We then estimated salmon growth across a broad range of water temperatures and daily rations of two primary prey, Daphnia and juvenile American shad (Alosa sapidissima) using a bioenergetics model. Laboratory feeding experiments yielded a Type-II functional response curve: C = 29.858 P *(4.271 + P)-1 indicating that salmon consumption (C) of Daphnia was not affected until Daphnia densities (P) were < 30 · L-1. Past field studies documented Daphnia densities in lower Columbia River reservoirs of < 3 · L-1 in July but as high as 40 · L-1 in August. Bioenergetics modeling indicated that subyearlings could not achieve positive growth above 22°C regardless of prey type or consumption rate. When feeding on Daphnia, subyearlings could not achieve positive growth above 20°C (water temperatures they commonly encounter in the lower Columbia River during summer). At 16–18°C, subyearlings had to consume about 27,000 Daphnia · day-1 to achieve positive growth. However, when feeding on juvenile American shad, subyearlings had to consume 20 shad · day-1 at 16–18°C, or at least 25 shad · day-1 at 20°C to achieve positive growth. Using empirical consumption rates and water temperatures from summer 2013, subyearlings exhibited negative growth during July (-0.23 to -0.29 g · d-1) and August (-0.05 to -0.07 g · d-1). By switching prey from Daphnia to juvenile shad which have a higher energy density, subyearlings can partially compensate for the effects of higher water temperatures they experience in the lower Columbia River during summer. However, achieving positive growth as piscivores requires subyearlings to feed at higher consumption rates than they exhibited empirically. While our results indicate compromised growth in reservoir habitats, the long-term repercussions to salmon populations in the Columbia River Basin are unknown.