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There is an emerging viewpoint that the sediment flux at a given point on the landscape may be influenced by landscape properties in a region extending away from the point of interest. Using a general sediment transport model that incorporates this non‐local nature via fractional derivatives, we find a strong asymmetry in the direction in which the non‐locality affects a given point: For erosional landscapes, physically plausible elevation profiles are obtained only when the spatial influence is restricted to the region upstream of the point of interest. By contrast, in depositional landscapes, the non‐local model is guaranteed to produce physically plausible topographic profiles only when the spatial influence is restricted to the region downstream of the point of interest. These results suggest that information flows downstream in an erosional landscape and upstream in depositional landscapes. Key Points The choice of direction in a non‐local model has physical consequences The flow of information reverses between erosional and depositional landscapes Results provide a means of validating fractional calculus landscape models

Hillslope sediment transport models express the sediment flux at a point as a function of some topographic attributes of the system, such as slope, curvature, soil thickness, etc., at that point only (referred here as “local” transport models) or at an appropriately defined vicinity of that point (referred here as “nonlocal” transport models). Typically, topographic attributes are computed from digital elevation data (DEMs) and thus their estimates depend on the DEM resolution (1 m, 10 m, 90 m, etc.) rendering any sediment flux computation scale‐dependent. Often calibration compensates for this scale‐dependence resulting in effective parameterizations with limited physical meaning. In this paper, we demonstrate the scale‐dependence of local nonlinear hillslope sediment flux models and derive a subgrid scale closure via upscaling. We parameterize the subgrid scale closure in terms of the low resolution, resolved topographic attributes of the landscape, thus allowing the reliable computation of a scale‐independent sediment flux from low resolution digital elevation data. We also show that the accuracy of the derived subgrid scale closure model depends on the dimensionless erosion rate and the dimensionless relief of any given basin. Finally, we present theoretical arguments and demonstrate that the recently proposed nonlocal sediment flux models are scale‐independent. These concepts are demonstrated via an application on a small basin (MR1) of the central Oregon Coast Range using high‐resolution lidar topographic data. Key Points Scale‐dependence of nonlinear hillslope sediment transport models Derivation of a subgrid scale closure for nonlinear hillslope transport model Preliminary investigation of scale‐independence of non‐local transport model

It is not a compelling argument, solely on the basis of a better fit to solute breakthrough curve (BTC) data, that a temporally nonlocal model is necessary to simulate transport in an advection‐dominated system. One may counter that the classical advection‐dispersion equation (ADE) is a valid model at some small scale and that the detailed hydraulic conductivity (K) data must be well‐represented: then the nonlocality is only a result of upscaling and loss of information. But is the nonlocal model demonstrably necessary at all scales? We examine the experiment conducted by Klise et al. (2008) in which a 30.5 × 30.5 cm slab of relatively homogeneous, cross‐bedded sandstone was exhaustively sampled for K. The slab was sealed, saturated with potassium iodide, and X‐rayed 10 times while being flushed with fresh water. The 8,649 air‐permeameter measurements were down‐ and upscaled to make finer and coarser grids on which the velocity field was solved and the ADE applied. The optimized parameters in the ADE were found to scale predictably, most notably the longitudinal dispersivity (), which grew linearly with upscaling. But at all levels of up‐ and downscaling, including the original K measurement scale of 0.33 cm, the ADE did not adequately represent the late‐time tails. The temporally nonlocal, time‐fractional ADE (t‐FADE) was applied and the optimized parameters ( and the immobile capacity ) did not depend on scale. The better fit provided by the t‐FADE in the late BTC tails did not bring about a sacrificed fit elsewhere in the BTC. Furthermore, the optimized ADE and t‐FADE solutions do not converge at the smallest scale, directly implying that the temporal nonlocality is a necessary model component. We conclude that the logical inference “if the ADE is valid in heterogeneous material, then there is tailing in the BTC” is not a proof that the reverse is true. We provide a clear counterexample. A corollary is that a mismatch between data and a discretized solution to the ADE does not imply that more data will improve fits or predictive ability. Key Points Temporal non‐locality not only better but required Complete characterization of hydraulic conductivity leads to insufficient model Scaling effects removed

Ongoing climate change is causing rapid changes in biodiversity and has ecological impacts on coastal marine systems. Predicting scenarios of how these pressures will affect bioturbation, a process vital to marine communities and human well-being, has become an important task. A first step is a better understanding of the interaction between macrofauna and the surrounding environment. Here, bioturbation (local and non-local sediment mixing) was surveyed using a high-resolution depth distribution of macrobenthos in the southwestern Baltic Sea, which is characterized by different sediment types and faunal communities. The distribution of local and non-local mixing with increasing non-local transports from west to east was explained by comparing vertical chlorophyll profiles and organisms’ depth distributions. The main bioturbators were identified based on chlorophyll profiles and community bioturbation potential (BPc) and by categorizing the main species into functional groups. Diastylis rathkei is most important for local sediment mixing, and bivalves, e.g. Arctica islandica and Limecola balthica, together with polychaetes, e.g. Nephtys hombergii and Scoloplos armiger, are most important for non-local transports. Significant correlations between modeled local and non-local mixing intensities and calculated BPc (105−1298 m−2) indicate that BPc is a suitable bioturbation indicator; however, it does not provide information on the different modes of mixing. Some species categorized as bio diffusors in the literature were found to cause non-local mixing according to their feeding behavior (e.g. L. balthica), size (e.g. Abra alba) or biomass (e.g. A. islandica).

Variability of bioturbation on different spatial scales was revealed through a survey at 6 stations in the southwestern Baltic Sea with different sediment types, salinities and macrozoobenthic communities. At each station, 6 sampling locations were investigated with 4 cores each (24 cores per station). The cores were analyzed for vertical chlorophyll (chl) profiles, which were modeled with both a local (tracer distribution decreasing exponentially with depth indicative of diffusive transport, D B) and a non-local (presence of subsurface maximum of the tracer, injection flux J and ingestion rate r) mixing model developed by Soetaert et al. (1996; J Mar Res 54: 1207–1227). Degradation of chl was determined experimentally by an incubation of fresh sediment under anoxic, dark conditions and provided decay constants k D of 0.01 d−1 for mud and 0.02 d−1 for sand. Mixing depths reach 7.1 ± 1.6 cm at stations in the west (except Lübeck Bay, LB), 2 cm deeper than at stations in the east, which reach 5.2 ± 1.7 cm (including LB), mainly depending on the macrozoobenthic community present. Bioturbation intensities indicate high variability between closely located sampling sites as well as across the southern Baltic Sea, and depend on the food supply from the water column. Stations indicate a difference in local mixing (D B) of a factor of 20 and in non-local processes (J) of 6. Non-local transports account for 33 to 50% of the investigated area in the west and for 70 to 100% in the east. The statistical description of the results indicates the necessity of high sampling effort when using chl as a particle tracer.

A double-distribution-function lattice Boltzmann model for large-eddy simulations of a passive scalar field in a neutrally stratified turbulent flow is described. In simulations of the scalar turbulence within and above a homogeneous plant canopy, the model’s performance is found to be comparable with that of a conventional large-eddy simulation model based on the Navier–Stokes equations and a scalar advection–diffusion equation in terms of the mean turbulence statistics, budgets of the second moments, power spectra, and spatial two-point correlation functions. For a top-down scalar, for which the plant canopy serves as a distributed sink, the variance and flux of the scalar near the canopy top are predominantly determined by sweep motions originating far above the canopy. These sweep motions, which have spatial scales much larger than the canopy height, penetrate deep inside the canopy and cause scalar sweep events near the canopy floor. By contrast, scalar ejection events near the canopy floor are induced by coherent eddies generated near the canopy top. The generation of such eddies is triggered by the downward approach of massive sweep motions to existing wide regions of weak ejective motions from inside to above the canopy. The non-local transport of scalars from above the canopy to the canopy floor, and vice versa, is driven by these eddies of different origins. Such non-local transport has significant implications for the scalar variance and flux budgets within and above the canopy, as well as the transport of scalars emitted from the underlying soils to the atmosphere.

Improving the performance of proton exchange membrane fuel cells (PEMFCs) requires deep understanding of the reactive transport processes inside the catalyst layers (CLs). In this study, a particle-overlapping model is developed for accurately describing the hierarchical structures and oxygen reactive transport processes in CLs. The analytical solutions derived from this model indicate that carbon particle overlap increases ionomer thickness, reduces specific surface areas of ionomer and carbon, and further intensifies the local oxygen transport resistance ( R other ). The relationship between R other and roughness factor predicted by the model in the range of 800-1600 s m -1 agrees well with the experiments. Then, a multiscale model is developed by coupling the particle-overlapping model with cell-scale models, which is validated by comparing with the polarization curves and local current density distribution obtained in experiments. The relative error of local current density distribution is below 15% in the ohmic polarization region. Finally, the multiscale model is employed to explore effects of CL structural parameters including Pt loading, I/C , ionomer coverage and carbon particle radius on the cell performance as well as the phase-change-induced (PCI) flow and capillary-driven (CD) flow in CL. The result demonstrates that the CL structural parameters have significant effects on the cell performance as well as the PCI and CD flows. Optimizing the CL structure can increase the current density and further enhance the heat-pipe effect within the CL, leading to overall higher PCI and CD rates. The maximum increase of PCI and CD rates can exceed 145%. Besides, the enhanced heat-pipe effect causes the reverse flow regions of PCI and CD near the CL/PEM interface, which can occupy about 30% of the CL. The multiscale model significantly contributes to a deep understanding of reactive transport and multiphase heat transfer processes inside PEMFCs. Highlights A particle-overlapping model for reactive transport process in catalyst layers. A multiscale model coupling particle-overlapping model with cell-scale models. The model is rigorously validated from nanoscale to commercial-cell scale. Effects of catalyst layer structures on cell performance are evaluated. Phase-change-induced and capillary-driven flows in catalyst layers are studied.

This paper focuses on the use of vague discourse in planning. Early contributions identified vagueness as a ‘problem’ to be solved so as to avoid potential misunderstandings and conflicts. This paper adopts the complementary point of view whereby vagueness can also be a ‘resource’, that is, a strategy used by actors in adverse circumstances. A systematic analysis of the texts and illustrations of 36 urban transport plans shows that vagueness is an essential ingredient. It is used mainly as a way to hedge against unwanted public commitments in the context of major uncertainties and tension between actors.

Climate change poses governance challenges at diverse scales and across the dimensions of risk and responsibility. Local governments are central to the delivery of action on both decarbonisation and adapting to the risks of climate change. Yet there are likely to be significant differences across local governments in terms of their capacity to act on climate change. This research documents and explains differences in the capacity to act within response spaces to risks to transport infrastructure and systems. We examine 80 Transport Plans across local governments in England, specifically their efforts to incorporate climate change adaptation. Data are generated from content analysis of the 80 documents and key informant interviews in a sample of 15% of authorities. The results show significant disparities across authorities. We explain differential outcomes as dependent on internal coordination, local prioritisation processes and political opposition. The results highlight that there are significant governance barriers associated with differential response capacity in the face of climate change risks.

Investigating the oxygen transport law within the Membrane Electrode Assembly at intermediate temperatures (80–120 °C) is crucial for enhancing fuel cell efficiency. This study analyzed the resistance to oxygen transport within the Membrane Electrode Assembly at intermediate temperatures using limiting current density and electrochemical impedance spectroscopy. The study findings reveal that, as temperature progressively increases, the Ostwald ripening effect leads to a 34% rise in the local oxygen transport resistance (Rlocal) in relation to the pressure-independent resistance (Rnp) within the cathode catalytic layer. Concurrently, the total transport resistance (Rtotal) decreases from 27.8% to 37.5% due to an increase in the gas diffusion coefficient and molecular reactivity; additionally, there is a decrease in the amount of liquid water inside the membrane electrode. A three-dimensional multiphysics field steady-state model was also established. The model demonstrates that the decrease in oxygen partial pressure can be mitigated effectively by increasing the back pressure at intermediate temperatures to ensure the cell’s performance.