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  1. Abstract

    Floating treatment wetlands are new ecological infrastructures for stormwater treatment. Despite a recent proliferation in their usage, their contaminant removal efficiencyecontinues to draw research attention. Here, theefrom idealized FTWs is numerically computed across a wide range of flow and geometric conditions while accommodating joint contributions of advection, turbulent dispersion, and vegetation removal. The emerging mathematical structure describingebears resemblance to a simplified plug flow model and supports an empirical shallow-basin model from long-term field measurements. The present model indicates thateremains significantly influenced by a Dämkohler number that quantifies the effects of both vegetation and flow properties. The impacts oneof the underflow region and contaminant blockage on the removal mechanisms are also investigated.

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  2. Abstract

    How large turbulent eddies influence non‐closure of the surface energy balance is an active research topic that cannot be uncovered by the mean continuity equation in isolation. It is demonstrated here that asymmetric turbulent flux transport of heat and water vapor by sweeps and ejections of large eddies under unstable atmospheric stability conditions reduce fluxes. Such asymmetry causes positive gradients in the third‐order moments in the turbulent flux budget equations, primarily attributed to substantially reduced flux contributions by sweeps and sustained large flux contributions by ejections. Small‐scale surface heterogeneity in heating generates ejecting eddies with larger air temperature variance than sweeping eddies, causing asymmetric flux transport in the atmospheric surface layer. Changes in asymmetry with increasing instability are congruent with observed increases in the surface energy balance non‐closure. To assess the contributions of asymmetric flux transport by large eddies to the non‐closure requires two eddy covariance systems on the tower to measure the gradients of the turbulent heat flux and other third‐order moments.

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  3. Abstract

    In the atmospheric surface layer (ASL), a characteristic wavelength marking the limit between energy‐containing and inertial subrange scales can be defined from the vertical velocity spectrum. This wavelength is related to the integral length scale of turbulence, used in turbulence closure approaches for the ASL. The scaling laws describing the displacement of this wavelength with changes in atmospheric stability have eluded theoretical treatment and are considered here. Two derivations are proposed for mildly unstable to mildly stable ASL flows one that only makes use of normalizing constraints on the vertical velocity variance along with idealized spectral shapes featuring production to inertial subrange regimes, while another utilizes a co‐spectral budget with a return‐to‐isotropy closure. The expressions agree with field experiments and permit inference of the variations of the wavelength with atmospheric stability. This methodology offers a new perspective for numerical and theoretical modeling of ASL flows and for experimental design.

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  4. Abstract

    The sensitivity of soil carbon dynamics to climate change is a major uncertainty in carbon cycle models. Of particular interest is the response of soil biogeochemical cycles to variability in hydroclimatic states and the related quantification of soil memory. Toward this goal, the power spectra of soil hydrologic and biogeochemical states were analyzed using measurements of soil temperature, moisture, oxygen, and carbon dioxide at two sites. Power spectra indicated multiscale power law scaling across subhourly to annual timescales. Precipitation fluctuations were most strongly expressed in the soil biogeochemical signals at monthly to annual timescales. Soil moisture and temperature fluctuations were comparable in strength at one site, while temperature was dominant at the other. The effect of soil hydrologic, thermal, and biogeochemical processes on gas concentration variability was evidenced by low spectral entropy relative to the white noise character of precipitation. A full mass balance model was unable to capture high‐frequency soil temperature influence, indicating a gap in commonly used model assumptions. A linearized model was shown to capture the main features of the observed and modeled gas concentration spectra and demonstrated how the means and variances of soil moisture and temperature interact to produce the gas concentration spectra. Breakpoints in the spectra corresponded to the mean rate of gas efflux, providing a first‐order estimate of the soil biogeochemical integral timescale (∼1 min). These methods can be used to identify biogeochemical system dynamics to develop robust, process‐based soil biogeochemistry models that capture variability in addition to long‐term mean values.

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  5. Abstract

    Turbulent mixing of scalars within canopies is investigated using a flume experiment with canopy‐like rods of heighthmounted to the channel bed. The data comprised a time sequence of high‐resolution images of a dye recorded in a plane parallel to the bed atz/h= 0.2. Image processing shows that von Kármán wakes shed by canopy drag and downward turbulent transport from upper canopy layers impose distinct scaling regimes on the scalar spectrum. Measures from information theory are then used to explore the dominant directionality of the interaction between small and large scales underlying these two spectral regimes, showing that the arrival of sweeps from aloft establishes an inertial‐range spectrum with forward “information” cascade. In contrast, wake growth with downstream distance leads to persistent upscale transfer (inverse cascade) of scalar variance, which hints at their nondiffusive character and the significance of the stem diameter as an active length scale in canopy turbulence.

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  6. Abstract

    Anisotropic turbulence is ubiquitous in atmospheric and oceanic boundary layers due to differences in energy injection mechanisms. Unlike mechanical production that injects energy in the streamwise velocity component, buoyancy affects only the vertical velocity component. This anisotropy in energy sources, quantified by the flux Richardson numberRif, is compensated by a “return to isotropy” (RTI) tendency of turbulent flows. Describing RTI in Reynolds‐averaged models and across scales continues to be a challenge in stratified turbulent flows. Using phenomenological models for spectral energy transfers, the necessary conditions for which the widely‐used Rotta model captures RTI across variousRifand eddy sizes are discussed for the first time. This work unravels adjustments to the Rotta constant, withRifand scale, necessary to obtain consistency between RTI models and the measured properties of the atmospheric surface layer for planar‐homogeneous and stationary flows in the absence of subsidence. A range ofRifand eddy sizes where the usage of a conventional Rotta model is prohibited is also found. Those adjustments lay the groundwork for new closure schemes.

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  7. Abstract

    The main source of soil moisture variability in savanna ecosystems is pulsed rainfall. Rainfall pulsing impacts water‐stress durations, soil moisture switching between wet‐to‐dry and dry‐to‐wet states, and soil moisture spectra as well as derived measures from it such as soil moisture memory. Rainfall pulsing is also responsible for rapid changes in grassland leaf area and concomitant changes in evapotranspirational (ET) losses, which then impact soil moisture variability. With the use of a hierarchy of models and soil moisture measurements, temporal variability in root‐zone soil moisture and water‐stress periods are analysed at four African sites ranging from grass to miombo savannas. The normalized difference vegetation index (NDVI) and potential ET (PET)‐adjusted ET model predict memory timescale and dry persistence in agreement with measurements. The model comparisons demonstrate that dry persistence and mean annual dry periods must account for seasonal and interannual changes in maximum ET represented by NDVI and to a lesser extent PET. Interestingly, the precipitation intensity and soil moisture memory were linearly related across three savannas with ET/infiltration ∼ 1.0. This relation and the variability of length and timing of dry periods are also discussed.

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  8. Abstract

    Evidence is mounting that temperate‐zone reforestation cools surface temperature (Tsurf), mitigating deleterious effects of climate warming. WhileTsurfdrives many biophysical processes, air temperature (Ta) is an equally important target for climate mitigation and adaptation. Whether reductions inTsurftranslate to reductions inTaremains complex, fraught by several nonlinear and intertwined processes. In particular, forest canopy structure strongly affects near‐surface temperature gradients, complicating cross‐site comparison. Here the influence of reforestation onTais assessed by targeting temperature metrics that are less sensitive to local canopy effects. Specifically, we consider the aerodynamic temperature (Taero), estimated using a novel procedure that does not rely on the assumptions of Monin‐Obukhov similarity theory, as well as the extrapolated temperature into the surface layer (Textrap). The approach is tested with flux tower data from a grass field, pine plantation, and mature hardwood stand co‐located in the Duke Forest (North Carolina, USA). During growing season daytime periods,Tsurfis 4–6 °C cooler, andTaeroand near‐surfaceTextrapare 2–3 °C cooler, in the forests relative to the grassland. During the dormant season, daytime differences are smaller but still substantial. At night, differences inTaeroare small, and near‐surfaceTextrapis warmer over forests than grasslands during the growing season (by 0.5 to 1 °C). Finally, the influence of land cover onTextrapat the interface between the surface and mixed layer is small. Overall, reforestation appears to provide a meaningful opportunity for adaption to warmer daytimeTain the southeastern United States, especially during the growing season.

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  9. Abstract

    Floating treatment wetlands (FTWs) are efficient at wastewater treatment; however, data and physical models describing water flow through them remain limited. A two‐domain model is proposed dividing the flow region into an upper part characterizing the flow through suspended vegetation and an inner part describing the vegetation‐free zone. The suspended vegetation domain is represented as a porous medium characterized by constant permeability thereby allowing Biot's Law to be used to describe the mean velocity and stress profiles. The flow in the inner part is bounded by asymmetric stresses arising from interactions with the suspended vegetated (porous) base and solid channel bed. An asymmetric eddy viscosity model is employed to derive an integral expression for the shear stress and the mean velocity profiles in this inner layer. The solution features an asymmetric shear stress index that reflects two different roughness conditions over the vegetation‐induced auxiliary bed and the physical channel bed. A phenomenological model is then presented to explain this index. An expression for the penetration depth into the porous medium defined by 10% of the maximum shear stress is also derived. The predicted shear stress profile, local mean velocity profile, and bulk velocity agree with the limited experiments published in the literature.

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  10. <bold>SUMMARY</bold>

    While the adverse effects of elevated salinity levels on leaf gas exchange in many crops are not in dispute, representing such effects on leaf photosynthetic rates (A) continues to draw research attention. Here, an optimization model for stomatal conductance (gc) that maximizesAwhile accounting for mesophyll conductance (gm) was used to interpret new leaf gas exchange measurements collected for five irrigation water salinity levels. A function between chloroplastic CO2concentration (cc) and intercellular CO2concentration (ci) modified by salinity stress to estimategmwas proposed. Results showed that with increased salinity, the estimatedgmand maximum photosynthetic capacity were both reduced, whereas the marginal water use efficiencyλincreased linearly. Adjustments ofgm,λand photosynthetic capacity were shown to be consistent with a large corpus of drought‐stress experiments. The inferred model parameters were then used to evaluate the combined effects of elevated salinity and atmospheric CO2concentration (ca) on leaf gas exchange. For a given salinity level, increasingcaincreasedAlinearly, but these increases were accompanied by mild reductions ingcand transpiration. Thecalevel needed to ameliorateAreductions due to increased salinity is also discussed using the aforementioned model calculations.

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