M. B. Siqueira

The Effect of the Screen on the Mass, Momentum, and Energy Exchange Rates of a Uniform Crop Situated in an Extensive Screenhouse

The area of crops cultivated in extensive screenhouses is rapidly growing, especially in semi-arid and arid regions. Water vapour, carbon dioxide, and sensible heat released or taken up by crops within such protected environments can substantially alter the immediate micro-environment, which in turn, affects these fluxes. This amplified interaction between plants and their microclimate challenges simple assessments on how partially covering the crop by a screen modifies plant water uptake and photosynthesis. Via a newly proposed higher-order closure model, the effects of a screen on the mean flow field, turbulent stresses, radiative and energy fluxes, as well as scalar sources, sinks, fluxes, and mean scalar concentration within screenhouses are explored. As a starting point, an extensive screenhouse is assumed thereby reducing the sensitivity of the model results to the precise geometric configuration of the screenhouse. The model findings for the screenhouse are presented and referenced against their open field counterpart. The radiation modulation and changes to turbulent transport due to the presence of the screen are investigated. In general, the presence of a screen results in a warmer and more humid environment inside the screenhouse, promoting reductions in both canopy photosynthesis and transpiration. However, the overall effect of the screen is to enhance water-use efficiency thereby resulting in water savings for the same amount of gross primary production.

Biotic and abiotic factors act in coordination to amplify hydraulic redistribution and lift.

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The Cooling Trend of Canopy Temperature During the Maturation, Succession, and Recovery of Ecosystems

The maximum exergy dissipation theory provides a theoretical basis for using surface temperature to measure the status and development of ecosystems, which could provide an early warning of rapid evaluation of ecosystem degradation. In the present study, we used the radiation balance of ecosystems to demonstrate this hypothesis theoretically. Further, we used empirical data to verify whether ecosystems gain more radiation, while lowering their surface temperatures, as they develop naturally. We analyzed 12 chronosequences from the FLUXNET database using meteorological data and heat fluxes. We included age, disturbance, and successional chronosequences across six climate zones. Net radiation (Rn) and the ratio of net radiation to global radiation (Rn/Rg) were used to measure the energy gain of the ecosystems. The maximum daily air temperature above the canopy (Tmax) and thermal response number (TRN) were used to analyze the surface temperature trends with ecosystem natural development. The general trends of Tmax, TRN, Rn, and Rn/Rg demonstrated that ecosystems become cooler and more stable, yet gain more energy, throughout their natural development. Among the four indicators, TRN showed the most consistent trends and highest sensitivity to ecosystem growth, succession, and recovery. Moreover, TRN was not significantly influenced by precipitation or wind. We propose that TRN can be used to rapidly evaluate or warn of ecosystem disturbance, senescence, and degradation without prior knowledge of species composition, nutrient status, and complex ecosystem processes.

Footprint Estimation for Multi-Layered Sources and Sinks Inside Canopies in Open and Protected Environments

A multi-layered flux footprint model is developed for a canopy situated within a protected environment such as a screenhouse. The model accounts for the vertically distributed sources and sinks within the canopy as well as modifications introduced by the screen on the flow field and micro-environment. The effect of the screen on fetch as a function of its relative height above the canopy is then studied and compared to the case where the screen is absent. It is found that the required fetch is not appreciably affected by the vertical source–sink distribution in open and protected environments, but changes with the canopy density. Moreover, the fetch-to-height ratio is increased by the presence of the screen, at least when compared to the open environment case. How footprint analysis can be employed to estimate the ratio between above-canopy measured flux and vertically-integrated canopy source–sink strengths in a prototypical screenhouse is illustrated and further evaluated against eddy-covariance measurements from two screenhouse experiments.

A Velocity–Dissipation Lagrangian Stochastic Model for Turbulent Dispersion in Atmospheric Boundary-Layer and Canopy Flows

An extended Lagrangian stochastic dispersion model that includes time variations of the turbulent kinetic energy dissipation rate is proposed. The instantaneous dissipation rate is described by a log-normal distribution to account for rare and intense bursts of dissipation occurring over short durations. This behaviour of the instantaneous dissipation rate is consistent with field measurements inside a pine forest and with published dissipation rate measurements in the atmospheric surface layer. The extended model is also shown to satisfy the well-mixed condition even for the highly inhomogeneous case of canopy flow. Application of this model to atmospheric boundary-layer and canopy flows reveals two types of motion that cannot be predicted by conventional dispersion models: a strong sweeping motion of particles towards the ground, and strong intermittent ejections of particles from the surface or canopy layer, which allows these particles to escape low-velocity regions to a high-velocity zone in the free air above. This ejective phenomenon increases the probability of marked fluid particles to reach far regions, creating a heavy tail in the mean concentration far from the scalar source.