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  • 1
    ISSN: 1573-5052
    Keywords: Canopy ; Evaporation ; Leaf area index ; Scaling ; Surface conductance ; Stomata
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract We examine conductances for evaporation from both vegetation and soil in response to environmental variables. Data from a vertically-structured pristine forest of Nothofagus are presented as an example of the effects of biodiversity on the scaling of conductances between tiers of plant organisation. Available data sets of maximum leaf stomatal conductances (g lmax ) and bulk vegetation surface conductances (G smax ) are compared. Overall, the ratio G smax /g lmax is consistently close to 3 for seven major vegetation types of diverse structure. An analytical model accounts for this close relationship, and in particular how G smax is conservative against changes in leaf area index because of the compensating decrease in plant canopy transpiration and increase in soil evaporation as leaf area index diminishes. The model is also successfully tested by comparison with canopy conductances of emergent trees measured in the Nothofagus forest. The constraint of vegetation surface conductance and evaporation via environmental regulation by irradiance, air saturation deficit and root zone water supply are discussed.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1573-0867
    Keywords: direct-seeding ; nitrogen fertilizer ; micrometeorology ; gas exchange ; irrigation ; volatilization ; denitrification
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract Ammonia loss from urea applied to dry-seeded rice, determined using a micrometeorological technique, varied considerably depending on the time of application. Ammonia volatilization was negligible, before and after flooding, when urea was applied to the dry soil surface two days before permanent flood. Before flooding, the urea prills remained undissolved and urea hydrolysis could not proceed. Thus there was no source of fertilizerderived ammonia for volatilization to occur. Upon flooding, the urea prills were washed into cracks in the soil which subsequently closed. Therefore the movement of soluble nitrogen into the floodwater was prevented, and again there was no ammonia source for the volatilization process. When urea was broadcast into the floodwater a few days after permanent flood, ammonia losses were high and varied from 11–21% of the nitrogen applied. These losses were associated with high floodwater pHs and high wind speeds near the water surface. However, when urea was applied into the floodwater at panicle initiation, ammonia losses were low (3–8% of the applied nitrogen). At this stage of growth the plant canopy shaded the floodwater, inhibiting algal photosynthesis and consequent pH elevation, thus resulting in low ammonia gas concentrations at the floodwater surface. In addition, the plant canopy restricted air movement at the water surface, thereby reducing ammonia transport away from the air-water interface. These findings provide basic information required for improving current fertilizer management practices.
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  • 3
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Methods of calibrating infrared CO2 analysers for sensitivity to CO2 and water vapour are described. Equations to correct eddy covariance CO2 flux measurements are presented for: (i) analyser cross-sensitivity to water vapour and the effects of density fluctuations arising from atmospheric fluxes of water vapour and sensible heat, (ii) flux losses caused by signal processing and limited instrument frequency response for open- and closed-path CO2 analysers, and (iii) flux losses resulting from damping of concentration fluctuations in a tube used to sample air for closed-path CO2 analysers. Examples of flux corrections required for typical instruments are presented.
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  • 4
    ISSN: 1573-1472
    Keywords: Mass balance ; Flux-gradient ; Boundary-layer budgeting ; Enteric fermentation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The paper examines the strengths and weaknesses of a rangeof meteorological flux measurement techniques that mightbe used to verify predictions of greenhouse gas inventories.Recent research into emissions of methane (CH4)produced by enteric fermentation in grazing cattle and sheepis used to illustrate various methodologies. Quantifying thisimportant source presents special difficulties because the animalsconstitute moving, heterogeneously distributed, intermittent, pointsources. There are two general approaches: one, from the bottom up,involves direct measurements of emissions from a known number ofanimals, and the other, from the top down, infers areal emissions ofCH4 from its atmospheric signature. A mass-balance methodproved successful for bottom-up verification. It permits undisturbedgrazing, has a simple theoretical basis and is appropriate for fluxmeasurements on small plots and where there are scattered pointsources. The top-down methodologies include conventional flux-gradientapproaches and convective and nocturnal boundary-layer (CBL and NBL)budgeting schemes. Particular attention is given to CBL budget methods inboth differential and integral form. All top-down methodologies require ideal weather conditions for their application, and they suffer from the scattered nature of the source, varying wind directions and low instrument resolution. As for mass-balance, flux-gradient micrometeorological measurements were in good agreement with inventory predictions of CH4 production by livestock, but the standard errors associated with both methods were too large to permit detection of changes of a few per cent in emission rate, which might be important for inventory, regulatory or research purposes. Fluxes calculated by CBL and NBL methods were of the same order of magnitude as inventory predictions, but more improvement is needed before their use can be endorsed. Opportunities for improving the precision of both bottom-up and top-down methodologies are discussed.
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Boundary layer meteorology 20 (1981), S. 445-457 
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract An anemometer based upon measurement of the tangential windspeed around a sphere with hot-film probes is described. The anemometer determined the windspeed with a root-mean-square (rms) error of 5%, and the direction with an rms error of 5.6 °. A comparison between omnidirectional and sonic anemometers in the field gave practically identical results for the vertical sensible heat flux using eddy correlation procedures. Other turbulence statistics are also reported. The new instrument should be useful for measurements in canopies, where turbulence intensities are often large.
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  • 6
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Flux densities of carbon dioxide were measured over an arid, vegetation-free surface by eddy covariance techniques and by a heat budget-profile method, in which CO2 concentration gradients were specified in terms of mixing ratios. This method showed negligible fluxes of CO2, consistent with the bareness of the experimental site, whereas the eddy covariance measurements indicated large downward fluxes of CO2. These apparently conflicting observations are in quantitative agreement with the results of a recent theory which predicts that whenever there are vertical fluxes of sensible or latent heat, a mean vertical velocity is developed. This velocity causes a mean vertical convective mass flux (= ρ cw for CO2, in standard notation). The eddy covariance technique neglects this mean convective flux and measures only the turbulent flux ρ′ c w′. Thus, when the net flux of CO2 is zero, the eddy covariance method indicates an apparent flux which is equal and opposite to the mean convective flux, i.e., ρ′ c w′ = −ρ c w. Corrections for the mean convective flux are particularly significant for CO2 because ρ cw and ρ′ c w′ are often of similar magnitude. The correct measurement of the net CO2 flux by eddy covariance techniques requires that the fluxes of sensible and latent heat be measured as well.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Eddy fluxes of CO2 estimated using a sonic anemometer and a closed-path analyser were, on average, 16% lower than those obtained with the same anemometer and an adjacent open-path CO2 analyser. Covariances between vertical windspeed and CO2 density from the closed-path analyser were calculated using data points for CO2 that were delayed relative to anemometer data by the time required for a parcel of air to travel from the tube inlet to the CO2 sensor. Air flow in the intake tube was laminar. Densities of CO2 that had been corrected for spurious fluctuations arising from fluctuations in temperature and humidity were used in the flux calculations. Corrections for the cross-sensitivity of CO2 analysers to water vapour were also incorporated. Spectral analysis of the corrected CO2 signal from the closed-path analyser showed that damping of fluctuations in the sampling tube at frequencies f 〉 0.1 Hz caused the apparent loss in flux. The measured losses can be predicted accurately using theory that describes the damping of oscillations in a sampling tube. High-frequency response of the closed-path system can be improved substantially by ensuring turbulent flow in the tube, using a combination of high volumetric flow rate and small tube diameter. The analysis of attenuation of turbulent fluctuations in flow through tubes is applicable to the measurement of fluxes of other minor atmospheric constituents using the eddy covariance method.
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  • 9
    ISSN: 1573-1472
    Keywords: Source/sink distributions ; Lagrangian dispersion ; Canopy models ; Canopy distributions ; Atmospheric stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Source/sink distributions of heat, water vapour andCO2 within a rice canopy were inferred using aninverse Lagrangian dispersion analysis and measuredmean profiles of temperature, specific humidity andCO2 mixing ratio. Monin–Obukhov similarity theorywas used to account for the effects of atmosphericstability on σw(z), the standard deviation ofvertical velocity and τL(z), the Lagrangian timescale of the turbulence. Classical surface layer scaling was applied in the inertial sublayer (z 〉 zruf)using the similarity parameter ζ = (z - d)/L, where z is height above ground, d is the zero plane displacementheight for momentum, L is the Obukhov length,and zruf ≈ 2.3hc, where hc iscanopy height. A single length scale hc, was usedfor the stability parameter 3 = hc/L in the height range 0.25 〈 z/hc 〈 2.5. This choice is justified by mixing layer theory, which shows that within the roughness sublayer there is one dominant turbulence length scaledetermined by the degree of inflection in the windprofile at the canopy top. In the absence of theoretical or experimental evidence for guidance,standard Monin–Obukhov similarity functions, withζ = hc/L, were used to calculate the stabilitydependence of σw(z) and τL(z) in the roughness sublayer. For z/hc 〈 0.25 the turbulence length and time scales are influenced by the presence of the lowersurface, and stability effects are minimal. With theseassumptions there was excellent agreement between eddycovariance flux measurements and deductions from theinverse Lagrangian analysis. Stability correctionswere particularly necessary for night time fluxes whenthe atmosphere was stably stratified. The inverse Lagrangian analysis provides a useful toolfor testing and refining multilayer canopy models usedto predict radiation absorption, energy partitioningand CO2 exchanges within the canopy and at thesoil surface. Comparison of model predictions withsource strengths deduced from the inverse analysisgave good results. Observed discrepancies may be dueto incorrect specification of the turbulent timescales and vertical velocity fluctuations close to theground. Further investigation of turbulencecharacteristics within plant canopies is required toresolve these issues.
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  • 10
    ISSN: 0168-1923
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Geography , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition , Physics
    Type of Medium: Electronic Resource
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