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861.
The computational method of Large-Eddy Simulations has been used to study the weak, neutrally stable drainage flow within
tree canopies. The computational results show that a secondary velocity maximum that resembles a jet is formed within the
canopy under the nocturnal flow conditions. This jet-like flow is important in the analysis and measurements of the net ecosystem-atmosphere
exchange (NEE) for carbon dioxide (CO2). A uniformly distributed, plane source was placed within the canopy in order to simulate the nocturnal production of CO2. The NEE is calculated as the sum of the integration of the rate of change of the concentration of CO2 over the computational domain, the vertical turbulent flux measured directly by eddy-covariance (EC) method, and the advection
terms, which are not taken into account in the EC method. Numerical results of the velocity and concentration fields, within
and above the canopy, are presented and their impact on the CO2 transport is investigated in detail. The computational results show that 15–20% of NEE is drained out by the advection process
under the canopy. The results also show that the turbulent fluctuations in the lateral direction are also significant and
may result in 2–5% CO2 transport. 相似文献
862.
国际高等职业教育的比较研究 总被引:3,自引:2,他引:3
本文分析了高等职业教育产生的背景,从办学定位、办学方式、培养目标、课程开发、培养途径、师资队伍、经费保障、法律保障等方面,比较研究了发达国家或地区的高等职业教育,最后提出了发展我国高等职业教育的若干建议。 相似文献
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864.
SPACSYS: Integration of a 3D root architecture component to carbon, nitrogen and water cycling—Model description 总被引:1,自引:0,他引:1
L. Wu M.B. McGechan N. McRoberts J.A. Baddeley C.A. Watson 《Ecological modelling》2007,200(3-4):343-359
It is an ongoing challenge to develop and demonstrate management practices that increase the sustainability of agricultural systems. Soil carbon and nitrogen dynamics directly affect soil quality, crop productivity and environmental impacts. Root systems are central to the acquisition of water and nutrients by plants, but are also a major pathway for the inputs of carbon and nutrients to soil. The complexity of both biotic and abiotic interactions, combined with stochastic changes in root architecture, makes it difficult to understand below-ground dynamics on the basis of experimentation alone. The integration of dynamic models of above-ground growth, three-dimensional root system demography, and interactions between plants and the environment, into one single model is a major challenge because of the complexity of the systems.In order to understand the interaction between a plant and the environment, it is advantageous to develop a model framework to integrate submodels that simulate various plant and environmental components. The objective of this paper is to outline a mechanistic and process-based model, which is capable of simulating interactions among environmental conditions around plants, plant growth and development, nitrogen and carbon cycles, with a three-dimensional root system submodel as an interface.The model presented in this paper is a mixed dimensional, multi-layer, field scale, weather-driven and daily time-step dynamic simulation model. The current version includes a plant growth and development component, a nitrogen cycling component, a carbon cycling component, plus a soil water component that includes representation of water flow to field drains as well as downwards through the soil layers, together with a heat transfer component. The components themselves and linkage among components are designed using object-oriented techniques, which makes the model robust, understandable and reusable. The components are implemented in the C++ programming language, and inputs and outputs of all components are organised as a database in either Microsoft® SQL Server 2000, Access 2000 or MySQL5.0. Root architecture is visualised by using the OpenGL graphics system. Preliminary validation with two separate experimental datasets shows that the model can reasonably simulate root systems, nitrogen cycling, water movement and plant growth. 相似文献
865.
Morrill PL Lacrampe-Couloume G Slater GF Sleep BE Edwards EA McMaster ML Major DW Sherwood Lollar B 《Journal of contaminant hydrology》2005,76(3-4):279-293
Stable isotope analysis of chlorinated ethene contaminants was carried out during a bioaugmentation pilot test at Kelly Air Force Base (AFB) in San Antonio Texas. In this pilot test, cis-1,2-dichloroethene (cDCE) was the primary volatile organic compound. A mixed microbial enrichment culture, KB-1, shown in laboratory experiments to reduce chlorinated ethenes to non-toxic ethene, was added to the pilot test area. Following bioaugmentation with KB-1, perchloroethene (PCE), trichloroethene (TCE) and cDCE concentrations declined, while vinyl chloride (VC) concentrations increased and subsequently decreased as ethene became the dominant transformation product. Shifts in carbon isotopic values up to 2.7 per thousand, 6.4 per thousand, 10.9 per thousand and 10.6 per thousand were observed for PCE, TCE, cDCE and VC, respectively, after bioaugmentation, consistent with the effects of biodegradation. While a rising trend of VC concentrations and the first appearance of ethene were indicative of biodegradation by 72 days post-bioaugmentation, the most compelling evidence of biodegradation was the substantial carbon isotope enrichment (2.0 per thousand to 5.0 per thousand) in ?13C(cDCE). Fractionation factors obtained in previous laboratory studies were used with isotope field measurements to estimate first-order cDCE degradation rate constants of 0.12 h(-1) and 0.17 h(-1) at 115 days post-bioaugmentation. These isotope-derived rate constants were clearly lower than, but within a factor of 2-4 of the previously published rate constant calculated in a parallel study at Kelly AFB using chlorinated ethene concentrations. Stable carbon isotopes can provide not only a sensitive means for early identification of the effects of biodegradation, but an additional means to quantify the rates of biodegradation in the field. 相似文献
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868.
Ongoing deliberations on the regulation of semivolatile organic chemicals require the assessment of chemical transport in atmospheric and marine systems. The characteristic travel distance was proposed as a measure for the transport potential in air and water. However, the existing definition treats the transport processes separately. It is shown that combined transport in coupled air-ocean systems can accelerate the overall transport into remote regions. Concentration ratios in air and water change with distance from sources depending on the initial concentration ratio and on the difference between the transport velocities. A measure is suggested facilitating the chemical screening with respect to transport potentials in such air-ocean systems. A case study for alpha and gamma-hexachlorocyclohexane shows that the suggested measure qualitatively reveals the transport potentials of these chemicals and exemplifies possible concentration patterns. 相似文献
869.
细水雾灭火技术的研究 总被引:12,自引:3,他引:9
细水雾灭火技术主要通过汽化隔氧、冷却燃料和氧化剂以及吸收部分热辐射等效应与火相互作用,降低燃烧化学反应速率和火焰传播速率,达到控制和扑灭火灾的目的,不会产生“二次性环境污染”,可以达到火灾防治洁净化目标。为此,对细水雾抑制火灾的过程和机理进行了实验研究和数值模拟,以促进细水雾灭火技术的发展 相似文献
870.