Water flux measurement and prediction in young cashew trees using sap flow data     

Philip G. Oguntunde  Nick van de Giesen 《水文研究》2005,19(16):3235-3248

Measurements of sap flow, meteorological parameters, soil water content and tension were made for 4 months in a young cashew (Anacardium occidentale L.) plantation during the 2002 rainy season in Ejura, Ghana. This experiment was part of a sustainable water management project in West Africa. The Granier system was used to measure half‐hourly whole‐tree sap flow. Weather variables were observed with an automatic weather station, whereas soil moisture and tension were measured with a Delta‐T profile probe and tensiometers respectively. Clearness index (CI), a measure of the sky condition, was significantly correlated with tree transpiration (r² = 0·73) and potential evaporation (r² = 0·86). Both diurnal and daily stomata conductance were poorly correlated with the climatic variables. Estimated daily canopy conductance g_c ranged from 4·0 to 21·2 mm s⁻¹, with a mean value of 8·0 ± 3·3 mm s⁻¹. Water flux variation was related to a range of environmental variables: soil water content, air temperature, solar radiation, relative humidity and vapour pressure deficit. Linear and non‐linear regression models, as well as a modified Priestley–Taylor formula, were fitted with transpiration, and the well‐correlated variables, using half‐hourly measurements. Measured and predicted transpiration using these regression models were in good agreement, with r² ranging from 0·71 to 0·84. The computed measure of accuracy δ indicated that a non‐linear model is better than its corresponding linear one. Furthermore, solar radiation, CI, clouds and rain were found to influence tree water flux. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

Turbulence Closures In Neutral Boundary Layer Over Complex Terrain     

Silvia Trini Castelli  Enrico Ferrero  Dominico Anfossi 《Boundary-Layer Meteorology》2001,100(3):405-419

In this work, three turbulence closure models, Mellor andYamada level 2.5, E - l and E - implemented in a circulation model, are compared in neutral condition over complex terrain. They are firstly applied to a one-dimensional case on flat terrain and then to a schematic two-dimensional valley. The simulation results, in terms of wind field and turbulent kinetic energy, are tested against measurements from a wind-tunnel experiment. The empirical constants defining the characteristic length scales of the closures are modified based on turbulence parameters estimated in the experiment. The formulation of the diffusion coefficients is analysed to explain the differences among the various closures in the simulation results. Regarding the mean flow, both on flat and complex terrain, all the closures yield satisfactory results. Concerning the turbulent kinetic energy, the best results are obtained by E - l and E - closures.  相似文献   

A Simple Single-Layer Urban Canopy Model For Atmospheric Models: Comparison With Multi-Layer And Slab Models   总被引：26，自引：7，他引：26  

Hiroyuki Kusaka  Hiroaki Kondo  Yokihiro Kikegawa  Fujio Kimura 《Boundary-Layer Meteorology》2001,101(3):329-358

We developed a simple, single-layer urban canopy model, and comparedit to both multi-layer and slab models. Our single-layer model has thefollowing features: (a) It is a column model of energy and momentumexchange between an urban surface and the atmosphere, (b) it includesthe influence of street canyons, which are parameterized to representthe urban geometry, (c) it includes shadowing from buildings andreflection of radiation, and (d) it estimates both the surfacetemperatures of, and heat fluxes from, three surface types: roof, wall,and road. In the simulation of the single-layer model, the roof washottest during the daytime, but coolest from midnight to early morning.This is consistent with output from the multi-layer model and fieldobservations at a residential area on a clear, summer day. The diurnalvariation of the energy budget from the single-layer model agrees wellwith that from the multi-layer model. Our single-layer model'sperformance is nearly that of a multi-layer model for studyingmesoscale heat islands. Nevertheless, it is simply parameterized,and thus easily included in larger-scale atmospheric models. The slabmodel has the largest nighttime cooling rate of the three models. Toovercome this, it needs more adjustments than for the canopy models.  相似文献   

青岛地区边界层结构的数值模拟   总被引：8，自引：1，他引：8  

王卫国  蒋维楣 《大气科学》1996,20(2):229-234

本文建立了一个非静力的三维细网格边界层模式，对青岛地区复杂下垫面条件下的边界层结构和湍流特征作了数值模拟，模式采用能量闭合方案，舍弃了静力近似，以提高空间分辨率和模式精度。模拟以实测资料为初始输入，对该地区风场和湍流场作了较细致的模拟分析，与观测资料相比，结果合理地一致。结果表明了陡峭地形和不规则海岸线对局地风场和湍流场影响很大。  相似文献   

The Control Of Coherent Eddies In Vegetation Canopies: Streamwise Structure Spacing, Canopy Shear Scale And Atmospheric Stability   总被引：9，自引：9，他引：0  

Y. Brunet  M. R. Irvine 《Boundary-Layer Meteorology》2000,94(1):139-163

An analogy has been established between a plane mixing layer and the atmospheric flow near the top of a vegetation canopy. It is based on a common feature, a strong inflection in the mean velocity profile, responsible for hydrodynamical instabilities that set the pattern for the coherent eddies and determine the turbulence length scales. In an earlier study, this analogy was tested using a small data set from thirteen experiments, all in near-neutral conditions. It provided a good prediction of the streamwise spacing _w of the dominant canopy eddies (evaluated from time series of vertical velocity) that appears to depend on a shear length scale L_s = U(h)/U'(h), where h is canopy height, U is mean velocity and U' the vertical gradient dU/dz. The present analysis utilizes an extensive data set of approximately 700 thirty-minute runs, from six experiments on two forest sites and a maize crop, with a large range of stability conditions. _w was estimated for each run using the wavelet transform as an objective, automated detection method. First, the variations of _w and L_s with atmospheric stability are discussed. Neutral and unstable values exhibit a large scatter whereas in stable conditions both variables decrease with increasing stability. It is subsequently found that _w is directly related to L_s, in a way close to the neutral prediction _w /h = 8.1L_s/h.The Strouhal number S_tr = L_s /_w is then shown to vary with atmospheric stability, weakly in unstable conditions, more significantly in stable conditions. Altogether these results suggest that, to some extent, the plane mixing-layer analogy can be extended to non-neutral conditions. It is argued that the primary effect of atmospheric stability, at least in stable conditions, is to modify the shear length scale L_s through changes in U(h) and U'(h), which in turn determines the streamwise spacing of the active, coherent motions.  相似文献   

Response of canopy quantum yield of alpine meadow to temperature under low atmospheric pressure on Tibetan Plateau     

《中国科学D辑(英文版)》2006,(Z2)

An open-path eddy covariance system was set up in Damxung rangeland station to measure the carbon flux from July to October, 2003. The canopy quantum yield (α) of alpine meadow was calculated by the linear function between the net ecosystem carbon dioxide exchange (NEE) and the photosynthetic active radiation (PAR) under low light, and how it was influenced by the temperature was also discussed. Results showed that the canopy or decreased almost linearly with temperature, with the decrease in every 1℃increase of temperature similar to those measured on leaf level of C3 plant. At the beginning, the decrease of canopyαwith temperature was 0.0005 umol CO2·μmol-1 PAR; while it increased to 0.0008μmol CO2·μmol-1 PAR in September, showing a rising trend with plant growing stages. Compared with the canopy a calculated with rectangular hyperbola function, the value in the paper was lower. However, the method advanced here has the advantages in examining the relationship betweenαand the key environmental factors, such as temperature.  相似文献   

Remotely sensed estimation of forest canopy density: A comparison of the performance of four methods     

Chudamani Joshi  Jan De Leeuw  Andrew K. Skidmore  Iris C. van Duren  Henk van Oosten 《International Journal of Applied Earth Observation and Geoinformation》2006

In recent years, a number of alternative methods have been proposed to predict forest canopy density from remotely sensed data. To date, however, it remains difficult to decide which method to use, since their relative performance has never been evaluated. In this study the performance of: (1) an artificial neural network, (2) a multiple linear regression, (3) the forest canopy density mapper and (4) a maximum likelihood classification method was compared for prediction of forest canopy density using a Landsat ETM+ image. Comparison of confusion matrices revealed that the regression model performed significantly worse than the three other methods. These results were based on a z-test for comparison of weighted kappa statistics, which is an appropriate statistic for analysis of ranked categories. About 89% of the variance of the observed canopy density was explained by the artificial neural networks, which outperformed the other three methods in this respect. Moreover, the artificial neural networks gave an unbiased prediction, while other methods systematically under or over predicted forest canopy density. The choice of biased method could have a high impact on canopy density inventories.  相似文献   

建筑群对城市冠层动力、热力效应的数值试验研究     

宋金暖  王振会  何金海  许建明 《气象与减灾研究》2006,29(3):5-11

利用中尺度模式MM5,分别耦合传统的边界层参数化方案和城市冠层参数化方案,模拟了2001年冬季北京冠层大气的动力、热力特征,并和观测资料进行了对比分析.分析结果表明,城市冠层参数化方案更能细致描述建筑群对城市冠层大气的动力、热力作用.由于建筑群的动力、热力作用,使北京城区的温度升高,风速减小,湍流动能增强,形成了明显的增温区、阻风区和湍能增强区,中心均位于城区高大建筑群附近,并向郊区递减.  相似文献   

基于植被-土壤二向反射模型的土壤含水量遥感   总被引：1，自引：0，他引：1  

宋承运  邓孺孺  王中挺 《国土资源遥感》2006,17(3):29-31

以北京地区为例，利用像元信息分解法，定量提取出区域植被盖度，由植被盖度得到区域叶面积指数，在只考虑一次散射的情况下，利用植被-土壤二向反射模型，提取出下层湿润土壤反射率。通过引入粗糙度因子建立起粗糙地表下土壤反射率与叶面积指数的函数关系，进而得出土壤含水量。  相似文献   

Buoyancy and The Sensible Heat Flux Budget Within Dense Canopies   总被引：1，自引：8，他引：1  

D. Cava  G. G. Katul  A. Scrimieri  D. Poggi  A. Cescatti  U. Giostra 《Boundary-Layer Meteorology》2006,118(1):217-240

In contrast to atmospheric surface-layer (ASL) turbulence, a linear relationship between turbulent heat fluxes (F_T) and vertical gradients of mean air temperature within canopies is frustrated by numerous factors, including local variation in heat sources and sinks and large-scale eddy motion whose signature is often linked with the ejection-sweep cycle. Furthermore, how atmospheric stability modifies such a relationship remains poorly understood, especially in stable canopy flows. To date, no explicit model exists for relating F_T to the mean air temperature gradient, buoyancy, and the statistical properties of the ejection-sweep cycle within the canopy volume. Using third-order cumulant expansion methods (CEM) and the heat flux budget equation, a “diagnostic” analytical relationship that links ejections and sweeps and the sensible heat flux for a wide range of atmospheric stability classes is derived. Closure model assumptions that relate scalar dissipation rates with sensible heat flux, and the validity of CEM in linking ejections and sweeps with the triple scalar-velocity correlations, were tested for a mixed hardwood forest in Lavarone, Italy. We showed that when the heat sources (S_T) and F_T have the same sign (i.e. the canopy is heating and sensible heat flux is positive), sweeps dominate the sensible heat flux. Conversely, if S_T and F_T are opposite in sign, standard gradient-diffusion closure model predict that ejections must dominate the sensible heat flux.  相似文献