Canopy-atmosphere Interface Research Articles

Seasonal variations and drivers of energy fluxes and partitioning along an aridity gradient in temperate grasslands of Northern China

Surface energy exchange is vital because it affects microclimate, water cycle and biogeochemical processes in the canopy-atmosphere interface. In this study, seasonal variations and regulators of energy fluxes and partitioning were investigated based on a long-term eddy flux observation dataset (12 towers, 82 site-years in total) along an aridity gradient in temperate grasslands of Northern China. Our results showed that the net radiation (Rn) and all energy components, including latent heat (LE), sensible heat (H) and ground heat (G0) fluxes, presented unimodal seasonal patterns across different grassland types, with Rn and LE peaked in the middle growing season but H and G0 at the beginning of the growing season. The energy partitioning shifted from the H-dominated to the LE-dominated period accompanied vegetation phenological development, thus a U-shaped seasonal pattern of Bowen ratio (β, H/LE) was observed. Prolonged LE-dominated period and great peak of LE flux enhanced LE/Rn proportion and decreased β from desert steppes to typical steppes and meadow steppes. Climate and vegetation factors co-regulated the seasonal patterns of energy fluxes and partitioning, with enhanced effects of LAI and canopy surface conductance (gc) from desert steppes to typical and meadow steppes, but a reverse pattern of land-atmosphere coupling via vegetation physiological regulation. Meanwhile, enhanced sensitivity of β in response to variations of climate and vegetation factors was also observed with increasing aridity. Our founding highlighted the role of vegetation especially physiological regulation in determining the seasonal patterns of energy fluxes and partitioning in water-limited grasslands. Whereas, the great sensitivity of energy partitioning to vegetation changing with increasing aridity also emphasized that it is vital to pay attention to the response of energy exchange in the desert steppes to the greening trend in Northern China under future climate scenarios.

Vertical propagation of submeso and coherent structure in a tall and dense amazon forest in different stability conditions. PART II: Coherent structures analysis

Observations of the vertical structure of the turbulent flow in different stability regimes above and within the Amazon Forest at the Amazon Tall Tower Observatory (ATTO) site have been presented in Part I. Here, the influence of stability on the inception and development of coherent structures is investigated. According to the mixing-layer analogy the coherent vortices that dominate the turbulent flows at the canopy-atmosphere interface are generated by hydrodynamical instabilities triggered by an inflection in the vertical profile of the mean wind speed at or near the canopy top. The coherent motions time, Tpeak and separation length scale, Λ, depend on the shear length scale, Ls, close to the canopy top. The present analysis studies the characteristics of Ls, Tpeak and Λ in the five stability regimes defined in Part I.The behaviour of Ls with stability is evaluated and parameterized. Ls increases with decreasing stability, presenting two asymptotes for large unstable and stable stratification and a linear behaviour close to neutral stratification. Coherent structures and their timescale are detected with an original method based on the autocorrelation functions of 5-min subsets of turbulent quantities. The vertical time scale is larger in neutral conditions and decreases for both increasing and decreasing stability. At the canopy top the separation length scale presents a linear dependence on Ls, whose slope is maximum in neutral conditions and decreases departing from neutrality. An original parameterization of the dependence of this slope on h/L, where h is the canopy height and L is the Obukhov length, is shown. Combining the parameterizations, the dependence of the separation length scale is finally presented. Λ has a maximum for slightly unstable conditions, linearly decreases towards neutral and weakly stable stratifications and then it tends to zero for very intense unstable and stable stratifications.

Seasonal variations in water flux compositions controlled by leaf development: isotopic insights at the canopy-atmosphere interface.

Water-stable isotopes provide a valuable tool for tracing plant-water interactions, particularly evapotranspiration (ET) partitioning and leaf water dynamics at the plant-atmosphere interface. However, process-based investigations of plant/leaf development and the associated isotopic dynamics of water fluxes involving isotope enrichment at plant-atmosphere interfaces at the ecosystem scale remain challenging. In this study, in situ isotopic measurements and tracer-aided models were used to study the dynamic interactions between vegetation growth and the isotopic dynamics of water fluxes (ET, soil evaporation, and transpiration) involving isotope enrichment in canopy leaves in a multispecies grassland ecosystem. The day-to-day variations in the isotopic compositions of ET flux were mainly controlled by plant growth, which could be explained by the significant logarithmic relationship determined between the leaf area index and transpiration fraction. Leaf development promoted a significant increase in the isotopic composition of ET and led to a slight decrease in the isotopic composition of water in canopy leaves. The transpiration (evaporation) isoflux acted to increase (decrease) the δ18O of water vapor, and the total isoflux impacts depended on the seasonal tradeoffs between transpiration and evaporation. The isotopic evidence in ET fluxes demonstrates the biotic controls on day-to-day variations in water/energy flux partitioning through transpiration activity. This study emphasizes that stable isotopes of hydrogen and oxygen are effective tools for quantitative evaluations of the hydrological component partitioning of ecosystems and plant-climate interactions.

北京山区侧柏林冠层-大气蒸腾导度模拟及环境因子响应

蒸腾导度模型是衡量冠层-大气界面水汽输出的重要阻力模型，研究其特征及对环境因子的响应，为揭示森林冠层-大气界面水汽输出阻力机制提供理论依据。以首都圈森林生态系统定位观测研究站侧柏林为研究对象，采用TDP热探针法测定侧柏林树干液流密度，同步监测光合有效辐射、饱和水汽压差、气温、风速等主要环境因子，分析冠层导度和空气动力学导度的动态变化，构建冠层-大气蒸腾导度模型并模拟，明确冠层-大气蒸腾导度对各环境因子的响应关系。结果表明：蒸腾导度季节变化表现为非生长季与冠层导度趋势一致，生长季与空气动力学导度趋势一致，全年均为单峰趋势。冬季蒸腾导度与冠层导度保持较稳定差值（45 mol m^-2 s^-1左右），其他季节蒸腾导度与冠层导度、空气动力学导度的最大差值，均在各季节冠层导度、空气动力学导度的峰值水平。全年日均蒸腾导度冬季最大（86.92 mol m^-2 s^-1），其他季节较小且稳定（40-50 mol m^-2 s^-1之间）。在非生长季各环境因子对蒸腾导度的影响与对冠层导度的影响基本一致，温度为主要影响因子（r=-0.198），其他环境因子影响较小（r<0.1）；在生长季中风速为主要影响因子（r=0.488），光合有效辐射（r=0.228）和饱和水汽压差（r=-0.299）的影响明显升高，温度的影响降低（r=0.114）。蒸腾导度模型较好的模拟了冠层-大气界面侧柏蒸腾不同季节的变化规律，阐明了各环境因子和冠层导度、空气动力学导度对蒸腾导度的影响机制，证实在生长季应重视空气动力学导度对蒸腾的影响。;Transpiration conductance model is an important resistance model to measure water vapor output at the canopy atmosphere interface. Our study foucused on its characteristics and response to environmental factors, which provided a theoretical basis for clarifying the water vapor output mechanism at the canopy atmosphere interface. In this paper, Platycladus orientalis forest observation station of forest ecosystem in the capital circle was taken as the research object. Through analyzing the dynamic changes of canopy conductance and aerodynamics conductance by TDP thermoprobe, we measured the density of sap flow in the trunk of Platycladus orientalis forest and monitoring the main environmental factors such as photosynthetic effective radiation, saturated water vapor pressure difference, air temperature, and wind speed. And the model of canopy-atmospheric transpiration conductance was derived and calculated so as to build a simulation, which clarified the response of canopy atmospheric transpiration conductance to various environmental factors. The results showed that the trend of transpiration conductance in non-growing season was consistent with that in canopy; the growing season was consistent with the trend of aerodynamic conductance, and the trend was single peak throughout the year. The difference between transpiration conductance and canopy conductance in winter remained stable (around 45 mol m^-2 s^-1);the maximum difference value among transpiration conductance, canopy conductance and aerodynamic conductance in other seasons was at the peak level of canopy conductance and aerodynamic conductance; the annual average transpiration conductance was the largest in winter (86.92 mol m^-2 s^-1) and it was smaller and stable in other seasons (40-50 mol m^-2 s^-1); in the non-growing season, the effect of environmental factors on transpiration conductance was basically the same as that on canopy conductance, and temperature was the most important factor (r=-0.198) while other environmental factors had little impact (r<0.1); in the growing season, wind speed was the most important factor (r=0.488); the influence of photosynthetic effective radiation (r=0.228) and vapor pressure deficit (r=-0.299) was significantly increased, and the influence of temperature was reduced (r=0.114). The model of transpiration conductance simulated the change rule of Platycladus orientalis transpiration in different seasons, clarified the influence mechanism of environmental factors, canopy conductance and aerodynamic conductance on transpiration conductance, and confirmed that we should pay attention to the influence of aerodynamic conductance on transpiration in growing season.

Leaf Area Index evaluation in vineyards using 3D point clouds from UAV imagery

The Leaf Area Index (LAI) is an ecophysiology key parameter characterising the canopy-atmosphere interface where most of the energy fluxes are exchanged. However, producing maps for managing the spatial and temporal variability of LAI in large croplands with traditional techniques is typically laborious and expensive. The objective of this paper is to evaluate the reliability of LAI estimation by processing dense 3D point clouds as a cost-effective alternative to traditional LAI assessments. This would allow for high resolution, extensive and fast mapping of the index, even in hilly and not easily accessible regions. In this setting, the 3D point clouds were generated from UAV-based multispectral imagery and processed by using an innovative methodology presented here. The LAI was estimated by a multivariate linear regression model using crop canopy descriptors derived from the 3D point cloud, which account for canopy thickness, height and leaf density distribution along the wall. For the validation of the estimated LAI, an experiment was conducted in a vineyard in Piedmont: the leaf area of 704 vines was manually measured by the inclined point quadrant approach and six UAV flights were contextually performed to acquire the aerial images. The vineyard LAI estimated by the proposed methodology showed to be correlated with the ones obtained by the traditional manual method. Indeed, the obtained R2 value of 0.82 can be considered fully adequate, compatible to the accuracy of the reference LAI manual measurement.

Wheat leaf area index retrieval using RISAT-1 hybrid polarized SAR data

Leaf Area Index (LAI) is a key parameter to characterize the canopy–atmosphere interface, where most of the energy fluxes exchange. Space-borne satellite images have shown their relevance for various applications including LAI retrieval over large areas. Although optical data have been used for this purpose in previous studies, the constraints to acquire optical data during extreme weather conditions due to the presence of clouds, haze, smoke etc. hinders its use for uninterrupted monitoring. This study aims to analyze the relationships of C-band RISAT-1 hybrid polarized SAR data (σ˚RH and σ˚RV) with wheat LAI. The results have shown the correlation coefficient (|r|) of 0.57 and 0.73 for RH and RV backscatter, respectively, using non-linear regression approach. It is also observed that the accuracy of LAI retrieval has been significantly improved with |r| and RMSE of 0.81 and 0.54 (m2/m2), respectively, by considering both RH and RV backscatter as inputs for support vector machine-based model.

Characteristics of canopy stomatal conductance of Platycladus orientalis and its responses to environmental factors in the mountainous area of North China

Canopy stomatal conductance (gs) is an important biological parameter to measure water vapor flux in canopy atmosphere interface. Exploring characteristics of canopy stomatal conductance and its responses to environmental factors can offer theoretical evidence for conducting mechanism based study of vapor exchange process in forest canopy. SF-L thermal dissipation probe was adopted to measure sap flow density (Js) of Platycladus orientalis in 2014. The environmental factors including photosynthetically active radiation (PAR), vapor pressure deficit (VPD) and air temperature (T) were simultaneously observed. The canopy stomatal conductance was calculated, and the responses of gs to environmental factors were analyzed. The results showed that the daily variation of sap flow density was of two peak pattern curve, and the Js in growing season was higher than in non-growing season. And the bigger DBH was, the higher Js would be. The daily variation of canopy stomatal conductance was similar to the canopy transpiration of per leaf area (EL), which was of two peak pattern curve. The gs and EL in growing season were higher than those in non growing season. The canopy stomatal conductance and air temperature presented parabolic relation, and gs reached peak valley at around 10 ℃. Taking the value of 400 μmol·m-2·s-1 as the PAR thre shold, when PAR was under this threshold, gs was positively correlated with PAR. When PAR was beyond this threshold, the correlation between gs and PAR would be small. The relationship between canopy stomatal conductance and vapor pressure deficit was negative logarithmic function. As VPD increasing, gs decreased gradually. Higher air temperature and photosynthetically active radiation, and lower vapor pressure deficit contributed to higher canopy stomatal conductance of P. orientalis and could promote greater canopy transpiration.

Modeling seasonal surface temperature variations in secondary tropical dry forests

Secondary tropical dry forests (TDFs) provide important ecosystem services such as carbon sequestration, biodiversity conservation, and nutrient cycle regulation. However, their biogeophysical processes at the canopy-atmosphere interface remain unknown, limiting our understanding of how this endangered ecosystem influences, and responds to the ongoing global warming. To facilitate future development of conservation policies, this study characterized the seasonal land surface temperature (LST) behavior of three successional stages (early, intermediate, and late) of a TDF, at the Santa Rosa National Park (SRNP), Costa Rica. A total of 38 Landsat-8 Thermal Infrared Sensor (TIRS) data and the Surface Reflectance (SR) product were utilized to model LST time series from July 2013 to July 2016 using a radiative transfer equation (RTE) algorithm. We further related the LST time series to seven vegetation indices which reflect different properties of TDFs, and soil moisture data obtained from a Wireless Sensor Network (WSN). Results showed that the LST in the dry season was 15–20K higher than in the wet season at SRNP. We found that the early successional stages were about 6–8K warmer than the intermediate successional stages and were 9–10K warmer than the late successional stages in the middle of the dry season; meanwhile, a minimum LST difference (0–1K) was observed at the end of the wet season. Leaf phenology and canopy architecture explained most LST variations in both dry and wet seasons. However, our analysis revealed that it is precipitation that ultimately determines the LST variations through both biogeochemical (leaf phenology) and biogeophysical processes (evapotranspiration) of the plants. Results of this study could help physiological modeling studies in secondary TDFs.

Characteristics of canopy turbulence over a deciduous forest on complex terrain

Canopy turbulence plays an important role in mass and energy exchanges at the canopy-atmosphere interface. Despite extensive studies on canopy turbulence over a flat terrain, less attention has been given to canopy turbulence in a complex terrain. The purpose of this study is to scrutinize characteristics of canopy turbulence in roughness sublayer over a hilly forest terrain. We investigated basic turbulence statistics, conditionally sampled statistics, and turbulence spectrum in terms of different atmospheric stabilities, wind direction and vertical structures of momentum fluxes. Similarly to canopy turbulence over a homogeneous terrain, turbulence statistics showed coherent structure. Both quadrant and spectrum analysis corroborated the role of intermittent and energetic eddies with length scale of the order of canopy height, regardless of wind direction except for shift of peak in vertical wind spectrum to relatively high frequency in the down-valley wind. However, the magnitude of the momentum correlation coefficient in a neutral condition was smaller than typical value over a flat terrain. Further scrutiny manifested that, in the up-valley flow, temperature skewness was larger and the contribution of ejection to both momentum and heat fluxes was larger compared to the downvalley flow, indicating that thermal instability and weaker wind shear in up-valley flow asymmetrically affect turbulent transport within the canopy.

Turbulent Pressure and Velocity Perturbations Induced by Gentle Hills Covered with Sparse and Dense Canopies

How the spatial perturbations of the first and second moments of the velocity and pressure fields differ for flow over a train of gentle hills covered by either sparse or dense vegetation is explored using large-eddy simulation (LES). Two simulations are investigated where the canopy is composed of uniformly arrayed rods each with a height that is comparable to the hill height. In the first simulation, the rod density is chosen so that much of the momentum is absorbed within the canopy volume yet the canopy is not dense enough to induce separation on the lee side of the hill. In the second simulation, the rod density is large enough to induce recirculation inside the canopy on the lee side of the hill. For this separating flow case, zones of intense shear stress originating near the canopy-atmosphere interface persist all the way up to the middle layer, ‘contaminating’ much of the middle and outer layers with shear stress gradients. The implications of these persistent shear-stress gradients on rapid distortion theory and phase relationships between higher order velocity statistics and hill-induced mean velocity perturbations (Δu) are discussed. Within the inner layer, these intense shear zones improve predictions of the spatial perturbation by K-theory, especially for the phase relationships between the shear stress (~ ∂Δu/∂z) and the velocity variances, where z is the height. For the upper canopy layers, wake production increases with increasing leaf area density resulting in a vertical velocity variance more in phase with Δu than with ∂Δu/∂z. However, background turbulence and inactive eddies may have dampened this effect for the longitudinal velocity variance. The increase in leaf area density does not significantly affect the phase relationship between mean surface pressure and topography for the two simulations, though the LES results here confirm earlier findings that the minimum mean pressure shifts downstream from the hill crest. The increase in leaf area density and associated flow separation simply stretches this difference further downstream. This shift increases the pressure drag, the dominant term in the overall drag on the hill surface, by some 15%. With regards to the normalized pressure variance, increasing leaf area density increases \({\sigma_p/u_{*}^{2}}\) near the canopy top, where u* is the longitudinally averaged friction velocity at the canopy top and σp is the standard deviation of the pressure fluctuations. This increase is shown to be consistent with a primitive scaling argument on the leading term describing the mean-flow turbulent interaction. This scaling argument also predicts the spatial variations in σp above the canopy reasonably well for both simulations, but not inside the canopy.

The Effects of Vegetation Density on Coherent Turbulent Structures within the Canopy Sublayer: A Large-Eddy Simulation Study

Terrestrial ecosystems are characterized by a wide range of canopy vegetation density, which is known to affect turbulent transport processes across the canopy–atmosphere interface. In the presence of a dense and horizontally homogeneous canopy, the canopy sublayer has been described as resembling a plane mixing layer. At the other extreme, where the canopy is essentially absent, the canopy sublayer is typically assumed to be similar to a turbulent boundary layer over a rough surface. However, it remains unclear how the canopy turbulence changes from boundary-layer-like to mixing-layer-like as the vegetation density increases. We use large-eddy simulation to study five different vegetation densities varying from an extremely sparse canopy to an extremely dense canopy. This investigation draws on the study of flow statistics as well as large-scale coherent turbulent structures within the canopy sublayer. The coherent structures are identified through the use of proper orthogonal decomposition. The results of skewness of velocity components and characteristic length scales suggest that, as the vegetation density increases, the canopy turbulence gradually undergoes a transition from resembling a rough-wall boundary layer to being similar to a mixing layer. As demonstrated by others, we found that the coherent structures within the canopy sublayer consist of a strong sweep/ejection motion framed by a counter-rotating vortex pair with elliptical cross-sections. As the canopy becomes denser, these important structures are shown here to be more elevated. Vegetation density does not appear to have a significant effect on the percentage of the total turbulent kinetic energy that is represented by the coherent structures.

Radiation balance and evaporation partitioning in a narrow-row soybean canopy

Seeding rate and row spacing of agricultural crops are managed to maximize yield but also have significant implications for energy partitioning and canopy microclimate. The objective of this study was to measure radiation budget components in a narrow-row soybean [ Glycine max (L.) Merr.] canopy, determine soil water evaporation ( E) as the difference between measured evapotranspiration (ET) and plant transpiration ( T), and compare measured E and T with estimates from an energy balance approach. The study was completed in a production soybean field near Ames, IA, with 350,000 plants ha −1 in 0.38 m wide rows. The field was planted on 8 May 2004 and continuous measurements were made from 8 June to 27 September. Shortwave and longwave radiation components were measured above the canopy with hemispherical radiometers. Net all-wave and incoming and reflected shortwave radiation were measured beneath the canopy with line radiometers and downwelling longwave radiation was calculated from a radiation balance. ET was measured with an eddy covariance system and T was measured with sapflow stem gauges. On sunny days under full canopy conditions, nearly 90% of the shortwave and net radiation was attenuated by the canopy with over 80% of the available energy utilized by ET. E accounted for 8–12% of the ET under full canopy conditions although these percentages may be overestimates due to evaporation of dew present on the canopy. With such a large proportion of the available energy consumed by ET, fluxes of sensible heat ( H) were very low and vertical temperature gradients across the soil–canopy–atmosphere interface were only 2–3 °C. A Priestley–Taylor energy balance approach used to estimate E and T tended to underestimate E and overestimate T. The E underestimate may be due in part to E including evaporation of dew. Conversely, T was overestimated by 25 and 14% on sunny days under full canopy conditions even with direct measurement of R n interception by the canopy. Use of shortwave extinction coefficients to estimate R n interception failed to improve T estimates significantly. The high-population, narrow-row planting strategy resulted in a dense canopy that, under full canopy conditions, resulted in very little light penetration or E.

Parameterization of the Urban Energy Budget with the Submesoscale Soil Model

AbstractThe thermal component of the Soil Model for Submesoscales, Urbanized Version (SM2-U), is described. SM2-U is an extension on a physical basis of the rural Interactions between Soil, Biosphere, and Atmosphere (ISBA) soil model to urban areas. It evaluates the turbulent energy, moisture, and radiative fluxes at the urban canopy–atmosphere interface to provide lower boundary conditions of high-resolution mesoscale models. Unlike previous urban canopy schemes, SM2-U integrates in a simple way the physical processes inside the urban canopy: the building wall influence is integrated in the pavement temperature equation, allowing the model to compute directly the energy budget of street canyons. The SM2-U model is evaluated on the Marseille, France, city-center energy-budget components measured during the field experiments to constrain models of atmospheric pollution and transport of emissions [Expérience sur Site pour Contraindre les Modèles de Pollution Atmosphérique et de Transport d’Emissions (ESCOMPTE)] urban boundary layer (UBL) campaign (June–July 2001). The observed behavior of net radiation and heat fluxes is reproduced by SM2-U with a high level of quality, demonstrating that the influence of building walls may be well modeled by modifying the pavement temperature equation. A sensitivity analysis shows that the accurate account of wall area and the parameterization of both the fast response of artificial materials to environmental forcing variations and their heat storage capacity are essential for mesoscale simulations of the urban boundary layer; they are probably more important than accurate but complex computation of radiative trapping (effective albedo and emissivity)

Organised Motion in a Tall Spruce Canopy: Temporal Scales, Structure Spacing and Terrain Effects

This study investigates the organised motion near the canopy-atmosphere interface of a moderately dense spruce forest in heterogeneous, complex terrain. Wind direction is used to assess differences in topography and surface properties. Observations were obtained at several heights above and within the canopy using sonic anemometers and fast-response gas analysers over the course of several weeks. Analysed variables include the three-dimensional wind vector, the sonic temperature, and the concentration of carbon dioxide. Wavelet analysis was used to extract the organised motion from time series and to derive its temporal scales. Spectral Fourier analysis was deployed to compute power spectra and phase spectra. Profiles of tem- poral scales of ramp-like coherent structures in the vertical and longitudinal wind components showed a reversed variation with height and were of similar size within the canopy. Temporal scales of scalar fields were comparable to those of the longi- tudinal wind component suggesting that the lateral scalar transport dominates. The existence of a -1 power law in the longitudinal power spectra was confirmed for a few cases only, with a majority showing a clear 5/3 decay. The variation of effective scales of organised motion in the longitudinal velocity and temperature were found to vary with atmospheric stability, suggesting that both Kelvin-Helmholtz instabili- ties and attached eddies dominate the flow with increasing convectional forcing. The canopy mixing-layer analogy was observed to be applicable for ramp-like coherent structures in the vertical wind component for selected wind directions only. Depar- tures from the prediction of m = � wL −1 s = 8-10 (wherew is the streamwise spacing of coherent structures in the vertical wind w and Ls is a canopy shear length scale) were caused by smaller shear length scales associated with large-scale changes in the terrain as well as the vertical structure of the canopy. The occurrence of linear gravity

Parameterization of the Urban Water Budget with the Submesoscale Soil Model

Abstract This paper presents the hydrological component of the Submesoscale Soil Model, urbanized version (SM2-U). This model is an extension of the rural Interactions between Soil, Biosphere, and Atmosphere (ISBA) soil model to urban surfaces. It considers in detail both rural and urban surfaces. Its purpose is to compute the sensible heat and humidity fluxes at the canopy–atmosphere interface for the computational domain lower boundary condition of atmospheric mesoscale models in order to simulate the urban boundary layer in any weather conditions. Because it computes separately the surface temperature of each land use cover mode while the original model computes a unique temperature for the soil and vegetation system, the new version is first validated for rural grounds by comparison with experimental data from the Hydrological Atmospheric Pilot Experiment-Modélisation du Bilan Hydrique (HAPEX-MOBILHY) and the European Field Experiment in a Desertification Threatened Area (EFEDA). The SM2-U water budget is then evaluated on the experimental data obtained at a suburban site in the Nantes urban area (Rezé, France), both on an annual scale and for two stormy events. SM2-U evaluates correctly the water flow measured in the drainage network (DN) at the annual scale and for the summer storm. As for the winter storm, when the soil is saturated, the simulation shows that water infiltration from the soil to the DN must be taken care of to evaluate correctly the DN flow. Yet, the addition of this soil water infiltration to the DN does not make any difference in the simulated surface fluxes that are the model outputs for simulating the urban boundary layer. Urban hydrological parameters are shown to largely influence the available water on artificial surfaces for evaporation and to influence less the evapotranspiration from natural surfaces. The influence of the water budget and surface structure on the suburban site local climatology is demonstrated.

Ground-based measurements of leaf area index: a review of methods, instruments and current controversies

Leaf area index (LAI) is the total one-sided area of leaf tissue per unit ground surface area. It is a key parameter in ecophysiology, especially for scaling up the gas exchange from leaf to canopy level. It characterizes the canopy-atmosphere interface, where most of the energy fluxes exchange. It is also one of the most difficult to quantify properly, owing to large spatial and temporal variability. Many methods have been developed to quantify LAI from the ground and some of them are also suitable for describing other structural parameters of the canopy. This paper reviews the direct and indirect methods, the required instruments, their advantages, disadvantages and accuracy of the results. Analysis of the literature shows that most cross-validations between direct and indirect methods have pointed to a significant underestimation of LAI with the latter techniques, especially in forest stands. The two main causes for the discrepancy, clumping and contribution of stem and branches, are discussed and some recent theoretical or technical solutions are presented as potential improvements to reduce bias or discrepancies. The accuracy, sampling strategy and spatial validity of the LAI measurements have to be assessed for quality assurance of both the measurement and the modelling purposes of all the LAI-dependent ecophysiological and biophysical processes of canopies.

Modelling the discrimination of13CO2above and within a temperate broad-leaved forest canopy on hourly to seasonal time scales

Fluxes and concentrations of carbon dioxide and 13CO2 provide information about ecosystem physiological processes and their response to environmental variation. The biophysical model, CANOAK, was adapted to compute concentration profiles and fluxes of 13CO2 within and above a temperate deciduous forest (Walker Branch Watershed, Tennessee, USA). Modifications to the model are described and the ability of the new model (CANISOTOPE) to simulate concentration profiles of 13CO2, its flux density across the canopy–atmosphere interface and leaf-level photosynthetic discrimination against 13CO2 is demonstrated by comparison with field measurements. The model was used to investigate several aspects of carbon isotope exchange between a forest ecosystem and the atmosphere. During the 1998 growing season, the mean photosynthetic discrimination against 13CO2, by the deciduous forest canopy (Δcanopy), was computed to be 22·4‰, but it varied between 18 and 27‰. On a diurnal basis, the greatest discrimination occurred during the early morning and late afternoon. On a seasonal time scale, the greatest diurnal range in Δcanopy occurred early and late in the growing season. Diurnal and seasonal variations in Δcanopy resulted from a strong dependence of Δcanopy on photosynthetically active radiation and vapour pressure deficit of air. Model calculations also revealed that the relationship between canopy-scale water use efficiency (CO2 assimilation/transpiration) and Δcanopy was positive due to complex feedbacks among fluxes, leaf temperature and vapour pressure deficit, a finding that is counter to what is predicted for leaves exposed to well-mixed environments.

Dominate time scale characteristics of the turbulence across canopy-atmosphere interface of a mixed broadleaved-Korean pine forest in Changbai Mountains

The measurement and observation for this study were carried out at Forest Ecosystem Opened Research Station of Changbai Mountains (128°28′E and 42°24′ N, Jilin Province, P. R. China). Characteristics of Dominant time scales (DTSes) and dominant time scales contribution ratios (DCRs) across canopy-atmosphere interface and controlling factors were analyzed with multi-scale method. DTS of stream wise and lateral velocity components were larger than that of vertical velocity component. While DCR of vertical velocity component was larger than that of the stream wise and lateral velocity components. Effect of atmospheric stability on DCR and DTS was nonlinear. DTS under unstable conditions was larger than that under stable conditions; DTS and DCR were determined upon the original scales characteristics of upwind flow under strong stability. Canopy structure would influence DTS and DCR across canopy-atmosphere interface remarkably.

Modelling Vegetation-Atmosphere Co2 Exchange By A Coupled Eulerian-Langrangian Approach

A Eulerian-Lagrangian canopy microclimate model was developed with the aim of dis- cerning physical from biophysical controls of CO2 and H2O fluxes. The model couples radiation attenuation with mass, energy, and momentum exchange at different canopy levels. A unique feature of the model is its ability to combine higher order Eulerian closure approaches that compute velocity statistics with Lagrangian scalar dispersion approaches within the canopy volume. Explicit account- ing for within-canopy CO2 ,H 2O, and heat storage is resolved by considering non-steadiness in mean scalar concentration and temperature. A seven-day experiment was conducted in August 1998 to investigate whether the proposed model can reproduce temporal evolution of scalar (CO2 ,H 2 Oa nd heat) fluxes, sources and sinks, and concentration profiles within and above a uniform 15-year old pine forest. The model reproduced well the measured depth-averaged canopy surface temperature, and CO2 concentration profiles within the canopy volume, CO 2 storage flux, net radiation above the canopy, and heat and mass fluxes above the canopy, as well as the velocity statistics near the canopy-atmosphere interface. Implications for scaling measured leaf-level biophysical functions to ecosystem scale are also discussed.