Aerodynamic Roughness Parameters Research Articles

Assessment of aerodynamic roughness parameters of turbulent boundary layers over barnacle-covered surfaces

Full-scale drag penalty predictions of flows over rough walls require surface roughness characterisation from laboratory experiments or numerical simulations. In either approach, it is necessary to determine the so-called equivalent sand-grain roughness height (k_s). There are several steps involved in determining this aerodynamic roughness lengthscale, but its procedure typically includes a combination of measurement of wall-shear stress (tau _w) using direct or indirect methods as well as analysis of velocity profiles. Indirect methods usually rely on assumptions made about flow and its scaling including the validity of universal outer-layer similarity. However, the implications of the underlying assumptions involved in full-scale drag prediction are unclear. In this work, we carry out wind tunnel measurements over a realistic rough surface (from a fouled ship-hull) to evaluate the impact of different methods with an emphasis on using the outer-layer similarity hypothesis for full-scale drag predictions. Wall-shear stress is measured using an in-house floating-element drag balance (DB), and velocity profiles are obtained using particle image velocimetry (PIV), allowing the evaluation of k_s, and the associated wake parameters through several methods. The aerodynamic roughness parameters hence obtained are used for full-scale drag penalty calculations. It is observed that the predicted drag penalty can vary by over 15% among the different methods highlighting the care that should be taken when employing such methods.

Large-scale coherent structure and canopy-level turbulence in the convective boundary layer over urban areas

Thermally unstable stratification alters the winds over urban areas and excites organized, large-scale motions (LSMs) in the atmospheric boundary layer (ABL). The current understanding of the interaction between urban turbulence and buoyancy-induced coherent structures is rather limited due to their multiscale, multiphysics nature. Nine sets of large-eddy simulation (LES) are performed to critically examine the influence of atmospheric instability on turbulent boundary layers (TBLs) interacting with idealized urban roughness elements. Varying the stratification from neutral to almost free convective conditions enables the coherent structures to transit from horizontal rolls to open cellular patterns. The aerodynamic roughness parameters tend to decrease in more unstable stratification, potentially a result of buoyancy effects that might enhance organized ejections and suppress energetic sweeps within the roughness sublayers (RSLs). Unexpectedly, roughness-induced sweeps dominate the vertical momentum fluxes within urban canopies even under the most convective condition. However, decoupling the momentum and heat transports in highly convective states results in a subtle influence of urban roughness on the vertical heat fluxes therein. The spatial distribution of canopy-level turbulent momentum fluxes is found to be strongly modulated by the overlying large-scale coherent structures. The modulation is enhanced with increasing instability. In addition, urban-like surfaces elevate the large-scale rolls and assist them in forming under a weaker stratification than in canonical settings.

Time‐lapse photogrammetry reveals hydrological controls of fine‐scale High‐Arctic glacier surface roughness evolution

AbstractIn a warming Arctic, as glacier snowlines rise, short‐ to medium‐term increases in seasonal bare‐ice extent are forecast for the next few decades. These changes will enhance the importance of turbulent energy fluxes for surface ablation and glacier mass balance. Turbulent energy exchanges at the ice surface are conditioned by its topography, or roughness, which has been hypothesized to be controlled by supraglacial hydrology at the glacier scale. However, current understanding of the dynamics in surface topography, and the role of drainage development, remains incomplete, particularly for the transition between seasonal snow cover and well‐developed, weathered bare‐ice. Using time‐lapse photogrammetry, we report a daily timeseries of fine (millimetre)‐scale supraglacial topography at a 2 m2 plot on the Lower Foxfonna glacier, Svalbard, over two 9‐day periods in 2011. We show traditional kernel‐based morphometric descriptions of roughness were ineffective in describing temporal change, but indicated fine‐scale albedo feedbacks at depths of ~60 mm contributed to conditioning surface topography. We found profile‐based and two‐dimensional estimates of roughness revealed temporal change, and the aerodynamic roughness parameter, z0, showed a 22–32% decrease from ~1 mm following the exposure of bare‐ice, and a subsequent 72–77% increase. Using geostatistical techniques, we identified ‘hole effect’ properties in the surface elevation semivariograms, and demonstrated that hydrological drivers control the plot‐scale topography: degradation of superimposed ice reduces roughness while the inception of braided rills initiates a subsequent development and amplification of topography. Our study presents an analytical framework for future studies that interrogate the coupling between ice surface roughness and hydro‐meteorological variables and seek to improve parameterizations of topographically evolving bare‐ice areas.

Automated terrain generation for precise atmospheric boundary layer simulation in the wind tunnel

This study presents a two-stage framework to characterize boundary layer wind tunnel (BLWT) approach flows naturally developed over grid roughness for partial atmospheric boundary layer (ABL) simulation. The first stage applies curve fitting techniques to a comprehensive series of high-resolution spatially-averaged velocity profile measurements to estimate aerodynamic roughness parameters (ARPs) for a wide range of homogeneous (i.e., equal height) roughness element configurations. For this study, an automated (i.e., computer-controlled) 62 ​× ​18 roughness element array called the Terraformer was used to generate 33 unique roughness element fields. The mean flow structure was captured downwind to the Terraformer, where key ARPs—i.e., the urban canopy attenuation coefficient, zero-plane displacement height, shear (friction) velocity, roughness length, and Coles’ wake strength coefficient—were estimated. In contrast to previous ABL modeling methods that primarily focused on curve fitting of the inertial sublayer (ISL), the proposed approach applies the urban canopy exponential profile within the roughness sublayer (RSL), the log law in the ISL, and the law of the wake in the outer wake layer to model full-depth (i.e., floor to freestream) rough-wall turbulent boundary layers. Further, the method explicitly captures potential variability of Reynolds shear stress in the ISL and the wake strength in the outer layer to generalize characterization of naturally-developed BLs produced by traditional tunnel designs. The second stage applies a morphometric model for each ARP—calibrated with estimates from Stage 1—to predict flow characteristics for a wide range of roughness element configurations, with the goal of producing a deterministic solution for selecting an element configuration to satisfy user-specified aerodynamic objectives for the approach flow. The calibrated models effectively interpolate between estimates, e.g., ARPs estimated for open and suburban terrains can be applied in the second stage model calibration to predict ARPs for a “rough-open” condition without further experimentation. The findings of this study demonstrate that coupling the proposed framework with a mechanized roughness element grid can significantly reduce the trial-and-error required to commission a BLWT, while improving the quality of flow characterization.

Urban morphology parameters from global digital elevation models: Implications for aerodynamic roughness and for wind-speed estimation

Urban morphology and aerodynamic roughness parameters are derived from three global digital elevation models (GDEM): Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), Shuttle Radar Topography Mission (SRTM), and TanDEM-X. Initially, each is compared to benchmark elevation data in London (UK). A moving window is used to extract ground heights from the GDEMs, generating terrain models with root-mean-square accuracy of up to 3 m. Subtraction of the ground heights extracted from each GDEM provides roughness-element heights only, allowing for calculation of morphology parameters. The parameters are calculated for eight directional sectors of 1 km grid-squares. Apparent merging of roughness elements in all GDEMs causes height-based parameter underestimation, whilst plan and frontal areas are over- and under-estimated, respectively. Combined, these lead to an underestimation of morphometrically-derived aerodynamic roughness parameters. Parameter errors are least for the TanDEM-X data. Further comparison in five cities (Auckland, Greater London, New York, Sao Paulo, Tokyo) provides basis for empirical corrections to TanDEM-X-derived geometric parameters. These reduce the error in parameters across the cities and for a separate location. Meteorological observations in central London give insight to wind-speed estimation accuracy using roughness parameters from the different elevation databases. The proposed corrections to TanDEM-X parameters lead to improved wind-speed estimates, which combined with the improved spatial representation of parameters across cities demonstrates their potential for use in future studies.

Assessing methods to extrapolate the vertical wind-speed profile from surface observations in a city centre during strong winds

Knowledge of the vertical wind-speed profile in cities is important for the construction and insurance industries, wind energy predictions, and simulations of pollutant and toxic gas release. Here, five methods to estimate the spatially- and temporally-averaged wind-speed profile are compared in London: the logarithmic wind law (LOG); the Deaves and Harris equilibrium (DHe) and non-equilibrium (DHv) models; an adaptation of the power law (PL) and the Gryning et al. (GR) profile. Using measurements at 2.5 times the average building height, a source area model is used to determine aerodynamic roughness parameters using two morphometric methods, which assume homogeneous and variable roughness-element heights, respectively. Hourly-averaged wind speeds are extrapolated to 200 m above the canopy during strong wind conditions, and compared to wind speeds observed with Doppler lidar. Wind speeds are consistently underestimated if roughness-element height variability is not considered during aerodynamic parameter determination. Considering height variability, the resulting estimations with the DHe and GR profiles are marginally more similar to observations than the DHv profile, which is more accurate than the LOG and PL methods. An exception is in directions with more homogeneous fetch and a gradual reduction in upwind roughness, where the LOG and PL profiles are more appropriate.

Aerodynamic roughness variation with vegetation: analysis in a suburban neighbourhood and a city park

Local aerodynamic roughness parameters (zero-plane displacement, zd, and aerodynamic roughness length, z0) are determined for an urban park and a suburban neighbourhood with a new morphometric parameterisation that includes vegetation. Inter-seasonal analysis at the urban park demonstrates zd determined with two anemometric methods is responsive to vegetation state and is 1–4 m greater during leaf-on periods. The seasonal change and directional variability in the magnitude of zd is reproduced by the morphometric methods, which also indicate z0 can be more than halved during leaf-on periods. In the suburban neighbourhood during leaf-on, the anemometric and morphometric methods have similar directional variability for both zd and z0. Wind speeds at approximately 3 times the average roughness-element height are estimated most accurately when using a morphometric method which considers roughness-element height variability. Inclusion of vegetation in the morphometric parameterisation improves wind-speed estimation in all cases. Results indicate that the influence of both vegetation and roughness-element height variability are important for accurate determination of local aerodynamic parameters and the associated wind-speed estimates.

Wind tunnel study of turbulent flow past an urban canyon model

Modeling dispersion in urban area requires appropriate input parameters, in par- ticular aerodynamic roughness parameters. A low-speed wind tunnel was deployed to study flow patterns over an urban canyon model with three aspect ratios and three flow speeds of 2, 5, and 10m/s with the objective of obtaining these parameters. Flow speed, standard devi- ation, and turbulence intensity profiles were determined with a single directional hot-wire anemometeratseveralpositionsacrosstheurbancanyonmodel.Theaerodynamicparameters u∗, z0,andd0 wereobtainedfromflowspeedprofileviaanon-linearfitafterasuitablechoice of the initial value of d0 for which all aerodynamic parameters converge. Flow speed and standard deviation profiles do not change significantly with the position across the canyon, but are much affected by the free flow speed. The regular way they respond to the free flow speed suggested a normalization for which all profiles collapse onto a single profile, which dependsonlyonthecanyonaspectratio.Thenormalizationcriterionrevealedtobeimportant for obtaining convergent dimensionless profiles. To describe the general profiles characteris- tics a simple new parameterization is proposed, in which a single-valued function (Gaussian curve) describing the flow speed profile is used in a flux-gradient relationship for describing the standard deviation profiles. This parameterization works well down to z/ h ∼ 0.25 -0.50.

Calibration of aerodynamic roughness over the Tibetan Plateau with Ensemble Kalman Filter analysed heat flux

Abstract. Aerodynamic roughness height (Zom) is a key parameter required in several land surface hydrological models, since errors in heat flux estimation are largely dependent on optimization of this input. Despite its significance, it remains an uncertain parameter which is not readily determined. This is mostly because of non-linear relationship in Monin-Obukhov similarity (MOS) equations and uncertainty of vertical characteristic of vegetation in a large scale. Previous studies often determined aerodynamic roughness using a minimization of cost function over MOS relationship or linear regression over it, traditional wind profile method, or remotely sensed vegetation index. However, these are complicated procedures that require a high accuracy for several other related parameters embedded in serveral equations including MOS. In order to simplify this procedure and reduce the number of parameters in need, this study suggests a new approach to extract aerodynamic roughness parameter from single or two heat flux measurements analyzed via Ensemble Kalman Filter (EnKF) that affords non-linearity. So far, to our knowledge, no previous study has applied EnKF to aerodynamic roughness estimation, while the majority of data assimilation study have paid attention to updates of other land surface state variables such as soil moisture or land surface temperature. The approach of this study was applied to grassland in semi-arid Tibetan Plateau and maize on moderately wet condition in Italy. It was demonstrated that aerodynamic roughness parameter can be inversely tracked from heat flux EnKF final analysis. The aerodynamic roughness height estimated in this approach was consistent with eddy covariance method and literature value. Through a calibration of this parameter, this adjusted the sensible heat previously overestimated and latent heat flux previously underestimated by the original Surface Energy Balance System (SEBS) model. It was considered that this improved heat flux estimation especially during the summer Monsoon period, based upon a comparison with precipitation and soil moisture field measurement. For an advantage of this approach over other previous methodologies, this approach is useful even when eddy covariance data are absent at a large scale and is time-variant over vegetation growth, as well as is not directly affected by saturation problem of remotely sensed vegetation index.

Surface roughness analysis of a conifer forest canopy with airborne and terrestrial laser scanning techniques

Two digital Canopy Height Models (CHMs) were generated using the novel Terrestrial Laser Scanning (TLS) technique combined with Airborne Laser Scanning (ALS) data, acquired over a conifer forest. The CHMs were used to extract cross-sections in order to derive surface geometric parameters. Different morphometric models were applied to estimate aerodynamic roughness parameters: the roughness length (z0) and the displacement height (d0). The CHMs were also used to derive the area–height relationship of the canopy surface. In order to estimate roughness parameters the observed canopy area–height relationship was modelled by uniform roughness elements of paraboloid or conical shape. The estimated average obstacle density varies between 0.14 and 0.24 for both CHMs. The canopy height distribution is approximately Gaussian, with average heights of about 26m and 21m for CHMs generated with data from TLS and ALS respectively. The estimated values of z0 and d0 depend very much on the selected model. It was observed that the Raupach models with parameters tuned to resemble the forest structure of the study area can be applied to a wide range of roughness densities. The cumulative area–height modelling approach also yielded results which are compatible with other models. The results confirm that, to model the upper canopy surface of the conifer forest, both the cone and the paraboloid shapes are fairly appropriate.

LES Analysis of the Aerodynamic Surface Properties for Turbulent Flows over Building Arrays with Various Geometries

AbstractThis paper describes aerodynamic roughness properties for turbulent flows over various building arrays that represent realistic urban surface geometries. First, building morphological characteristics such as roughness density λf and building height variability Vh, defined respectively as the ratio of total frontal area of roughness elements to the total surface area and the ratio of standard deviation in building height to the average building height of the study site, were investigated. Next, large-eddy simulations (LESs) of turbulent flows over building arrays were performed with various surface geometries characterized by a wide range of values for both λf and Vh, based on this building morphological analysis. Third, aerodynamic roughness parameters such as roughness length z0 and drag coefficient were evaluated for the central Tokyo area from the values of z0 and Vh using the LES results. The values of z0 and as a function of both λf and Vh were comparable to those found in earlier studies. The values of z0 and evaluated by a conventional method using only λf were underestimated, particularly for densely built-up areas. This indicates that the present approach to estimating aerodynamic roughness parameters, taking account of both roughness density and building height variability, is more appropriate than conventional approaches when applied to actual urban areas. The roughness aerodynamic parameters as a function of λf and Vh obtained from the LES results will be useful in incorporating urban effects into weather forecasting models.

Observational verification of urban surface roughness parameters derived from morphological models

AbstractThis study focuses on how to verify the aerodynamic roughness parameters in an urban area by using observational data collected in the roughness sub‐layer. Anemometrical observations were made in the downtown area of Nanjing, China, in summer and winter. Data were collected using both slow and fast response anemometers mounted on a 36 m tower. The friction velocity was observed just above the top of the urban canopy layer, which is at the height of 19.7 m, while the mean wind speed was observed at a relatively low level that is generally thought to be in the roughness sub‐layer, of which the top usually ranges from 2 to 5 times the height of the urban canopy layer. The results of the measurements show that the dimensionless friction velocities (normalized by the mean wind speed at an upper level) maintain approximately a constant (0.20) and the fluctuations are small ( ± 0.02) in a period of 21 days. The results imply that the similarity relation for the mean wind profile is still valid and that the effect of stratification is negligibly small in the urban roughness sub‐layer.Based on the measurement results, the logarithmic wind profile was applied to verify the aerodynamic roughness parameters empirically derived from four morphological models: the rule of thumb (Rt) method, the Bottema (Ba) method, the Macdonald (Ma) method and the Raupach (Ra) method. The results show that they are not very different from each other. The good performance of the Rt method may be due to the fact that the distribution of buildings in the study area is regular. The Ba and Ra methods are likely to be better since they can give reasonable estimates of roughness parameters. Evidence indicates that the Ma method underestimates the roughness length. Copyright © 2008 Royal Meteorological Society

Two aerodynamic roughness maps derived from Mars Orbiter Laser Altimeter (MOLA) data and their effects on boundary layer properties in a Mars general circulation model (GCM)

Mechanical (forced convective) and free convective turbulent heat and momentum transfer in the lower atmosphere of a terrestrial planet has some dependence on the roughness characteristics of the surface, often quantified in terms of a single roughness parameter which is then used to calculate the coefficients that govern heat and momentum transport between the surface and the boundary layer. We take two different approaches for deriving this aerodynamic roughness parameter for Martian surfaces using data from the Mars Orbiter Laser Altimeter. We then use these two different roughness maps to force the boundary layer in a Mars general circulation model, primarily investigating differences in temperatures and the pressure cycle between the two simulations. While the pressure cycle does not vary significantly, spring and summer high‐latitude temperatures are somewhat sensitive to the input roughness conditions. Daytime temperatures may vary up to 10 K seasonally, though zonally and annually averaged daytime temperatures vary only by ∼1 K. Our results can be explained by the dominance of mechanical over convective turbulent heat transfer processes on Mars. These simulations, however, use a prescribed atmospheric dust distribution and thus only provide a minimum estimate of the uncertainty in boundary layer temperatures because of this plausible range of aerodynamic roughness parameters. Since surface roughness determines the threshold wind velocity for dust lifting we anticipate a much larger effect of the aerodynamic roughness parameter on temperatures when the dust distribution is allowed to vary according to predicted lifting and transport.

Parâmetros de rugosidade aerodinâmica sobre vegetação esparsa em região semi-árida

Utilizando observações micrometeorológicas e da estrutura física da vegetação do experimento HAPEX-Sahel foi possível avaliar alguns métodos para determinar o comprimento de rugosidade (z0) e o deslocamento do plano zero (d). Propõe-se também uma alternativa para estimar esses parâmetros em escoamentos atmosféricos sobre vegetação esparsa em região semi-árida. O método proposto combinou o perfil logarítmico do vento com z0 = lambda (h - d), em que, h é a altura da vegetação e lambda depende de sua estrutura física. Valores de lambda foram estimados em função da estrutura física da vegetação ou obtido na literatura. A altura média da vegetação era 2,06 (± 0,47) m. O perfil do vento foi caracterizado por medidas a quatro níveis acima da superfície (3,0; 4,1; 5,3 e 8,5 m), com a velocidade de fricção (u*) determinada por correlações dos turbilhões a 9 m. O método proposto estimou adequadamente a rugosidade aerodinâmica da superfície com lambda = 0,188 e 0,190, determinados pela estrutura da vegetação local, o que não ocorreu para lambda = 0,166, da literatura. Os parâmetros de rugosidade determinados pelos métodos que utilizam u* na solução da lei logarítmica do vento descreveram melhor a rugosidade da superfície. Nos períodos de observações a rugosidade da área foi descrita satisfatoriamente por d = 0,95 m = 0,46 h e z0 = 0,204 m = 0,1 h.

Improvement of a Method for Estimation of Aerodynamic Roughness Parameter Using Synthetic Aperture Radar Data

The backscattering coefficients observed by synthetic aperture radar (SAR) are influenced by surface geometric roughness obstacles. Using this attributes, spatial distributions of aerodynamic roughness parameter are estimated from the SAR images obtained by the Japanese Earth Resources Satellite (JERS-1). By the method, it is possible to make roughness maps which are very similar to roughness maps derived by the method proposed by Kondo & Yamazawa (1986) which uses the National Land Numerical Information.

Aerodynamic roughness parameters for semi-arid natural shrub communities of Wyoming, USA

Estimates of aerodynamic roughness length ( z 0) were calculated at nine sites for natural sagebrush ( Artemisia tridentata spp.), saltbush ( Atriplex nuttallii) and greasewood ( Sarcobatus vermiculatus) plant communities in two semi-arid basins in Wyoming, USA. Estimates were based on wind and temperature profiles measured above the plant canopies during summer (August) of 1994 and fall (September and October) of 1995. Values of z 0 were estimated for periods of near-neutral stability (fully forced convection) for four scenarios of displacement height ( d) at each site, and for a fifth scenario based on using the low level drag coefficient for constraining friction velocity ( u ∗ ). Vegetation canopy height, fractional cover, shrub density, average canopy area (per shrub), and average biomass/area were measured for all sites. Additionally, leaf and plant area indices (LAI, PAI) were measured at three of the sites, and average shrub height was measured at the six sites. Roughness lengths calculated using an iterative method with unconstrained u ∗ averaged 0.01 m for the saltbush sites, 0.02 m for the sagebrush sites, and 0.07 m for the greasewood site. z 0/canopy height ratios ( z 0 h c ) averaged 0.04, 0.04 and 0.13, respectively, for the three shrub types, although the average is misleading for saltbush, which showed considerable variation between sites. Roughness length appears to be related to shrub structure, as expressed by the dominant species, and by shrub density at the sites, although differences were large depending on which calculation method was used. When u ∗ was constrained, calculated z 0 and z 0 h c were smaller and less consistent in terms of relationships to vegetation structure, suggesting that further constraints on the iterations may be necessary. The results highlight the importance of improving aerodynamic roughness parameterization of natural vegetation communities.

Aeolian sediment flux decay: Non‐linear behaviour on developing deflation lag surfaces

AbstractWind tunnel simulations of the effect of non‐erodible roughness elements on sediment transport show that the flux ratio q/qs, shear velocity U*, and roughness density λ are co‐dependent variables. Initially, the sediment flux is enhanced by kinetic energy retention in relatively elastic collisions that occur at the roughness element surfaces, but at the same time, the rising surface coverage of the immobile elements reduces the probability of grain ejection. A zone of strong shearing stress develops within 0·03 to 0·04 m of the rough bed because of a relative straightening of velocity profiles which are normally convex with saltation drag. This positive influence on fluid entrainment is opposed by declining shear stress partitioned to the sand bed. Similarly, because the free stream velocity Uf is fixed while U* increases, velocity at height z and particle momentum gain from the airstream decline, leading eventually to lower numbers of particles ejected on average at each impact. When the ratio of the element basal area to frontal area σ is approximately equal to 3·5, secondary flow effects appear to become significant, so that the dimensionless aerodynamic roughness parameter Z0/h and shear stress on the exposed sand bed Ts decrease. It is at this point that grain supply to the airstream and saltation drag appear to be significantly reduced, thereby intensifying the reduction in U*. The zone of strong fluid shear near the bed dissipates.

Preliminary analysis of Shuttle Radar Laboratory (SRL-1) data to study aeolian features and processes

In April 1994, the Shuttle Radar Laboratory (SRL-1) obtained quad-polarization data in L-band (/spl lambda/=24 cm) and C-band (/spl lambda/=5.6 cm) and VV-polarized data in X-band (/spl lambda/=3 cm). Preliminary analyses show that radar backscatter coefficients (/spl sigma//sup 0/) can be used to map the aerodynamic roughness parameter, z/sub 0/, over large areas from orbit, as demonstrated by comparisons with measurements of z/sub 0/ obtained in the field. Although all wavelengths and polarizations show fairly good correlations with z/sub 0/ (r=0.78), L-band HV shows the best relationship (r=0.81). This result has potential use in regional studies of sand and dust transport, modeling atmospheric circulation, and general climate studies. SRL images of aeolian landforms were also studied, including dunes, sand sheets, and wind streaks. Dunes in the Stovepipe Wells area of Death Valley were clearly visible on L-band HV images, but not detected on C-band images. In the same area, L-band penetrated a sand sheet to reveal a buried fault scarp. L-band images in Namibia showed the classic linear dunes near Gobabeb and also revealed previously undetected sets of "cross dunes". Wind streaks were imaged in several areas, including Namibia where streaks are associated with the impact crater, Roter Kamm. These features enable mapping of near surface wind patterns.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Research on the impact of forest stand structure on atmospheric deposition

Dry and wet deposition onto thirty forest stands in relation to stand structure is studied by sampling throughfall and bulk precipitation. Nine measurement sites are situated in Pseudotsuga menziesii stands, ten in Pinus sylvestris and eleven in Quercus robur stands. All stands are situated within a radius of 1·2 km to assure a more or less equal air pollution load. In each stand, detailed forest structure inventories are made to determine aerodynamic roughness, collecting efficiency and surface area parameters. Measurements to data cover a four month period (April–July 1990). First results show relatively high throughfall deposition in Pseudotsuga menziesii stands. Lowest throughfall fluxes are recorded for Quercus robur and intermediate values for Pinus sylvestris stands. There are indications of a relatively strong canopy exchange in Quercus robur stands during the measurement period. Many results from forest stand structure inventories are not available yet. However, a strong relation is observed between throughfall deposition in Pseudotsuga menziesii stands and total crown volume.

Evapotranspiration from Isolated Stands of Hydrophytes: Cattail and Bulrush

Evapotranspiration coefficients and aerodynamic roughness and bulk stomatal resistance parameters were developed for predicting evapotranspiration (Et) from narrow, isolated stands of cattail and bulrush (hydrophytes) using measurements from drainage lysimeters. Peak ratios of Et from vegetation in the center of 6 m wide stands of hydrophytes to Et from alfalfa averaged 1.6 for cattails and 1.8 for bulrushes. A ratio of momentum roughness height to vegetative height of 0.3 for use in the Penman-Monteith equation best fit experimental data. Errors between prediction equations and lysimeter measurements were minimized using linear, segmented evapotranspiration coefficient curves as compared to using a fitted Penman-Monteith approach.