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Knowledge of paleoclimates and past climate change is important to put recent and future climate change in perspective. In the absence of welldeveloped methodology to reconstruct paleoprecipitation the majority of climate reconstructions focus on temperature, whereas precipitation is an equally important climate parameter. This paper explores the possibility of inferring paleoprecipitation from lake-level records by inverse hydrological modelling. Pollen spectra of a lacustrine sediment core were used to infer changes in past temperatures and lake levels during the past 14,000 years. A hydrological model that calculates lake levels using meteorological parameters and a digital terrain model were developed for the catchment area of Lake La Cocha. After calibration the model accurately simulated modern lake levels. A sensitivity analysis shows that the model results are most sensitive to temperature and precipitation. This hydrological model was subsequently used to estimate mean annual precipitation needed to reproduce the pollen-based reconstructed lake levels (inverse modelling). The lake currently discharges through the permanent Guamue´s River, with a modelled mean annual discharge of 3.6 m3 s-1. However, past lake levels and hydrological modelling results suggest that Lake La Cocha has been free of discharge during most of the Holocene, and after an intermittent phase only recently started discharging permanently. The uncertainty in the inferred precipitation during the discharge-free period is estimated at *22 mm. Quasi stable lake levels seem to justify using equilibrium conditions when reconstructing precipitation. Early Holocene lake levels were *10 m lower than modern values, implying that precipitation must have been 30-40 % less than today.

The biomisation method is used to reconstruct Latin American vegetation at 6000±500 and 18 000±1000 radiocarbon years before present (14C yr BP) from pollen data. Tests using modern pollen data from 381 samples derived from 287 locations broadly reproduce potential natural vegetation. The strong temperature gradient associated with the Andes is recorded by a transition from high altitude cool grass/shrubland and cool mixed forest to mid-altitude cool temperate rain forest, to tropical dry, seasonal and rain forest at low altitudes. Reconstructed biomes from a number of sites do not match the potential vegetation due to local factors such as human impact, methodological artefacts and mechanisms of pollen representivity of the parent vegetation. At 6000±500 14C yr BP 255 samples are analysed from 127 sites. Differences between the modern and the 6000±500 14C yr BP reconstruction are comparatively small; change relative to the modern reconstruction are mainly to biomes characteristic of drier climate in the north of the region with a slight more mesic shift in the south. Cool temperate rain forest remains dominant in western South America. In northwestern South America a number of sites record transitions from tropical seasonal forest to tropical dry forest and tropical rain forest to tropical seasonal forest. Sites in Central America show a change in biome assignment, but to more mesic vegetation, indicative of greater plant available moisture, e.g. on the Yucatán peninsula sites record warm evergreen forest, replacing tropical dry forest and warm mixed forest presently recorded. At 18 000±1000 14C yr BP 61 samples from 34 sites record vegetation reflecting a generally cool and dry environment. Cool grass/shrubland is prevalent in southeast Brazil whereas Amazonian sites record tropical dry forest, warm temperate rain forest and tropical seasonal forest. Southernmost South America is dominated by cool grass/shrubland, a single site retains cool temperate rain forest indicating that forest was present at some locations at the LGM. Some sites in Central Mexico and lowland Colombia remain unchanged in the biome assignments of warm mixed forest and tropical dry forest respectively, although the affinities that these sites have to different biomes do change between 18 000±1000 14C yr BP and present. The "unresponsive" nature of these sites results from their location and the impact of local edaphic influence.

Soil loss caused by wind erosion is a widespread phenomenon in the Sahelian zone of West Africa. According to Sahelian farmers, scattered vegetation standing in amongst the crop has the potential for a wind erosion control strategy. This study was conducted to study the effect of single vegetation elements on the pattern of average wind speed and sediment transport. This was done by two experiments that were carried out during the rainy seasons of 2002 and 2003 in north Burkina Faso, West Africa. Wind speeds were measured using three sonic anemometers, at a sampling frequency of 16 Hz. Sediment transport was determined by calculating the mass fluxes from 17 MWAC catchers. In this study, a shrub was defined as a vegetation element with branches until ground and a tree as a vegetation element with a distinctive trunk below a canopy.Behind shrubs wind speed near the soil surface was reduced up to approximately seven times the height of the shrub. The observed reduction in wind speed in the area where wind speed was reduced was 15 per cent on average. At the sides of the shrub, wind speed was increased, by on average 6 per cent. As the area of increase in wind speed is one-third of the area of decrease in wind speed, the net effect of a shrub is a reduction in wind speed. A similar pattern was visible for the pattern of sediment transport around a shrub. Downwind of a shrub, sediment transport was diminished up to seven times the height of the shrub. Probably most of this material was trapped by the shrub. Trees showed a local increase of wind around the trunk, which is expected to relate to an increase in sediment transport around the trunk. Mass flux measurements of sediment transport were not made, but visual observations in the field substantiate this. Behind the canopy of a tree, a tree acts similarly to a shrub regarding its effects on average wind speed, but as a tree is generally a larger obstacle than a shrub the extent of this effect is larger than for shrubs. Thus, whereas shrubs are more effective than trees regarding their direct effect on soil loss by trapping sand particles near the soil surface, trees are more effective in affecting soil loss indirectly by reducing the wind speed downwind more effectively than shrubs. Therefore, to reduce soil loss in an area, the presence of both trees and shrubs is crucial.

This paper discusses a model which simulates dune development resulting from aeolian saltation transport. The model was developed for application to coastal foredunes, but is also applicable to sandy deserts with transverse dunes. Sediment transport is calculated using published deterministic and empirical relationships, describing the influence of meteorological conditions, topography, sediment characteristics and vegetation. A so-called adaptation length is incorporated to calculate the development of transport equilibrium along the profile. Changes in topography are derived from the predicted transport, using the continuity equation. Vegetation height is incorporated in the model as a dynamic variable. Vegetation can be buried during transport events, which results in important changes in the sediment transport rates. The sediment transport model is dynamically linked to a second-order closure air flow model, which predicts friction velocities over the profile, influenced by topography and surface roughness. Modelling results are shown for (a) the growth and migration of bare, initially sine-shaped dunes, and (b) dune building on a partly vegetated and initially flat surface. Results show that the bare symmetrical dunes change into asymmetric shapes with a slipface on the lee side. This result could only be achieved in combination with the secondorder closure model for the calculation of air flow. The simulations with the partly vegetated surfaces reveal that the resulting dune morphology strongly depends on the value of the adaptation length parameter and on the vegetation height. The latter result implies that the dynamical interaction between aeolian activity and vegetation (reaction to burial, growth rates) is highly relevant in dune geomorphology and deserves much attention in future studies. Copyright © 1999 John Wiley & Sons, Ltd.

This paper discusses a two-dimensional second-order closure model simulating air flow and turbulence across transverse dunes. Input parameters are upwind wind speed, topography of the dune ridge and surface roughness distribution over the ridge. The most important output is the distribution of the friction velocity over the surface. This model is dynamically linked to a model that calculates sand transport rates and the resulting changes in elevation. The sand transport model is discussed in a separate paper. The simulated wind speeds resemble patterns observed during field experiments. Despite the increased wind speed over the crest, the friction velocity at the crest of a bare dune is reduced compared to the upstream value, because of the effect of stream line curvature on turbulence. These curvature effects explain why desert dunes can grow in height. In order to obtain realistic predictions of friction velocity it was essential to include equations for the turbulent variables in the model. In these equations streamline curvature is an important parameter. The main flaw of the model is that it cannot deal with flow separation and the resulting recirculation vortex. As a result, the increase of the wind speed and friction velocity after a steep dune or a slipface will be too close to the dune foot. In the sand transport model this was overcome by defining a separation zone. Copyright # 1999 John Wiley & Sons, Ltd.

A five-year monitoring study has been carried out to examine the combined effects of grazing and atmospheric nitrogen deposition on water and solute fluxes in dry coastal dune grasslands. Two vegetation types were studied: (a) a short, species-rich stand on calcareous sand (foredune site) and (b) a short, species-poor stand on partly decalcified soil on calcareous sand (innerdune site). In each stand four experimental plots were created: (1) control, (2) fertilized with nitrogen, (3) excluded from grazing by rabbits and (4) combination of fertilization and exclusion of grazing by rabbits. Due to the large spatial variability of the soil water content, no differences between the treatments could be measured. Average soil water content at 10 cm depth is very small (3–5%) from May until October and does not increase after rainfall. However, measured soil water content at 20 cm and 50 cm depth increased after rainfall. In winter, nearly all measured soil water contents increase upon rainfall, although sometimes one soil water content remained dry till the end of the next summer. In summer it was impossible to sample soil water for the estimation of the solute concentrations due to the very small soil water content. Therefore, only solute concentrations of the winter period could be evaluated. Without fertilization, fluxes of nitrogen out of the soil system are below the incoming flux, due to storage in the biomass and in the soil compartment. When fertilized, 70% of the added NH4 +-N was leached from the foredune soil profile as NO3 --N, due to nitrification. Conversely, at the grazed innerdune site most of the added nitrogen remained in the system. Here, nitrification rates will be small due to the decalcified topsoil and NH4 +-N is not easily leached out of the soil compartment. At the exclosures of the innerdune site, about 15% of the amount of the added fertilizer N was leached, after added NH4 + is taken up by the plants and partly washed out as nitrate after mineralization and nitrification of dead biomass.

In an inner dune area in the Dutch coastal dunes several artificially stabilized blowouts were reactivated. The purpose was to investigate whether these reactivated blowouts could remain active despite the increased atmospheric deposition of nutrients, how much area would be affected by sand from the blowouts, whether the vegetation would respond to the deposition of sand, and whether the reactivation of blowouts could be a measure against the effects of acidification and eutrophication.

Wind speed and temperature were measured within a corn row canopy to investigate horizontal and vertical variability of the mean wind speed and temperature. It appears that the mean wind speed can vary between 20% and 30% from its horizontal mean value. In the narrow row crop, the horizontal mean air temperature varies between 0.1°C (night-time) and 0.35°C (daytime) from the spatial mean value. Exceptions occur around noon in daytime when direct irradiation dominates and where the direct beam illuminates the within-row space of the canopy. Then, deviations of 1°C or more from the horizontal mean value are observed. Attention was focused on finding adequate scaling parameters of the within-canopy wind speed and air temperature profiles under various atmospheric stratification states. During daytime and night-time, the above-canopy friction velocity appears to be a good scaling parameter. Clear nights, however, are exceptions, when the above-crop wind speed drops to a very low value. Then, the within-canopy free convection velocity scale appears to be an appropriate scaling parameter for within-canopy processes. During daytime, the within-canopy temperature profiles scale well with the above-canopy temperature scale, T∗, for stationary irradiation and wind speed regimes. On calm nights, however, the relative within-canopy temperature profile scales very well with the within-canopy free convection temperature scale, Θ∗.

In two saucer-shaped blowouts wind velocity was measured at 0.18 m height and compared to the wind velocity data from the nearest standard meteorological station at Valkenburg about 3.5 km to the northeast. In blowout 1 relative velocities are highest when the wind blows from directions between E and SSE. In blowout 2 winds blowing from directions between S and WSW are relatively stronger. In general the wind speed increases as its moves along the surface inside the blowout, because the aerodynamic roughness of the blowout surface is less than that of the surrounding area. Less important are the influence of the configuration of the terrain surrounding the blowout and the influence of the slopes within the blowout. The frequency distribution of the general wind at Valkenburg is analysed and combined with the horizontal wind velocity pattern within blowouts. A simulation model, producing the spatial distribution of the wind power shows that the prevalence of algal crusts at the SW side of blowouts agrees well with the location which reaches the lowest wind power values. In other parts of the blowouts algal crusts are damaged by moving sand at times of high wind velocity.

Transport properties near the Earth's surface are strongly influenced by the thermal stratification of the atmosphere. Until now, no distinction has been made between thermal stability parameters within and above a plant canopy, and it has been usual to classify canopy transport processes in terms of above-canopy stability parameters only. The question arises, however, whether such parameters adequately describe within-canopy properties because it is often the case that thermal stratification differs considerably between air layers above and below the top of the canopy. In the present study, two within-canopy thermal stratification parameters have been defined and tested to determine whether they yield additional information about canopy turbulence. It appears that a within-canopy bulk Richardson number provides useful information under low-wind nocturnal conditions. Strongly unstable conditions inside dense canopies commonly occur at night when the air layers above the canopy are very stable, resulting in a decoupling between the above- and within-canopy regions. A local within-canopy Obukhov length proved to be less useful, perhaps because the sensible heat flux within the canopy was nearly always directed upwards, regardless of the temperature gradient. A penetration length scale, defined for daytime conditions only, was of the order of the height of the canopy. This suggests that the height of the canopy is a suitable length scale for within-canopy processes.

Extremely important to the climate in any region are the radiation balance and the exchange processes of heat, water vapour and momentum. Most climatological parameters (e.g. temperature, humidity, wind speed, cloudiness and precipitation) are the direct or indirect result of the radiation balance and these exchange processes. The weather of the West European coast from Tarifa (Spain) to Skagen (Denmark) is especially suitable for the formation of dunes. Often a wind is blowing, varying widely in force and direction. The conditions are optimal for the formation of high and wide dune complexes, given a large supply of sand by the sea. The annual precipitation surplus is considerable for most of this coast. This favours the establishment of vegetation, and thereby it enhances dune formation. The short distance to the land-sea border causes strong gradients in several climatological parameters. These gradients lead to mesoscale effects, such as land-sea breezes and coastal fronts. The varying vegetation cover and the presence of slopes in all directions induce a strongly varying microclimate. However, this microclimate is not unique to the coastal dunes. Unique is the interaction with the wide range of ambient weather, which is inherent to the coast. It is not possible to be conclusive about the effects of climatic change on coastal dunes because climate models are not yet able to predict the changes adequately and because these models supply information on the expected mean climate, but not on the actual weather.

A fast-response temperature sensor for measuring atmospheric temperature was constructed and is described. The sensor was based on the thermocouple principle, connected to a thermocouple conditioner (AD595): the cold junction was compensated via an electrical reference and the signal amplified. This reference compensation was built into the sensor itself. The time constant of the thermocouple was decreased by rolling out a circular wire. The mean temperatures measured by the sensor were compared with those measured by an accurately calibrated Pt 100 resistance thermometer. The agreement between both sensors for outdoor measurements gave a standard error of estimate of 0.20 K. The fast outdoor temperature excursions around the running mean, measured by the sensor, were compared with those measured by a fast-response sonic thermometer. The agreement of the temperature variances between both sensors was better than 2% (standard error of estimate 0.05 K) and was dependent on measuring height and mean windspeed. The 3 dB point of the instrument was about 2 Hz.

Blowouts are the main features of aeolian activity in many dune areas. To assess the impact of future climatic change on the geomorphological processes prevailing in a dune landscape it is essential to understand blowout formation and identify the meteorological parameters which are important.The development, that is, local erosion and accumulation, of six blowouts in a dune terrain along the Dutch coast has been related to wind velocity and wind direction, as measured at a nearby standard meteorological station. Blowout changes correlate best with wind velocities between 6.25 and 12.5 m/s (measured at 10 mheight) which are the critical wind velocities for moving particles in the 0.15 to 0.42 mm range. These winds mostly blow from the southwest. Consequently, the blowouts are elongated in the same direction.Extreme aeolian events such as northwestern storms have little effect on blowout development compared to events which have a lower magnitude but occur with a higher frequency. An eventual shift towards higher effective wind velocities would probably result not in larger blowouts but in a break-down of the whole system, especially if this shift were accompanied by a change in wind direction. The accumulation of sand in blowouts during storms should be seen as a first step of adaptation to a higher energy level.

Wind speed measurements within a maize row canopy were carried out to investigate the horizontal and vertical variability of the mean wind speed and its standard deviation. Attention was given to finding adequate scaling parameters of the within-canopy wind speed profiles under various atmospheric stratification states. A validation of existing model simulations was also carried out. The horizontal mean wind speed and its standard deviation can vary about 20% from its spatial mean value. During day time and night time the friction velocity appears to be a good scaling parameter. Clear nights, however, are exceptions, when the wind speed above the crop drops to a very low value. Then the free convection velocity appears to be an appropriate scaling parameter for the within-canopy processes. The canopy models of Wilson & Shaw [Journal of Applied Meteorology (1977) 16, 1197-1205] and Li et al. [Boundary-Layer Meteorology (1985) 33, 77-84] were found to simulate the spatially averaged mean wind profile within the range of the horizontal variability. (Abstract retrieved from CAB Abstracts by CABI’s permission)

The effect of several wind characteristics on sand transport was studied in three experiments in north Burkina Faso, West Africa. The first experiment is used to analyse the relation between wind speed and shear stress fluctuations across height. The second experiment is used to study the relation of these wind characteristics with saltation transport for fourteen convective storms, registered during the rainy seasons of 2002 and 2003. The effect of sampling time is studied for two of these convective storms. The third experiment relates the turbulent structures of four convective storms to saltation transport. Wind speed measurements were undertaken with two sonic anemometers and sediment transport was measured by two saltiphones. The sampling frequency was either 8 or 16 Hz. The sonic frame of reference was rotated according to a triple rotation.Horizontal fluctuations showed a (fairly) good correlation with height because the wind speed at both sensors was affected by the same vortices. The correlation coefficients ranged from 0.42 (when the distance between the sensors was 1.75 m) to 0.92 (when the distance was 0.25 m). The instantaneous Reynolds¿ stress had the weakest correlation (correlation coefficient of 0.05 at 1.75 m between the sensors and 0.56 at 0.25 m between the sensors), because the momentum at 2 m above the soil surface is transported by different eddies than those close to the ground. This also explains the fairly good correlation coefficients between the horizontal components of the wind and saltation compared to the poor correlations between instantaneous Reynolds¿ stress and saltation. An increase in sampling time did not have much impact on these correlation coefficients up to sampling periods of about 30 s. However, this sampling interval would be too coarse to describe the vertical wind component adequately. The classification of the moments of shear stress into the turbulent structures, sweeps, ejections, inward and outward interactions, showed that the mean saltation flux is higher at sweeps and outward interactions than at ejections and inward interactions. Also, saltation occurred more often during sweeps and outward interactions than during ejections and inward interactions.

Fast-response wind and turbulence instruments, including sonic anemometers, are used more and more in aeolian sediment transport research. These instruments give information on mean wind, but also on fluctuations and turbulent statistics, such as the uw covariance, which is a direct measure of Reynolds' stress (RS) and friction velocity. This paper discusses the interpretation of sonic anemometer data, the transformations needed to get proper results and turbulence spectra, and how they are influenced by instrument size, sampling frequency, and measurement height. Turbulence spectra characterize how much the different frequencies in the turbulent signals contribute to the variance of wind speed, or to the covariance of horizontal and vertical wind speed. They are important in determining the measurement strategy when working with fast-response instruments, such as sonic anemometers, and are useful for interpreting the measurement results. Choices on the type of sonic anemometer, observation height, sampling period, sampling frequency, and filtering can be made on the basis of expected high and low-frequency losses in turbulent signals, which are affected by those variables, as well as wind speed and atmospheric stability. Friction velocity and RS, important variables in aeolian sediment transport research, are very sensitive to tilt or slope errors. During a field experiment, the slope sensitivity of the RS was established as 9% per degree of slope, which is 1.5 times the value reported in literature on the basis of theoretical considerations. An important reason for the difference probably is the large influence of streamline curvature on turbulence statistics and thereby on the slope sensitivity of the RS. An error of 9% per degree of slope in the RS will translate into an error of approximately 4% per degree of slope in the calculated friction velocity. Space-time correlation of the horizontal wind speed is much larger than that of the vertical wind speed and the instantaneous RS. This largely explains why, in previous studies, a poor correlation was found between instantaneous RS measured at 3 in height and saltation flux near the surface, whereas the correlation between wind speed at some height and saltation flux was much better. Therefore, the poor correlation between RS away from the surface and saltation flux does not contradict that saltation flux is caused by RS. (C) 2003 Elsevier B.V. All rights reserved.

Suspended sand is sampled at several heights and positions on a beach and foredune, providing detailed insight into the vertical and horizontal variation in sand content in the air during landward transport. Grain-size analysis is used to study the changes in grain-size distribution during landward transport. Mean grain size and sorting decrease during transport. Changes in textural parameters follow a gradual and regular path when the sediment is transported into the foredune. Sediment trapped on the seaward slope at a height of 30-50 cm above the surface closely resembles the sediment trapped landward, which implies that changes in the direction of transport are related to vertical changes within the sediment transport profiles. The movement of sand over the vegetated foredunes is induced by turbulent forces created by the air flowing across the vegetation and the foredune, leading to a change from saltation on the beach to modified saltation and suspension on the foredune. Small grains are lifted higher and fall more slowly than coarse grains and therefore are transported further landward, resulting in a gradual decrease in grain size of the landward-deposited sediment.

Vegetation density on foredunes exerts an important control on aeolian sediment transport and deposition, and therefore on profile development. In a long-term monitoring field experiment, three plots were planted with regular grids of reed bundles in three different densities: 4, 2 and 1 bundles per m2. This study reports on the differences in profile development under the range of vegetation densities. Topographic profiles were measured between May 1996 and April 1997. Results indicate important differences in profile development for the three reed bundle densities: in the highest density plot a distinct, steep dune developed, while in the lowest density a more gradual and smooth sand ramp was deposited. When the stems had been completely buried, differences in profile evolution vanished. After a second planting of reed stems in January 1997 the process was repeated. In May 1997, all plots had gained a sand volume ranging from 11·5 to 12·3 m3 m−1, indicating that the sediment budget is relatively constant, regardless of the particular profile evolution. The field evidence is compared with simulations of profile development, generated by the foredune development model SAFE. The model successfully reproduces the overall profile development, but in general, the equations used for vegetation–transport interaction overestimate the effect of vegetation. This causes some deviations between field and model results. Several reasons for this are discussed. Based on the experiments reported here, recommendations are given for further research. Copyright © 2001 John Wiley & Sons, Ltd.

The effect of turbulent flow structures on saltation sand transport was studied during two convective storms in Niger, West Africa. Continuous, synchronous measurements of saltation fluxes and turbulent velocity fluctuations were made with a sampling frequency of 1 Hz. The shear stress production was determined from the vertical and streamwise velocity fluctuations. The greatest stress-bearing events were classified as turbulent structures, with sweep, ejection, inward interaction, and outward interaction described according to the quadrant technique. The classified turbulent structures accounted for 63·5 per cent of the average shear stress during the first storm, and 56·0 per cent during the second storm. The percentage of active time was only 20·6 per cent and 15·8 per cent, respectively. High saltation fluxes were associated with sweeps and outward interactions. These two structures contribute positively (sweeps) and negatively (outward interactions) to the shear stress, but have in common that the streamwise velocity component is higher than average. Therefore, the horizontal drag force seems primarily responsible for saltation sand transport, and not the shear stress. This was also reflected by the low correlation coefficients (r) between shear stress and saltation flux (0·12 and 0·14, respectively), while the correlation coefficients between the streamwise velocity component and saltation flux were much higher (0·65 and 0·57, respectively).

An intensive measurement campaign within and above a maize row canopy was carried out to investigate flow characteristics within this vegetation. Attention was given to finding adequate scaling parameters of the within-canopy windspeed and air temperature profiles under above-canopy stable stratification. During clear and calm nights the within-canopy condition differs considerably from the abovecanopy state. In contrast to the daytime, the windspeed and temperature profiles do not scale with the above-canopy friction velocity,u * , and the scaling temperature,T * , respectively. A free convection flow regime is generated, forced by the soil heat flux at the canopy floor and by cooling at the top of the canopy. However, the windspeed and temperature profiles appear to scale well with the free convective velocity scale,w * , and the free convective temperature scale,T f , respectively. The free convective state within the canopy agrees well with the free convection criterion Gr>16Re2(u * ), where Gr is the Grashof number and Re(u * ) the Reynolds number, a criterion often used in technical flow problems. Also it is shown that under within-canopy free convection, there is a unique relation between the Grashof number, Gr, and the Reynolds number if the latter is based on the free convective velocity scale. Under within-canopy free convective conditions, it appears that within the canopy the fluxes of heat and water vapour can be estimated well with the relatively simple variance technique. Under these conditions, the Grashof, or Rayleigh number, represents a measure for the kinetic energy of the turbulence within the canopy.

During a whole growing season, the evolution of the displacement height, d, and roughness length, z0, of a maize crop has been estimated by a measurement programme. The results have been used to check different types of existing models to calculate these parameters from canopy characteristics only; a simple geometric model and two matching models have been investigated. A geometric model is based on geometric features of the surface only. After a simple modification, the geometric model gives good results for the displacement height as well as for the roughness length.A matching model, based on gradient-diffusion theory, yields good results for the displacement height. The roughness parameter, however, is overestimated by 17%. By a simple modification, the model results could be improved considerably.A matching model, based on a second-order closure procedure, yields excellent results for the displacement height and good results for the roughness length. But it appears that, when applying this model, the plant density index and plant area density distribution as a function of height must be well known.

The feasibility of assessing the sampling efficiency of aerosol samplers for large windborn particles in the open air was determined in a pilot study. The experiments with 17 μm diameter monodisperse aerosols show the low sampling efficiency of widely used instruments such as the EPA high volume sampler and LIB sampler for a coarse aerosol. A new developed tunnel sampler shows a high sampling efficiency for this aerosol relative to the Rotorod as a reference sampler. Additional experiments with different aerosol sizes and under varying meteorological situations are planned.

A surface energy balance model with a multilayer canopy representation has been developed to simulate the vertical temperature distribution of the foliage elements and the soil underneath. This model allows one to simulate the radiative temperature of the complex vegetation-covered surface as a function of the angle of observation. The model was implemented for a two-layer canopy and has been tested by comparing simulated radiative surface temperatures (observed under a look angle of 45°), latent heat, sensible heat, momentum, and ground heat fluxes, and soil temperatures at different depths, with observed data, gathered in a maize field. The fluxes above the canopy were measured by eddy correlation methods. This paper describes the model structure and simulation results as compared with the field measurements. Except for the ground heat fluxes, all the simulated and measured parameters match reasonably well. Further, the model was used to study the sensitivity of the radiative temperature of the complex surface to variations in the roughness length z0, the leaf area index LAI, and the drag coefficient Cd. The radiative surface temperature turned out to be very insensitive to variations in Cd and LAI, and it was found that resulting variations in the radiative surface temperature fall well within the detection error of the remotely sensed surface temperature.

The Sahelian zone ofAfricais the region that is globally most subjected to land degradation, with wind erosion being the most important soil degradation process. By using control measures, the negative effects of wind erosion can be reduced. At present, adoption of wind erosion control measures by Sahelian farmers is low, as most recommended measures do not fit into the farming systems. Therefore, the possibilities of using the local agro-forestry system, i.e. scattered vegetation of trees and shrubs as a wind erosion control strategy were explored in this study. The study area was located in the Sahelian zone of Burkina Faso. A survey among farmers revealed that they generally have a good knowledge of wind erosion processes and the possible wind erosion protection by natural vegetation. Detailed field measurements of wind speed and sediment transport revealed that the fluctuations in horizontal wind speed mainly cause wind erosion. Measurements around isolated vegetation elements revealed that these elements reduce wind speeds and sediment transport, and they are effective in trapping material already in transport. The effectiveness in reducing wind speed and sediment transport of scattered vegetation depends on the number of vegetation elements, the type of vegetation element and the height, width and porosity of the canopy of the element. A model was developed to simulate field-scale wind erosion with different types and arrangements of vegetation elements. This model can be used to develop optimal vegetation cover densities and spatial arrangements for wind erosion control. Overall it can be concluded that the use of the local agro-forestry system as a wind erosion control strategy is promising.