Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    The Effect of Single Vegetation Elements on Wind Speed and Sediment Transport in the Sahelian Zone of Burkina Faso
    Leenders, J.K. ; Boxel, J. van; Sterk, G. - \ 2007
    Earth Surface Processes and Landforms 32 (2007)10. - ISSN 0197-9337 - p. 1454 - 1474.
    erosion control - farmers perceptions - turbulent-flow - sand transport - crop residues - air-flow - windbreaks - field - saltation - tunnel
    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
    Wind erosion control with scattered vegetation in the Sahelian zone of Burkina Faso
    Leenders, J.K. - \ 2006
    Wageningen University. Promotor(en): Leo Stroosnijder, co-promotor(en): Geert Sterk; J.H. van Boxel. - [S.l. ] : S.n. - ISBN 9789085044000 - 170
    winderosie - erosiebestrijding - bouwland - vegetatie - windsnelheid - bodemdegradatie - simulatiemodellen - sahel - burkina faso - wind erosion - erosion control - arable land - vegetation - wind speed - soil degradation - simulation models - sahel - burkina faso
    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.
    Wind forces and related saltation transport
    Leenders, J.K. ; Boxel, J.H. van; Sterk, G. - \ 2005
    Geomorphology 71 (2005)3-4. - ISSN 0169-555X - p. 357 - 372.
    atmospheric surface-layer - boundary-layer - sediment transport - sea
    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.
    Sonic anemometers in aeolian sediment transport research
    Boxel, J.H. van; Sterk, G. ; Arens, S.M. - \ 2004
    Geomorphology 59 (2004). - ISSN 0169-555X - p. 131 - 147.
    wind-tunnel - frequency-response - transverse dunes - sand transport - turbulent-flow - air-flow - fluxes - saltation - heat - covariance
    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.
    W-learning voor Geo-informatie
    Boxel, P. van; Wentzel, P. ; Lammeren, R.J.A. van; Boschma, S. ; Molendijk, M. ; Bruin, S. de - \ 2004
    Amsterdam : Vrije Universiteit (GISPY-rapport 2004-11) - 17
    onderwijs - geografie - leeractiviteiten - onderwijsvernieuwing - mobiele uitrusting - draagbare instrumenten - telecommunicatie - datacommunicatie - education - geography - learning activities - educational innovation - mobile equipment - portable instruments - telecommunications - data communication
    Interactions between turbulent wind flow and saltation sand transport
    Sterk, G. ; Boxel, J. van; Zuurbier, R. - \ 2002
    In: Proceedings of ICAR 5/GCTE-SEN Joint Conference, Lubbock, Texas, U.S.A., Lubbock, Texas, 2002 / Lee, J.A., Zobeck, T.M., - p. 63 - 65.
    Sonic anemometers in aeolian sediment transport research
    Boxel, J.H. van; Sterk, G. ; Arens, S.M. - \ 2002
    In: Proceedings of ICAR 5/GCTE-SEN Joint Conference : GCTE-SEN joint conference, 22-25 July 2002, Texas, 2002 / Lee, J.A., Zobeck, T.M., Lubbock, Texas, U.S.A. : - p. 132 - 136.
    The effect of turbulent flow structures on saltation sand transport in the atmospheric boundary layer.
    Sterk, G. ; Jacobs, A.F.G. ; Boxel, J.H. van - \ 1998
    Earth Surface Processes and Landforms 23 (1998). - ISSN 0197-9337 - p. 877 - 887.
    A quantification of the turbulent transport coefficient during the quiescence period.
    El-Kilani, R.M.M. ; Boxel, J.H. van - \ 1996
    In: Proc. 22nd Conf. on Agricultural & Forest Meteorology, Atlanta, Georgia, USA - p. 210 - 213.
    Nighttime exchange processes near the soil surface of a maize canopy.
    Jacobs, A.F.G. ; Boxel, J.H. van; Nieveen, J.P. - \ 1996
    Agricultural and Forest Meteorology 82 (1996). - ISSN 0168-1923 - p. 155 - 169.
    Vertical and horizontal distribution of windspeed and air temperature in a dense vegetation.
    Jacobs, A.F.G. ; Boxel, J.H. van; El-Kilani, R.M.M. - \ 1995
    Journal of Hydrology 166 (1995). - ISSN 0022-1694 - p. 313 - 326.
    Intermittent canopy turbulent transport, correlations time domain maps and the resulting inherent inadequacy of using large time averaged second order closure to describe canopy turbulent transport processes.
    El-Kilani, R.M.M. ; Jacobs, A.F.G. ; Boxel, J.H. van - \ 1994
    In: Proc. 21st Conf. Agricultural and forest meteorology, San Diego, California, USA - p. 80 - 83.
    An intermittent model for describing heat and mass transfer within plant canopies.
    El-Kilani, R.M.M. ; Jacobs, A.F.G. ; Boxel, J.H. van - \ 1994
    In: Proc. 21st Conf. Agricultural and forest meteorology, San Diego, California, USA - p. 76 - 79.
    Nighttime free convection characteristics within a plant canopy.
    Jacobs, A.F.G. ; Boxel, J.H. van; El-Kilani, R.M.M. - \ 1994
    Boundary-Layer Meteorology 71 (1994). - ISSN 0006-8314 - p. 375 - 391.
    Wind speed and air temperature characteristics within a dense vegetation canopy.
    Jacobs, A.F.G. ; Boxel, J.H. van; Shaw, R.H. - \ 1994
    In: 21st Conf. Agricultural and forest meteorology, San Diego, California, USA - p. 309 - 312.
    Horizontal and vertical distribution of air temperature in a vegetation canopy
    Jacobs, A.F.G. ; Boxel, J.H. van; Shaw, R.H. - \ 1992
    Netherlands Journal of Agricultural Science 40 (1992). - ISSN 0028-2928 - p. 359 - 372.
    luchttemperatuur - grenzen - warmte - microklimaat - planten - temperatuur - air temperature - boundaries - heat - microclimate - plants - temperature
    Flow patern within a row canopy.
    Jacobs, A.F.G. ; Boxel, J.H. van; Kroon, L.J.M. - \ 1992
    In: Precipitation scavenging and atmosphere surface exchange / Schwartz, S.E., Slinn, W.G.N., - p. 753 - 762.
    The dependence of canopy layer turbulence on within-canopy thermal stratification.
    Jacobs, A.F.G. ; Boxel, J.H. van; Shaw, R.H. - \ 1992
    Agricultural and Forest Meteorology 58 (1992). - ISSN 0168-1923 - p. 247 - 256.
    grenzen - microklimaat - planten - boundaries - microclimate - plants
    Vertical and horizontal windspeed and temperature in a dense vegetation canopy.
    Jacobs, A.F.G. ; Boxel, J.H. van; El-Kilani, R.M.M. - \ 1992
    Annales geophysicae 10 (1992)suppl. 2. - ISSN 0755-0685 - p. C223 - C223.
    Horizontal and vertical distribution of wind speed in a vegetation canopy.
    Jacobs, A.F.G. ; Boxel, J.H. van - \ 1991
    Netherlands Journal of Agricultural Science 39 (1991). - ISSN 0028-2928 - p. 165 - 178.
    grenzen - meteorologische factoren - microklimaat - planten - turbulentie - wind - boundaries - meteorological factors - microclimate - plants - turbulence - wind
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