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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Condens op diffuus glas: Invloed op lichttransmissie?
Dueck, T.A. ; Janse, J. ; Moerenhout, S. - \ 2012
Wageningen : Wageningen UR Glastuinbouw (Rapport / Wageningen UR Glastuinbouw ) - 19
glastuinbouw - lichtdoorlating - kassen - diffuus glas - waterdamp - greenhouse horticulture - light transmission - greenhouses - diffused glass - water vapour
In dit verkennende onderzoek wordt gekeken naar de invloed van condens tegen het kasdek op de lichttransmissie. Verkennende metingen worden gedaan in het lab en in een proefkas.
Measuring water-vapour and carbon-dioxide fluxes at field scales with scintillometry
Kesteren, A.J.H. van - \ 2012
Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): Oscar Hartogensis. - [S.l.] : s.n. - ISBN 9789461734235 - 212
scintillometrie - waterdamp - kooldioxide - meteorologie - meettechnieken - scintillometry - water vapour - carbon dioxide - meteorology - measurement techniques

Scintillometry is a measurement technique that has proven itself to be of great value for measuring spatial-averaged fluxes of sensible heat, momentum, and evapotranspiration. Furthermore, for crop fields (field scales), scintillometry has been shown to accurately determine the sensible-heat and momentum flux over time intervals as short as 6 seconds. As a consequence, interests in scintillometry are growing and scintillometers that determine sensible-heat fluxes and momentum fluxes have become commercially available.

This thesis deals with two aspects of scintillometry. First, after a general introduction of scintillometry, measurement errors that have been observed in the large-aperture scintillometer from Kipp&Zonen and in the SLS field-scale scintillometer from Scintec are evaluated. For both scintillometer types, we discuss the variability in the measurement errors among different instruments and, where possible, we give solutions to remove these errors. Furthermore, we present the results of a prototype scintillometer that was developed as part of the research project. With our proposed design, we aim to overcome the measurement errors in the Scintec scintillometer and extend the applicability of the field-scale scintillometer to paths that are longer than 200 m.

Second, we extend the application of field-scale scintillometry to the flux measurements of latent-heat, carbon-dioxide, and other passive scalars. Until now, scintillometers could not be used for measuring passive-scalar fluxes over crop fields and we show that with our extended methodology these fluxes can be accurately determined over time intervals as short as 1 minute. The methodology is based on a combination of scintillometer measurements and additional high-frequency scalar measurements and works under conditions of homogeneous turbulence, i.e. single crop fields. We introduce four methods, notably the energy-balance method, the Bowen-variance method, the flux-variance method, and the structure-parameter method. Using several validation methods, we show that the energy-balance method is unsuitable for estimating scalar fluxes over 1-min averaging intervals. The Bowen-variance and flux-variance method perform better and the structure-parameter method accurately resolves 1-minute fluxes. Thus, with this methodology fluxes can be resolved with a high temporal resolution, making it possible to study vegetation in a natural environment under non-stationary conditions. This allows us to show that the wheat vegetation affects fluxes upon changes in solar radiation in time periods clearly shorter than 30 minutes and that the canopy resistance can change significantly within several minutes.

Eddy covariance and scintillation measurements of atmospheric exchange processes over different types of vegetation
Nieveen, J.P. - \ 1999
Agricultural University. Promotor(en): J. Goudriaan; A.F.G. Jacobs. - S.l. : Nieveen - ISBN 9789058080288 - 122
vegetatietypen - atmosfeer - turbulentie - gasuitwisseling - bladoppervlakte-index - kooldioxide - waterdamp - covariantie - analytische methoden - vegetation types - atmosphere - turbulence - gas exchange - leaf area index - carbon dioxide - water vapour - covariance - analytical methods

Introduction and objectives

Good comprehension of the energy and mass cycles and their effect on climate dynamics is crucial to understanding, predicting and anticipating ecological changes due to possible future climate perturbations. Here direct and long-term flux density measurements of greenhouse gases from various ecosystems provide means to supply such fundamental knowledge. For the global water vapour and carbon cycles, however, the interactions between different spatial scales become important, where extrapolating from canopy flux density measurements to global budgets lead to practical and theoretical problems. This thesis focuses on the direct and long-term measurement of surface flux densities and interaction processes at the canopy (< 1 km scale within the framework of the Surface Layer Integration Measurements and Modelling (SLIMM) project. Furthermore, some characteristics and limitations of the scintillation technique are studied in two field experiments in New Zealand.

As indicated in Chapter 1, the first objective of this project was the direct and continuous long-term measurement of the surface flux densities of radiation, momentum, heat, water vapour and carbon dioxide (CO 2 ) to study the effect of biological and climatic processes that regulate carbon dioxide exchange of this ecosystem at the canopy scale. At the same time these data were used to study the effect of plant related and environmental conditions on the interaction of carbon dioxide and water vapour exchange, to satisfy the second objective of the thesis. The third objective focussed on the prospect of obtaining both the spatial averaged sensible heat flux density and momentum flux density from scintillation measurements.

Generally, a compromising point measurement of the mean horizontal wind speed or friction velocity is used to calculate the sensible heat flux density from the temperature structure parameter. By using two scintillometers at two heights, point measurements to obtain the atmospheric stability can be omitted. The fourth objective of this thesis was to study the influence of absorption fluctuations on the average sensible heat flux density derived from the scintillation technique.

Carbon dioxide exchange and the effect of biological and climatic processes

Carbon dioxide exchange was measured, using the eddy covariance technique, during a one and a half-year period in 1994 and 1995. The measurements took place over a former true raised bog, characterised by a shallow peat layer and tussock vegetation dominated by Molinia caerulea . Peat soils in the Northern Hemisphere's wetlands contain about one third of the worlds carbon pool. Many regions in the arctic tundra, however, have changed from sinks to sources for CO 2 over the past decade but this can not simply be generalised.

The growing season extended from May until late October, with a maximum LAI in August of 1.7. The carbon balance showed a net release of 97 g CO 2 m -2y -1(265 kg C ha -1y -1) from the peat bog ecosystem to the atmosphere. During June, July and August there was net consumption of CO 2 , while during the rest of the year there was net production of CO 2 . The maximum daytime net exchange rates were about -0.5 mg CO 2 m -2s -1(-11.3μmol CO 2 m -2s -1) with an average peak exchange rate of -0.2 mg CO 2 m -2s -1(-4.5μmol CO 2 m -2s -1), in a period where the LAI ranged between 1 and 1.7. A high vapour pressure deficit (>15 hPa) corresponding with high temperature was found to reduce the net CO 2 exchange rate by on average 50%.

Apart from these factors, LAI and the soil temperature co-determined the net exchange of CO 2 . The total nocturnal respiration during the growing season was within the same order as the average daytime net photosynthetic rate. Temperature was found to be the main factor controlling soil respiration, with a Q 10 of 4.8.

The effect of plant related and environmental conditions on the interaction of CO 2 and H 2 O exchange

The tussock grassland, dominated by Molinea caerulea , was covered with a dense layer of dead organic material from the previous growing seasons. During the summer months, the daytime carbon dioxide uptake often showed a single early morning maximum and a decline in uptake during the rest of the day. Surprisingly, maximum water vapour flux densities were not greatly reduced. The surface cover and the small value of the leaf area index were the main reasons for this phenomenon.

The layer of dead organic material acted as an insulating blanket to the transport of water vapour from the soil to the atmosphere. Furthermore, the canopy was far from closed with a peak leaf area index of 1.7 in early August. For both low vapour pressure deficit (< 15 hPa) and high vapour pressure deficit (> 20 hPa) at high surface temperatures, the vegetation showed similar behaviour resulting in a clear reduction of the daytime CO 2 uptake. Temperature was therefore inferred to be main the reason for a reduction in CO 2 exchange. The response of the stomata to atmospheric humidity was deduced to be small possibly due to the abundant availability of soil water. Instead transpiration increased with increasing vapour pressure deficit. The latter was stimulated by the surface temperature, which often exceeded the optimum temperature for photosynthesis and led to an increase in the atmospheric evaporative demand.

The scintillation technique

An optical or electromagnetic wave propagating through a turbulent atmosphere exhibits fluctuations in intensity known as 'scintillations'. In atmospheric turbulence, fluctuations in temperature, humidity and pressure cause density fluctuation and with it fluctuations in the refractive index ( n ). These refractive index fluctuations cause random refraction and absorption of electromagnetic (EM) radiation passing through the turbulent atmosphere, changing the characteristics of the wave. Scintillation of light is related to these phenomena and is experienced at a receiver as fluctuations in the light intensity caused by interference of refracted light and absorption of the light. Scintillometers measure the turbulent intensity of the refractive index fluctuations of the air from the intensity fluctuations of a received signal expressed in the refractive index structure parameter, C n2.

The measured C n2value is related to the structure parameters of temperature C T2, humidity C Q2and a covariant term C TQ , respectively. To calculate the sensible heat flux density from C n2compromising point measurements of the Bowen ratio,β, and friction velocity, u * , are necessary. Generally, the deficiency in the available spatial measurement of u * is overcome by using a point measure of the average wind speed, u and surface roughness, z 0 , but the necessity forβoften remains unresolved.

By using two scintillometers at different heights above the surface, a spatial measurement of the Obukhov length, L o , and u * can be derived without incorporating compromising point measurements of the friction velocity or alternatively the average wind speed combined with a measure of the roughness length. The presumption that such measurements are representative of the entire transect usually holds for homogeneous surface cover but may not be valid for patchwork terrain. The two-scintillometer technique is referred to as the C T2-profile method.

Refraction is the result of normal and anomalous dispersion. If, however, the frequency of the emitted EM wave is close to a resonance frequency (absorption lines) of atmospheric constituents, like water vapour and carbon dioxide, absorption becomes important. To quantitatively describe the combined effect of refraction and absorption, a complex refractive index structure parameter, C n2, is introduced. Here the phenomenon of absorption is represented by the imaginary part of the refractive index and is solely determined by single absorption lines and their corresponding absorption coefficients (β i ), resulting in a total absorption coefficient for a band of lines (Hill et al. , 1980). The absorption line strength is temperature dependent, while the absorption line width is temperature, humidity and pressure dependent.

The contribution of absorption fluctuations to C n2is generally neglected, that means to have a real component only. In reality C n2includes both a real part, C nR2, due to refraction and an imaginary part, C nI2, attributable to the absorption mechanism. Any additional source of scintillation such as a contribution from absorption fluctuations could conceivably corrupt the estimation of the sensible heat flux.

Measuring sensible heat flux density over pasture using the C T2- profile method

Two large aperture scintillometers were positioned at heights ( z ) of 10 and 1.5 m with beams propagating horizontally over pasture for distances of 3.1 km and 141 m respectively. From each scintillometer a half-hourly average value of the path-averaged, temperature structure parameter ( C T2) was obtained in unstable atmospheric conditions. The result suggested C T2to scale with height as z -2/3. Using the C T2- profile method, a path averaged measure of the Obukhov length ( L o ) was calculated for each half hour period. L o was used to determine the friction velocity and the surface layer temperature scaling parameter, T * . The scintillometer sensible heat flux density, H sc , was then calculated from H sc = -ρC p u *T * . A time series of half-hourly averaged H sc compared to H ec obtained by the eddy covariance method agreed to within 10%, with R 2= 0.67, for a range of unstable conditions (-0.2≤( z/L o )≤-0.01).

Using a Large Aperture Scintillometer to measure absorption and refractive index fluctuations

The contribution of refraction and absorption fluctuations to the measured scintillation were observed for a near-infrared absorption region using a NOAA designed large aperture scintillometer. The logarithm amplitude spectra were shown to decay with a frequency as f-8/3for both the absorption and scattering mechanism. For the absorption mechanism this is in line with similar observations made at microwave and infrared frequencies. However, for finite transmitting and receiving apertures, theory predicts a stronger decay of the scattering mechanism due to aperture averaging. The spectral shape is characterised by a region of low frequency absorption, higher frequency refraction separated by a flattish transition zone. The upper observed corner frequency ( f C2 ), compared well with the calculated values using the measured transverse wind speed ( v ) for a known aperture radius. The lower corner frequency ( f C1 ) position was shown to be sensitive to the ratio of the real and imaginary part of the refractive index structure parameter, ( C nR2/C nI2) 3/8and v . The part of the spectrum associated with the absorption scintillations was observed to be much less than that due to refraction until the evening when decreasing C nR2caused C nR2/C nI2to decrease and absorption to become significant. If absorption is ignored, this may have consequences for calculating nocturnal surface heat flux densities. During unstable, daytime conditions the large aperture scintillometer is most sensitive to refractive scintillations despite having an infrared source transmitting in a lossy atmosphere. But also under these conditions, the low frequency absorption part of the spectrum is observable.

Leaf diffusion resistance to water vapour and its direct measurement: Results regarding the improved diffusion porometer in growth rooms and fields of Indian corn (Zea mays)
Stigter, C.J. ; Lammers, B. - \ 1974
Wageningen : Veenman (Mededelingen / Landbouwhogeschool Wageningen 74-21) - 76
emissie - waterdamp - bladeren - emission - water vapour - leaves
The epidermal resistance to diffusion of water vapour : an improved measuring method and field results in Indian corn (Zea mays)
Stigter, C.J. - \ 1974
Landbouwhogeschool Wageningen. Promotor(en): J. Schenk, co-promotor(en): C.T. de Wit. - Wageningen : Pudoc - ISBN 9789022005446 - 26
plantkunde - gegevens verzamelen - emissie - schatting - bladeren - maïs - meting - registreren - waterdamp - Zea mays - botany - data collection - emission - estimation - leaves - maize - measurement - recording - water vapour - Zea mays
The resistance of the epidermis of the leaf to diffusion of water vapour can be measured most accurately in the field by closed diffusion porometers. It was possible to overcome the problems related to calibration and to dynamical use of a LiCl humidity sensor in the porometer. The dynamic behaviour could be represented by one calibration volume provided a special measuring scheme is applied. Presence or absence of device influence on the opening situation of the stomata is demonstrated. Measurements on corn leaves yielded permitted clamping times of 1½ to 2 minutes. All sunlit places of corn leaves, sampled throughout a dense crop on a moist soil, showed on the average a basic equality in resistance. Sampling shaded places separately this equality was also found in the average reaction of resistance to different light density levels.
Leaf diffusion resistance to water vapour and its direct measurement: Design, calibration and pertinent theory of an improved leaf diffusion resistance meter
Stigter, C.J. ; Birnie, J. ; Lammers, B. - \ 1973
Wageningen : Veenman (Mededelingen / Landbouwhogeschool Wageningen 73-15) - 55
emissie - waterdamp - bladeren - emission - water vapour - leaves
Leaf diffusion resistance to water vapour and its direct measurement: Introduction and review concerning relevant factors and methods
Stigter, C.J. - \ 1972
Wageningen : Veenman (Mededelingen Landbouwhogeschool 72-3) - 47
emissie - waterdamp - bladeren - emission - water vapour - leaves
De potentiele verdampingsfactor van een gewas
Makkink, G.F. ; Heemst, H.D.J. van - \ 1970
Wageningen : [s.n.] (Mededeling / Instituut voor biologisch en scheikundig onderzoek van landbouwgewassen no. 417) - 10
emissie - waterdamp - emission - water vapour
Transpiration and production of crops in relation to climate and irrigation
Rijtema, P.E. - \ 1966
Wageningen : I.C.W. (Technical bulletin / Institute for land water management research no. 44) - 25
bodemwatergehalte - oogsttoename - oogstverliezen - opbrengsten - irrigatie - meteorologie - emissie - waterdamp - soil water content - yield increases - yield losses - yields - irrigation - meteorology - emission - water vapour
An analysis of actual evapotranspiration
Rijtema, P.E. - \ 1965
Wageningen University. Promotor(en): F. Hellinga. - Wageningen : Pudoc - 107
meteorologie - evaporatie - evapotranspiratie - bodem - hydrologie - emissie - waterdamp - meteorology - evaporation - evapotranspiration - soil - hydrology - emission - water vapour
The main interacting factors determining the actual evapotranspiration from crops were analysed.

The apparent diffusion resistance of the crop was introduced because of the geometry of the evaporative surface. Its relation to light intensity and to suction in the leaves had been estimated for grass.

The precipitation intercepted by the crop increased evapotranspiration when the diffusion resistance of the crop was not zero.

The relation between capillary conductivity and suction was discussed. The relation between diffusion resistance of the crop and the soil physical conditions was calculated with a flow model.

A mathematical model of the factors determining evapotranspiration was derived. The possibility of relating evapotranspiration from crops and evaporation from a free water surface was discussed, and the transfer of such a relation from one area to another different in climate was considered.

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