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.

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Understanding growth of East Africa highland banana: experiments and simulation
Nyombi, K. - \ 2010
University. Promotor(en): Ken Giller, co-promotor(en): Peter Leffelaar; P.J.A. van Asten. - [S.l. : S.n. - ISBN 9789085855507 - 196
musa - bananen - bladoppervlakte-index - lichtpenetratie - straling - simulatiemodellen - kunstmeststoffen - gebruiksefficiëntie - plantenvoeding - middelgebergte - uganda - oost-afrika - bananas - leaf area index - light penetration - radiation - simulation models - fertilizers - use efficiency - plant nutrition - upland areas - east africa
Key words: leaf area; radiation interception; QUEFTS model; fertilizer recovery fractions; nutrient mass fractions; crop growth; calibration; validation; radiation use efficiency; sensitivity analysis

East Africa Highland banana yields on smallholder farms in the Great Lakes region are small (11−26 Mg ha−1 cycle−1 in Uganda, 21−43 Mg ha−1 cycle−1 in Burundi and 25−53 Mg ha−1 cycle−1 in Rwanda). The major causes of poor yields are declining soil fertility and soil moisture stress. In order to improve production, knowledge on highland banana physiology, growth patterns and response to fertilization is important, to establish the potential yield of the crop, to quantify the yield gaps between potential and actual yield, and to explore options for closing the yield gaps.
Measurements of plant morphological characteristics, radiation interception and biomass (by destructive harvesting) were taken in experimental fields in central and southwest Uganda. Results showed that total leaf area can be estimated by using height and girth (used to estimate middle leaf area) and number of functional leaves. The light extinction coefficient, k determined from photosynthetically active radiation (PAR) measurements over the entire day was 0.7. Banana plants partitioned more dry matter (DM) to the leaves during first phase of vegetative growth, with the pseudostem becoming the dominant sink later with 58% of total DM at flowering, and the bunch at harvest with 53% of the total DM. Changes in dry matter partitioning influenced the allometric relationships between above-ground biomass (AGB in kg DM) and girth (cm), the relationship following a power function during the vegetative phase (AGB = 0.0001 (girth)2.35), and exponential functions at flowering (AGB = 0.325 e0.036 (girth)) and at harvest (AGB = 0.069 e0.068 (girth)). This thesis shows that allometric relationships can be derived and used to estimate biomass and bunch weights.
In fertilizer trials, yield increases above the control (13.0 Mg ha−1 yr−1) ranged from 2.2−11.2 Mg ha−1 yr−1 at Kawanda, to more than double at Ntungamo, 7.0−29.5 Mg ha−1 yr−1 (control 7.9 Mg ha−1 yr−1). The limiting nutrients at both sites were in the order K>P>N. Differences in soil moisture availability and texture resulted in higher yields and total nutrient uptakes (K>N>P) at Ntungamo, compared with Kawanda. Per unit dry matter yield, highland bananas take up a similar amount of N (49.2 kg finger DM kg−1 N), half the amount of P (587 kg finger DM kg−1 P), and five times the amount of K (10.8 kg finger DM kg−1 K), when compared with cereal grain. Calibration results of the static nutrient response model QUEFTS using data from Ntungamo were fair (R2 = 0.57, RMSE = 648 kg ha−1). The calibrated QUEFTS model predicted yields well using data from Mbarara southwest Uganda (R2 = 0.68, RMSE = 562 kg ha−1).
A new dynamic radiation and temperature-driven growth model, LINTUL BANANA 1 was developed to the compute potential yields of East Africa highland banana. The model considers (i) the physiology of the highland banana crop; (ii) the plant dynamics (i.e. three plant generations, Plant 1, 2 and 3 at different stages of growth constituting a mat); and (iii) three canopy levels formed by the leaves of the three plants. Average computed potential bunch dry and fresh matter were slightly higher at Ntungamo (20 Mg ha−1 DW; 111 Mg ha−1 FW), compared with Kawanda (18.25 Mg ha−1 DW; 100 Mg ha−1 FW), and values compared well with banana yields under optimal situations at comparable leaf area index values (20.3 Mg ha−1 DW; 113 Mg ha−1 FW). Sensitivity analysis was done to assess the effects of changes in parameters (light use efficiency, LUE; the light extinction coefficient, k; specific leaf area, SLA; the relative death rate of leaves, rd; relative growth rate of leaf area, RGRL; and the initial dry matter values) on bunch dry matter, leaf dry matter and leaf area index (L) at flowering. Sensitivity results for Kawanda and Ntungamo showed that changes in LUE1 resulted in more than proportional increase in bunch DM (1.30 and 1.36), a higher leaf DM (0.60 and 0.67) and L at flowering (0.60 and 0.67). Changes in rd1 values reduced bunch dry matter, leaf dry matter and L at flowering. Changes in SLA1 reduced only leaf DM, whereas both leaf DM and L at flowering were reduced by changes in k1 at both sites. Initial dry matter values had a small effect (sensitivity < 0.0263) for bunch DM, leaf DM and L at flowering. Based on the model results, it is clear that the potential yield of East Africa highland bananas is more than 18 Mg ha−1 DW. Management options that increase LUE and reduce the relative death rate of leaves, and improvements in parameters related to light interception (SLA and k) are important to increase yield.

Tillering in spring wheat : a 3D virtual plant modelling study
Evers, J.B. - \ 2006
University. Promotor(en): Paul Struik, co-promotor(en): Jan Vos; B. Andrieu. - Wageningen : s.n. - ISBN 9085043778 - 159
triticum aestivum - tarwe - plantenontwikkeling - uitstoeling - plantenmorfologie - modellen - licht - verrood licht - lichtrelaties - optische eigenschappen - bladoppervlakte-index - grondbedekking - wheat - plant development - tillering - plant morphology - models - light - far red light - light relations - optical properties - leaf area index - ground cover - cum laude
cum laude graduation (with distinction)
Optimaal bladoppervlak levert geld op : voorzichtig zijn met teveel blad plukken bij tomaat
Heuvelink, E. ; Kierkels, T. - \ 2005
Onder Glas 2 (2005)2. - p. 14 - 15.
tomaten - solanum lycopersicum - plantenontwikkeling - bladoppervlakte - bladoppervlakte-index - ontbladering - assimilatie - netto-assimilatiesnelheid - glastuinbouw - groenten - tomatoes - plant development - leaf area - leaf area index - defoliation - assimilation - net assimilation rate - greenhouse horticulture - vegetables
Uit onderzoek blijkt dat sommige tomatentelers tot 10% meer licht weten te vangen dan andere telers. In principe betekent dat ook een 10% hogere productie. Het onderzoek bewijst: zorg voor een hoge lichtonderschepping door het gewas. Dat zorgt voor een optimale productie en optimale benutting van duur assimilatielicht. Gegevens in bijgaande grafiek: De hoeveelheid licht die het gewas onderschept loopt op naarmate het aantal vierkante meters blad per grondoppervlak groter is, tot een bepaalt maximum is bereikt
Energiebesparing door bladplukken bij paprika: haalbaarheidsstudie op basis van modellen
Grashoff, C. ; Stanghellini, C. ; Kempkes, F.L.K. ; Elings, A. ; Marcelis, L.F.M. - \ 2003
Wageningen : Plant Research International (Nota / Plant Research International 310) - 15
capsicum annuum - bladoppervlakte - bladoppervlakte-index - kassen - teelt onder bescherming - nederland - paprika - energiebesparing - leaf area - leaf area index - greenhouses - protected cultivation - netherlands - sweet peppers - energy saving
Met behulp van simulatiemodellen werd een haalbaarheidsstudie uitgevoerd naar de effecten van bladplukken op energiebesparing en rendementsverbetering bij paprika. Deze studie leverde het volgende op: 蛮 Eenmalige bladpluk (van LAI=6 terug naar LAI=3) geeft een waterbesparing van 11%, een energiebesparing van 5% bij een opbrengststijging van 1%. Het netto financiële resultaat hangt sterk af van de prijzen voor gas, arbeid (voor bladplukken) en product. Voor de berekeningen die het meest overeenkomen met huidig prijspeil schatten wij dat het financiële resultaat ligt tussen € 1050, - winst per hectare (bij een uurloon van € 15,- ) en € 820,- verlies per hectare (bij een uurloon van € 20,-). 蛮 Het gunstigste moment voor bladplukken is in augustus. 蛮 Deze resultaten zijn gebaseerd op de klimaatinstellingen van een teler die nu reeds energiezuinig teelt. Een gemiddelde tuinder gebruikt 10% meer energie, maar de berekende resultaten van bladplukken zijn vergelijkbaar. 蛮 Wekelijks bladplukken lijkt minder perspectiefvol. Weliswaar is de energiebesparing dan groter (8%), maar het positieve effect op productie is kleiner. De kosten voor bladplukken zullen naar verwachting hoger zijn. De resultaten zijn op 11 december 2003 besproken met de tuinders in een vergadering van de Landelijke commissie paprika van LTO. Daarbij is gediscussieerd over de toepassingsmogelijkheden in de praktijk en hebben we de tuinders gevraagd of ze perspectief zien in een nadere toetsing in de praktijk. De tuinders ondersteunen een praktijktoetsing, waarbij ze enkele randvoorwaarden hebben aangegeven. De reacties van de tuinders zijn weergegeven in dit rapport.
Het verwerken van informatie uit Remote Sensing-opnamen en het gebruik ervan in een gewasgroeimodel
Heijne, M.S.M. - \ 2002
Wageningen : Plant Research International - 68
veldgewassen - simulatiemodellen - remote sensing - bladoppervlakte-index - precisielandbouw - agro-ecologie - field crops - simulation models - leaf area index - precision agriculture - agroecology
Analysis and simulation of growth and yield of cut chrysanthemum
Lee, J.H. - \ 2002
University. Promotor(en): H. Challa; Ep Heuvelink. - S.l. : S.n. - ISBN 9789058087621 - 120
chrysanthemum - groei - gewasopbrengst - snijbloemen - groeianalyse - simulatiemodellen - systeemanalyse - bladoppervlakte-index - teelt onder bescherming - kassen - growth - crop yield - growth analysis - leaf area index - protected cultivation - greenhouses - simulation models - systems analysis - cut flowers
<strong><em><font size="3"><p>Key words</strong></em> : chrysanthemum, crop growth, development, explanatory model, expolinear growth, dry mass, dry matter partitioning, fresh mass, leaf area index, light interception, light use efficiency, plant density, season, simulation, validation, year-round.</p><p> </p><p>Seasonal variation in daily light integral naturally leads to seasonal variation in production and plant quality in year-round cut chrysanthemum ( <em>Chrysanthemum</em> , Indicum group). Growers try to deliver constant product quantity and quality throughout the year by adapting plant density, day length, duration of long-day periods and supplementary lighting. Optimising crop management is very complex and requires a great deal of knowledge that is best represented by a generic model. There is, however, no such model available and there is also a lack of information on the dynamics of crop performance, i.e. dry mass production and leaf area index.</p><p>In this thesis, growth and yield of cut chrysanthemum were analysed and quantified, as related to radiation, planting date and plant density. A generic model, CHRYSIMv1.0, was derived from an existing photosynthesis-driven model, calibrated and validated for year-round cut chrysanthemum.</p><p>Final plant fresh and dry mass and number of flowers per plant decreased with plant density and this decrease was larger in summer than in winter. The observed variation in plant fresh mass can be described by a linear relationship with cumulative incident photosynthetically active radiation (PAR) per plant.</font><font size="3">Dynamics of aboveground total dry mass per m <sup>2</SUP>(TDM) and leaf area index (LAI) were well described by the expolinear growth equation. Moreover a modified expolinear growth equation, formulated as a function of accumulated daily incident PAR was used to derive the light use efficiency (LUE, g MJ <sup>-1</SUP>) for closed canopies. LUE showed a hyperbolic relation to daily incident PAR and LUE was used to determine the maximum crop growth rate (g m <sup>-2</SUP>d <sup>-1</SUP>) at closed canopy. However, it was hard to generalise growth and yield of cut chrysanthemum under a wide range of crop growing conditions and in this approach measured LAI is still required as an input. These limitations lead to the development of a model for predicting LAI and use of it with a generic model for predicting growth and yield of year-round cut chrysanthemum.</p><p>Dynamics of LAI can be simulated by the combination of increase in leaf dry mass (LDM) and specific leaf area of new leaves (SLA <sub>n</sub> ). Dynamics of LDM could be adequately described using a Gompertz function to describe dry mass partitioning to the leaves. SLA <sub>n</sub> was linearly related to the inverse of the daily incident PAR, to temperature and to plant density. Dynamics of LAI were satisfactorily simulated for independent experiments and for commercially-grown crops.</p><p>CHRYSIMv1.0 was validated, using measured LAI and dry matter partitioning to the organs as an input, to check first possible errors in crop photosynthesis, maintenance respiration and dry mass conversion efficiency. Global radiation outside, inside greenhouse temperature and CO <sub>2</sub> concentration were also input to the model. Simulated TDM was equal to measured TDM in summer (natural light) only, whereas a large under-estimation occurred at constant shade, and in winter. At low light LUE was largely underestimated by CHRYSIMv1.0. Calibrating the parameters of the photosynthesis light response curve of the leaves, i.e. initial light use efficiency,<FONT FACE="Symbol">e</font>, and maximum leaf photosynthetic rate, P <sub>gmax</sub> , based on one experiment resulted in largely improved simulations of TDM in all experiments, but unrealistic parameter values. Finally some limitations for application of CHRYSIMv1.0 are discussed in this thesis.
Manipulating the physiological quality of in vitro plantlets and transplants of potato
Mehari, T. - \ 2000
Agricultural University. Promotor(en): Paul Struik; W.J.M. Lommen. - S.l. : S.n. - ISBN 9789058083302 - 230
solanum tuberosum - aardappelen - in vitro kweek - microvermeerdering - zaadproductie - transplantaten - groei - groeianalyse - bladoppervlakte-index - bladoppervlakte - lichtpenetratie - ophoping van drogestof - stikstof - temperatuur - acclimatisatie - potatoes - in vitro culture - micropropagation - seed production - transplants - growth - growth analysis - leaf area index - leaf area - light penetration - dry matter accumulation - nitrogen - temperature - acclimatization
<p><em>In vitro</em> techniques have been introduced in potato seed production systems in recent years. This research project aimed at studying the morphological and physiological changes in plants and crops in the last three phases of a seed production system that included an <em>in vitro</em> multiplication, an <em>in vitro</em> normalisation (growing cuttings to rooted plantlets), a transplant production, and a tuber production (field) phase.</p><p>Leaf area was identified as an important plant parameter for plant growth in the normalisation and transplant production phases. Explants and plantlets with larger initial leaf area performed better than those with smaller initial leaf area. <em>In vitro</em> treatments mainly affected leaf area of transplants through their effects on early above-ground leaf area. Leaf area increase was better described by logistic than by exponential or expolinear curves in all phases of growth, suggesting restriction of leaf area increase in all phases.</p><p>Low temperature decreased leaf and stem dry weights in all phases, and increased tuber fresh and dry yields, average tuber weight, leaf/stem ratio, specific leaf area and harvest index in the tuber production phase. Growing <em>in vitro</em> plants at low normalisation temperatures increased leaf and total plant dry weights early in the transplant production and tuber production phases. It resulted in higher tuber yields, heavier individual tubers and higher harvest index.</p><p>Fertilising plants with higher nitrogen (40 versus 10 mg N per plant) during transplant production resulted in plants with higher groundcover in the field. This led to higher interception of solar radiation and higher tuber yield in one of the two experiments. Growing plants at higher temperature (26/20 versus 12/18 °C) during transplant production increased leaf area at the end of the transplant production phase. After transplanting to the field, it resulted in crops with higher groundcover, which intercepted more incoming solar radiation. Yield tended to be higher, but differences could not be assessed as statistically significant. A glasshouse experiment showed that high temperature during transplant production increased leaf and stem dry weights in the tuber production phase, but reduced tuber dry weights and harvest index when temperatures during tuber production were high. Thus, high temperature during transplant production may favour haulm growth and light interception in the field, but may also reduce dry matter partitioning to tubers.</p><p>Conditions in the tuber production phase were found to be of greater importance for final yield than conditions and treatments in earlier phases.</p><p>Strategies to optimise the production and use of propagules and transplants should focus on achieving leafy starting material, reducing stress during changes in environment and optimising conditions during tuber production. Production of transplants should be adjusted to the expected growth conditions in the tuber production phase.</p><p><strong>Key words:</strong><em>Solanum tuberosum</em> L., <em>in vitro</em> plantlet, seed production, normalisation, transplant production, tuber production, acclimatisation, leaf area, groundcover, logistic growth, temperature, nitrogen, dry matter production, specific leaf area, harvest index, radiation interception, radiation use efficiency.</p>
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
<p>Introduction and objectives</p><p>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 (&lt; 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.</p><p>As indicated in Chapter 1, the <em>first objective</em> 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 <sub>2</sub> ) 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 <em>second objective</em> of the thesis. The <em>third objective</em> focussed on the prospect of obtaining both the spatial averaged sensible heat flux density and momentum flux density from scintillation measurements.</p><p>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 <em>fourth objective</em> of this thesis was to study the influence of absorption fluctuations on the average sensible heat flux density derived from the scintillation technique.</p><p><strong>Carbon dioxide exchange and the effect of biological and climatic processes</strong></p><p>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 <em>Molinia caerulea</em> . 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 <sub>2</sub> over the past decade but this can not simply be generalised.</p><p>The growing season extended from May until late October, with a maximum <em>LAI</em> in August of 1.7. The carbon balance showed a net release of 97 g CO <sub>2</sub> m <sup>-2</SUP>y <sup>-1</SUP>(265 kg C ha <sup>-1</SUP>y <sup>-1</SUP>) from the peat bog ecosystem to the atmosphere. During June, July and August there was net consumption of CO <sub>2</sub> , while during the rest of the year there was net production of CO <sub>2</sub> . The maximum daytime net exchange rates were about -0.5 mg CO <sub>2</sub> m <sup>-2</SUP>s <sup>-1</SUP>(-11.3μmol CO <sub>2</sub> m <sup>-2</SUP>s <sup>-1</SUP>) with an average peak exchange rate of -0.2 mg CO <sub>2</sub> m <sup>-2</SUP>s <sup>-1</SUP>(-4.5μmol CO <sub>2</sub> m <sup>-2</SUP>s <sup>-1</SUP>), in a period where the <em>LAI</em> ranged between 1 and 1.7. A high vapour pressure deficit (&gt;15 hPa) corresponding with high temperature was found to reduce the net CO <sub>2</sub> exchange rate by on average 50%.</p><p>Apart from these factors, <em>LAI</em> and the soil temperature co-determined the net exchange of CO <sub>2</sub> . 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 <em>Q <sub>10</em></sub> of 4.8.</p><p><strong>The effect of plant related and environmental conditions on the interaction of CO <sub>2</sub> and H <sub>2</sub> O exchange</strong></p><p>The tussock grassland, dominated by <em>Molinea caerulea</em> , 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.</p><p>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 (&lt; 15 hPa) and high vapour pressure deficit (&gt; 20 hPa) at high surface temperatures, the vegetation showed similar behaviour resulting in a clear reduction of the daytime CO <sub>2</sub> uptake. Temperature was therefore inferred to be main the reason for a reduction in CO <sub>2</sub> 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.</p><p><strong>The scintillation technique</strong></p><p>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 ( <em>n</em> ). 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, <em>C <sub>n</sub><sup>2</em></SUP>.</p><p>The measured <em>C <sub>n</sub><sup>2</em></SUP>value is related to the structure parameters of temperature <em>C <sub>T</sub><sup>2</em></SUP>, humidity <em>C <sub>Q</sub><sup>2</em></SUP>and a covariant term <em>C <sub>TQ</em></sub> , respectively. To calculate the sensible heat flux density from <em>C <sub>n</sub><sup>2</em></SUP>compromising point measurements of the Bowen ratio,β, and friction velocity, <em>u <sub>*</em></sub> , are necessary. Generally, the deficiency in the available spatial measurement of <em>u <sub>*</em></sub> is overcome by using a point measure of the average wind speed, <em>u</em> and surface roughness, <em>z <sub>0</em></sub> , <em><sub></em></sub> but the necessity forβoften remains unresolved.</p><p>By using two scintillometers at different heights above the surface, a spatial measurement of the Obukhov length, <em>L <sub>o</em></sub> , and <em>u <sub>*</em></sub> 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 <em>C <sub>T</sub><sup>2</em></SUP>-profile method.</p><p>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, <em>C <sub>n</sub><sup>2</em></SUP>, 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 <em><sub>i</em></sub> ), resulting in a total absorption coefficient for a band of lines (Hill <em>et al.</em> , 1980). The absorption line strength is temperature dependent, while the absorption line width is temperature, humidity and pressure dependent.</p><p>The contribution of absorption fluctuations to <em>C <sub>n</sub><sup>2</em></SUP>is generally neglected, that means to have a real component only. In reality <em>C <sub>n</sub><sup>2</em></SUP>includes both a real part, <em>C <sub>nR</sub><sup>2</em></SUP>, due to refraction and an imaginary part, <em>C <sub>nI</sub><sup>2</em></SUP>, 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.</p><p><strong>Measuring sensible heat flux density over pasture using the C <sub>T</sub><sup>2</SUP>- profile method</strong></p><p>Two large aperture scintillometers were positioned at heights ( <em>z</em> ) 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 ( <em>C <sub>T</sub><sup>2</em></SUP>) was obtained in unstable atmospheric conditions. The result suggested <em>C <sub>T</sub><sup>2</em></SUP>to scale with height as <em>z <sup>-2/3</em></SUP>. Using the <em>C <sub>T</sub><sup>2</em></SUP>- profile method, a path averaged measure of the Obukhov length ( <em>L <sub>o</em></sub> ) was calculated for each half hour period. <em>L <sub>o</em></sub> was used to determine the friction velocity and the surface layer temperature scaling parameter, <em>T <sub>*</em></sub> . The scintillometer sensible heat flux density, <em>H <sub>sc</em></sub> , was then calculated from <em>H <sub>sc</sub></em> = <em>-ρC <sub>p</sub> u <sub>*</em></sub><em>T <sub>*</em></sub> . A time series of half-hourly averaged <em>H <sub>sc</em></sub> compared to <em>H <sub>ec</em></sub> obtained by the eddy covariance method agreed to within 10%, with R <sup>2</SUP>= 0.67, for a range of unstable conditions (-0.2≤( <em>z/L <sub>o</em></sub> )≤-0.01).</p><p><strong>Using a Large Aperture Scintillometer to measure absorption and refractive index fluctuations</strong></p><p>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 <em>f</em><sup>-8/3</SUP>for 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 ( <em>f <sub>C2</em></sub> ), compared well with the calculated values using the measured transverse wind speed ( <em>v</em> ) for a known aperture radius. The lower corner frequency ( <em>f <sub>C1</em></sub> ) position was shown to be sensitive to the ratio of the real and imaginary part of the refractive index structure parameter, ( <em>C <sub>nR</sub><sup>2</SUP>/C <sub>nI</sub><sup>2</em></SUP>) <sup>3/8</SUP>and <em>v</em> . 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 <em>C <sub>nR</sub><sup>2</em></SUP>caused <em>C <sub>nR</sub><sup>2</SUP>/C <sub>nI</sub><sup>2</em></SUP>to 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.</p>
LAI estimation by means of the WDVI: a sensitivity analysis with a combined PROSPECT-SAIL model.
Clevers, J.G.P.W. ; Verhoef, W. - \ 1993
Remote sensing reviews 7 (1993)1. - ISSN 0275-7257 - p. 43 - 64.
landbouw - toepassingen - bladoppervlakte-index - remote sensing - agriculture - applications - leaf area index
Multispectrale luchtfotografie van de akkerbouwproefboerderij Vredepeel
Bakker, J.G.M. ; Boer, R.J. de - \ 1991
Lelystad : PAGV - 46
luchtfotografie - monitoring - bodemkarteringen - gewassen - bladoppervlakte-index - nederland - spectra - landbouwkundig onderzoek - aerial photography - soil surveys - crops - leaf area index - netherlands - agricultural research
The application of a weighted infrared-red vegetation index for estimating leaf area index by correcting for soil moisture.
Clevers, J.G.P.W. - \ 1989
Remote Sensing of Environment 29 (1989)1. - ISSN 0034-4257 - p. 25 - 37.
remote sensing - applications - absorption - reflection - soil water - leaf area index - models - research - toepassingen - absorptie - reflectie - bodemwater - bladoppervlakte-index - modellen - onderzoek
The simplified reflectance model described earlier (Clevers, 1988b) for estimating leaf area index (LAI) is further simplified. In this model the nearinfrared reflectance was corrected for soil background (in particular differences in soil moisture content) and subsequently used for estimating LAI by applying the inverse of a special case of the Mitscherlich function. In the specific situation that the ratio of the reflectances of bare soil in the red and near-infrared is constant for a given soil background, the corrected near-infrared reflectance is now ascertained as a weighted difference of total measured near-infrared and red reflectances (socalled WDVI = weighted difference vegetation index). The validity of this approach is confirmed by simulations with the SAIL model. The above concept was tested at the experimental farm of the Agricultural University Wageningen, by using reflectance factors measured in field trials by means of multispectral aerial photography. The soil type at the experimental farm yielded constant ratios between green, red, and near-infrared reflectances independent of soil moisture content (that is, as a function of time). The difference between measured near-infrared and red reflectances provided a satisfactory approximation of the corrected near-infrared reflectance. The estimation of LAI by this corrected near-infrared reflectance for real field data yielded good results in this study, resulting in the ascertainment of treatment effects with larger precision than by means of the LAI measured in the field by conventional field sampling methods.
Modelvorming en synergie optische en microgolf-remote sensing.
Clevers, J.G.P.W. ; Hoekman, D.H. - \ 1989
[Delft] : BCRS
toepassingen - in de grond doordringende radar - bladoppervlakte-index - plantenecologie - remote sensing - scannen - microgolfstraling - applications - ground-penetrating radar - leaf area index - plant ecology - scanning - microwave radiation
Influence of day and night temperature on the growth of young tomato plants.
Heuvelink, E. - \ 1989
Scientia Horticulturae 39 (1989). - ISSN 0304-4238 - p. 11 - 22.
gewassen, groeifasen - groeistadia - warmte - bladoppervlakte-index - solanum lycopersicum - teelt onder bescherming - temperatuur - tomaten - crop growth stage - growth stages - heat - leaf area index - protected cultivation - temperature - tomatoes
The Heiligenschijn reflectance meter : active hot-spot reflectance compared with passive reflectance from crop canopies for the determination of crop parameters
Boer, T. de; Bouman, B.A.M. ; Kasteren, H.J.W. van - \ 1988
Wageningen : CABO (CABO-verslag nr. 78) - 67
remote sensing - absorptie - reflectie - landbouw - veldgewassen - toepassingen - licht - akkerbouw - instrumenten (meters) - bladeren - bladoppervlakte-index - absorption - reflection - agriculture - field crops - applications - light - arable farming - instruments - leaves - leaf area index
Application of remote sensing to agricultural field trials
Clevers, J.G.P.W. - \ 1986
Landbouwhogeschool Wageningen. Promotor(en): L.C.A. Corsten; M. Molenaar. - Wageningen : Clevers - 227
proefvelden - experimenteel veldonderzoek - proeven - remote sensing - toepassingen - bodemkunde - absorptie - reflectie - veldgewassen - akkerbouw - bladoppervlakte-index - nederland - experimental plots - field experimentation - trials - applications - soil science - absorption - reflection - field crops - arable farming - leaf area index - netherlands
<p>Remote sensing techniques enable quantitative information about a field trial to be obtained instantaneously and non-destructively. The aim of this study was to identify a method that can reduce inaccuracies in field trial analysis, and to identify how remote sensing can support and/or replace conventional field measurements in field trials.<p>In the literature there is a certain consensus that the best bands from which characteristic spectral information about vegetation can be extracted are those in the visible (green and red) and infrared regions of the electromagnetic spectrum. This was confirmed in the present study by an analysis of multispectral scanner data ('Daedalus scanner') from field trials with cereals. The optimal bands that were thereby selected for explaining grain yield mostly contained the channels 5 (550- 600 nm), 7 (650-700 nm), and 9 (800-890 nm).<p>Multispectral aerial photography was found to be most appropriate for recording extensive field trials in a short period. In the present study, recordings were carried out with a single-engine aircraft, using two Hasselblad cameras for obtaining vertical photographs on black and white 70-mm aerial films. In this way, costs stayed within acceptable limits. The recording scale chosen, given the dimensions of the trials at the experimental farm of the Wageningen Agricultural University, where the research was carried out, was 1:8 000. Photographs were taken approximately fortnightly to keep in step with conventional field sampling. The film/filter combinations selected for obtaining a high spectral resolution and for matching bands 5, 7 and 9 of the Daedalus scanner, resulted in the following passbands:<br/>green : 555-580 nm;<br/>red :665-700 nm;<br/>infrared :840-900 nm,<p>The densities of the objects on the film were measured by means of an automated Macbeth TD-504 densitometer. An aperture with a diameter of 0.25 mm was selected for the densitometer, in order to obtain a high spatial resolution at the scale of 1:8000, applicable to field trials with plots 3 metres wide. The measured densities were converted into exposure values, corrected for light falloff, and then a linear function was applied to convert them into reflectance factors. In this linear function the exposure time, relative aperture, transmittance of the optical system, irradiance, path radiance and atmospheric attenuation were incorporated. Reference targets with known reflectance characteristics were set up in the field during missions and recorded at the same camera setting and under the same atmospheric conditions as the field trials, in order to ascertain the parameters of the linear function.<p>Information about crop reflectance obtained from the literature suggested that reflectances in the visible region of the electromagnetic spectrum (green or red) would be most suitable for estimating soil cover, whereas reflectances in the infrared might be most suitable for estimating leaf area index (LAI). Other plant characteristics, such as dry matter weight or yield, may be estimated indirectly from reflectances. Field trials with cereals analysed during the present study showed that treatment effects shown by green and red reflectances tended to be opposite to those shown by LAI. Treatment effects shown by infrared reflectance tended to be similar to those shown by LAI, even at large LAI (6-8). The treatment effects manifest in reflectances were more stable in time than those for LAI. Coefficients of variation of residuals resulting from analyses of variance were systematically smaller for reflectances than for the LAI in all experiments: those for the infrared reflectance were particularly small. In general, critical levels in testing for treatment effects were smaller for the infrared reflectance than for the LAI, which indicates that the power for infrared reflectance was larger than for LAI.<p>Soil moisture content is not constant during the growing season and differences in soil moisture content greatly influence soil reflectance. Since a multitemporal analysis of remote sensing data was required, a correction had to be made for soil background when ascertaining the relationship between reflectances and crop characteristics. In the literature no index or reflectance model stood out as being suitable for estimating crop characteristics in agricultural field trials. Thus, in this monograph an appropriate simplified reflectance model is presented for estimating soil cover and LAI for green vegetation. First of all, soil cover is redefined as: the vertical projection of green vegetation and the relative area of shadows included, seen by a sensor pointing vertically downwards, relative to the total soil area (in this definition soil cover depends on the position of the sun). Then, the simplified reflectance model is based on the expression of the measured reflectance as a composite reflectance of plants and soil: the measured reflectance in the various passbands is a linear combination of soil cover and its complement, with the reflectances of the plants and of the soil as coefficients, respectively.<p>By using this model, it should, theoretically, be possible to correct for soil background when estimating soil cover by combination of measurements in the green and red passbands. In practice, however, all the procedures derived yielded poor results because the difference between green and red reflectances was so small. Thus, attention was focussed on estimating LAI.<p>For estimating LAI a corrected infrared reflectance was calculated by subtracting the contribution of the soil from the measured reflectance. Theoretically, combining the reflectance measurements obtained in the green, red and infrared passbands, enables the corrected infrared reflectance to be calculated, without knowing soil reflectances. The main assumption was that there is a constant ratio between the reflectances of bare soil in different passbands, independent of soil moisture content: this assumption is valid for many soil types. For the soil type at the experimental farm of the Agricultural University, the corrected infrared reflectance can be approximated by the difference between total measured infrared and red reflectances. Subsequently this corrected infrared reflectance was used for estimating LAI according to the inverse of a special case of the Mitscherlich function. This function contains two parameters that have to be ascertained empirically. Model simulations with the SAIL model (introduced by Verhoef, 1984) confirmed the potential of this simplified, semi-empirical, reflectance model for estimating LAI.<p>Analogous derivations were applied for a generative canopy (cereals) with yellowing leaves.<p>The estimation of LAI by reflectances yielded good results for the field trials with cereals analysed in this study. The presence of treatment effects could be shown with larger power and the coefficients of variation were smaller for this estimated LAI than for the one measured in the field. Regression curves of LAI on corrected infrared reflectance differed significantly in different trials with the same crop, particularly for the generative stage. This may have been caused by large systematic discrepancies between LAI measurements obtained with the conventional sampling techniques for two field trials, because of subjectivity in separating green from yellow leaves. To date, the best approach is to ascertain regression curves of LAI on corrected infrared reflectance for each field trial by incorporating a few additional plots, in which both the LAI and the reflectances are measured.
Simulatie van de droge stof-produktie en de Leaf area index van cassave
Gijzen, H. - \ 1985
Wageningen : Gijzen (Verslag / Vakgroep Tropische Plantenteelt, Landbouwhogeschool ) - 67
cassave - computersimulatie - bladoppervlakte-index - manihot esculenta - simulatie - simulatiemodellen - oogsttoename - oogstverliezen - opbrengsten - cassava - computer simulation - leaf area index - simulation - simulation models - yield increases - yield losses - yields
Photosynthesis of leaf canopies
Wit, C.T. de - \ 1965
Wageningen : Pudoc (Agricultural research reports 663) - 57
bladeren - fotosynthese - fotosynthese van het kroondak - lichtpenetratie - bladoppervlakte-index - leaves - photosynthesis - canopy photosynthesis - light penetration - leaf area index
The development of a procedure to calculate the effect of certain environmental factors on the rate of photo-synthesis imposed mainly geometrical problems, which were solved in such a way that the actual calculation could be carried out by means of a computer. The calculation procedures have been used to study the. relative importance of the variables under various conditions. The results for a standard set of conditions, have been summarized in order to make it possible to estimate the daily photosynthesis at any time and place for a wide range of photosynthesis functions without a computer.
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