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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    Evaluating the performance of commonly used gas analysers for methane eddy covariance flux measurements: the InGOS inter-comparison field experiment
    Peltola, O. ; Hensen, A. ; Helfter, C. ; Belelli Marchesini, L. ; Bosveld, F.C. ; Bulk, W.C.M. van de; Elbers, J.A. ; Haapanala, S. ; Holst, J. ; Laurila, T. ; Lindroth, A. ; Nemitz, E. ; Röckmann, T. ; Vermeulen, A.T. ; Mammarella, I. - \ 2014
    Biogeosciences 11 (2014). - ISSN 1726-4170 - p. 3163 - 3186.
    water-vapor - atmospheric methane - mixing-ratio - wpl terms - path - ch4 - attenuation - accuracy - strategy - quality
    The performance of eight fast-response methane (CH4) gas analysers suitable for eddy covariance flux measurements were tested at a grassland site near the Cabauw tall tower (Netherlands) during June 2012. The instruments were positioned close to each other in order to minimise the effect of varying turbulent conditions. The moderate CH4 fluxes observed at the location, of the order of 25 nmol m-2 s-1, provided a suitable signal for testing the instruments' performance. Generally, all analysers tested were able to quantify the concentration fluctuations at the frequency range relevant for turbulent exchange and were able to deliver high-quality data. The tested cavity ringdown spectrometer (CRDS) instruments from Picarro, models G2311-f and G1301-f, were superior to other CH4 analysers with respect to instrumental noise. As an open-path instrument susceptible to the effects of rain, the LI-COR LI-7700 achieved lower data coverage and also required larger density corrections; however, the system is especially useful for remote sites that are restricted in power availability. In this study the open-path LI-7700 results were compromised due to a data acquisition problem in our data-logging setup. Some of the older closed-path analysers tested do not measure H2O concentrations alongside CH4 (i.e. FMA1 and DLT-100 by Los Gatos Research) and this complicates data processing since the required corrections for dilution and spectroscopic interactions have to be based on external information. To overcome this issue, we used H2O mole fractions measured by other gas analysers, adjusted them with different methods and then applied them to correct the CH4 fluxes. Following this procedure we estimated a bias of the order of 0.1 g (CH4) m-2 (8% of the measured mean flux) in the processed and corrected CH4 fluxes on a monthly scale due to missing H2O concentration measurements. Finally, cumulative CH4 fluxes over 14 days from three closed-path gas analysers, G2311-f (Picarro Inc.), FGGA (Los Gatos Research) and FMA2 (Los Gatos Research), which were measuring H2O concentrations in addition to CH4, agreed within 3% (355–367 mg (CH4) m-2) and were not clearly different from each other, whereas the other instruments derived total fluxes which showed small but distinct differences (±10%, 330–399 mg (CH4) m-2).
    Agricultural peatlands: towards a greenhouse gas sink - a synthesis of a Dutch landscape study
    Schrier-Uijl, A.P. ; Kroon, P.S. ; Hendriks, D.M.D. ; Hensen, A. ; Huissteden, J. van; Leffelaar, P.A. ; Berendse, F. ; Veenendaal, E.M. - \ 2014
    Biogeosciences 11 (2014). - ISSN 1726-4170 - p. 4559 - 4576.
    anemometer (co)sine response - covariance flux measurements - cut-away peatland - eddy covariance - carbon balance - water-vapor - n2o - exchange - meadow - soil
    It is generally known that managed, drained peatlands act as carbon (C) sources. In this study we examined how mitigation through the reduction of the intensity of land management and through rewetting may affect the greenhouse gas (GHG) emission and the C balance of intensively managed, drained, agricultural peatlands. Carbon and GHG balances were determined for three peatlands in the western part of the Netherlands from 2005 to 2008 by considering spatial and temporal variability of emissions (CO2, CH4 and N2O). One area (Oukoop) is an intensively managed grass-on-peatland area, including a dairy farm, with the ground water level at an average annual depth of 0.55 (±0.37) m below the soil surface. The second area (Stein) is an extensively managed grass-on-peatland area, formerly intensively managed, with a dynamic ground water level at an average annual depth of 0.45 (±0.35) m below the soil surface. The third area is a (since 1998) rewetted former agricultural peatland (Horstermeer), close to Oukoop and Stein, with the average annual ground water level at a depth of 0.2 (±0.20) m below the soil surface. During the measurement campaigns we found that both agriculturally managed sites acted as C and GHG sources and the rewetted former agricultural peatland acted as a C and GHG sink. The ecosystem (fields and ditches) total GHG balance, including CO2, CH4 and N2O, amounted to 3.9 (±0.4), 1.3 (±0.5) and -1.7 (±1.8) g CO2-eq m-2 d-1 for Oukoop, Stein and Horstermeer, respectively. Adding the farm-based emissions to Oukoop and Stein resulted in a total GHG emission of 8.3 (±1.0) and 6.6 (±1.3) g CO2-eq m-2 d-1, respectively. For Horstermeer the GHG balance remained the same since no farm-based emissions exist. Considering the C balance (uncertainty range 40–60%), the total C release in Oukoop and Stein is 5270 and 6258 kg C ha-1 yr-1, respectively (including ecosystem and management fluxes), and the total C uptake in Horstermeer is 3538 kg C ha-1 yr-1. Water bodies contributed significantly to the terrestrial GHG balance because of a high release of CH4. Overall, this study suggests that managed peatlands are large sources of GHGs and C, but, if appropriate measures are taken, they can be turned back into GHG and C sinks within 15 years of abandonment and rewetting. The shift from an intensively managed grass-on-peat area (Oukoop) to an extensively managed one (Stein) reduced the GHG emissions mainly because N2O emission and farm-based CH4 emissions decreased.
    Estimation of the refractive index structure parameter from single-level daytime routine weather data
    Boer, A. van de; Moene, A.F. ; Graf, A. ; Simmer, C. ; Holtslag, A.A.M. - \ 2014
    Applied Optics 53 (2014)26. - ISSN 1559-128X - p. 5944 - 5960.
    obukhov similarity functions - water-vapor - optical turbulence - sonic anemometer - surface fluxes - energy-balance - temperature - radiation - land - heat
    Atmospheric scintillations cause difficulties for applications where an undistorted propagation of electromagnetic radiation is essential. These scintillations are related to turbulent fluctuations of temperature and humidity that are in turn related to surface heat fluxes. We developed an approach that quantifies these scintillations by estimating Cn2 from surface fluxes that are derived from single-level routine weather data. In contrast to previous methods that are biased to dry and warm air, our method is directly applicable to several land surface types, environmental conditions, wavelengths, and measurement heights (lookup tables for a limited number of site-specific parameters are provided). The approach allows for an efficient evaluation of the performance of, e.g., infrared imaging systems, laser geodetic systems, and ground-to-satellite optical communication systems.We tested our approach for two grass fields in central and southern Europe, and for a wheat field in central Europe. Although there are uncertainties in the flux estimates, the impact on Cn2 is shown to be rather small. The Cn2 daytime estimates agree well with values determined from eddy covariance measurements for the application to the three fields. However, some adjustments were needed for the approach for the grass in southern Europe because of non-negligible boundary-layer processes that occur in addition to surface-layer processes.
    Role of the residual layer and large-scale subsidence on the development and evolution of the convective boundary layer
    Blay-Carreras, E. ; Pino, D. ; Vilà-Guerau de Arellano, J. ; Boer, A. van de; Coster, O. de; Darbieu, C. ; Hartogensis, O.K. ; Lohou, F. ; Lothon, M. ; Pietersen, H.P. - \ 2014
    Atmospheric Chemistry and Physics 14 (2014). - ISSN 1680-7316 - p. 4515 - 4530.
    large-eddy-simulation - morning transition - carbon-dioxide - mixed-layer - water-vapor - order-jump - part i - turbulence - entrainment - inversion
    Observations, mixed-layer theory and the Dutch Large-Eddy Simulation model (DALES) are used to analyze the dynamics of the boundary layer during an intensive operational period (1 July 2011) of the Boundary Layer Late Afternoon and Sunset Turbulence campaign. Continuous measurements made by remote sensing and in situ instruments in combination with radio soundings, and measurements done by remotely piloted aircraft systems and two manned aircrafts probed the vertical structure and the temporal evolution of the boundary layer during the campaign. The initial vertical profiles of potential temperature, specific humidity and wind, and the temporal evolution of the surface heat and moisture fluxes prescribed in the models runs are inspired by some of these observations. The research focuses on the role played by the residual layer during the morning transition and by the large-scale subsidence on the evolution of the boundary layer. By using DALES, we show the importance of the dynamics of the boundary layer during the previous night in the development of the boundary layer at the morning. DALES numerical experiments including the residual layer are capable of modeling the observed sudden increase of the boundary-layer depth during the morning transition and the subsequent evolution of the boundary layer. These simulations show a large increase of the entrainment buoyancy flux when the residual layer is incorporated into the mixed layer. We also examine how the inclusion of the residual layer above a shallow convective boundary layer modifies the turbulent kinetic energy budget. Large-scale subsidence mainly acts when the boundary layer is fully developed, and, for the studied day, it is necessary to be considered to reproduce the afternoon observations. Finally, we also investigate how carbon dioxide (CO2) mixing ratio stored the previous night in the residual layer plays a fundamental role in the evolution of the CO2 mixing ratio during the following day.
    Variability of the Structure Parameters of Temperature and Humidity Observed in the Atmospheric Surface Layer Under Unstable Conditions
    Braam, M. ; Moene, A.F. ; Beyrich, F. - \ 2014
    Boundary-Layer Meteorology 150 (2014)3. - ISSN 0006-8314 - p. 399 - 422.
    sonic anemometer - water-vapor - aperture scintillometer - heterogeneous surface - local-structure - boundary-layer - sensible heat - fluxes - turbulence - momentum
    The structure parameters of temperature and humidity are important in scintillometry as they determine the structure parameter of the refractive index of air, the primary atmospheric variable obtained with scintillometers. In this study, we investigate the variability of the logarithm of the Monin-Obukhov-scaled structure parameters (denoted as log(2s )) of temperature and humidity. We use observations from eddy-covariance systems operated at three heights (2.5, 50, and 90 m) within the atmospheric surface layer under unstable conditions. The variability of log(C2 s ) depends on instability and on the size of the averaging window over which log(C2 s ) is calculated. If instability increases, differences in log(C2s ) between upward motions (large C2 s ) and downward motions (small C2 s ) increase. The differences are, however, not sufficiently large to result in a bimodal probability density function. If the averaging window size increases, the variances of log(C2 s ) decrease. A linear regression of the variances of log(C2 s ) versus the averaging window size for various stability classes shows an increase of both the offset and slope (in absolute sense) with increasing instability. For temperature, data from the three heights show comparable results. For humidity, in contrast, the offset and slope are larger at 50 and 90 m than at 2.5 m. In the end we discuss how these findings could be used to assess whether observed differences in C2 s along a scintillometer path or aircraft flight leg are just within the range of local variability in C2 s or could be attributed to surface heterogeneity. This is important for the interpretation of data measured above a heterogeneous surface.
    Should we use a simple or complex model for moisture recycling and atmospheric moisture tracking?
    Ent, R.J. van der; Tuinenburg, O.A. ; Knoche, H.R. ; Kunstmann, H. ; Savenije, H.H.G. - \ 2013
    Hydrology and Earth System Sciences 17 (2013)12. - ISSN 1027-5606 - p. 4869 - 4884.
    water-vapor - deuterium excess - hydrologic-cycle - climate model - part ii - precipitation - rainfall - regions - deforestation - dispersion
    This paper compares state-of-the-art atmospheric moisture tracking models. Such models are typically used to study the water component of coupled land and atmosphere models, in particular quantifying moisture recycling and the source-sink relations between evaporation and precipitation. There are several atmospheric moisture tracking methods in use. However, depending on the level of aggregation, the assumptions made and the level of detail, the performance of these methods may differ substantially. In this paper, we compare three methods. The RCM-tag method uses highly accurate 3-D water tracking (including phase transitions) directly within a regional climate model (online), while the other two methods (WAM and 3D-T) use a posteriori (offline) water vapour tracking. The original version of WAM is a single-layer model, while 3D-T is a multi-layer model, but both make use the "well-mixed" assumption for evaporation and precipitation. The a posteriori models are faster and more flexible, but less accurate than online moisture tracking with RCM-tag. In order to evaluate the accuracy of the a posteriori models, we tagged evaporated water from Lake Volta in West Africa and traced it to where it precipitates. It is found that the strong wind shear in West Africa is the main cause of errors in the a posteriori models. The number of vertical layers and the initial release height of tagged water in the model are found to have the most significant influences on the results. With this knowledge small improvements have been made to the a posteriori models. It appeared that expanding WAM to a 2-layer model, or a lower release height in 3D-T, led to significantly better results. Finally, we introduced a simple metric to assess wind shear globally and give recommendations about when to use which model. The "best" method, however, very much depends on the research question, the spatial extent under investigation, as well as the available computational power.
    Towards the integration of research and monitoring at forest ecosystems in Europe
    Danielewska, A. ; Paoletti, E. ; Clarke, N. ; Olejnik, J. ; Urbaniak, M. ; Baran, M. ; Siedlecki, P. ; Hansen, K. ; Lundin, L. ; Vries, W. de - \ 2013
    Forest Systems 22 (2013)3. - ISSN 2171-5068 - p. 535 - 545.
    eddy covariance measurements - carbon balance - climate-change - air-pollution - mixed forest - heat fluxes - water-vapor - co2 - exchange - boreal
    Aim of study: The main aim of the work was to summarize availability, quality and comparability of on-going European Research and Monitoring Networks (ERMN), based on the results of a COST FP0903 Action questionnaire carried out in September 2010 and May 2012. Area of study: The COST Action FP0903 involves 29 European countries and 4 non-COST institutions from USA, Morocco and Tunisia. In this study, the total of 22 replies to the questionnaire from 18 countries were included. Materials and methods: Based on the feedback from the Action FP0903 countries, the most popular European Networks were identified. Thereafter, the access to the network database, available quality assurance/quality control procedures and publication were described. Finally, the so-called “Supersites” concept, defined as a “highly instrumented research infrastructure, for both research and monitoring of soil-plant-atmosphere interactions” was discussed. Main results: The result of the survey indicate that the vast majority of the Action FP0903 countries participate in the International Cooperative Programme on Assessment and Monitoring of Air Pollution Effects on Forest (ICP Forest). The multi-disciplinary International Cooperative Programme on Integrated Monitoring of Air Pollution Effects on Ecosystems (ICPIM) is the second most widespread forest programme. Research highlights: To fully understand biochemical cycles in forest ecosystems, long-term monitoring is needed. Hence, a network of “Supersites”, is proposed. The application of the above infrastructure can be an effective way to attain a better integration of research and monitoring networks at forest sites in Europe
    Synergistic process design: Reducing drying energy consumption by optimal adsorbent selection
    Atuonwu, J.C. ; Straten, G. van; Deventer, H.C. van; Boxtel, A.J.B. van - \ 2013
    Industrial & Engineering Chemistry Research 52 (2013)18. - ISSN 0888-5885 - p. 6201 - 6210.
    water-vapor - adsorption dryer - optimization - efficiency - algorithm - behavior
    This work analyzes the synergy between two complementary unit operations - adsorbent dehumidification and drying - and presents a mixed integer nonlinear programming approach to optimize energy performance in a two-stage system. Combined with active constraint analysis, the adsorbent properties that promote energy performance are derived. Microporous adsorbents with higher sorption capacities at low vapor pressures and requiring higher regeneration temperatures are preferred for ambient air dehumidification in the first stage. For exhaust air dehumidification, mesoporous adsorbents with lower regeneration temperatures are preferred such that the exhaust air from the first regeneration stage can sufficiently regenerate them. For drying below 50 C, energy consumption reductions of about 70% are achieved compared to conventional dryers without adsorbent dehumidification depending on adsorbent properties. The results demonstrate the usefulness of superstructure optimization in matching the drying process with the capabilities of the adsorbents to enhance process synergy for improved energy efficiency. © 2013 American Chemical Society
    Derivation of Structure Parameters of Temperature and Humidity in the Convective Boundary Layer from Large-Eddy Simulations and Implications for the Interpretation of Scintillometer Observations
    Maronga, B. ; Moene, A.F. ; Dinther, D. van; Raasch, S. ; Bosveld, F.C. ; Gioli, B. - \ 2013
    Boundary-Layer Meteorology 148 (2013)1. - ISSN 0006-8314 - p. 1 - 30.
    large-aperture scintillometer - heterogeneous land-surface - flevoland field experiment - index-structure parameter - refractive-index - optical scintillometer - water-vapor - fluxes - model - heat
    We derive the turbulent structure parameters of temperature C 2 T and humidity C 2 q from high-resolution large-eddy simulations (LES) of a homogeneously-heated convective boundary layer. Boundary conditions and model forcing were derived from measurements at Cabauw in The Netherlands. Three different methods to obtain the structure-parameters from LES are investigated. The shape of the vertical structure-parameter profiles from all three methods compare well with former experimental and LES results. Depending on the method, deviations in the magnitude up to a factor of two are found and traced back to the effects of discretization and numerical dissipation of the advection scheme. Furthermore, we validate the LES data with airborne and large-aperture scintillometer (LAS) measurements at Cabauw. Virtual path measurements are used to study the variability of C 2 T in the mixed layer and surface layer and its implications for airborne and LAS measurements. A high variability of C 2 T along a given horizontal path in the LES data is associated with plumes (high values) and downdrafts (low values). The path average of C 2 T varies rapidly in time due to the limited path length. The LES results suggest that measured path averages require sufficient temporal averaging and an adequate ratio of path length to height above the ground for the LAS in order to approach the domain average of C 2 T .
    A critical revision of the estimation of the latent heat flux from two-wavelength scintillometry
    Ward, H.C. ; Evans, J.G. ; Hartogensis, O.K. ; Moene, A.F. ; Debruin, H.A.R. ; Grimmond, C.S.B. - \ 2013
    Quarterly Journal of the Royal Meteorological Society 139 (2013)676. - ISSN 0035-9009 - p. 1912 - 1922.
    large-aperture scintillometer - heterogeneous land-surface - temperature-humidity correlation - refractive-index - water-vapor - structure parameters - regional advection - eddy-covariance - scintillation - fluctuations
    Simultaneous scintillometer measurements at multiple wavelengths (pairing visible or infrared with millimetre or radio waves) have the potential to provide estimates of path-averaged surface fluxes of sensible and latent heat. Traditionally, the equations to deduce fluxes from measurements of the refractive index structure parameter at the two wavelengths have been formulated in terms of absolute humidity. Here, it is shown that formulation in terms of specific humidity has several advantages. Specific humidity satisfies the requirement for a conserved variable in similarity theory and inherently accounts for density effects misapportioned through the use of absolute humidity. The validity and interpretation of both formulations are assessed and the analogy with open-path infrared gas analyser density corrections is discussed. Original derivations using absolute humidity to represent the influence of water vapour are shown to misrepresent the latent heat flux. The errors in the flux, which depend on the Bowen ratio (larger for drier conditions), may be of the order of 10%. The sensible heat flux is shown to remain unchanged. It is also verified that use of a single scintillometer at optical wavelengths is essentially unaffected by these new formulations. Where it may not be possible to reprocess two-wavelength results, a density correction to the latent heat flux is proposed for scintillometry, which can be applied retrospectively to reduce the error.
    Sensitivity and uncertainty of analytical footprint models according to a combined natural tracer and ensemble approach
    Boer, A. van de; Moene, A.F. ; Schüttemeyer, D. ; Graf, A. - \ 2013
    Agricultural and Forest Meteorology 169 (2013). - ISSN 0168-1923 - p. 1 - 11.
    flux measurements - sonic anemometer - carbon-dioxide - surface-layer - water-vapor - exchange - heat - validation - efflux - fetch
    Evaluations of analytical footprint models using data from several stations located in different land use types are still scarce, but valuable for defining the spatial context of the measurements. Therefore, we evaluated two analytical footprint models by applying a ‘forward’ and an ‘inversion’ method. We used eddy covariance measurements from a flat agricultural landscape in western Germany in the summer of 2009, with seven eddy covariance systems over three different land use types with contrasting sensible heat fluxes. We found that the model of Hsieh et al. (2000. Adv. Water Resour. 23, 765–772) and of Kormann and Meixner (2001. Boundary Layer Meteorol. 99, 207–224) are both overestimating the distance of the peak contribution of the footprint. In our evaluation, the former model performs slightly better, independent of whether the crosswind dispersion was used from the latter model, or from the proposed model by Detto et al. (2006. Water Resour. Res. 42, 1–16).
    Measuring H2O and CO2 fluxes at field scales with scintillometry: Part I – Introduction and validation of four methods
    Kesteren, A.J.H. van; Hartogensis, O.K. ; Dinther, D. van; Moene, A.F. ; Bruin, H.A.R. de - \ 2013
    Agricultural and Forest Meteorology 178-179 (2013). - ISSN 0168-1923 - p. 75 - 87.
    small aperture scintillometer - monin-obukhov similarity - temperature-humidity correlation - energy-balance closure - stable boundary-layer - surface-layer - water-vapor - sensible heat - structure parameter - heterogeneous surface
    This study introduces four methods for determining turbulent water vapour and carbon dioxide flux densities, the evapotranspiration and CO2 flux respectively. These methods combine scintillometer measurements with point-sampling measurements of scalar quantities and consequently have a faster statistical convergence than the eddy-covariance method. The scintillometer measures the friction velocity and stability averaged over space, allowing the time averaging to be a minute or less in homogenous conditions. This paper aims to thoroughly test the methods by analysing their sensitivity to the variables that go into the method and validate the methods with 30-min eddy-covariance data. Introduced are: the Bowen-variance method, the flux-variance method, the structure-parameter method, and the energy-balance method. Sensitivity analysis shows that each method is sensitive to the turbulence measurements of the scalar quantities that are specific to the method, as well as to the friction velocity. This demonstrates that the accuracy of the flux results from a correct representation of the turbulence variables used by the methods. Furthermore, a 30-min flux validation shows that the methods compare well to the independent eddy-covariance fluxes. We found that the structure-parameter method performs best – a low scatter (the correlation coefficient, r = 0.99) and a 5% underestimation were observed. Also the other methods perform well, although the energy-balance did not close, because storage terms and CO2 flux were neglected. Furthermore, during the night the variance methods were influenced by non-stationarity in the measurement signal. Finally, we suggest using the correlation coefficients between temperature and scalar quantities to acquire the sign of the fluxes. Data for this study were gathered in May–June 2009 over a wheat field near Merken, Germany, in the framework of the TransRegio32 program.
    Swiss prealpine Rietholzbach research catchment and lysimeter: 32 year time series and 2003 drought event
    Seneviratne, S.I. ; Lehner, I. ; Gurtz, J. ; Teuling, A.J. ; Lang, H. ; Moser, U. ; Grebner, D. ; Menzel, L. ; Schroff, K. ; Vitvar, T. ; Zappa, M. - \ 2012
    Water Resources Research 48 (2012)6. - ISSN 0043-1397 - 20 p.
    soil-moisture memory - european summer - united-states - heat-wave - groundwater recharge - spatial variability - carbon-dioxide - climate-change - water-vapor - evapotranspiration
    The prealpine Rietholzbach research catchment provides long-term continuous hydroclimatological measurements in northeastern Switzerland, including lysimeter evapotranspiration measurements since 1976, and soil moisture measurements since 1994. We analyze here the monthly data record over 32 years (1976–2007), with a focus on the extreme 2003 European drought. In particular, we assess whether the well-established hypothesis that the 2003 event was due to spring precipitation deficits is valid at the site. The Rietholzbach measurements are found to be internally consistent and representative for a larger region in Switzerland. Despite the scale discrepancy (3.14 m2 versus 3.31 km2), the lysimeter seepage and catchment-wide streamflow show similar monthly dynamics. High correlations are further found with other streamflow measurements within the Thur river basin (1750 km2) and—for interannual anomalies—also in most of northern Switzerland. Analyses for 2003 confirm the occurrence of extreme heat and drought conditions at Rietholzbach. However, unlike findings from regional-scale modeling studies, they reveal a late onset of the soil moisture deficit (from June onward), despite large precipitation deficits from mid-February to mid-April. These early spring deficits were mostly compensated for by decreased runoff during this period and excess precipitation in the preceding weeks to months (including in the 2002 fall). Our results show that evapotranspiration excess in June 2003 was the main driver initiating the 2003 summer drought conditions in Rietholzbach, contributing 60% of the June 2003 water storage deficit. Finally, long-lasting drought effects on the lysimeter water storage due to rewetting inhibition were recorded until spring 2004.
    What eddy-covariance measurements tell us about prior land flux errors in co 2-flux inversion schemes
    Chevallier, F. ; Wang, T. ; Ciais, P. ; Maignan, F. ; Bocquet, M. ; Moors, E.J. - \ 2012
    Global Biogeochemical Cycles 26 (2012)1. - ISSN 0886-6236 - 9 p.
    carbon-dioxide exchange - interannual variability - soil respiration - atmospheric co2 - pine forests - water-vapor - oak forest - assimilation - uncertainty - grassland
    To guide the future development of CO2-atmospheric inversion modeling systems, we analyzed the errors arising from prior information about terrestrial ecosystem fluxes. We compared the surface fluxes calculated by a process-based terrestrial ecosystem model with daily averages of CO2flux measurements at 156 sites across the world in the FLUXNET network. At the daily scale, the standard deviation of the model-data fit was 2.5 gC·m-2·d-1; temporal autocorrelations were significant at the weekly scale (>0.3 for lags less than four weeks), while spatial correlations were confined to within the first few hundred kilometers (
    On Monin–Obukhov Scaling in and Above the Atmospheric Surface Layer: The Complexities of Elevated Scintillometer Measurements
    Braam, M. ; Bosveld, F.C. ; Moene, A.F. - \ 2012
    Boundary-Layer Meteorology 144 (2012)2. - ISSN 0006-8314 - p. 157 - 177.
    large-aperture scintillometer - boundary-layer - structure parameters - sonic anemometer - refractive-index - sensible heat - water-vapor - temperature - fluxes - humidity
    In scintillometry Monin–Obukhov similarity theory (MOST) is used to calculate the surface sensible heat flux from the structure parameter of temperature (CT2)(CT2) . In order to prevent saturation a scintillometer can be installed at an elevated level. However, in that case the observation level might be located outside the atmospheric surface layer (ASL) and thus the validity of MOST questioned. Therefore, we examine two concepts to determine the turbulent surface sensible heat flux from the structure parameter at elevated levels with data obtained at 60-m height on the Cabauw tower (the Netherlands). In the first concept (MOSTs) CT2CT2 is still scaled with the surface flux, whereas in the second (MOSTl) CT2CT2 is scaled with the local sensible heat flux. The CT2CT2 obtained from both concepts is compared with direct observations of CT2CT2 using a sonic anemometer/thermometer. In the afternoon (when the measurement height is located within the ASL) both concepts give results that are comparable to the directly observed values of CT2CT2 . In the morning (data outside the ASL), our data do not unequivocally support either of the two concepts. First, the peak in CT2CT2 that occurs when the measurement height is located in the entrainment zone disqualifies the use of MOST. Second, during the morning transition, local scaling shows the correct pattern (zero flux and a minimum in CT2CT2) but underestimates CT2CT2 by a factor of ten. Third, from the best linear fit a we found that the slope of MOSTl gave better results, whereas the offset is closer to zero for MOSTs. Further, the correlation between the direct observations and MOST-scaled results is low and similar for the two concepts. In the end, we conclude that MOST is not applicable for the morning hours when the observation level is above the ASL.
    Evapotranspiration of deforested areas in central and southwestern Amazonia
    Randow, R.C.S. von; Randow, C. ; Hutjes, R.W.A. ; Tomasella, J. ; Kruijt, B. - \ 2012
    Theoretical and Applied Climatology 109 (2012)1-2. - ISSN 0177-798X - p. 205 - 220.
    large-aperture scintillometer - net ecosystem exchange - tropical deforestation - surface conductance - sensible heat - water-vapor - rain-forest - long-term - pasture - fluxes
    Considering the high rates of evapotranspiration of Amazonian forests, understanding the impacts of deforestation on water loss rates is important for assessing those impacts on a regional and global scale. This paper quantifies evapotranspiration rates in two different pasture sites in Amazonia and evaluates the differences between the sites. In both places, measured evapotranspiration varies seasonally, decreasing during the dry season. The decrease is higher at the southwestern Amazonia site, while at the central Amazonia site, the decrease is less pronounced. During the dry season, average values of evapotranspiration are around 2.2¿±¿0.6 mm day-1 in central Amazonia and 2.4¿±¿0.6 mm day-1 in southwestern Amazonia, while during the wet season, those values are 2.1¿±¿0.6 mm day-1 in central Amazonia and 3.5¿±¿0.8 mm day-1 in southwestern Amazonia. On an annual basis, the pasture in southwestern Amazonia has higher evapotranspiration than in central Amazonia. We conclude that the main reason for this difference is the lower available energy in the wet season at the central Amazonian site, combined with a lower leaf area index at this site during the whole year. Still, the evapotranspiration is significantly controlled by the vegetation, which is well coupled with the local moisture conditions in the dry season
    On the temporal upscaling of evapotranspiration from instantaneous remote sensing measurements to 8-day mean daily-sums
    Ryu, Y. ; Baldocchi, D.D. ; Black, T.A. ; Moors, E.J. - \ 2012
    Agricultural and Forest Meteorology 152 (2012). - ISSN 0168-1923 - p. 212 - 222.
    carbon-dioxide exchange - net ecosystem productivity - co2 exchange - energy fluxes - water-vapor - heterogeneous landscape - temperate grassland - daily evaporation - surface fluxes - boundary-layer
    The regular monitoring of evapotranspiration from satellites has been limited because of discontinuous temporal coverage, resulting in snapshots at a particular point in space and time. We developed a temporal upscaling scheme using satellite-derived instantaneous estimates of evapotranspiration to produce a daily-sum evapotranspiration averaged over an 8-day interval. We tested this scheme against measured evapotranspiration data from 34 eddy covariance flux towers covering seven plant functional types from boreal to tropical climatic zones. We found that the ratio of a half-hourly-sum of potential solar radiation (extraterrestrial solar irradiance on a plane parallel to the Earth’s surface) between 10:00 hh and 14:00 hh to a daily-sum of potential solar radiation provides a robust scaling factor to convert a half-hourly measured evapotranspiration to an estimate of a daily-sum; the estimated and measured daily sum evapotranspiration showed strong linear relation (r2 = 0.92) and small bias (-2.7%). By comparison, assuming a constant evaporative fraction (the ratio of evapotranspiration to available energy) during the daytime, although commonly used for temporal upscaling, caused 13 underestimation of evapotranspiration on an annual scale. The proposed temporal upscaling scheme requires only latitude, longitude and time as input. Thus it will be useful for developing continuous evapotranspiration estimates in space and time, which will improve continuous monitoring of hydrological cycle from local to global scales.
    Importance of crop varieties and management practices: evaluation of a process-based model for simulating CO2 and H2O fluxes at five European maize (Zea mays L.) sites
    Li, L. ; Vuichard, N. ; Viovy, N. ; Ciais, P. ; Wang, T. ; Ceschia, E. ; Jans, W.W.P. ; Wattenbach, M. ; Beziat, P. ; Gruenwald, T. ; Lehuger, S. ; Bernhofer, C. - \ 2011
    Biogeosciences 8 (2011)6. - ISSN 1726-4170 - p. 1721 - 1736.
    carbon-dioxide exchange - eddy covariance technique - net ecosystem exchange - rain-fed maize - primary productivity - agricultural soils - nitrogen balances - generic model - water-vapor - wheat
    This paper is a modelling study of crop management impacts on carbon and water fluxes at a range of European sites. The model is a crop growth model (STICS) coupled with a process-based land surface model (ORCHIDEE). The data are online eddy-covariance observations of CO2 and H2O fluxes at five European maize cultivation sites. The results show that the ORCHIDEE-STICS model explains up to 75% of the observed daily net CO2 ecosystem exchange (NEE) variance, and up to 79% of the latent heat flux (LE) variance at five sites. The model is better able to reproduce gross primary production (GPP) variations than terrestrial ecosystem respiration (TER) variations. We conclude that structural deficiencies in the model parameterizations of leaf area index (LAI) and TER are the main sources of error in simulating CO2 and H2O fluxes. A number of sensitivity tests, with variable crop variety, nitrogen fertilization, irrigation, and planting date, indicate that any of these management factors is able to change NEE by more than 15%, but that the response of NEE to management parameters is highly site-dependent. Changes in management parameters are found to impact not only the daily values of NEE and LE, but also the cumulative yearly values. In addition, LE is shown to be less sensitive to management parameters than NEE. Multi-site model evaluations, coupled with sensitivity analysis to management parameters, thus provide important information about model errors, which helps to improve the simulation of CO2 and H2O fluxes across European croplands.
    Spatial and temporal dynamics in eddy covariance observations of methane fluxes at a tundra site in Northeastern Siberia
    Parmentier, F.J.W. ; Huissteden, J. van; Molen, M.K. van der; Dolman, A.J. ; Schaepman-Strub, G. ; Karsanaev, S.A. ; Maximov, T.C. - \ 2011
    Journal of Geophysical Research: Biogeosciences 116 (2011). - ISSN 2169-8953 - 14 p.
    carbon-dioxide - arctic tundra - climate-change - water-vapor - net carbon - exchange - atmosphere - co2 - emission - system
    In the past two decades, the eddy covariance technique has been used for an increasing number of methane flux studies at an ecosystem scale. Previously, most of these studies used a closed path setup with a tunable diode laser spectrometer (TDL). Although this method worked well, the TDL has to be calibrated regularly and cooled with liquid nitrogen or a cryogenic system, which limits its use in remote areas. Recently, a new closed path technique has been introduced that uses off-axis integrated cavity output spectroscopy that does not require regular calibration or liquid nitrogen to operate and can thus be applied in remote areas. In the summer of 2008 and 2009, this eddy covariance technique was used to study methane fluxes from a tundra site in northeastern Siberia. The measured emissions showed to be very dependent on the fetch area, due to a large contrast in dry and wet vegetation in between wind directions. Furthermore, the observed short- and long-term variation of methane fluxes could be readily explained with a nonlinear model that used relationships with atmospheric stability, soil temperature, and water level. This model was subsequently extended to fieldwork periods preceding the eddy covariance setup and applied to evaluate a spatially integrated flux. The model result showed that average fluxes were 56.5, 48.7, and 30.4 nmol CH4 m-2 s-1 for the summers of 2007 to 2009. While previous models of the same type were only applicable to daily averages, the method described can be used on a much higher temporal resolution, making it suitable for gap filling. Furthermore, by partitioning the measured fluxes along wind direction, this model can also be used in areas with nonuniform terrain but nonetheless provide spatially integrated fluxes
    Low Temperature Drying With Air Dehumidified by Zeolite for Food Products: Energy Efficiency Aspect Analysis
    Djaeni, M. ; Asselt, C.J. van; Bartels, P.V. ; Sanders, J.P.M. ; Straten, G. van; Boxtel, A.J.B. van - \ 2011
    International Journal of Food Engineering 7 (2011)6. - ISSN 1556-3758
    water-vapor - adsorption - multistage - dryer - performance - simulation - flow
    Developments in low temperature drying of food products are still an interesting issue; especially with respect to the energy efficiency. This research studies the energy efficiency that can be achieved by a dryer using air which is dehumidified by zeolite. Experimental results are fitted to a dynamic model to find important variables for the drying operation. The results show that ambient air temperature as well as the ratio between air flow for drying and air flow for regeneration, affect the energy efficiency significantly. Relative humidity of used air, and shift time have a minor effect on the dryer performance. From the total work, it can be noted that the dryer efficiency operated at 50-60°C achieves 75 percent, which is attractive for drying of food products.
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