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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    Predicting the consequence of natural and chemical dispersion for oil slick size over time
    Zeinstra-Helfrich, Marieke ; Koops, Wierd ; Murk, Albertinka J. - \ 2017
    Journal of Geophysical Research: Oceans 122 (2017)9. - ISSN 2169-9275 - p. 7312 - 7324.
    dispersion - oil slick size - oil spill modeling - oil viscosity - shear spreading - wind speed
    Application of dispersants aims to enhance the natural dispersion process in order to reduce the size of the slick and the amount of oil at the surface. This study presents an approach for modeling the development of the surface oil slick as a function of the wind speed, oil viscosity, and dispersant application. We modeled the oil slick mass distribution across a transect through the slick over time taking into account the continuous entrainment of oil, resurfacing process of the different oil droplet size classes and horizontal transport. Outcomes show distinctively different oil slick features, depending on how favorable conditions are for dispersion. A large comet-shaped slick is formed in the case of suboptimal dispersion. Optimal dispersion yields a small surface oil slick, with a large mass of oil suspended. The benefit of dispersants is limited to in conditions with suboptimal natural dispersion, with the exception of extremely unfavorable conditions in which the slick size would be increased. The oil slick length, fraction of oil still floating, lifetime of the slick, and wind drift are highly influenced by wind speed and related mixing conditions, and to a lesser extent by oil properties. In the newly defined “Dispersibility Factor” (DF) the oil slick properties and environmental conditions can be combined into one value that correlates with the simulation outcomes and therefore can be used as an indicator of favorability of natural dispersion and likelihood of added value of chemical dispersion.
    Oil slick fate in 3D : predicting the influence of (natural and chemical) dispersion on oil slick fate
    Zeinstra-Helfrich, Marieke - \ 2016
    Wageningen University. Promotor(en): Tinka Murk, co-promotor(en): W. Koops. - Wageningen : Wageningen University - ISBN 9789462579279 - 174
    oil spills - pollution - adverse effects - oils - dispersion - models - thickness - olieverontreinigingen - verontreiniging - nadelige gevolgen - oliën - dispersie - modellen - dikte

    In certain conditions, (part of) an oil spill can disappear from the water surface through a process called natural dispersion. One available oil spill response option is to enhance this process by addition of dispersants (chemical dispersion). An informed decision for such response requires insight in the oil slick size WITH and WITHOUT treatment. This thesis aims to enable such assessment of net effectiveness, by providing a strategy for modelling the dispersion process.

    A plunging jet test was developed for investigating entrainment and droplet breakup. Using this set up the relevance of oil layer thickness was proven and an algorithm to model droplet sizes of dispersed oil was defined. The findings were applied in a model simulating dispersion and resurfacing as well as the wind-driven differential transport between the floating slick and suspended droplets. The simulation outputs help assess the added value (or not) of dispersant application in reducing the surface oil slick size for different oil types and conditions.

     

    Consequences of mixing assumptions for time-variable travel time distributions
    Velde, Y. van der; Heidbüchel, I. ; Lyon, S.W. ; Nyberg, L. ; Rodhe, A. ; Bishop, K. ; Troch, P.A. - \ 2015
    Hydrological Processes 29 (2015)16. - ISSN 0885-6087 - p. 3460 - 3474.
    solute transport - stream chemistry - stable-isotopes - residence time - transit times - water storage - catchment - model - age - dispersion
    The current generation of catchment travel time distribution (TTD) research, integrating nearly three decades of work since publication of Water's Journey from Rain to Stream, seeks to represent the full distribution in catchment travel times and its temporal variability. Here, we compare conceptualizations of increasing complexity with regards to mixing of water storages and evaluate how these assumptions influence time-variable TTD estimates for two catchments with contrasting climates: the Gårdsjön catchment in Sweden and the Marshall Gulch catchment in Arizona, USA. Our results highlight that, as long as catchment TTDs cannot be measured directly but need to be inferred from input-output signals of catchments, the inferred catchment TTDs depend strongly on the underlying assumptions of mixing within a catchment. Furthermore, we found that the conceptualization of the evapotranspiration flux strongly influences the inferred travel times of stream discharge. For the wet and forested Gårdsjön catchment in Sweden, we inferred that evapotranspiration most likely resembles a completely mixed sample of the water stored in the catchment; however, for the drier Marshall Gulch catchment in Arizona, evapotranspiration predominantly contained the younger water stored in the catchment. For the Marshall Gulch catchment, this higher probability for young water in evapotranspiration resulted in older water in the stream compared to travel times inferred with assumptions of complete mixing. New observations that focus on the TTD of the evapotranspiration flux and the actual travel time of water through a catchment are necessary to improve identification of mixing and consequently travel times of stream water. Copyright © 2014 John Wiley & Sons, Ltd.
    DIMO, a plant dispersal model
    Wamelink, G.W.W. ; Jochem, R. ; Greft, J.G.M. van der; Franke, J. ; Malinowska, A.H. ; Geertsema, W. ; Prins, A.H. ; Ozinga, W.A. ; Hoek, D.C.J. van der; Grashof-Bokdam, C.J. - \ 2014
    Wageningen : Statutory Research Tasks Unit for Nature & the Environment (WOT Natuur & Milieu) (WOt-paper 37) - 12
    habitatverbindingszones - vegetatie - dispersie - landschapsecologie - modellen - habitat corridors - vegetation - dispersion - landscape ecology - models
    Due to human activities many natural habitats have become isolated. As a result the dispersal of many plant species is hampered. Isolated populations may become extinct and have a lower probability to become reestablished in a natural way. Moreover, plant species may be forced to migrate to new areas due to climate change. Species survival in these cases may depend on increasing the connectivity of the landscape by engineering. To investigate and to predict the effects of isolation on the dispersal abilities of plant species, to increase spatial cohesion of a habitat network, to advise policy makers and spatial planners, we developed a simple GIS based dispersal model, DIMO. The model simulates dispersal and establishment of plant populations over a period of time in heterogeneous landscapes on a yearly basis. The model includes proxies for dispersal by wind, animals, water, and self-dispersal. Species establishment is based on habitat suitability maps and simulations include the effect of seed dormancy and generation time. A sensitivity analysis and validation were carried out. The model was validated with Juncus tenuis, an introduced species in the Netherlands. On average the difference between observed and simulated dispersal distance was 9.8 km for a distance of 155 km. The model was applied for a designed corridor in the South of the Netherlands. Model runs indicate that three of the five tested species were able to use the corridor. Two species could not, both due to lack of suitable habitat and one of them also due to lack of dispersal capacity. The results suggest that DIMO is able to evaluate the effectiveness of corridors, but also made clear that besides connectivity the present and future availability of suitable habitats in a corridor is of great importance. The model could be helpful for evaluating policy plans but also for policy making. It may be used for defining and implementation of adaptation measures to climate change on regional to continental scale. Key-words: dispersal, germination, spatial-explicit modeling, climate change, landscape fragmentation, ecological networks
    A model-based approach to studying changes in compositional heterogeneity
    Baeten, L. ; Warton, D. ; Calster, H. van; Frenne, P. De; Verstraeten, G. ; Bonte, D. ; Bernhardt-Romermann, M. ; Cornelis, R. ; Decocq, G. ; Eriksson, O. ; Hommel, P.W.F.M. - \ 2014
    Methods in Ecology and Evolution 5 (2014)2. - ISSN 2041-210X - p. 156 - 164.
    with-standards forest - biotic homogenization - beta-diversity - plant-communities - deciduous forest - vegetation - turnover - dissimilarity - nestedness - dispersion
    1. Non-random species loss and gain in local communities change the compositional heterogeneity between communities over time, which is traditionally quantified with dissimilarity-based approaches. Yet, dissimilarities summarize the multivariate species data into a univariate index and obscure the species-level patterns of change, which are central to understand the causes and consequences of the community changes. 2. Here, we propose a model-based approach that looks for species-level effects of time period and construct a multiple-site metric as a sum across species to test the consistency of the individual species responses. Species fall into different response types, showing how they influence the changes in community heterogeneity. 3. In a comparison with other multiple-sitemetrics, we illustrate the properties of our method and the differences and similarities with other approaches. For instance, ourmetric estimates the total variation in a community data set based on species-level contributions, not the compositional dissimilarities between particular sites. Similar to some other approaches, we can distinguish between heterogeneity derived from turnover or richness differences. 4. Our approach was applied to a set of 23 forest understorey resurvey studies spread across Europe. We show the species gains and lossesmay as well decrease or increase levels of community heterogeneity. Although species occurrences and communities have not changed in a consistent way along continental-scale environmental gradients such as climatic conditions, several species shifted in a similar way across the different data sets. 5. Testing the significance of shifts in species prevalence over time to infer corresponding changes in the compositional heterogeneity among sites provides a very intuitive tool for community resurvey studies. The main strengths of our framework are the explicit consideration of the relative roles of species gains and losses and the straightforward generalization to different sets of hypotheses related to community changes. Key-words: biodiversity, community composition, biotic homogenization, binomial deviance, dissimilarity, beta diversity,multivariate analysis,meta-analysis, forest understorey
    Synthesis of ecosystem vulnerability to climate change in the Netherlands shows the need to consider environmental fluctuations in adaptation measures
    Bodegom, P.M. van; Verboom-Vasiljev, J. ; Witte, J.P.M. ; Vos, C.C. ; Batholomeus, R.P. ; Geertsema, W. ; Cormont, A. ; Veen, M. van der; Aerts, R. - \ 2014
    Regional Environmental Change 14 (2014)3. - ISSN 1436-3798 - p. 933 - 942.
    klimaatverandering - ecosystemen - dispersie - ecohydrologie - climatic change - ecosystems - dispersion - ecohydrology - plant traits - species distribution - range shifts - water-flow - models - vegetation - biodiversity - conservation - conductance - terrestrial
    Climate change impacts on individual species are various and range from shifts in phenology and functional properties to changes in productivity and dispersal. The combination of impacts determines future biodiversity and species composition, but is difficult to evaluate with a single method. Instead, a comparison of mutually independent approaches provides information and confidence in patterns observed beyond what may be achieved in individual approaches. Here, we carried out such comparison to assess which ecosystem types in the Netherlands appear most vulnerable to climate change impacts, as arising from changes in hydrology, nutrient conditions and dispersal limitations. We thus combined meta-analyses of species range shifts with species distribution modelling and ecohydrological modelling with expert knowledge in two respective impact studies. Both impact studies showed that nutrient-poor ecosystems and ecosystem types with fluctuating water tables—like hay meadows, moist heathlands and moorlands—seem to be most at risk upon climate change. A subsequent meta-analysis of species–environmental stress relations indicated that particularly endangered species are adversely affected by the combination of drought and oxygen stress, caused by fluctuating moisture conditions. This implies that adaptation measures should not only aim to optimise mean environmental conditions but should also buffer environmental extremes. Major uncertainties in the assessment included the quantitative impacts of vegetation-hydrology feedbacks, vegetation adaptation and interactions between dispersal capacity and traits linked to environmental selection. Once such quantifications become feasible, adaptation measures may be tailor-made and optimised to conserve vulnerable ecosystem types
    Effects of loading and synthesis method of titania-supported cobalt catalysts for Fischer–Tropsch synthesis
    Eschemann, T.O. ; Bitter, J.H. ; Jong, K.P. de - \ 2014
    Catalysis today 228 (2014). - ISSN 0920-5861 - p. 89 - 95.
    deposition-precipitation - model catalysts - mn promotion - selectivity - dispersion - co/sio2 - reducibility - impregnation - precursors - impact
    Because of their high activity and selectivity to C5+ hydrocarbons in the Fischer–Tropsch, process, titania-supported cobalt catalysts have received great interest from industrial and academic, institutions. Here, we report on three catalyst preparation procedures, incipient wetness impregnation (IWI), deposition precipitation using urea hydrolysis (DPU) or ammonia evaporation (DPA) to prepare, Co/TiO2 catalysts with 4–24 wt.% Co. The results reveal a strong impact of the catalyst preparation, procedure on dispersion and distribution of cobalt over the titania surface. IWI led to clustered cobalt, oxide particles of ~10 nm, while for the DPA catalysts, similarly sized nanoparticles were found to be, well-distributed over the support surface. Co/TiO2 catalysts prepared by DPU showed the presence of, ~2 nm supported cobalt oxide particles beside large unsupported particles. Catalytic tests under, industrially relevant conditions show that the DPA catalysts displayed a superior activity and C5+, selectivity for the entire cobalt loading range studied.
    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.
    The impact of aquifer heterogeneity on the performance of aquifer thermal energy storage
    Sommer, W.T. ; Valstar, J.R. ; Gaans, P. van; Grotenhuis, J.T.C. ; Rijnaarts, H. - \ 2013
    Water Resources Research 49 (2013)12. - ISSN 0043-1397 - p. 8128 - 8138.
    heat-transport - solute transport - porous-media - hydraulic conductivity - geothermal systems - field - simulation - flow - macrodispersion - dispersion
    Heterogeneity in hydraulic properties of the subsurface is not accounted for in current design calculations of aquifer thermal energy storage (ATES). However, the subsurface is heterogeneous and thus affects the heat distribution around ATES wells. In this paper, the influence of heterogeneity on the performance of a doublet well system is quantified using stochastic heat transport modeling. The results show that on average, thermal recovery decreases with increasing heterogeneity, expressed as the lognormal standard deviation of the hydraulic conductivity field around the doublet. Furthermore, heterogeneity at the scale of a doublet ATES system introduces an uncertainty in the amount of expected thermal interference between the warm and cold storage. This results in an uncertainty in thermal recovery that also increases with heterogeneity and decreases with increasing distance between ATES wells. The uncertainty in thermal balance due to heterogeneity can reach values near 50 percent points in case of regional groundwater flow in excess of 200 m/yr. To account for heterogeneity whilst using homogeneous models, an attempt was made to express the effect of heterogeneity by an apparent macrodispersivity. As expected, apparent macrodispersivity increases with increasing heterogeneity. However, it also depends on well-to-well distance and regional groundwater velocity. Again, the uncertainty in thermal recovery is reflected in a range in the apparent macrodispersivity values. Considering the increasing density of ATES systems, we conclude that thermal interference limits the number of ATES systems that can be implemented in a specific area, and the uncertainty in the hydraulic conductivity field related to heterogeneity should be accounted for when optimizing well-to-well distances.
    Analytical solution of electrohydrodynamic flow and transport in rectangular channels: inclusion of double layer effects
    Joekar-Niasar, V. ; Schotting, R. ; Leijnse, A. - \ 2013
    Computational Geosciences 17 (2013)3. - ISSN 1420-0597 - p. 497 - 513.
    small zeta potentials - electroosmotic flow - electrokinetic flow - drug-delivery - ph - elctroosmosis - capillaries - remediation - dispersion - geometries
    Upscaling electroosmosis in porous media is a challenge due to the complexity and scale-dependent nonlinearities of this coupled phenomenon. “Pore-network modeling” for upscaling electroosmosis from pore scale to Darcy scale can be considered as a promising approach. However, this method requires analytical solutions for flow and transport at pore scale. This study concentrates on the development of analytical solutions of flow and transport in a single rectangular channel under combined effects of electrohydrodynamic forces. These relations will be used in future works for pore-network modeling. The analytical solutions are valid for all regimes of overlapping electrical double layers and have the potential to be extended to nonlinear Boltzmann distribution. The innovative aspects of this study are (a) contribution of overlapping of electrical double layers to the Stokes flow as well as Nernst–Planck transport has been carefully included in the analytical solutions. (b) All important transport mechanisms including advection, diffusion, and electromigration have been included in the analytical solutions. (c) Fully algebraic relations developed in this study can be easily employed to upscale electroosmosis to Darcy scale using pore-network modeling.
    Individual Investor Perceptions and Behavior During the Financial Crisis
    Hoffmann, A.O.I. ; Post, T. ; Pennings, J.M.E. - \ 2013
    Journal of Banking and Finance 37 (2013)1. - ISSN 0378-4266 - p. 60 - 74.
    common-stock investment - subprime crisis - risk-taking - market - overconfidence - returns - fluctuations - performance - dispersion - sentiment
    Combining monthly survey data with matching trading records, we examine how individual investor perceptions change and drive trading and risk-taking behavior during the 2008–2009 financial crisis. We find that investor perceptions fluctuate significantly during the crisis, with risk tolerance and risk perceptions being less volatile than return expectations. During the worst months of the crisis, investors’ return expectations and risk tolerance decrease, while their risk perceptions increase. Towards the end of the crisis, investor perceptions recover. We document substantial swings in trading and risk-taking behavior that are driven by changes in investor perceptions. Overall, individual investors continue to trade actively and do not de-risk their investment portfolios during the crisis.
    Passant wolf?
    Groot Bruinderink, Geert - \ 2012
    wolves - dispersion - migration - animal behaviour - germany
    Response to recharge variation of thin rainwater lenses and their mixing zone with underlying saline groundwater
    Eeman, S. ; Zee, S.E.A.T.M. van der; Leijnse, A. ; Louw, P.G.B. de; Maas, C. - \ 2012
    Hydrology and Earth System Sciences 16 (2012). - ISSN 1027-5606 - p. 3535 - 3549.
    grondwater - watervoerende lagen - zoet water - neerslag - zoutwaterindringing - kustgebieden - groundwater - aquifers - fresh water - precipitation - salt water intrusion - coastal areas - sea-level rise - climate-change - coastal aquifer - water interface - fresh - transport - dispersion - discharge - islands - surface
    In coastal zones with saline groundwater, fresh groundwater lenses may form due to infiltration of rain water. The thickness of both the lens and the mixing zone, determines fresh water availability for plant growth. Due to recharge variation, the thickness of the lens and the mixing zone are not constant, which may adversely affect agricultural and natural vegetation if saline water reaches the root zone during the growing season. In this paper, we study the response of thin lenses and their mixing zone to variation of recharge. The recharge is varied using sinusoids with a range of amplitudes and frequencies. We vary lens characteristics by varying the Rayleigh number and Mass flux ratio of saline and fresh water, as these dominantly influence the thickness of thin lenses and their mixing zone. Numerical results show a linear relation between the normalised lens volume and the main lens and recharge characteristics, enabling an empirical approximation of the variation of lens thickness. Increase of the recharge amplitude causes increase and the increase of recharge frequency causes a decrease in the variation of lens thickness. The average lens thickness is not significantly influenced by these variations in recharge, contrary to the mixing zone thickness. The mixing zone thickness is compared to that of a Fickian mixing regime. A simple relation between the travelled distance of the centre of the mixing zone position due to variations in recharge and the mixing zone thickness is shown to be valid for both a sinusoidal recharge variation and actual records of daily recharge data. Starting from a step response function, convolution can be used to determine the effect of variable recharge in time. For a sinusoidal curve, we can determine delay of lens movement compared to the recharge curve as well as the lens amplitude, derived from the convolution integral. Together the proposed equations provide us with a first order approximation of lens characteristics using basic lens and recharge parameters without the use of numerical models. This enables the assessment of the vulnerability of any thin fresh water lens on saline, upward seeping groundwater to salinity stress in the root zone.
    Parameterizing the Leaching Surface by Combining Curve-Fitting for Solute Breakthrough and for Spatial Solute Distribution
    Bloem, E. ; Gee, M. de; Rooij, G.H. de - \ 2012
    Transport in Porous Media 92 (2012)3. - ISSN 0169-3913 - p. 667 - 685.
    unsaturated flow - transport models - saturated soil - water - dispersion - variability - validation - zone
    Multi-compartment samplers (MCSs) measure unsaturated solute transport in space and time at a given depth. Sorting the breakthrough curves (BTCs) for individual compartments in descending order of total solute amount and plotting in 3D produces the leaching surface. The leaching surface is a useful tool to organize, present, and analyze MCS data. We present a novel method to quantitatively characterize leaching surfaces. We fitted a mean pore-water velocity and a dispersion coefficient to each BTC, and then approximated their values by functions of the rank order of the BTCs. By combining the parameters of these functions with those of the Beta distribution fitted to the spatial distribution of solutes, we described an entire leaching surface by four to eight parameters. This direct characterization method allows trends to be subtracted from the observations, and incorporates the effects of local heterogeneity. The parametric fit creates the possibility to quantify concisely the leaching behavior of a soil in a given climate under given land use, and eases the quantitative comparison of spatio-temporal leaching behavior in different soils and climates.
    Core-shell particles : experiments and self-consistent field modelling
    Iakovlev, P.A. - \ 2011
    Wageningen University. Promotor(en): Martien Cohen Stuart; Frans Leermakers, co-promotor(en): Mieke Kleijn. - [S.l.] : S.n. - ISBN 9789085858683 - 211
    afdeklagen - acrylaten - monomeren - nanotechnologie - dispersie - coatings - acrylics - monomers - nanotechnology - dispersion
    Effect of local weather on butterfly flight behaviour, movement, and colonization: significance for dispersal under climate change
    Cormont, A. ; Malinowska, A.H. ; Kostenko, O. ; Radchuk, V. ; Hemerik, L. ; Wallis de Vries, M.F. ; Verboom, J. - \ 2011
    Biodiversity and Conservation 20 (2011)3. - ISSN 0960-3115 - p. 483 - 503.
    klimaatverandering - weersgegevens - diergedrag - vliegen - lepidoptera - migratie - dispersie - climatic change - weather data - animal behaviour - flight - lepidoptera - migration - dispersion - british butterflies - range margins - habitat - landscapes - metapopulations - temperature - responses - search - models - wind
    Recent climate change is recognized as a main cause of shifts in geographical distributions of species. The impacts of climate change may be aggravated by habitat fragmentation, causing regional or large scale extinctions. However, we propose that climate change also may diminish the effects of fragmentation by enhancing flight behaviour and dispersal of ectothermic species like butterflies. We show that under weather conditions associated with anticipated climate change, behavioural components of dispersal of butterflies are enhanced, and colonization frequencies increase. In a field study, we recorded flight behaviour and mobility of four butterfly species: two habitat generalists (Coenonympha pamphilus; Maniola jurtina) and two specialists (Melitaea athalia; Plebejus argus), under different weather conditions. Flying bout duration generally increased with temperature and decreased with cloudiness. Proportion of time spent flying decreased with cloudiness. Net displacement generally increased with temperature. When butterflies fly longer, start flying more readily and fly over longer distances, we expect dispersal propensity to increase. Monitoring data showed that colonization frequencies moreover increased with temperature and radiation and decreased with cloudiness. Increased dispersal propensity at local scale might therefore lower the impact of habitat fragmentation on the distribution at a regional scale. Synergetic effects of climate change and habitat fragmentation on population dynamics and species distributions might therefore appear to be more complex than previously assumed
    Analysis of the thickness of a fresh water lens and of the transition zone
    Eeman, S. ; Leijnse, A. ; Raats, P.A.C. ; Zee, S.E.A.T.M. van der - \ 2011
    Advances in Water Resources 34 (2011)2. - ISSN 0309-1708 - p. 291 - 302.
    geohydrologie - watervoerende lagen - zoet water - zout water - delta's - geohydrology - aquifers - fresh water - saline water - deltas - submarine groundwater discharge - porous-media - coastal aquifer - brine transport - barrier-island - interface - flow - infiltration - dispersion - intrusion
    In regions with saline groundwater, fresh water lenses may develop due to rainwater infiltration. The amount of fresh water that is available for e.g. agricultural crops depends on the thickness of the lens and the extent of mixing between fresh and saline water. In this paper, we consider the mixing of fresh water and upward moving saline ground water in low-lying deltaic areas. The parameters that dominate the flow and transport problem are investigated using dimensionless groups and scaled sensitivities. We characterize the numerically simulated thicknesses of the lens and of the mixing zone by spatial moments. Rayleigh number and mass flux ratio, which is the ratio of the salt water seepage and the precipitation, determine the thickness of the fresh water lens. The local thickness of the mixing zone is mainly influenced by the dispersive/diffusive groups and the mass flux ratio. In addition, convergence of streamlines towards an outflow boundary affects the thickness, particularly in the vicinity of this boundary. Analytical and numerical steady state solutions for lens thickness are compared, taking into account upward seepage, for the two cases with and without a density difference between lens and underlying groundwater. Agreement between the numerical and analytical solutions for the lens thickness is good except when the mass flux ratio becomes small. For zero mass flux ratio, it is implicitly assumed in the analytical solution that salt water is stagnant, and that is unrealistic. Relative contributions of longitudinal and transversal hydrodynamic dispersion and diffusion to the thickness of the mixing zone are quantified numerically for different phases of lens formation. Longitudinal dispersion dominates in the early stages of lens formation, while diffusion and transversal dispersion dominate at steady state
    Belang van dispersie bij herstel van waternatuur
    Didderen, K. ; Verdonschot, P.F.M. - \ 2010
    De Levende Natuur 111 (2010)3. - ISSN 0024-1520 - p. 24 - 29.
    hydrobiologie - migratie - duikers - waterdieren - dispersie - macrofauna - hydrobiology - migration - culverts - aquatic animals - dispersion - macrofauna
    Resultaten van herstelmaatregelen in wateren vallen vaak tegen, omdat de gewenste fauna niet (terug)komt. Een belangrijke reden hiervoor is dat (her)kolonisatie van waterfauna moet plaatsvinden vanuit bronpopulaties. Om meer te weten te komen over de dispersie van macrofauna en hun verplaatsing door het water is de barrièrewerking van duikers onderzocht. Dit leidt tot een aantal aanbevelingen.
    Flow characteristics and exchange in complex biological systems as observed by pulsed-field-gradient magnetic-resonance imaging
    Homan, N. ; Venne, B.B. ; As, H. van - \ 2010
    Physical Review. E, Statistical nonlinear, and soft matter physics 82 (2010)2. - ISSN 1539-3755
    distance water transport - pfg nmr - porous-media - membrane-permeability - spatial correlations - self-diffusion - mri - relaxation - dispersion - sequence
    Water flow through model porous media was studied in the presence of surface relaxation, internal magnetic field inhomogeneities and exchange with stagnant water pools with different relaxation behavior, demonstrating how the apparent flow parameters average velocity, volume flow and flow conducting area in these situations depend on the observation time. To investigate the water exchange process a two component biological model system consisting of water flowing through a biofilm reactor (column packed with methanogenic granular sludge beads) was used, before and after a heat treatment to introduce exchange. We show that correction of the stagnant fluid signal amplitude for relaxation at increasing observation time using the observed relaxation times reveals exchange between the two fractions in the system. Further it is demonstrated how this exchange can be quantified
    Averaging hydraulic head, pressure head, end gravitational head in subsurface hydrology, and implications for averaged fluxes, and hydraulic conductivity
    Rooij, G.H. de - \ 2009
    Hydrology and Earth System Sciences 13 (2009)7. - ISSN 1027-5606 - p. 1123 - 1132.
    heterogeneous porous-media - 2-phase flow - stratified system - catchment scale - darcys-law - transport - soils - dispersion - derivation - equation
    Current theories for water flow in porous media are valid for scales much smaller than those at which problem of public interest manifest themselves. This provides a drive for upscaled flow equations with their associated upscaled parameters. Upscaling is often achieved through volume averaging, but the solution to the resulting closure problem imposes severe restrictions to the flow conditions that limit the practical applicability. Here, the derivation of a closed expression of the effective hydraulic conductivity is forfeited to circumvent the closure problem. Thus, more limited but practical results can be derived. At the Representative Elementary Volume scale and larger scales, the gravitational potential and fluid pressure are treated as additive potentials. The necessary requirement that the superposition be maintained across scales is combined with conservation of energy during volume integration to establish consistent upscaling equations for the various heads. The power of these upscaling equations is demonstrated by the derivation of upscaled water content-matric head relationships and the resolution of an apparent paradox reported in the literature that is shown to have arisen from a violation of the superposition principle. Applying the upscaling procedure to Darcy's Law leads to the general definition of an upscaled hydraulic conductivity. By examining this definition in detail for porous media with different degrees of heterogeneity, a series of criteria is derived that must be satisfied for Darcy's Law to remain valid at a larger scale
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