1,107 research outputs found
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Band 13
No full textLevensschetsen en portretten, bijeengebracht door N.C. Balsem
Leo XIII
Johann Caspar Bluntschli
Louis Pasteur
G. J. Mulder
Dean Stanley
Joseph Viktor von Scheffel + Scheffel's werken
Berthold Auerbach
J. E. Goudsmit
William Ewart Gladstone
From the Editor. Decription of author John Neal, his book Portland Illustrate
No full textFrom the Editor. Decription of author John Neal, his book Portland Illustrated (published in 1874), and his relationships with art critic N.C. Willis, Lady Blessington, and painter Charles Codman
Le Research Triangle Park, N.C. triangle d’or de la recherche américaine?
No full textThe Research Triangle Park, N.C. : The Golden Triangle of U.S. Research ?
The Research Triangle Park (RTP), N.C. is the largest scientific park in the South of the US, in which government agencies, big business and universities conduct joint research. This article analyses in particular the activities of the Research Triangle Institute, the linchpin of the Park and the author wonders if the undisputable scientific success of the RTP can act as an economic and social locomotive for the rest of North Carolina.Le Research Triangle Park (RTP) de Caroline du Nord est le parc scientifique le plus important du sud des Etats-Unis où agences gouvernementales, grandes entreprises et universités poursuivent conjointement leurs recherches. Cet article analyse en particulier l'activité du Research Triangle Institute qui en est la cheville ouvrière et se demande si la réussite scientifique du Park peut servir de moteur à la vie économique et sociale de Caroline du Nord.Rivière Jean. Le Research Triangle Park, N.C. triangle d’or de la recherche américaine?. In: Revue Française d'Etudes Américaines, N°23, février 1985. Aspects du Sud aujourd'hui. pp. 115-124
Recollections of Clement C. Moore, author of "A Visit from St. Nicholas"
No full textIncludes 2 poems by Clement C Moore, including "A Visit from St. Nicholas." Part of the Nancy H. Marshall Night before Christmas collection. Swem Library copy includes and undated letter about the book by Margaret N.C. Bradley, niece of the author
Moisture in the topsoil: From large-scale observations to small-scale process understanding
No full textThe boundary that separates the earth from the atmosphere is a crucial zone of study for meteorology and hydrology. Here, solar energy is partitioned into sensible heat which drives atmospheric circulation, latent heat needed for evaporation from the soil and transpiration of vegetation, and soil heat which warms the subsurface. Precipitation is partitioned into interception that evaporates directly into the atmosphere, surface runoff that discharges quickly into water courses and infiltration which resides longer in the subsurface. Soil moisture influences all these processes and is therefore considered a key variable in land-atmosphere interaction. In order to obtain a better understanding of the heat and water balance of topsoil, observations are key, but challenging with in situ point sensors. Recent rapid developments in remote sensing have tremendously increased our ability to observe the boundary between soil and atmosphere. Retrieving state variables such as soil temperature and moisture from remote sensing is far from trivial: detected signals originate not only from the soil, but also from the atmosphere and vegetation, the depth of the detection is a function of the soil moisture itself, and pixels are large and heterogeneous. Field validation is difficult, because of scale disparity between in situ point sensors and remote sensing pixels. Still, given the limitations, remote sensing provides an opportunity to improve understanding of heat and moisture transfer in the topsoil. The central question of this research is: What can be learnt from (remote sensing) observations about the heat and moisture balance of the topsoil? First a cross validation of different soil moisture products based on remote sensing was performed to investigate similarities and differences between these products. The differences were significant and could be attributed to differences in land use and vegetation, but not fully explained. This illustrated that retrieval algorithms for soil moisture are far from converged. One prerequisite for improving retrieval algorithms is ground truth, ground observations at scales relevant for remote sensing. Second, a field technique was developed that can potentially be used for bridging the observation gap between point sensors and remote sensing pixels. This technique uses Distributed Temperature Sensing (DTS) over horizontal extents up to kilometers to infer soil moisture at this intermediate scale. Propagation of variations in atmospheric temperature and radiation with depth is a function of soil moisture. By using DTS observations at three depths, it is possible to infer soil moisture, assuming that heat conduction is the dominant heat transfer mechanism. The heat diffusion equation is inverted to obtain estimates of soil heat diffusivity and soil moisture. Since this technique relies on observations of the passive thermal response of the soil to atmospheric temperature and radiation variations, this technique is called passive SoilDTS in contrast to active soil DTS, which relies on active heat pulses. The feasibility of passive SoilDTS for soil moisture estimation was asserted in a field experiment conducted in Monster in the Netherlands. The analysis of the experimental results of this feasibility study pointed out a number of technical and modeling issues that needed to be investigated further in order for passive SoilDTS to be used for soil moisture estimation and scaling. Some soil moisture estimates were not reasonable due to uncertainties in cable depths and heat transfer mechanisms. To separate the technical issue of cable depth from the modeling challenges, the same methodology used to infer soil moisture from passive SoilDTS was applied to profile data of temperature and soil moisture obtained with point sensors. The depth of these point sensors could be determined with far greater accuracy than the cable depth. Analysis of the point observations challenged the common assumption that conduction is the dominant heat transfer mechanism in soil. Evaporation seemed to play a dominant role in heat transfer on dry days. Yet evaporation rates found were higher than would be expected if mass diffusion would be the dominant transfer mechanism of water vapor. Vapor diffusion appeared to be enhanced. Enhancement of vapor diffusion is a long-studied phenomenon, subject to debate on the explanations of underlying mechanism. In an extensive literature review on vapor enhancement in soils, the plausibility of various mechanisms was assessed. We reviewed mechanisms based on (combinations of) diffusive, viscous, buoyant, capillary and external pressure forces including: thermodiffusion, dispersion, Stefan’s flow, Knudsen diffusion, liquid island effect, hydraulic lift, free convection, double diffusive convection and forced convection. The analysis of the order of magnitude of the mechanisms based on first principles clearly distinguishes between plausible and implausible mechanisms. Thermodiffusion, Stefan’s flow, Knudsen effects, liquid islands do not significantly contribute to enhanced evaporation. Double diffusive convection seemed unlikely due to lack of experimental evidence, but could not be completely excluded from the list of potential mechanisms. Hydraulic lift, the mechanism that small capillaries lift liquid water to the surface where it evaporates, does significantly contribute to enhanced evaporation from soils, also from dryer soils. The experimental evidence for and the theoretical underpinnings of this mechanism are convincing. However, we sought mechanisms that both explain enhanced evaporation and steep temperature gradients in the soil during the daytime. These often observed gradients consist of a sharp decrease of temperature with a depth up to the depth of the evaporation front. Hydraulic lift cannot explain this because the evaporation front is located at the surface. One remaining mechanism is forced convection due to atmospheric pressure fluctuations, also referred to as wind pumping. Wind pumping causes displacement and flow velocities too small for significant convective and too small for significant dispersive transport, when steady state dispersion formulations are used. However, experiments do indicate significant dispersive transport that can be explained by dispersion under unsteady flow conditions. Forced convection due to pressure fluctuations seems to be the only mechanism that can explain both enhanced evaporation and the steep temperature gradients. We investigated under which conditions wind pumping can enhance water vapor transfer from the soil to the atmosphere and which mechanisms are responsible for this enhancement in a modeling study. Previous models of wind pumping relied on enhanced transfer due to enhanced mixing described with empirical macroscopic dispersion coefficients with weak physical foundations. We searched for better understanding of physical mechanisms driving enhanced mixing. With combination of order of magnitude analysis, phenomenological, empirical and analytical models, mechanisms were investigated. A model for surface pressure fluctuations was coupled with a pressure diffusion model, a pore flow velocity model and a dispersion model. Based on this coupled model, we propose that the enhancement is caused by mixing at the pore level due to flow instabilities. Fast pressure fluctuations at the soil-air interface make vortices in the soil unstable. Instabilities arise when the timescale of the pressure fluctuations is close to the timescale of viscous dissipation which is related to the pore size. In this case, vortices in the soil cannot increase, decrease and turn direction, in phase with the pressure fluctuations and instabilities occur in the form of ejections. The ejections of vortices enhance mixing and transport. Timescales of wind induced pressure fluctuations and pore sizes are such that this mechanism is considered likely in soils. Further research is needed to prove this mechanism and quantify it. The developed model is a hypothesis and should be tested with numerical and laboratory experiments. For estimating the effect of this vapor enhancement on the soil heat budget, a coupled heat and moisture transfer model should be developed. Such a model could also shed light on the relative importance of hydraulic lift and wind pumping for evaporation rates. Perhaps, because the topsoil forms the boundary between land and atmosphere, but also between two disciplines meteorology and hydrology, there are still many questions that remain about heat and moisture transfer in the upper few centimeters of the soil. Remote sensing soil moisture retrievals force the scientific community to revisit our understanding of the topsoil. As a result, remote sensing presents not only a challenge for ground validation, but also an opportunity for hydrological and meteorological model improvement. Observation is the beginning of most learning.Water ManagementCivil Engineering and Geoscience
Information Theory for Risk-based Water System Operation
No full textOperational management of water resources needs predictions of future behavior of water systems, to anticipate shortage or excess of water in a timely manner. Because the natural systems that are part of the hydrological cycle are complex, the predictions inevitably are subject to considerable uncertainty. Still, definitive decisions about e.g. hydropower reservoir releases or polder pump flows have to be made looking ahead into the uncertain future. This demands risk-based approach, in which, ideally, all possible future events should be considered, along with their probabilities that represent the information and uncertainty available at the time of decision. The thesis deals with water, but the flows studied are mostly those of information. Like the flow of water, also information flows obey certain fundamental laws. These are the laws of Information Theory, which also provide guidelines for developing models, handling data, and designing statistical procedures to make predictions and decisions. The information-theoretical perspective used in the thesis leads to the conclusion that predictions should necessarily be probabilistic and should be evaluated using a relative entropy measure, of which an intuitive decomposition into three components is presented. Other chapters in the thesis deal with the use of model predictive control and stochastic dynamic programming for operational water management, the time-dynamics of information, generation of weighted ensemble forecasts that balance uncertainty and information, and a perspective on data compression as philosophy of science. Recommendations for practice and further research indicate that entropy has a bright future, not only as an ever-increasing thermodynamic measure, but also as an information-theoretical measure of uncertainty that is useful in any field where predictions and decisions have to be made in a context of complex and largely unobservable systems.WatermanagementCivil Engineering and Geoscience
Impact of land-use intensity on evaporation and surface runoff: Processes and parameters for Eastern Burkina Faso, West Africa
No full textLand-use is one of the main factors affecting the hydrological cycle in the Volta river basin, where two major projects (GLOWA-Volta and VinVal) are dealing with the elaboration of a land-use planning and decision support system for sustainable agricultural production and water resource management. Both projects, especially GLOWA-Volta need information about the seasonal dynamics of the actual evaporation and surface runoff. Therefore, the inter-annual dynamics of the actual evaporation, effect of land-use on the evaporation and on land surface models were analyzed via surface flux measurements during the first long-term eddy covariance measurement over the savanna vegetation in West Africa. The investigation took place in eastern Burkina Faso; the analysis covers almost two years, including one relatively wet (2003) and one relatively dry (2004) year. Surface runoff and scale effect were also investigated in three adjacent watersheds with different land-use intensity during the 2004 rainy season.Civil Engineering and Geoscience
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