1,721,044 research outputs found
Errors in Water Retention Curves Determined with Pressure Plates
No full textPressure plates are commonly used to measure the soil water retention curve. Low plate and soil conductance, lack of plate–soil contact, and soil dispersion, however, make this method oft en
unreliable at low water potentials. We investigated how errors in the determination of the water retention curve affect the soil hydraulic properties and the computation of plant-available water.
We first determined soil water retention data for a silt loam soil using pressure plates only and a combination of pressure plates and a dew point meter. Th e two sets of soil water retention data
were then analyzed using different models for describing the hydraulic properties. Th e soil water retention curves determined with the two methods deviated at potentials less than about −20 m
H2O, with the pressure plate apparatus data yielding larger water contents than the dew point meter at the same water potentials. These results indicate that soil water retention curves determined from pressure plates may be in error at potentials less than −20 m H2O, which may lead to substantial errors in water flow calculations. These errors can be eliminated by using a combination of pressure plates and dew point measurements to determine the water retention curve
A thermodielectric analyzer to measure the freezing and moisture characteristic of porous media
No full textThe freezing and moisture characteristics of porous media are difficult to measure. We have designed an instrument to measure the freezing characteristic of a porous medium in the range -20 to 0°C. Because of the similarity between the freezing characteristic and the moisture characteristic, the data obtained can be used to infer the moisture characteristic from about -30 to -23,600 J kg-1. Temperatures and unfrozen water contents are measured with a thermistor and a spiral-shaped transmission line connected to an oscillator, respectively. Temperatures are converted to water potential using the Clapeyron equation. Experimental results obtained with the freezing technique were compared with vapor pressure methods. The four test media used, a silt loam soil, a silty-clay loam, a clay, and a sandstone, showed good agreement between freezing and standard methods. The freezing technique described here has a water potential resolution of about -11 J kg-1. An entire characteristic can be determined within 24 hours
Characterization of particle-size distribution in soils with a fragmentation model
No full textParticle-size distributions (PSDs) of soils are often used to estimate other soil properties, such as soil moisture characteristics and hydraulic conductivities. Prediction of hydraulic properties from soil texture requires an accurate characterization of PSDs. The objective of this study was to test the validity of a mass-based fragmentation model to describe PSDs in soils. Wet sieving, pipette, and light-diffraction techniques were used to obtain PSDs of 19 soils in the range of 0.05 to 2000 μm. Light diffraction allows determination of smaller particle sizes than the classical sedimentation methods, and provides a high resolution of the PSD. The measured data were analyzed with a mass-based model originating from fragmentation processes, which yields a power-law relation between mass and size of soil particles. It was found that a single power-law exponent could not characterize the PSD across the whole range of the measurements. Three main power-law domains were identified. The boundaries between the three domains were located at particle diameters of 0.51±0.15 and 85.3±25.3 μm. The exponent of the power law describing the domain between 0.51 and 85.3 μm was correlated with the clay and sand contents of the soil sample, indicating some relationship between power-law exponent and textural class. Two simple equations are derived to calculate the parameters of the fragmentation model of the domain between 0.51 and 85.3 μm from mass fractions of clay and silt
Reduction of transpiration through foliar application of chitosan
No full textIn this study, we investigate the potential of chitosan, a natural beta-1-4-linked glucosamine polymer, to reduce plant transpiration. Chitosan was applied foliarly to pepper plants and water use was monitored. Peppers were grown in pots in growth-chambers, where transpiration was measured by weighing pots. In an accompanying field study, water use was determined by monitoring soil moisture depletion with time domain reflectometry. An automated irrigation system replenished the water used each day. Plant biomass and yield were determined to calculate biomass-to-water ratios. Differences in canopy resistance between control and chitosan treated plants were analyzed with the aid of the Penman-Monteith equation. Scanning electron microscopy (SEM) and histochemical analyses demonstrated that chitosan induced closure of the plant's stomata, resulting in decreased transpiration. Foliar application of chitosan reduced water use of pepper plants by 26-43% while maintaining biomass production and yield. We suggest that chitosan might be an effective antitranspirant to conserve water use in agriculture. © 2001 Elsevier Science B.V
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Multifractal characterization of soil particle-size distributions
No full textA particle-size distribution (PSD) constitutes a fundamental soil property correlated to many other soil properties. Accurate representations of PSDs are, therefore, needed for soil characterization and prediction purposes. A power-law dependence of particle mass on particle diameter has been used to model soil PSDs, and such power-law dependence has been interpreted as being the result of a fractal distribution of particle sizes characterized with a single fractal dimension. However, recent studies have shown that a single fractal dimension is not sufficient to characterize a distribution for the entire range of particle sizes. The objective of this study was to apply multifractal techniques to characterize contrasting PSDs and to identify multifractal parameters potentially useful for classification and prediction. The multifractal spectra of 30 PSDs covering a wide range of soil textural classes were analyzed. Parameters calculated from each multifractal spectrum were: (i) the Hausdorff dimension,f(α); (ii) the singularities of strength, α (iii) the generalized fractal dimension, Dq; and (iv) their conjugate parameter the mass exponent, θ (q), calculated in the range of moment orders (q) of between -10 and +10 taken at 0.5 lag increments. Multifractal scaling was evident by an increase in the difference between the capacity D0 and the entropy D1 dimensions for soils with more than 10% clay content. Soils with <10% clay content exhibited single scaling. Our results indicate that multifractal parameters are promising descriptors of PSDs. Differences in scaling properties of PSDs should be considered in future studies
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
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