548 research outputs found
Modelling groundwater/surface-water interaction in a managed riparian chalk valley wetland
Understanding hydrological processes in wetlands may be complicated by management practices and complex groundwater/surface water interactions. This is especially true for wetlands underlain by permeable geology, such as chalk. In this study, the physically based, distributed model MIKE SHE is used to simulate hydrological processes at the Centre for Ecology and Hydrology River Lambourn Observatory, Boxford, Berkshire, UK. This comprises a 10-ha lowland, chalk valley bottom, riparian wetland designated for its conservation value and scientific interest. Channel management and a compound geology exert important, but to date not completely understood, influences upon hydrological conditions. Model calibration and validation were based upon comparisons of observed and simulated groundwater heads and channel stages over an equally split 20-month period. Model results are generally consistent with field observations and include short-term responses to events as well as longer-term seasonal trends. An intrinsic difficulty in representing compressible, anisotropic soils limited otherwise excellent performance in some areas. Hydrological processes in the wetland are dominated by the interaction between groundwater and surface water. Channel stage provides head boundaries for broad water levels across the wetland, whilst areas of groundwater upwelling control discrete head elevations. A relic surface drainage network confines flooding extents and routes seepage to the main channels. In-channel macrophyte growth and its management have an acute effect on water levels and the proportional contribution of groundwater and surface water. The implications of model results for management of conservation species and their associated habitats are discusse
Instream and riparian implications of weed cutting in a chalk river
Macrophyte growth is extensive in the iconic chalk streams that are concentrated in southern and eastern England. Widespread and frequent weed cutting is undertaken to maintain their key functions (e.g. flood water conveyance and maintenance of viable fisheries). In this study, a multidisciplinary approach was adopted to quantify coincident physico-chemical responses (instream and riparian) that result from weed cutting and
to discuss their potential implications. Three weed cuts were monitored at a site on the River Lambourn (The CEH River Lambourn Observatory) and major instream and riparian impacts were observed. Measurements clearly
demonstrated how weed cutting enhanced flood flow conveyance, reduced water levels (river and wetland), increased river velocities, and mobilised suspended sediment (with associated chemicals) and reduced the capacity for its retention within the river channel. Potential implications in relation to flood risk, water resources, downstream water quality, instream and riparian ecology, amenity value of the river, and wetland greenhouse gas emissions were considered. Provided the major influence of macrophytes on instream and riparian environments is fully understood then the manipulation of macrophytes represents an effective management tool that demonstrates the great potential of working with nature
Avian seed dispersal may be insufficient for plants to track future temperature change on tropical mountains
Avian seed dispersal may be insufficient for plants to track future temperature change on tropical mountains
Disordered Boson Systems: A Perturbative Study
A hard-core disordered boson system is mapped onto a quantum spin 1/2
XY-model with transverse random fields. It is then generalized to a system of
spins with an arbitrary magnitude S and studied through a 1/S expansion. The
first order 1/S expansion corresponds to a spin-wave theory. The effect of weak
disorder is studied perturbatively within such a first order 1/S scheme. We
compute the reduction of the speed of sound and the life time of the Bloch
phonons in the regime of weak disorder. Generalizations of the present study to
the strong disordered regime are discussed.Comment: 27 pages, revte
Methods for interpreting lists of affected genes obstained in a DNA microarray experiment
Background - The aim of this paper was to describe and compare the methods used and the results obtained by the participants in a joint EADGENE (European Animal Disease Genomic Network of Excellence) and SABRE (Cutting Edge Genomics for Sustainable Animal Breeding) workshop focusing on post analysis of microarray data. The participating groups were provided with identical lists of microarray probes, including test statistics for three different contrasts, and the normalised log-ratios for each array, to be used as the starting point for interpreting the affected probes. The data originated from a microarray experiment conducted to study the host reactions in broilers occurring shortly after a secondary challenge with either a homologous or heterologous species of Eimeria. Results - Several conceptually different analytical approaches, using both commercial and public available software, were applied by the participating groups. The following tools were used: Ingenuity Pathway Analysis, MAPPFinder, LIMMA, GOstats, GOEAST, GOTM, Globaltest, TopGO, ArrayUnlock, Pathway Studio, GIST and AnnotationDbi. The main focus of the approaches was to utilise the relation between probes/genes and their gene ontology and pathways to interpret the affected probes/genes. The lack of a well-annotated chicken genome did though limit the possibilities to fully explore the tools. The main results from these analyses showed that the biological interpretation is highly dependent on the statistical method used but that some common biological conclusions could be reached. Conclusion - It is highly recommended to test different analytical methods on the same data set and compare the results to obtain a reliable biological interpretation of the affected genes in a DNA microarray experimen
A review of the Dividend Discount Model: from deterministic to stochastic models
This chapter presents a review of the dividend discount models starting from
the basic models (Williams 1938, Gordon and Shapiro 1956) to more recent and
complex models (Ghezzi and Piccardi 2003, Barbu et al. 2017, D'Amico and De
Blasis 2018) with a focus on the modelling of the dividend process rather than
the discounting factor, that is assumed constant in most of the models. The
Chapter starts with an introduction of the basic valuation model with some
general aspects to consider when performing the computation. Then, Section 1.3
presents the Gordon growth model (Gordon 1962) with some of its extensions
(Malkiel 1963, Fuller and Hsia 1984, Molodovsky et al. 1965, Brooks and Helms
1990, Barsky and De Long 1993), and reports some empirical evidence. Extended
reviews of the Gordon stock valuation model and its extensions can be found in
Kamstra (2003) and Damodaran (2012). In Section 1.4, the focus is directed to
more recent advancements which make us of the Markov chain to model the
dividend process (Hurley and Johnson 1994, Yao 1997, Hurley and Johnson 1998,
Ghezzi and Piccardi 2003, Barbu et al. 2017, D'Amico and De Blasis 2018). The
advantage of these models is the possibility to obtain a different valuation
that depends on the state of the dividend series, allowing the model to be
closer to reality. In addition, these models permit to obtain a measure of the
risk of the single stock or a portfolio of stocks
Evidence Against Instanton Dominance of Topological Charge Fluctuations in QCD
The low-lying eigenmodes of the Dirac operator associated with typical gauge
field configurations in QCD encode, among other low-energy properties, the
physics behind the solution to the problem (i.e. the origin of the
mass), the nature of spontaneous chiral symmetry breaking, and the
physics of string-breaking, quark-antiquark pair production, and the OZI rule.
Moreover, the space-time chiral structure of these eigenmodes reflects the
space-time topological structure of the underlying gauge field. We present
evidence from lattice QCD on the local chiral structure of low Dirac eigenmodes
leading to the conclusion that topological charge fluctuations of the QCD
vacuum are not instanton-dominated. The result supports Witten's arguments that
topological charge is produced by confinement-related gauge fluctuations rather
than instantons.Comment: 35 pages, 11 figure
Nature of the quantum phase transitions in the two-dimensional hardcore boson model
We use two Quantum Monte Carlo algorithms to map out the phase diagram of the
two-dimensional hardcore boson Hubbard model with near () and next near
() neighbor repulsion. At half filling we find three phases: Superfluid
(SF), checkerboard solid and striped solid depending on the relative values of
, and the kinetic energy. Doping away from half filling, the
checkerboard solid undergoes phase separation: The superfluid and solid phases
co-exist but not as a single thermodynamic phase. As a function of doping, the
transition from the checkerboard solid is therefore first order. In contrast,
doping the striped solid away from half filling instead produces a striped
supersolid phase: Co-existence of density order with superfluidity as a single
phase. One surprising result is that the entire line of transitions between the
SF and checkerboard solid phases at half filling appears to exhibit dynamical
O(3) symmetry restoration. The transitions appear to be in the same
universality class as the special Heisenberg point even though this symmetry is
explicitly broken by the interaction.Comment: 10 pages, 14 eps figures, include
Longitudinal double-spin asymmetry and cross section for inclusive neutral pion production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV
We report a measurement of the longitudinal double-spin asymmetry A_LL and
the differential cross section for inclusive Pi0 production at midrapidity in
polarized proton collisions at sqrt(s) = 200 GeV. The cross section was
measured over a transverse momentum range of 1 < p_T < 17 GeV/c and found to be
in good agreement with a next-to-leading order perturbative QCD calculation.
The longitudinal double-spin asymmetry was measured in the range of 3.7 < p_T <
11 GeV/c and excludes a maximal positive gluon polarization in the proton. The
mean transverse momentum fraction of Pi0's in their parent jets was found to be
around 0.7 for electromagnetically triggered events.Comment: 6 pages, 3 figures, submitted to Phys. Rev. D (RC
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