231 research outputs found
An index-based framework for assessing patterns and trends in river fragmentation and flow regulation by global dams at multiple scales
The global number of dam constructions has increased dramatically over the
past six decades and is forecast to continue to rise, particularly in less
industrialized regions. Identifying development pathways that can deliver the
benefits of new infrastructure while also maintaining healthy and productive
river systems is a great challenge that requires understanding the
multifaceted impacts of dams at a range of scales. New approaches and advanced
methodologies are needed to improve predictions of how future dam construction
will affect biodiversity, ecosystem functioning, and fluvial geomorphology
worldwide, helping to frame a global strategy to achieve sustainable dam
development. Here, we respond to this need by applying a graph-based river
routing model to simultaneously assess flow regulation and fragmentation by
dams at multiple scales using data at high spatial resolution. We calculated
the cumulative impact of a set of 6374 large existing dams and 3377 planned or
proposed dams on river connectivity and river flow at basin and subbasin
scales by fusing two novel indicators to create a holistic dam impact matrix
for the period 1930–2030. Static network descriptors such as basin area or
channel length are of limited use in hierarchically nested and dynamic river
systems, so we developed the river fragmentation index and the river
regulation index, which are based on river volume. These indicators are less
sensitive to the effects of network configuration, offering increased
comparability among studies with disparate hydrographies as well as across
scales. Our results indicate that, on a global basis, 48% of river volume is
moderately to severely impacted by either flow regulation, fragmentation, or
both. Assuming completion of all dams planned and under construction in our
future scenario, this number would nearly double to 93%, largely due to major
dam construction in the Amazon Basin. We provide evidence for the importance
of considering small to medium sized dams and for the need to include
waterfalls to establish a baseline of natural fragmentation. Our versatile
framework can serve as a component of river fragmentation and connectivity
assessments; as a standardized, easily replicable monitoring framework at
global and basin scales; and as part of regional dam planning and management
strategies
High-pressure behavior and phase stability of Na2B4O6(OH)2·3H2O (kernite)
The high-pressure behavior of kernite [ideally Na2B4O6(OH)2\ub73H2O, a ~ 7.02 \u212b,
b ~ 9.16 \u212b, c ~ 15.68 \u212b, \u3b2 = 108.9\ub0, Sp Gr P21/c, at ambient conditions], an important
B-bearing raw material (with B2O3 48 51 wt%) and a potential B-rich aggregate
in radiation shielding materials, has been studied by single-crystal synchrotron X-ray
diffraction up to 14.6 GPa. Kernite undergoes an iso-symmetric phase transition at
1.6-2.0 GPa (to kernite-II). Between 6.6-7.5 GPa, kernite undergoes a second phase
transition, possibly iso-symmetric in character (to kernite-III). The crystal structure
of kernite-II was solved and refined. The isothermal bulk modulus (KV0 = \u3b2-1
P0,T0, where \u3b2P0,T0 is the volume compressibility coefficient) of the ambient-pressure polymorph
of kernite was found to be KV0 = 29(1) GPa and a marked anisotropic compressional
pattern, with K(a)0: K(b)0: K(c)0~1:3:1.5., was observed. In kernite-II, the
KV0 increases to 43.3(9) GPa and the anisotropic compressional pattern increases
pronouncedly. The mechanisms, at the atomic scale, which govern the structure deformation,
have been described
Spatiotemporal dynamics in a spatial plankton system
In this paper, we investigate the complex dynamics of a spatial plankton-fish
system with Holling type III functional responses. We have carried out the
analytical study for both one and two dimensional system in details and found
out a condition for diffusive instability of a locally stable equilibrium.
Furthermore, we present a theoretical analysis of processes of pattern
formation that involves organism distribution and their interaction of
spatially distributed population with local diffusion. The results of numerical
simulations reveal that, on increasing the value of the fish predation rates,
the sequences spots spot-stripe mixtures
stripes hole-stripe mixtures holes wave pattern is
observed. Our study shows that the spatially extended model system has not only
more complex dynamic patterns in the space, but also has spiral waves.Comment: Published Pape
Universal phase transitions of B1 structured stoichiometric transition-metal carbides
The high-pressure phase transitions of B1-structured stoichiometric
transition metal carbides (TMCs, TM=Ti, Zr, Hf, V, Nb, and Ta) were
systematically investigated using ab initio calculations. These carbides
underwent universal phase transitions along two novel phase-transition routes,
namely, B1\rightarrowdistorted TlI (TlI')\rightarrowTlI and/or
B1\rightarrowdistorted TiB (TiB')\rightarrowTiB, when subjected to pressures.
The two routes can coexist possibly because of the tiny enthalpy differences
between the new phases under corresponding pressures. Four new phases result
from atomic slips of the B1-structured parent phases under pressure. After
completely releasing the pressure, taking TiC as a representative of TMCs, only
its new TlI'-type phase is mechanically and dynamically stable, and may be
recovered.Comment: [email protected]
Thermomechanical response of thickly tamped targets and diamond anvil cells under pulsed hard x-ray irradiation
In the laboratory study of extreme conditions of temperature and density, the exposure of matter to high intensity radiation sources has been of central importance. Here, we interrogate the performance of multi-layered targets in experiments involving high intensity, hard x-ray irradiation, motivated by the advent of extremely high brightness hard x-ray sources, such as free electron lasers and 4th-generation synchrotron facilities. Intense hard x-ray beams can deliver significant energy in targets having thick x-ray transparent layers (tampers) around samples of interest for the study of novel states of matter and materials’ dynamics. Heated-state lifetimes in such targets can approach the microsecond level, regardless of radiation pulse duration, enabling the exploration of conditions of local thermal and thermodynamic equilibrium at extreme temperature in solid density matter. The thermal and mechanical responses of such thick layered targets following x-ray heating, including hydrodynamic relaxation and heat flow on picosecond to millisecond timescales, are modeled using radiation hydrocode simulation, finite element analysis, and thermodynamic calculations. Assessing the potential for target survival over one or more exposures and resistance to damage arising from heating and resulting mechanical stresses, this study doubles as an investigation into the performance of diamond anvil high pressure cells under high x-ray fluences. Long used in conjunction with synchrotron x-ray radiation and high power optical lasers, the strong confinement afforded by such cells suggests novel applications at emerging high intensity x-ray facilities and new routes to studying thermodynamic equilibrium states of warm, very dense matter
Pressure-induced structural change in liquid GaIn eutectic alloy
Synchrotron x-ray diffraction reveals a pressure induced crystallization at about 3.4 GPa and apolymorphic transition near 10.3 GPa when compressed a liquid GaIn eutectic alloy up to ~13 GPa atroom temperature in a diamond anvil cell. Upon decompression, the high pressure crystalline phaseremains almost unchanged until it transforms to the liquid state at around 2.3 GPa. The ab initiomolecular dynamics calculations can reproduce the low pressure crystallization and give some hints onthe understanding of the transition between the liquid and the crystalline phase on the atomic level.The calculated pair correlation function g(r) shows a non-uniform contraction reflected by the differentcompressibility between the short (1st shell) and the intermediate (2nd to 4th shells). It is concludedthat the pressure-induced liquid-crystalline phase transformation likely arises from the changes in localatomic packing of the nearest neighbors as well as electronic structures at the transition pressure
Hydrogen storage systems from waste Mg alloys
The production cost of materials for hydrogen storage is one of the major issues to be addressed in order to consider them suitable for large scale applications. In the last decades several authors reported on the hydrogen sorption properties of Mg and Mg-based systems. In this work magnesium industrial wastes of AZ91 alloy and Mg-10 wt.% Gd alloy are used for the production of hydrogen storage materials. The hydrogen sorption properties of the alloys were investigated by means of volumetric technique, in situ synchrotron radiation powder X-ray diffraction (SR-PXD) and calorimetric methods. The measured reversible hydrogen storage capacity for the alloys AZ91 and Mg-10 wt.% Gd are 4.2 and 5.8 wt.%, respectively. For the Mg-10 wt.% Gd alloy, the hydrogenated product was also successfully used as starting reactant for the synthesis of Mg(NH2)2 and as MgH2 substitute in the Reactive Hydride Composite (RHC) 2LiBH4 + MgH 2. The results of this work demonstrate the concrete possibility to use Mg alloy wastes for hydrogen storage purposes
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