2,183 research outputs found
Critical voltage of a mesoscopic superconductor
We study the role of the quasiparticle distribution function f on the
properties of a superconducting nanowire. We employ a numerical calculation
based upon the Usadel equation. Going beyond linear response, we find a
non-thermal distribution for f caused by an applied bias voltage. We
demonstrate that the even part of f (the energy mode f_L) drives a first order
transition from the superconducting state to the normal state irrespective of
the current
Runoff at the micro-plot and slope scale following wildfire, central Portugal
Through their effects on soil properties and vegetation/litter cover, wildfires can strongly enhance overland flow generation and accelerate soil erosion [1] and, thereby, negatively affect land-use sustainability as well as downstream aquatic and flood zones. Wildfires are a common phenomenon in present-day Portugal, devastating in an average year some 100.000 ha of forest and woodlands and in an exceptional year like 2003 over 400.000 ha. There therefore exists a clear need in Portugal for a tool that can provide guidance to post-fire land management by predicting soil erosion risk, on the one hand, and, on the other, the mitigation effectiveness of soil conservation measures. Such a tool has recently been developed for the Western U.S.A. [3: ERMiT] but its suitability for Portuguese forests will need to be corroborated by field observations.
Testing the suitability of existing erosion models in recently burned forest areas in Portugal is, in a nutshell, the aim of the EROSFIRE projects. In the first EROSFIRE project the emphasis was on the prediction of erosion at the scale of individual hill slopes. In the ongoing EROSFIRE-II project the spatial scope is extended to include the catchment scale, so that also the connectivity between hill slopes as well as channel and road processes are being addressed. Besides ERMiT, the principal models under evaluation for slope-scale erosion prediction are: (i) the variant of USLE [4] applied by the Portuguese Water Institute after the wildfires of 2003; (ii) the MorganâMorganâFinney model (MMF) [5]; (iii) MEFIDIS [6]. From these models, MEFIDIS and perhaps MMF will, after successful calibration at the slope scale, also be applied for predicting catchment-scale sediment yields of extreme events
Some thoughts about nonequilibrium temperature
The main objective of this paper is to show that, within the present
framework of the kinetic theoretical approach to irreversible thermodynamics,
there is no evidence that provides a basis to modify the ordinary Fourier
equation relating the heat flux in a non-equilibrium steady state to the
gradient of the local equilibrium temperature. This fact is supported, among
other arguments, through the kinetic foundations of generalized hydrodynamics.
Some attempts have been recently proposed asserting that, in the presence of
non-linearities of the state variables, such a temperature should be replaced
by the non-equilibrium temperature as defined in Extended Irreversible
Thermodynamics. In the approximations used for such a temperature there is so
far no evidence that sustains this proposal.Comment: 13 pages, TeX, no figures, to appear in Mol. Phy
Runoff and erosion at the micro-plot and slope scale in a small burnt catchment, central Portugal
Wildfires can have important impacts on hydrological processes and soil erosion in forest catchments, due to the destruction of vegetation cover and changes to soil properties. However, the processes involved are non-linear and not fully understood. This has severely limited the understanding on the impacts of wildfires, especially in the up-scaling from hillslopes to catchments; in consequence, current models are poorly adapted for burnt forest conditions.
The objective of this presentation is to give an overview of the hydrological response and sediment yield from the micro-plot to slope scale, in the first year following a wildfire (2008/2009) that burnt an entire catchment nearby the Colmeal village, central Portugal. The overview will focus on three slopes inside the catchment, with samples including:
âą Runoff at micro-plot scale (12 bounded plots) and slope scale (12 open plots); âą Sediments and Organic Matter loss at micro-plot scale (12 bounded plots) and slope scale (12 open plots plus 3 Sediment fences); âą Rainfall and Soil moisture data; âą Soil Water Repellency and Ground Cover data.
The analysis of the first year following the wildfire clearly shows the complexity of runoff generation and the associated sediment transport in recently burnt areas, with pronounced differences between hillslopes and across spatial scales as well as with marked variations through time.
This work was performed in the framework of the EROSFIRE-II project (PTDC/AGR-CFL/70968/2006) which has as overall aim to predict soil erosion risk in recently burnt forest areas, including common post-fire forest management practices; the project focuses on the simultaneous measurement of runoff and soil erosion at multiple spatial scales.The results to be presented in this session are expected to show how sediment is generated, transported and exported in the Colmeal watershed; and contribute to understand and simulate erosion processes in burnt catchments, including for model development and evaluation
Hydrological and erosion response at micro-plot to -catchment scale following forest wildfire, north-central Portugal
Wildfires can have important impacts on hydrological and soil erosion processes, due to the destruction of vegetation cover and changes to soil properties. According to Shakesby and Doerr (2006), these wildfire effects are: i) much better known at small spatial scales (especially erosion plots) than at the scale of catchments; ii) much better studied with respect to overland flow and streamflow (and, then, especially peak discharges) than to soil erosion. Following up on a precursor project studying runoff generation and the associated soil losses from micro-plot to slope-scale in Portuguese eucalypt forests, the EROSFIRE-II project addresses the connectivity of these processes across hillslopes as well as within the channel network.
This is done in the Colmeal study area in central Portugal, where the outlet of an entirely burnt catchment of roughly 10 ha was instrumented with a gauging station continuously recording water level and tubidity, and five slopes were each equipped with 4 runoff plots of < 0,5 m2 (âmicro-plotâ) and 4 slope-scale plots as well as 1 slope-scale sediment fence. Starting one month after the August 2008 wildfire, the plots were monitored at 1- to 2-weekly intervals, depending on the occurrence of rainfall. The gauging station became operational at the end of November 2008, since the in-situ construction of an H-flume required several weeks.
A preliminary analysis of the data collected till the end of 2008, focusing on two slopes with contrasting slope lengths as well as the gauging station: revealed clear differences in runoff and erosion between: (i) the micro-plot and slope-scale plots on the same hillslope; (ii) the two slopes; (iii) an initial dry period and a subsequent much wetter period; (iv) the slopes and the catchment-scale, also depending on the sampling period. These results suggest that the different processes govern the hydrological and erosion response at different spatial scales as well as for different periods, with soil water repellency playing a role during the initial post-fire period.
The current presentation will review these preliminary results based on the data collected during the first year after the wildfire
What is wrong with postâfire soil erosion modelling? A metaâanalysis on current approaches, research gaps, and future directions
In the near future, a higher occurrence of wildfires is expected due to climate change, carrying social, environmental, and economic implications. Such impacts are often associated with an increase of postâfire hydrological and erosive responses, which are difficult to predict. Soil erosion models have been proven to be a valuable tool in the decisionâmaking process, from emergency response to longâterm planning, however, they were not designed for postâfire conditions, so they need to be adapted to include fireâinduced changes.
In the recent years, there has been an increasing number of studies testing different models and adaptations for the prediction of postâfire soil erosion. However, many of these adaptations are being applied without field validation or model performance assessment. Therefore, this study aims to describe the scientific advances in the last twenty years in postâfire soil erosion modelling research and evaluate model adaptations to burned areas that aim to include: i) fireâinduced changes in soil and ground cover, ii) fireâinduced changes in infiltration, iii) burn severity, and iv) mitigation measures in their predictions. This study also discusses the strengths and weaknesses of those approaches, suggests potential improvements, and identifies directions for future research.
Results show that studies are not homogeneously distributed worldwide, neither according to the model type used, nor by regions most affected by wildfires. During calibration, 73% of the cases involved model adaptation to burned conditions, and only 21% attempted to accommodate new processes. Burn severity was addressed in 75% of the cases, whilst mitigation measures were simulated in 27%. Additionally, only a minor percentage of model predictions were validated with independent field data (17%) or assessed for uncertainties (13%). Therefore, further efforts are required on the adaptation of erosion models to burned conditions to be widely used for postâfire management decision.publishe
Possible experiment to check the reality of a nonequilibrium temperature
An experiment is proposed to check the physical reality of a nonequilibrium absolute temperature previously proposed from theoretical grounds in the framework of extended irreversible thermodynamics
Proximity-driven source of highly spin-polarized ac current on the basis of superconductor/weak ferromagnet/superconductor voltage-biased Josephson junction
We theoretically investigate an opportunity to implement a source of highly
spin-polarized ac current on the basis of superconductor/weak
ferromagnet/superconductor (SFS) voltage-biased junction in the regime of
essential proximity effect and calculate the current flowing through the probe
electrode tunnel coupled to the ferromagnetic interlayer region. It is shown
that while the polarization of the dc current component is generally small in
case of weak exchange field of the ferromagnet, there is an ac component of the
current in the system. This ac current is highly spin-polarized and entirely
originated from the non-equilibrium proximity effect in the interlayer. The
frequency of the current is controlled by the voltage applied to SFS junction.
We discuss a possibility to obtain a source of coherent ac currents with a
certain phase shift between them by tunnel coupling two probe electrodes at
different locations of the interlayer region.Comment: 8 pages, 5 figure
Reversible Diffusion-Limited Reactions: "Chemical Equilibrium" State and the Law of Mass Action Revisited
The validity of two fundamental concepts of classical chemical kinetics - the
notion of "Chemical Equilibrium" and the "Law of Mass Action" - are re-examined
for reversible \textit{diffusion-limited} reactions (DLR), as exemplified here
by association/dissociation reactions. We consider a
general model of long-ranged reactions, such that any pair of particles,
separated by distance , may react with probability , and
any may dissociate with probability into a geminate
pair of s separated by distance . Within an exact analytical
approach, we show that the asymptotic state attained by reversible DLR at is generally \textit{not a true thermodynamic equilibrium}, but rather
a non-equilibrium steady-state, and that the Law of Mass Action is invalid. The
classical picture holds \text{only} in physically unrealistic case when
for any value of .Comment: 4 page
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