Fitting and testing the significance of linear trends in Gumbel-distributed data

International audienceThe widely-used hydrological procedures for calculating events with T-year return periods from data that follow a Gumbel distribution assume that the data sequence from which the Gumbel distribution is fitted remains stationary in time. If non-stationarity is suspected, whether as a consequence of changes in land-use practices or climate, it is common practice to test the significance of trend by either of two methods: linear regression, which assumes that data in the record have a Normal distribution with mean value that possibly varies with time; or a non-parametric test such as that of Mann-Kendall, which makes no assumption about the distribution of the data. Thus, the hypothesis that the data are Gumbel-distributed is temporarily abandoned while testing for trend, but is re-adopted if the trend proves to be not significant, when events with T-year return periods are then calculated. This is illogical. The paper describes an alternative model in which the Gumbel distribution has a (possibly) time-variant mean, the time-trend in mean value being determined, for the present purpose, by a single parameter ? estimated by Maximum Likelihood (ML). The large-sample variance of the ML estimate ??MR is compared with the variance of the trend ?LR calculated by linear regression; the latter is found to be 64% greater. Simulated samples from a standard Gumbel distribution were given superimposed linear trends of different magnitudes, and the power of each of three trend-testing procedures (Maximum Likelihood, Linear Regression, and the non-parametric Mann-Kendall test) were compared. The ML test was always more powerful than either the Linear Regression or Mann-Kendall test, whatever the (positive) value of the trend ?; the power of the MK test was always least, for all values of ?. Keywords: Extreme value probability distribution, Gumbel distribution, statistical stationarity, trend-testing procedure

Hydrological prediction in a non-stationary world

International audienceThe paper discusses evidence that common assumptions in the analysis of hydrological time series (homogeneous variability in random fluctuations about a constant mean value) may not be appropriate for some South American drainage basins. Relatively rapid changes have occurred, and are occurring, as a consequence of replacing mature forest by short crops and urban development. Some research claims to have detected non-linear trends in streamflow in rivers draining the south-eastern part of the sub-continent, together with decadal fluctuations and interannual peaks at ENSO timescales. The paper discusses the implications of such changes for hydrological practices now in widespread and largely unquestioned use

Influence of Charge and Energy Imbalances on the Tunneling Current through a Superconductor-Normal Metal Junction

We consider quasiparticle charge and energy imbalances in a thin superconductor weakly coupled with two normal-metal electrodes via tunnel junctions at low temperatures. Charge and energy imbalances, which can be created by injecting quasiparticles at one junction, induce excess tunneling current $I_{\rm ex}$ at the other junction. We numerically obtain $I_{\rm ex}$ as a function of the bias voltage $V_{\rm det}$ across the detection junction. We show that $I_{\rm ex}$ at the zero bias voltage is purely determined by the charge imbalance, while the energy imbalance causes a nontrivial $V_{\rm det}$-dependence of $I_{\rm ex}$. The obtained voltage-current characteristics qualitatively agree with the experimental result by R. Yagi [Phys. Rev. B {\bf 73} (2006) 134507].Comment: 10 pages, 5 figure

Creating citizen-consumers? Public service reform and (un)willing selves

About the book: Postmodern theories heralded the "death of the subject", and thereby deeply contested our intuition that we are free and willing selves. In recent times, the (free) will has come under attack yet again. Findings from the neuro- and cognitive sciences claim the concept of will to be scientifically untenable, specifying that it is our brain rather than our 'self' which decides what we want to do. In spite of these challenges however, the willing self has come to take centre stage in our society: juridical and moral practices ascribing guilt, or the organization of everyday life attributing responsibilities, for instance, can hardly be understood without taking recourse to the willing subject. In this vein, the authors address topics such as the genealogy of the concept of willing selves, the discourse on agency in neuroscience and sociology, the political debate on volition within neoliberal and neoconservative regimes, approaches toward novel forms of relational responsibility as well as moral evaluations in conceptualizing autonomy

When images work faster than words: The integration of content-based image retrieval with the Northumbria Watermark Archive

Information on the manufacture, history, provenance, identification, care and conservation of paper-based artwork/objects is disparate and not always readily available. The Northumbria Watermark Archive will incorporate such material into a database, which will be made freely available on the Internet providing an invaluable resource for conservation, research and education. The efficiency of a database is highly dependant on its search mechanism. Text based mechanisms are frequently ineffective when a range of descriptive terminologies might be used i.e. when describing images or translating from foreign languages. In such cases a Content Based Image Retrieval (CBIR) system can be more effective. Watermarks provide paper with unique visual identification characteristics and have been used to provide a point of entry to the archive that is more efficient and effective than a text based search mechanism. The research carried out has the potential to be applied to any numerically large collection of images with distinctive features of colour, shape or texture i.e. coins, architectural features, picture frame profiles, hallmarks, Japanese artists stamps etc. Although the establishment of an electronic archive incorporating a CBIR system can undoubtedly improve access to large collections of images and related data, the development is rarely trouble free. This paper discusses some of the issues that must be considered i.e. collaboration between disciplines; project management; copying and digitising objects; content based image retrieval; the Northumbria Watermark Archive; the use of standardised terminology within a database as well as copyright issues

XMM-Newton Observation of the Northwest Radio Relic Region in Abell 3667

Abell 3667 is the archetype of a merging cluster with radio relics. The NW radio relic is the brightest cluster relic or halo known, and is believed to be due to a strong merger shock. We have observed the NW relic for 40 ksec of net XMM time. We observe a global decline of temperature across the relic from 6 to 1 keV, similar to the Suzaku results. Our new observations reveal a sharp change of both temperature and surface brightness near the position of the relic. The increased X-ray emission on the relic can be equivalently well described by either a thermal or nonthermal spectral model. The parameters of the thermal model are consistent with a Mach number M~2 shock and a shock speed of ~1200 km s^-1. The energy content of the relativistic particles in the radio relic can be explained if they are (re)-accelerated by the shock with an efficiency of ~0.2%. Comparing the limit on the inverse Compton X-ray emission with the measured radio synchrotron emission, we set a lower limit to the magnetic field in the relic of 3 muG. If the emission from the relic is non-thermal, this lower limit is in fact the required magnetic field.Comment: 11 pages, ApJ in pres

Absence of strong magnetic fluctuations in the iron phosphide superconductors LaFePO and Sr2ScO3FeP

We report neutron inelastic scattering measurements on polycrystalline LaFePO and Sr2ScO3FeP, two members of the iron phosphide families of superconductors. No evidence is found for any magnetic fluctuations in the spectrum of either material in the energy and wavevector ranges probed. Special attention is paid to the wavevector at which spin-density-wave-like fluctuations are seen in other iron-based superconductors. We estimate that the magnetic signal, if present, is at least a factor of four (Sr2ScO3FeP) or seven (LaFePO) smaller than in the related iron arsenide and chalcogenide superconductors. These results suggest that magnetic fluctuations are not as influential on the electronic properties of the iron phosphide systems as they are in other iron-based superconductors.Comment: 7 pages, 5 figure

Probabilidade de ocorrencia de cheia no Rio Paraguai, em Ladario, MS - Pantanal.

A previsao de cheia no Pantanal e de grande importancia para a socio-economia da regiao, pois possibilita o planejamento, nao so da pecuaria, principal atividade economica do Pantanal, como tambem do turismo, da pesca e da navegacao. Atualmente, o Departamento Nacional de Aguas e Energia Eletrica - DNAEE, em conjunto com a Companhia de Pesquisa de Recursos Minerais - CPRM, por intermedio do Projeto Pantanal, realizam previsoes dos niveis hidrometricos de alguns postos localizados na Bacia do Alto Paraguai (BAP). Dentre esses postos, o do rio Paraguai, em Ladario -MS, e considerado o principal referencial do regime hidrologico da BAP, pois possibilita ate mesmo a caracterizacao de um periodo como sendo de seca ou de cheia no Pantanal. Diante da grande demanda por previsao de cheia no Pantanal, o presente trabalho apresenta um novo metodo para previsao dos picos ou pontas de cheia do rio Paraguai, em Ladario. Trata-se de um metodo probabilistico baseado na comparacao dos niveis atuais com os registros de anos anteriores do rio Paraguai, em Ladario, para a mesma data do ano (dia e mes), levando-se em consideracao o que se sucedeu nos dias seguintes (pico de cheia). O metodo determina as probabilidades de ocorrerem pontas de cheia iguais ou superiores a 4,0, 4,5, 5,0, 5,5 e 6,0m, a partir do nivel do rio Paraguai, em Ladario em varias datas do ano (dias 1o, 11, 21, dos meses de janeiro a junho). Esse metodo determina tambem a data provavel (mes) de ocorrencia do pico de cheia. O metodo probabilistico foi utilizado em carater experimental nas cheias de 1995 (considerada a 3a maior do seculo) e de 1996. Os resultados obtidos pelo metodo proposto foram muito bons, pois sinalizaram com grande antecedencia as magnitudes das pontas de cheia e as datas (meses) de sua ocorrencia. O seu uso concomitante com outros metodos de previsao, baseados em informacoes nao so da regua de Ladario, bem como de postos hidrometricos localizados a montante (Projeto Pantanal, por exemplo), ira possibilitar uma predicao mais segura da cheia rio Paraguai, em Ladario.bitstream/item/37575/1/CT23.pd

Levantamento e estatistica descritiva dos niveis hidrometricos do rio Paraguai em Ladario, MS - Pantanal: periodo 1900-1994.

O nivel hidrometrico do rio Paraguai em Ladario, MS, medido pela regua instalada no 6. Distrito Naval da Marinha do Brasil, constitui o principal referencial do regime hidrologico da bacia do Alto Paraguai, possibilitando ate mesmo a caracterizacao de um dado periodo como sendo de seca ou de cheia no Pantanal. Quando o nivel maximo anual (pico ou ponta de cheia) e igual ou superior a cota de alerta de enchente do rio Paraguai em Ladario, ou seja 4,00 m, esse ano e considerado como sendo de cheia no Pantanal, caso contrario, como sendo de seca. Utilizando-se 34.607 registros diarios, relativos ao periodo de 01/01/1900 a 30/09/1994, foram realizados levantamentos e estudos estatisticos a nivel; diario, mensal e anual (ano hidrologico). Foram levantados valores minimos e maximos, calculadas medias, desvios-padrao e coeficientes de variacao e elaboradas tabelas de frequencia e histogramas.bitstream/item/37758/1/DOC14.pd