Thermodynamics and log–contrast analysis in fluid geochemistry

There are two principal chemical concepts that are important for studying the natural environment. The first one is thermodynamics, which describes whether a system is at equilibrium or can spontaneously change by chemical reactions. The second main concept is how fast chemical reactions (kinetics or rate of chemical change) take place whenever they start. In this work we examine a natural system in which both thermodynamics and kinetic factors are important in determining the abundance of NH+4 , NO−2 and NO−3 in superficial waters. Samples were collected in the Arno Basin (Tuscany, Italy), a system in which natural and antrophic effects both contribute to highly modify the chemical composition of water. Thermodynamical modelling based on the reduction-oxidation reactions involving the passage NH+4 -> NO−2 -> NO−3 in equilibrium conditions has allowed to determine the Eh redox potential values able to characterise the state of each sample and, consequently, of the fluid environment from which it was drawn. Just as pH expresses the concentration of H+ in solution, redox potential is used to express the tendency of an environment to receive or supply electrons. In this context, oxic environments, as those of river systems, are said to have a high redox potential because O2 is available as an electron acceptor. Principles of thermodynamics and chemical kinetics allow to obtain a model that often does not completely describe the reality of natural systems. Chemical reactions may indeed fail to achieve equilibrium because the products escape from the site of the rection or because reactions involving the trasformation are very slow, so that non-equilibrium conditions exist for long periods. Moreover, reaction rates can be sensitive to poorly understood catalytic effects or to surface effects, while variables as concentration (a large number of chemical species can coexist and interact concurrently), temperature and pressure can have large gradients in natural systems. By taking into account this, data of 91 water samples have been modelled by using statistical methodologies for compositional data. The application of log–contrast analysis has allowed to obtain statistical parameters to be correlated with the calculated Eh values. In this way, natural conditions in which chemical equilibrium is hypothesised, as well as underlying fast reactions, are compared with those described by a stochastic approach

Compositional Time Series: An Application

The composition of the labour force is an important economic factor for a country. Often the changes in proportions of different groups are of interest. I this paper we study a monthly compositional time series from the Swedish Labour Force Survey from 1994 to 2005. Three models are studied: the ILR-transformed series, the ILR-transformation of the compositional differenced series of order 1, and the ILRtransformation of the compositional differenced series of order 12. For each of the three models a VAR-model is fitted based on the data 1994-2003. We predict the time series 15 steps ahead and calculate 95 % prediction regions. The predictions of the three models are compared with actual values using MAD and MSE and the prediction regions are compared graphically in a ternary time series plot. We conclude that the first, and simplest, model possesses the best predictive power of the three models

A new distribution on the simplex containing the Dirichlet family

The Dirichlet family owes its privileged status within simplex distributions to easyness of interpretation and good mathematical properties. In particular, we recall fundamental properties for the analysis of compositional data such as closure under amalgamation and subcomposition. From a probabilistic point of view, it is characterised (uniquely) by a variety of independence relationships which makes it indisputably the reference model for expressing the non trivial idea of substantial independence for compositions. Indeed, its well known inadequacy as a general model for compositional data stems from such an independence structure together with the poorness of its parametrisation. In this paper a new class of distributions (called Flexible Dirichlet) capable of handling various dependence structures and containing the Dirichlet as a special case is presented. The new model exhibits a considerably richer parametrisation which, for example, allows to model the means and (part of) the variance-covariance matrix separately. Moreover, such a model preserves some good mathematical properties of the Dirichlet, i.e. closure under amalgamation and subcomposition with new parameters simply related to the parent composition parameters. Furthermore, the joint and conditional distributions of subcompositions and relative totals can be expressed as simple mixtures of two Flexible Dirichlet distributions. The basis generating the Flexible Dirichlet, though keeping compositional invariance, shows a dependence structure which allows various forms of partitional dependence to be contemplated by the model (e.g. non-neutrality, subcompositional dependence and subcompositional non-invariance), independence cases being identified by suitable parameter configurations. In particular, within this model substantial independence among subsets of components of the composition naturally occurs when the subsets have a Dirichlet distribution

Mixing compositions and other scales

Theory of compositional data analysis is often focused on the composition only. However in practical applications we often treat a composition together with covariables with some other scale. This contribution systematically gathers and develop statistical tools for this situation. For instance, for the graphical display of the dependence of a composition with a categorical variable, a colored set of ternary diagrams might be a good idea for a first look at the data, but it will fast hide important aspects if the composition has many parts, or it takes extreme values. On the other hand colored scatterplots of ilr components could not be very instructive for the analyst, if the conventional, black-box ilr is used. Thinking on terms of the Euclidean structure of the simplex, we suggest to set up appropriate projections, which on one side show the compositional geometry and on the other side are still comprehensible by a non-expert analyst, readable for all locations and scales of the data. This is e.g. done by defining special balance displays with carefully- selected axes. Following this idea, we need to systematically ask how to display, explore, describe, and test the relation to complementary or explanatory data of categorical, real, ratio or again compositional scales. This contribution shows that it is sufficient to use some basic concepts and very few advanced tools from multivariate statistics (principal covariances, multivariate linear models, trellis or parallel plots, etc.) to build appropriate procedures for all these combinations of scales. This has some fundamental implications in their software implementation, and how might they be taught to analysts not already experts in multivariate analysis

Modelling of Mercury’s surface composition and remote detection from the orbit with the BepiColombo Mercury Planetary Orbiter

It can be assumed that the composition of Mercury’s thin gas envelope (exosphere) is related to the composition of the planets crustal materials. If this relationship is true, then inferences regarding the bulk chemistry of the planet might be made from a thorough exospheric study. The most vexing of all unsolved problems is the uncertainty in the source of each component. Historically, it has been believed that H and He come primarily from the solar wind, while Na and K originate from volatilized materials partitioned between Mercury’s crust and meteoritic impactors. The processes that eject atoms and molecules into the exosphere of Mercury are generally considered to be thermal vaporization, photonstimulated desorption (PSD), impact vaporization, and ion sputtering. Each of these processes has its own temporal and spatial dependence. The exosphere is strongly influenced by Mercury’s highly elliptical orbit and rapid orbital speed. As a consequence the surface undergoes large fluctuations in temperature and experiences differences of insolation with longitude. We will discuss these processes but focus more on the expected surface composition and solar wind particle sputtering which releases material like Ca and other elements from the surface minerals and discuss the relevance of composition modelling

Progress in front propagation research

We review the progress in the field of front propagation in recent years. We survey many physical, biophysical and cross-disciplinary applications, including reduced-variable models of combustion flames, Reid's paradox of rapid forest range expansions, the European colonization of North America during the 19th century, the Neolithic transition in Europe from 13 000 to 5000 years ago, the description of subsistence boundaries, the formation of cultural boundaries, the spread of genetic mutations, theory and experiments on virus infections, models of cancer tumors, etc. Recent theoretical advances are unified in a single framework, encompassing very diverse systems such as those with biased random walks, distributed delays, sequential reaction and dispersion, cohabitation models, age structure and systems with several interacting species. Directions for future progress are outlined

Integro-difference equations for interacting species and the Neolithic transition

We introduce a set of sequential integro-difference equations to analyze the dynamics of two interacting species. Firstly, we derive the speed of the fronts when a species invades a space previously occupied by a second species, and check its validity by means of numerical random-walk simulations. As an example, we consider the Neolithic transition: the predictions of the model are consistent with the archaeological data for the front speed, provided that the interaction parameter is low enough. Secondly, an equation for the coexistence time between the invasive and the invaded populations is obtained for the first time. It agrees well with the simulations, is consistent with observations of the Neolithic transition, and makes it possible to estimate the value of the interaction parameter between the incoming and the indigenous populations

Fronts from integrodifference equations and persistence effects on the Neolithic transition

We extend a previous model of the Neolithic transition in Europe [J. Fort and V. Méndez, Phys. Rev. Lett. 82, 867 (1999)] by taking two effects into account: (i) we do not use the diffusion approximation (which corresponds to second-order Taylor expansions), and (ii) we take proper care of the fact that parents do not migrate away from their children (we refer to this as a time-order effect, in the sense that it implies that children grow up with their parents, before they become adults and can survive and migrate). We also derive a time-ordered, second-order equation, which we call the sequential reaction-diffusion equation, and use it to show that effect (ii) is the most important one, and that both of them should in general be taken into account to derive accurate results. As an example, we consider the Neolithic transition: the model predictions agree with the observed front speed, and the corrections relative to previous models are important (up to 70%)

Tecnologia: Imprevistos i incertesa tècnica

Conjunt d'articles amb anàlisis i reflexions sobre la incertesa i els imprevistos en l'àmbit de la tècnica des de diferents punts de vista

Incidencia de las exenciones tributarias aplicadas al impuesto a la renta en Colombia de 2008 a 2009. Un análisis.

Con esta monografía se busca describir y analizar la incidencia que tienen las exenciones tributarias, aplicadas al impuesto a la renta, específicamente en los años 2008 y 2009 en Colombia. Es un análisis transversal del proceso e impacto que tiene la inclusión de las exenciones en Colombia, partiendo no sólo del hecho de que estas representan un costo fiscal para la nación, sino también de que constituyen un proceso típico de toma de decisiones políticas, cuya incidencia va más allá de querer intervenir en la economía, formular la política fiscal o aumentar la inversión en determinado sector productivo. Por lo que se llegó a concluir que las exenciones tributarias, aplicadas al impuesto a la renta en Colombia, además de disminuir los ingresos de la nación, infringen principios tributarios como la equidad, la eficiencia y la neutralidad y se encuentran incentivando a las personas de mayores ingresos en el país.

Incidencia de las exenciones, aplicadas al impuesto a la renta en Colombia de 2008 a 2009. Un análisis

Con esta monografía se busca describir y analizar la incidencia que tienen las exenciones tributarias, aplicadas al impuesto a la renta, específicamente en los años 2008 y 2009 en Colombia. Es un análisis transversal del proceso e impacto que tiene la inclusión de las exenciones en Colombia, partiendo no sólo del hecho de que estas representan un costo fiscal para la nación, sino también de que constituyen un proceso típico de toma de decisiones políticas, cuya incidencia va más allá de querer intervenir en la economía, formular la política fiscal o aumentar la inversión en determinado sector productivo. Por lo que se llegó a concluir que las exenciones tributarias, aplicadas al impuesto a la renta en Colombia, además de disminuir los ingresos de la nación, infringen principios tributarios como la equidad, la eficiencia y la neutralidad y se encuentran incentivando a las personas de mayores ingresos en el país.

Impuesto a la renta en Colombia: Una aproximación desde la teoría de la tributación óptima

La política fiscal es un mecanismo con importantes implicaciones en el bienestar de la población. En particular, el impuesto de renta genera incentivos sobre la oferta laboral de los contribuyentes y esto tiene repercusiones sobre su consumo, ingreso y bienestar. En Colombia, el impuesto de renta a personas naturales tiene poca relevancia al interior de la estructura tributaria; sin embargo, es un impuesto que puede ser reformado para obtener mayor recaudo y progresividad de la política fiscal. Este artículo analiza, desde la perspectiva de la tributación óptima, cuál debe ser la estructura óptima del impuesto a la renta a personas naturales en Colombia. Utilizando simulaciones con datos trasversales de la Encuesta de Calidad de Vida se encuentra que el régimen óptimo de tributación de renta a personas naturales para este ejemplo metodológico está compuesto por tasas marginales decrecientes a lo largo de la distribución del ingreso laboral y que este es altamente sensible ante variaciones en la distribución de habilidades.

El proceso de descentralización intraterritorial en el Distrito Federal mexicano: la ambigüedad generada por la tensión entre descentralización y centralización en la gestión de los territorios

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El entorno empresarial y las condicionantes tributarias y fiscales en América Latina

La primera parte de este estudio versa sobre los temas del crecimiento y el desarrollo y sobre la caracterización de las políticas económicas. La segunda sección se dedica a una caracterización de las condicionantes fiscales con base en las cuales desempeñan sus funciones los Gobiernos de América Latina y el Caribe. En la primera sección se discuten las principales características de las políticas fiscales, monetarias y cambiarias, y con fundamento en ellas se exponen los conceptos centrales relacionados con el desarrollo humano. El principal argumento a sostener aquí es que, en general, los países de menor desarrollo relativo, los que tienen que enfrentar los desafíos sociales más significativos, son también los que poseen condiciones fiscales más débiles o vulnerables. Los intentos de alcanzar mayores niveles de desarrollo implican la necesidad de realizar cambios efectivos en las condiciones fiscales de varios países latinoamericanos.

Incidencia de los actores armados ilegales en la tributación minera de los municipios auríferos de Antioquia 2007 - I. 2012

Inmersos en un conflicto armado y guiados por un modelo político que busca aprovechar el boom minero en pro del desarrollo de Colombia, se han potencializado dinámicas ilegales donde los Actores Armados Ilegales (A.A.I), como ‘bandidos estacionarios’, se han ido adaptando a las nuevas dinámicas del mercado donde el lucro, la rentabilidad y la financiación son su objetivo central. En el departamento de Antioquia, esa situación data desde la formación de sus regiones, como son el caso del Bajo Cauca y Nordeste Antioqueño. Empero, lo novedoso en el conflicto armado, y que tiene relación directa con la minería aurífera es, la alta posibilidad que los A.A.I. estén viendo en esta actividad su principal fuente de financiación como consecuencia de dos hechos importantes: 1). El éxito que ha tenido la lucha contra el narcotráfico y por ende, contra los cultivos ilícitos. Y, 2) El elevado precio del oro en el mercado internacional puesto que, se estima que en los próximos años alcance entre $2.000 y $2.107 dólares la onza. En este orden de ideas, el lector encontrará como los actores armados ilegales actúan como bandidos estacionarios que ejerciendo la “minería criminal”, inciden en la política fiscal municipal a partir de la para-tributación o ‘impuesto de protección’, captación de regalías y el lavado de activos; donde el fin es financiar sus actividades criminales, afectando negativamente la política tributaria de los gobiernos municipales que se caracteriza por su baja capacidad de gestión.