Statistics of hot and cold events in Catalonia (NE Spain) for the recording period 1950–2004

Spatial patterns and statistical models for hot and cold events affecting Catalonia (NE Spain) are obtained from series of daily maximum and minimum temperatures recorded at 65 meteorological stations throughout the period 1950–2004. The study is based on the crossing theory, taking care that daily temperatures are normally distributed and previous data treatments (removal of trends and periodicities) assure their stationary character. With the aim of facilitating comparisons among different thermometric records, hot and cold events are defined as large departures, given in standard deviations, of daily temperatures from daily averages. From the statistical point of view, the assumption of a normal distribution of the initial date of the events is questionable and a simple assignment of hot events to summer and cold events to winter must be discarded according to the averaged initial dates and their standard deviations. The event magnitudes, defined as the absolute value of their maximum departures, follow an exponential distribution and event lengths can be modelled by an autoregressive Markov process with a Gaussian noise component. The number of events per year fits a Poisson distribution well only for high departures and the whole number of hot and cold events decays exponentially with the increasing departure for every temperature series. This property permits an estimation of the expected maximum departure for every thermometric station during the recording period. Even though spatial features observed for the number of events, their mean initial date, average event magnitude and average event length depict quite complex patterns due to the orography of the country and the vicinity to the Mediterranean Sea, the results obtained improve the knowledge on the hot and cold events in Catalonia.     

Statistical distributions of daily rainfall regime in Europe for the period 1951–2000

Amount and time distributions, X and Y, of daily rain amounts in Europe along the second half of 20th century have been studied from 267 rain gauge records. Different geographical features, such as latitude, vicinity to Mediterranean Sea or the Atlantic Ocean or altitude above sea level, cause the averages of daily rain and annual number of rainy days to vary within a wide range. The largest daily percentiles of amount and time distributions are reached at latitudes south of 50°N and in southwestern Norway. The amount of distribution, X, is well-modelled by the exponential function, with parameters derived from probability graphs. Time distributions, Y, are well-fitted by Pearson type III (Gamma) and Weibull models, their parameters being estimated by L-moments. Normalised rainfall curves (NRC) have been modelled by the analytical function $ X = Y \cdot \exp \left\{ { - b{{\left( {1 - Y} \right)}^c}} \right\} $ , with b and c parameters depicting spatial variability. Alternatively, the beta distribution also describes quite well the empirical NRCs, with parameters estimated by statistical moments. The coordinates of the average daily amount (X _r , Y _r ) and the values of X ^* and Y ^* , which are defined as the fraction of rain amount for a half of rainy days and the fraction of number of rainy days accounting for a half of total rain amount, respectively, depict very similar spatial distribution throughout Europe. In fact, X _r and X ^* keep a linear relationship, as well as Y _r and Y ^* , the four coordinates depending on the coefficient of variation of daily rain amounts. A similar linear relationship is found for the pair (X ^* , Y ^* ). Finally, the Average Linkage algorithm applied to the coordinates X _r , Y _r , X ^* and Y ^* characterising every one of the 267 NRCs permits to group the rain gauges into several spatial clusters, each of them related to a different normalised daily pluviometric regime.     

Monthly and annual statistical distributions of daily rainfall at the Fabra Observatory (Barcelona, NE Spain) for the years 1917–1999

Summary Three empirical distributions of the daily rainfall collected at the Fabra Observatory from 1917 to 1999 are fitted to different statistical models. The first two are designated as the distributions of cumulative amounts and cumulative times. The third distribution accounts for the time interval between two consecutive rainy days with rain amounts equalling or exceeding a threshold amount. Whereas the distribution of cumulative amounts follows an exponential model at monthly and annual scale, except for a few cases, the distribution of the cumulative times is well modelled by a Weibull function, whether monthly or annual scales are considered. The distribution of time intervals also follows a Weibull distribution for the different thresholds considered. In addition, the combination of the two first distributions leads to the normalised rainfall curve, NRC, which is also reproduced satisfactorily by a beta (type 1) distribution. It is worth mentioning that the NRCs follow the expected behaviour with respect to the coefficient of variation of daily rain amounts at monthly and annual scales. In addition, a better understanding of fluctuations and time trends affecting the daily pluviometric regime is achieved by analysing the annual NRCs. The impact of some features of this rain regime, developed for Barcelona, a crowded metropolitan area, on many human activities, may provides the focus of future interdisciplinary analyses.     

Probabilities of Repeated Long Dry Episodes Based on the Poisson Distribution. An Example for Catalonia (NE Spain)

Summary A formulation, similar to the procedure employed in seismic risk analysis, has allowed us to quantify probabilities concerning repeated long episodes of dry days for an arbitrary number of years and their return periods. This formulation is based on both a cumulative expression, similar to the well known Gutemberg-Richter seismic law, and the Poisson distribution. We will assume that this latter distribution is applicable, provided that some constraints affecting the length of the dry episodes and their average number recorded on a year are satisfied. In comparison with other methodologies, based on either Markov chains or Gumbell and Jenkinson formulations, we have been able to quantify probabilities concerning repeated long episodes, including extreme events, contributing significantly to the generation of drought episodes. The formulations that we introduce has been applied to sets of dry episodes obtained from daily pluviometric recordings belonging to 69 gauges of the Instituto Nacional de Meteorología located in Catalonia (NE Spain). The results obtained in terms of return period maps, return period-length curves and probabilities for repeated episodes and for an arbitrary number of years are in agreement with previous pluviometric studies and the spatial diversity of the region due to its orographic complexity. The more relevant synoptic situations causing the long dry episodes are summarized and briefly described. Received June 24, 1997 Revised October 28, 1997     

Periodicities and irregularities of indices describing the daily pluviometric regime of the Fabra Observatory (NE Spain) for the years 1917–1999

Summary Several patterns of a daily pluviometric regime are obtained from an homogeneous set of daily rainfall recorded at the Fabra Observatory (NE Spain) for the period 1917–1999. Power spectral analyses of four annual pluviometric indices, determined from different daily rain amount percentiles, are performed. Periodicities of the quasi-biannual oscillation (QBO), North Atlantic Oscillation (NAO) and sunspot cycles are correlated in some cases with empirical spectral peaks of the indices exceeding Markov red-noise with a confidence level of 95%. Depending on the rainy day percentiles, a set of short periods (2.1–4.6 years), a second group of intermediate periods (5.5 and 9.2 years) and, finally, a third group of long periods of 11.8, 20.8 and 41.5 years can be distinguished. These periods are compared with those derived for other European and African emplacements. Additionally, the consecutive irregularity of every annual series is quantified by means of a concept similar to the entropy.     

Spatial and temporal variability of groundwater dynamics in a sub‐Mediterranean mountain catchment

The temporal and spatial dynamics of groundwater was investigated in a small catchment in the Spanish Pyrenees, which was extensively used for agriculture in the past. Analysis of the water table fluctuations at five locations over a 6‐year period demonstrated that the groundwater dynamics had a marked seasonal cycle involving a wetting‐up period that commenced with the first autumn rainfall events, a saturation period during winter and spring and a drying‐down period from the end of spring until the end of the summer. The length of the saturation period showed great interannual variability, which was mainly influenced by the rainfall and evapotranspiration characteristics. There was marked spatial variability in the water table, especially during the wetting‐up period, which could be related to differences in slope and drainage area, geomorphology, soil properties and local topography. Areas contributing to runoff generation were identified within the catchment by field mapping of moisture conditions. Areas contributing to infiltration excess runoff were correlated with former cultivated fields affected by severe sheetwash erosion. Areas contributing to saturation excess runoff were characterized by a marked spatial dynamics associated with catchment wetness conditions. The saturation spatial pattern, which was partially related to the topographic index, was very patchy throughout the catchment, suggesting the influence of other factors associated with past agricultural activities, including changes in local topography and soil properties. The relationship between water table levels and stream flow was weak, especially during the wetting‐up period, suggesting little connection between ground water and the hydrological response, at least at some locations. The results suggest that in drier and human‐disturbed environments, such as sub‐Mediterranean mountains, saturation patterns cannot be represented only by the general topography of the catchment. They also suggest that groundwater storage and runoff is not a succession of steady‐state flow conditions, as assumed in most hydrological models. Copyright © 2013 John Wiley & Sons, Ltd.     

Estimation of potential failure risks in a mine slope using indicator kriging

Indicator kriging has been applied to the study of failure mechanisms in a mine slope in Minas Gerais, Brazil, to estimate potential failure risks in limited areas along this slope. Timbopeba Mine, Vale Company, is an open pit iron mine situated in the Quadrilátero Ferrífero, a very important mining district in Minas Gerais. A slope excavated in quartzite with a maximum height of 200 m at the time of this study, has presented many failure problems involving the sliding of blocks formed by discontinuities. These blocks are of limited size in comparison to the dimensions of the overall slope. They appear along the entire slope, wherever discontinuity orientations have led to the kinematic feasibility of these blocks. Geostatistics permits the estimation of local failure probability distributions associated to these local failures, which would not be possible with traditional statistical models. The geostatistical method employed in this study, indicator kriging, is quite suitable because it is unnecessary to assume a particular global distribution of the phenomena being modeled. The model was used for locating areas with a great tendency for sliding failure, as it considers the local spatial variability of discontinuity orientations. Copyright © 2010 John Wiley & Sons, Ltd.     

U–Pb–Hf isotopes and trace elements of metasomatic zircon delimit the evolution of neoproterozoic Capané ophiolite in the southern Brasiliano Orogen

The Capané ophiolite is a fragment of oceanic lithosphere obducted into the Ediacaran Porongos fold and thrust belt, southern Brasiliano Orogen. A studied rodingite blackwall contained in serpentinite has metasomatic zircon that displays multiple U–Pb ages from Tonian to Cryogenian (793 ± 0.9, 757 ± 2.1, 715 ± 2.2 Ma). The ages are interpreted as corresponding to multiple alteration events in the mantle. Multiple U–Pb–Hf isotopes and trace element analyses on the same crystals by laser ablation were controlled by backscatered electron images. Hf isotopes indicate zircon origin from a depleted mantle (εHf = +15 to +10.7), and trace elements point to an oceanic origin. The Capané ophiolite thus marks the evolution of the Adamastor ocean during the Tonian and Cryogenian, a significant result for the reconstruction of Rodinia and Gondwana supercontinents.     

Objective regionalization of Rayleigh wave dispersion data by clustering algorithms: an application to the Mediterranean basin

The main target of the present study is an objective and automated regionalization of Rayleigh wave dispersion data for the Mediterranean basin, without a priori seismotectonic constraints, and to determine the corresponding regional shear-velocity structures. The database used is formed by almost 200 Rayleigh wavetrains corresponding to 42 regional events, with surface-wave magnitude greater than 4.5, recorded at the MedNet very-broad-band stations in the Mediterranean area. Path-averaged group velocities for the Rayleigh wave fundamental mode are derived for each available epicentre-station trajectory crossing the Mediterranean basin. After this, a principal component analysis and a clustering process are applied to local group velocities, obtained for 13 different periods from 10 to 70 s, in order to classify the Mediterranean basin into several homogeneous regions. The stochastic inversion of the averaged group velocity dispersion curve obtained for each region provides the respective shear-velocity structures, down to a depth of 150–160 km. The characteristics of these areas and their possible correlation with the main seismotectonic features of the Mediterranean region are discussed. The regional models reveal significant lateral changes in the elastic structure, with the main differences concerning particularly the upper 35–40 km. Within this depth range, low shear velocities, varying from 2.8 to 3.9 km s⁻¹, characterize the Eastern Mediterranean, whereas higher velocities, ranging from 3.0 to 4.2 km s⁻¹, are deduced for the Western Mediterranean. These results suggest a thicker crust in the eastern part, but with a greater thickness of sedimentary layers. However, for depths of between 80 and 110 km, lower shear velocities are obtained in the Western part, while higher shear velocities are derived for the Eastern Mediterranean Sea, in the Aegean Sea, Greece, the south of Italy, Sicily and Tunisia. This velocity pattern suggests an averaged thicker lithosphere under the latter areas, as the top of the asthenosphere is detected at a mean depth of 75 km for the remaining regions. This thicker lithosphere can be related to processes associated with the convergence of the Eurasian and African plates and subduction under the Calabrian and Hellenic Arcs.     

Wind influence on surface current variability in the Ibiza Channel from HF Radar

Surface current variability is investigated using 2.5 years of continuous velocity measurements from an high frequency radar (HFR) located in the Ibiza Channel (Western Mediterranean Sea). The Ibiza Channel is identified as a key geographical feature for the exchange of water masses but still poorly documented. Operational, quality controlled, HFR derived velocities are provided by the Balearic Islands Coastal Observing and Forecasting System (SOCIB). They are assessed by performing statistical comparisons with current-meter, ADCP, and surface lagrangian drifters. HFR system does not show significant bias, and its accuracy is in accordance with previous studies performed in other areas. The main surface circulation patterns are deduced from an EOF analysis. The first three modes represent almost 70 % of the total variability. A cross-correlation analysis between zonal and meridional wind components and the temporal amplitudes of the first three modes reveal that the first two modes are mainly driven by local winds, with immediate effects of wind forcing and veering following Ekman effect. The first mode (37 % of total variability) is the response of meridional wind while the second mode (24 % of total variability) is linked primarily with zonal winds. The third and higher order modes are related to mesoscale circulation features. HFR derived surface transport presents a markedly seasonal variability being mostly southwards. Its comparison with Ekman-induced transport shows that wind contribution to the total surface transport is on average around 65 %.