Models for extremes using the extended three-parameter Burr XII system with application to flood frequency analysis / Modèles d’extrêmes utilisant le système Burr XII étendu à trois paramètres et application à l’analyse fréquentielle des crues

Abstract

Abstract A new theoretically-based distribution in frequency analysis is proposed. The extended three-parameter Burr XII distribution includes the generalized Pareto distribution, which is used to model the exceedences over threshold; log-logistic distribution, which is also advocated in flood frequency analysis; and Weibull distribution, which is a part of the generalized extreme value distribution used to model annual maxima as special cases. The extended Burr distribution is flexible to approximate extreme value distribution. Note that both the generalized Pareto and generalized extreme value distributions are limiting results in modelling the exceedences over threshold and block extremes, respectively. From a modelling perspective, generalization might be necessary in order to obtain a better fit. The extended three-parameter Burr XII distribution is therefore a meaningful candidate distribution in the frequency analysis. Maximum likelihood estimation for this distribution is investigated in the paper. The use of the extended three-parameter Burr XII distribution is demonstrated using data from China.     

Glacier shrinkage and negative mass balance in the Chilean Lake District (40°S) / Rétrécissement glaciaire et bilan massique négatif dans la Région des Lacs du Chili (40°S)

Abstract

Ice-capped volcanoes of the Chilean Lake District have shown significant glacier retreat during recent decades, probably in response to tropospheric warming and precipitation decrease. Volcán Mocho-Choshuenco (39°55′S, 72°02′W) is one of the main active volcanoes in this part of the country. A mass balance programme was initiated on its southeastern glacier in 2003, in view of its representative conditions as an ice body that is presumably not affected by current volcanic activity. The glaciers of this volcano have been retreating and shrinking in recent decades; by 2003 there had been a reduction of 40% of the original area of 28.4 km² in 1976. A maximum decrease of area was observed in the most recently analysed period, a rate of 0.45 km² year^-1 between 1987 and 2003. The glacier average net mass balance of 2003/04 yielded ?0.88 m w.e. (water equivalent) per year (±0.18), with an average net accumulation and ablation of 2.59 and ?3.47 m w.e. per year, respectively. This is the first direct measurement of glacier mass balance in southern Chile, where very little is known about glacier variations and glacier–volcano interactions.     

Approche régionale pour l’estimation des distributions ponctuelles des pluies journalières dans le Languedoc-Roussillon (France) / Regional approach for the estimation of low-frequency distribution of daily rainfall in the Languedoc-Roussillon region,France

Abstract

Abstract This paper aims to show the benefit of a regional approach for the estimation of rare daily rainfall. The studied region is Languedoc-Roussillon (south of France), where recent exceptional storms necessitate the revision of the statistical distributions, particularly their asymptotic tails over extreme values. The example of a large single-site time series of maximum daily rainfall at Marseille (1864–2002), very close to the studied region, shows a hyper-exponential behaviour for extreme events. At the regional scale, the homogenization process of daily maximum rainfall has been performed by considering that the coefficients of variation of the yearly maximum daily rainfall are stationary over the study zone. Two regional sample studies have been carried out on 15 and 23 gauges, randomly distributed in space, and a similar distribution could be fitted to both samples. As in the case of Marseille, the regional distribution shows a hyper-exponential asymptotic behaviour at the extreme values. The obtained regional distribution provides a systematic method for computation of rare daily rainfall that may be applied in every part of the studied region and, when compared with previous estimations, leads to a significant increase in the depth of rare rainfall.     

Statistics of extremes and estimation of extreme rainfall: I. Theoretical investigation / Statistiques de valeurs extrêmes et estimation de précipitations extrêmes: I. Recherche théorique

Abstract

Abstract The Gumbel distribution has been the prevailing model for quantifying risk associated with extreme rainfall. Several arguments including theoretical reasoning and empirical evidence are supposed to support the appropriateness of the Gumbel distribution. These arguments are examined thoroughly in this work and are put into question. Specifically, theoretical analyses show that the Gumbel distribution is quite unlikely to apply to hydrological extremes and its application may misjudge the risk, as it underestimates seriously the largest extreme rainfall amounts. Besides, it is shown that hydrological records of typical length (some decades) may display a distorted picture of the actual distribution, suggesting that the Gumbel distribution is an appropriate model for rainfall extremes while it is not. In addition, it is shown that the extreme value distribution of type II (EV2) is a more consistent alternative. Based on the theoretical analysis, in the second part of this study an extensive empirical investigation is performed using a collection of 169 of the longest available rainfall records worldwide, each having 100–154 years of data. This verifies the inappropriateness of the Gumbel distribution and the appropriateness of EV2 distribution for rainfall extremes.     

Analysis of a large inter-basin water transfer system in India / Analyse d’un grand système de transfert d’eau inter-bassins en Inde

Abstract

Abstract Inter-basin transfer of water in India is a long-term option to correct the spatial and temporal mismatch of water availability and demand, largely owing to the monsoon climate. This paper is concerned with analysis and preliminary design of a large inter-basin water transfer system in peninsular India. The system covers four major basins and involves operation of 13 major structures. The study was carried out in three stages. First, the surface water deficit in each basin was estimated. Then the net deficit was worked out by considering the availability of groundwater. Finally, the link systems were planned to transfer the amount of water needed to meet the demands with desired reliability.     

Effet auto-épurateur de la lithologie des affleurements géologiques dans un climat semi-aride: cas du bassin versant de l'Oued Mellègue (Nord-Ouest de la Tunisie)

Résumé

Le Nord-Ouest de la Tunisie renferme plus que 81% des eaux de surface et 95% des mines et gîtes miniers. Le bassin versant de l'Oued Mellègue, situé dans cette zone, englobe 13 mines de plomb, zinc, fer, strontium, fluor, barium et phosphates. L'impact des rejets de ces mines sur les eaux de surface a été apprécié grâce à plusieurs compagnes d’échantillonnage. Les résultats d'analyse par ICP/MS de ces échantillons montrent qu'ils renferment des quantités non négligeables d'Al, As, B, Ba, Br, Cd, Co, Cr, Cu, Hg, Mo, Ni, Sr, U et de Zn. L'analyse des ces éléments dans les eaux du Mellègue, de ses affluents et dans la retenue de Nebeur montre que leurs concentrations sont inférieures aux normes d'eau potable. Ce comportement pourrait être dû à la nature des affleurements géologiques formés par des carbonates des argiles et des évaporites, à l'absence de cours d'eau drainant directement les rejets, à la formation de minéraux secondaires caractérisés par une importante rétention des métaux et à l’étendue des bassins versants non pollués.     

4. Hydraulics

Abstract

A solid state event recorder and reader suitable for use with a tipping bucket raingauge are described. The recorder is a small battery operated unit with very low power consumption, capable of recording 64, 8 bit, words. The portable reader unit is used to interrogate it.     

A pore-throat model based on grain-size distribution to quantify gravity-dominated DNAPL instabilities in a water-saturated homogeneous porous medium

A pore-scale network model based on spherical pore bodies and cylindrical pore throats was developed to describe the displacement of water by DNAPL. The pore body size and the pore throat size were given by statistical distributions with user-specified values for the minimum, mean and maximum sizes. The numerical model was applied to a laboratory experiment conducted on a sand-filled glass column. The parameters relative to pore body size and pore throat size that were used in the construction of the equivalent network were derived from the discrete grain-size distribution of the real porous medium. The calculated arrival times of the DNAPL front were compared with those measured using optic fibre sensors placed at different points on the control section of the experimental device. Furthermore, the model simulated DNAPL pressure measured at the entrance section of the system. In general, the numerical results obtained with the model were in good agreement with the actual measurements.     

Iron Isotope Variations in Coastal Seawater Determined by Multicollector ICP-MS

In this paper, we applied a reliable technique for measuring Fe isotope variations in coastal seawater at nanomolar levels. Iron was directly pre-concentrated from acidified seawater samples onto a nitrilotriacetic acid chelating resin and further purified using anion-exchange resin. Sample recovery, determined using a standard addition method, was essentially quantitative. Iron was then determined using a high-resolution multicollector ICP-MS (Neptune) coupled to an ApexQ desolvation introduction system. The external precision for δ⁵⁶Fe values was 0.11‰ (2s) when using total a Fe quantity between 25 and 100 ng. We initially applied this technique to measure the Fe isotope composition of dissolved Fe from several coastal environments in the north-eastern United States and we observed a range of δ⁵⁶Fe values between -0.9‰ and 0.1‰ relative to the IRMM-14 reference material. Iron isotope compositions of several reference water materials for inter-laboratory comparisons were also reported. Our results suggest that iron in coastal seawater, derived from benthic diagenesis and/or groundwater has negative Fe isotopic signatures that are distinct from other iron sources such as atmospheric deposition and rivers.