Frequency analysis of maximum annual suspended sediment concentrations in North America / Analyse fréquentielle des maximums annuels de concentration en sédiments en suspension en Amérique du Nord

Abstract

Suspended sediments are a natural component of aquatic ecosystems, but when present in high concentrations they can become a threat to aquatic life and can carry large amounts of pollutants. Suspended sediment concentration (SSC) is therefore an important abiotic variable used to quantify water quality and habitat availability for some species of fish and invertebrates. This study is an attempt to quantify and predict annual extreme events of SSC using frequency analysis methods. Time series of daily suspended sediment concentrations in 208 rivers in North America were analysed to provide a large-scale frequency analysis study of annual maximum concentrations. Seasonality and the correlation of discharges and annual peak of suspended sediment concentration were also analysed. Peak concentrations usually occur in spring and summer. A significant correlation between extreme SSC and associated discharge was detected only in half of the stations. Probability distributions were fitted to station data recorded at the stations to estimate the return period for a specific concentration, or the concentration for a given return period. Selection criteria such as the Akaike and Bayesian information criterion were used to select the best statistical distribution in each case. For each selected distribution, the most appropriate parameter estimation method was used. The most commonly used distributions were exponential, lognormal, Weibull and Gamma. These four distributions were used for 90% of stations.     

Identification of homogeneous regional classes for flood frequency analysis in the light of regional taxonomy / Identification de classes régionales homogènes pour l'analyse fréquentielle des crues à la lumière d'une taxinomie régionale

Abstract

The paper analyses delineation of hydrological regional classes in the light of regional taxonomy. A brief review of terminological and methodological aspects of regional taxonomy is outlined. The analysis of identification of hydrological regional classes from the point of view of the definition of the basic spatial unit, formulation of the regional taxonomic problem and evaluation of the hydrological response of the physical regional classes is then followed. A more detailed delineation of physical regional classes and a marked separation concerning their hydrological response are achieved if the basic spatial unit is defined as a small basin. Formulation of a hydrological regionalization or regional typification by means of problems defined in regional taxonomy can remove ambiguous and inconsistent features in identifying regional classes. The physical regional classes formed for the purpose of regional flood frequency analysis are considered as regional also from the hydrological point of view only if they satisfy both conditions of intra-class similarity and of inter-class dissimilarity regarding the hydrological attributes.     

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.     

Trends in frequency of precipitation extremes in the Yangtze River basin,China: 1960–2003 / Tendances d'évolution de la fréquence des précipitations extrêmes entre 1960 et 2003 dans le bassin versant du Fleuve Yangtze (Chine)

Abstract

The spatial distribution and trends in the frequency of precipitation extremes over the last 44 years (1960–2003), especially since 1990, have been analysed using daily precipitation data from 147 stations in the Yangtze River basin. The research results are as follows: (1) The 15 mm precipitation isohyet approximately divides the precipitation extremes (corresponding to the 95th percentile) of the stations in the middle and lower Yangtze reaches (higher) from those of the upper Yangtze reaches (lower). Also the starting time of the precipitation extremes in the middle and lower Yangtze reaches is earlier than of those in the upper Yangtze reaches. Precipitation extremes are concentrated mostly in June in the middle and lower Yangtze reaches, and July in the upper Yangtze reaches. (2) During the period 1960–2003, the first two decades had fewer precipitation extremes than the last two decades. There have been significant increasing trends and step changes in frequency of annual total precipitation extremes and precipitation extremes with a 1–5 day gap in the middle and lower Yangtze reaches. Precipitation extremes occur more frequently in shorter periods, separated by a few days. Precipitation extremes are also becoming more concentrated in the month with the highest frequency of extremes (June) in the middle and lower Yangtze reaches. In the upper Yangtze reaches, there is an upward tendency of extreme events in June. Increasing precipitation extremes in June for both the middle and lower, and the upper Yangtze reaches will increase the probability of flooding if the observed trends of the last 40 years continue into the future.     

SIMULATION OF A SERIES OF DAILY AREAL MEAN RAINFALLS FOR A BASIN USING RAINFALL OBSERVATIONS AT A REFERENCE STATION LOCATED OUTSIDE THE BASIN / Simulation d'une série de précipitations journalières moyennes spatiales sur un bassin,en utilisant les hauteurs pluviométriques observées à une station de référence située en dehors du bassin

Abstract

A mathematical model is built which enables daily estimates to be made of the areal mean rainfalls for a basin using rainfall observations at a reference station of the same climature* but which may be situated at some distance from the basin. The method consists of:

1. (1) determining the probabilities of the characteristic types of association between areal mean rainfalls of varying magnitude at the basin and rainfall events of various types at the reference station.

2. (2) establishing the conditional distributions of areal mean rainfalls at the basin.

3. (3) estimating the areal mean rainfalls at the basin (a) by the Monte Carlo method and (b) by accepting that the mean basin rainfalls and the reference station rainfall are, in their respective statistical distributions, fractiles of the same order.

Finally, the validity of the simulated series is checked by means of various tests.