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1.
Sediment is fractionated by size during its cascade from source to sink in sediment routing systems. It is anticipated, therefore, that the grain size distribution of sediment will undergo down‐system changes as a result of fluvial sorting processes and selective deposition. We assess this hypothesis by comparing grain size statistical properties of samples from within the erosional source region with those that have undergone different amounts of transport. A truncated Pareto distribution describes well the coarser half of the clast size distribution of regolith, coarse channel bed sediment and proximal debris flows (particularly their levees), as well as the coarser half of the clast size distribution of gravels that have undergone considerable amounts of transport in rivers. The Pareto shape parameter a evolves in response to mobilization, sediment transport and, importantly, the selective extraction of particles from the surface flow to build underlying stratigraphy. A goodness of fit statistic, the Kolmogorov–Smirnov vertical difference, illustrates the closeness of the observed clast size distributions to the Pareto, Weibull and log‐normal models as a function of distance from the depositional apex. The goodness of fit of the particle size distribution of regolith varies with bedrock geology. Bedload sediment at catchment outlets is fitted well by the log‐normal and truncated Pareto models, whereas the exponential Weibull model provides a less good fit. In the Eocene Escanilla palaeo‐sediment routing system of the south‐central Pyrenees, the log‐normal and truncated Pareto models provide excellent fits for distances of up to 80 km from the depositional apex, whereas the Weibull fit progressively worsens with increasing transport distance. A similar trend is found in the Miocene–Pliocene gravels of the Nebraskan Great Plains over a distance of >300 km. Despite the large fractionation in mean grain size and gravel percentage from source region to depositional sink, particle size distributions therefore appear to maintain log‐normality over a wide range of transport distance. Use of statistical models enables down‐system fractionation of sediment released from source regions to be better understood and predicted and is a potentially valuable tool in source‐to‐sink approaches to basin analysis.  相似文献   

2.
Landslide mass size frequency distributions and mean slope-angle frequency distributions were calculated for slump, slide, and creep type landslides in the Higashikubiki area. Mean slope-angle frequency distributions closely approximated Weibull distributions. Size frequency distributions show power-law dependencies. Both can be explained by modeling landslides as linked uniform blocks in tensile force. Power coefficients for size frequency distributions were 2.01–2.32 (approximation to power functions) or 2.10–2.24 (approximation to Pareto distributions).  相似文献   

3.
Earlier methods of fitting Pareto–lognormal distributions to large samples of worldwide metal deposit size data are improved by using a sliding window method for estimating upper-tail Pareto coefficients and constructing best-fitting lognormal Q–Q plots with their corresponding probability-density curves. Lower-tail Pareto distributions are fitted to some extent as well. Copper and Zn deposits of the world are taken as example in this paper. Three principal statistical laws resulting in the basic lognormal with two Pareto tails are thought to underlie the generation of Pareto–lognormals for amounts of metal in primarily hydrothermal ore deposits. Historical trends in mining and exploration are thought to create an excess of smaller deposits with respect to the basic lognormal that decreases steadily with increasing deposit size until it changes into a deficit slightly before median size is reached. This deficit decreases for the largest metal deposit sizes for which the upper-tail Pareto and extrapolated basic lognormal show similar size frequencies again. The Pareto–lognormal model can also be used to describe metal size-frequency distributions for smaller geographically coherent regions on the continents. A new version of the original model of de Wijs is considered to help explain why regional Pareto–lognormal distributions with lesser logarithmic variances and Pareto coefficients can be combined to form worldwide size-frequency distributions of the same type.  相似文献   

4.
An important question in seismic hazard assessments is the frequency-size and recurrence interval statistics at a point on a fault. Does a point on a fault obey the same statistics as earthquakes in a region do? This is a difficult question to answer because the number of repetitive earthquakes on a particular fault that have been observed is small. In order to overcome this difficulty we consider slip events on the creeping section of the San Andreas fault in central California. Sequences of up to 100 events are obtained from creepmeter records. We compare the statistical distribution of recurrence times with the Brownian passage-time, lognormal, and Weibull distributions and using goodness-of-fit tests find that the Weibull is the preferred distribution. We also consider the frequency–amplitude distribution of slip events. We find that the data clearly do not obey a Gutenberg–Richter distribution. Instead there is a uniform distribution of event sizes for a large fraction of events.  相似文献   

5.
Recently, large worldwide databases with statistics on amounts of metal in mineral deposits have become available. Frequently, most metal is contained in the largest deposits for a metal. A major problem in meaningful modeling of the size–frequency distributions of the largest deposits is that they are very rare. Until now it was rather difficult to establish the exact form of their size–frequency distribution. However, because of the new very large databases it can now be concluded that two commonly used approaches (lognormal and Pareto) thought to be mutually incompatible in the past, are both correct with a high probability. One approach does not necessarily exclude validity of the other. Patiño-Douce (Nat Resour Res 25(1):97–124, 2016b) has shown that metal tonnage frequency distributions for worldwide metal deposits are approximately lognormal with similar standard deviations (σ) of log-transformed data. In this paper, it is assumed that worldwide metals satisfy both lognormal and Pareto models simultaneously. Copper and Au are taken for example for comparison with results previously obtained for these two metals in the Abitibi area of the Canadian Shield. Worldwide there are 2541 Cu deposits approximately satisfying a lognormal distribution. Total amount of Cu in these deposits is 2.319 × 109 tons of Cu. However, the 45 largest deposits, which together contain 1.281 × 109 tons of Cu, satisfy a Pareto distribution. If their lognormal model would apply in the upper tail as well, these 45 largest deposits should have contained only about 0.076 × 109 tons of Cu. It is shown in detail for Cu that the best statistical model for Cu deposits is a worldwide Pareto–lognormal model in which the basic lognormal size–frequency distribution is flanked by two juxtaposed Pareto distributions for the largest and smallest Cu deposits, respectively. Both Pareto distributions smoothly change into the central lognormal by means of bridge functions that can be determined separately. The worldwide Pareto–lognormal model also was found to be applicable to several other metals, especially Ag, Ni, Pb, and U. For Au, the model does not work as well for the upper tail Pareto distribution as it does for the other metals taken for example.  相似文献   

6.
This paper aims at the spatiotemporal distribution of rainfall in Ethiopia and developing stochastic daily rainfall model. Particularly, in this study, we used a Markov Chain Analogue Year (MCAY) model that is, Markov Chain with Analogue year (AY) component is used to model the occurrence process of daily rainfall and the intensity or amount of rainfall on wet days is described using Weibull, Log normal, mixed exponential and Gamma distributions. The MCAY model best describes the occurrence process of daily rainfall, this is due to the AY component included in the MC to model the frequency of daily rainfall. Then, by combining the occurrence process model and amount process model, we developed Markov Chain Analogue Year Weibull model (MCAYWBM), Markov Chain Analogue Year Log normal model (MCAYLNM), Markov Chain Analogue Year mixed exponential model (MCAYMEM) and Markov Chain Analogue Year gamma model (MCAYGM). The performance of the models is assessed by taking daily rainfall data from 21 weather stations (ranging from 1 January 1984-31 December 2018). The data is obtained from Ethiopia National Meteorology Agency (ENMA). The result shows that MCAYWBM, MCAYMEM and MCAYGM performs very well in the simulation of daily rainfall process in Ethiopia and their performances are nearly the same with a slight difference between them compared to MCAYLNM. The mean absolute percentage error (MAPE) in the four models: MCAYGM, MCAYWBM, MAYMEM and MCAYLNM are 2.16%, 2.27%, 2.25% and 11.41% respectively. Hence, MCAYGM, MCAYWBM, MAYMEM models have shown an excellent performance compared to MCAYLNM. In general, the light tailed distributions: Weibull, gamma and mixed exponential distributions are appropriate probability distributions to model the intensity of daily rainfall in Ethiopia especially, when these distributions are combined with MCAYM.  相似文献   

7.

The Pareto-lognormal frequency distribution, which can result from multifractal cascade modeling, previously was shown to be useful to describe the worldwide size-frequency distributions of metals including copper, zinc, gold and silver in ore deposits. In this paper, it is shown that the model also can be used for the size-frequency distributions of these metals in Canada which covers 6.6% of the continental crust. Like their worldwide equivalents, these Canadian deposits show two significant departures from the Pareto-lognormal model: (1) there are too many small deposits, and (2) there are too few deposits in the transition zone between the central lognormal and the upper tail Pareto describing the size-frequency distribution of the largest deposits. Probable causes of these departures are: (1) historically, relatively many small ore deposits were mined before bulk mining methods became available in the twentieth century, and (2) economically, giant and supergiant deposits are preferred for mining and these have strongest geophysical and geochemical anomalies. It is shown that there probably exist many large deposits that have not been discovered or mined. Although overall the samples of the size-frequency distributions are very large, frequencies uncertainties associated with the largest deposits are relatively small and it remains difficult to estimate more precisely how many undiscovered mineral deposits there are in the upper tails of the size-frequency distributions of the metals considered.

  相似文献   

8.
我们应用层次贝叶斯模型模拟大气二氧化碳浓度加倍可能对美国北卡罗来纳州西部威塔(Coweeta)流域水文的影响。这个模型整合了多重数据来源并且同时考虑了数据,参数和模型结构的不确定性。贝叶斯分析的预测分布显示流量和土壤含水量在秋季和夏季将明显下降,这将造成这两个季节更严重的干旱。同时我们用通用极值分布(Generalized Extreme Value distribution)和通用普拉托分布(Generalized Pareto distribution)分析预测流量,结果显示洪水频率也会增减。层次贝叶斯模型,和许多只能得到最佳参数估计的水文模型相比,能提供更丰富的信息,包括预测的不确定性。这将有助于可持续水资源管理的大前提下发展应对气候变化的措施。  相似文献   

9.
A number of studies have shown that cumulative distributions of turbidite bed thicknesses (as observed in boreholes or outcrops) follow a power law in the sense that the number of beds whose measured thickness is greater than η is proportional to η−β. The purpose of this paper is to investigate the relationship between the distribution of measured thicknesses of turbidite sand beds and the distribution of bed volumes. The distribution of sand bed volumes has practical implications to model hydrocarbon reservoirs in 3D and quantifies how the total amount of sand within a turbidite sequence is distributed among beds of different sizes. If the cumulative distribution of bed volumes v is proportional to v −c the few largest beds account for more and more of the total volume of sand and the sequence becomes more and more 'punctuated' as the exponent c decreases below 1.  The relationship between the exponents of the distribution of measured bed thicknesses ( β )and of bed volumes (c) can be investigated by assuming that the turbidite beds are disc-like bodies distributed in the 3D space of a basin, and that their thicknesses are sampled on a vertical line that does not in general intersect all the beds. The relationship between β and c can be shown to depend on how bed length scales with bed thickness (as noted by other authors) and on how the beds are distributed in the 3D space of the basin (which is a new result). The main conclusion is that it is not generally possible to make inferences on the distribution of turbidite bed volumes knowing only the distribution of bed thicknesses. On the other hand, if some independent information is available on the geometrical characteristics of the turbidite deposit, the observations of bed thicknesses can be converted to bed volumes, thus providing general information on the 3D characteristics of the sequence.  相似文献   

10.
The article presents a simple sediment deposition model, based on current knowledge of the processes of transport and deposition of suspended material. It indicates possibilities of interpreting flow conditions from grain-size distributions at two localities, situated along the water flowline. The model reveals new information on ancient hydrological and sedimentological processes and events. Previously the model has been applied to forecast trap efficiency of reservoirs and predict reservoir sedimentation. In this study the model is used as a palaeohydrological method in studies of annual varves.
In varve geochronology a major attention has been paid to varve correlations and varve thickness variations, whereas other sedimentological data have been more or less disregarded. In this article new information on the spatial distribution of particle sizes demonstrate that further knowledge can be gained by careful studies of the patterns of grain-size distributions.
Annual variations in water discharges during late glacial time were studied in the Indalsälven and Dalälven valleys in Sweden. Water discharges during the deglaciation seem to have been of the same order of magnitude as in present-day rivers during normal years. During some years catastrophic events occurred, resulting in exceptional water discharges. The magnitudes of these discharges were estimated, and drainage directions were established. It appears from the studies that there are promising prospects of finding out both the magnitude and frequency of processes that until now have been described only in qualitative terms.  相似文献   

11.
A recent paper (Dearing et al . 1996) on the use of frequency dependence of magnetic susceptibility (Xfd) in magnetic grain-size investigations of environmental materic has proposed a fundamentally new model for the magnetic susceptibility of single-domain (SD) grains measured at different frequencies, and is divergent from the no well-established theory of SD behaviour. This new model appears to be founded on a confusion about the behaviour of SD grains, as predicted by theory, and magnetic measurements of real materials, which naturally have a distribution of magnetic grain sizes. Here I try to clear up this confusion by showing how different log-normal distributions of magnetic grain sizes affect the frequency dependence of magnetic susceptibility measurement. This analysis highlights that the limiting maximum Xfd value of ˜ 15 per cent obtained for environmental materials can be explained simply as a constraint imposed by the magnetic grain-size distribution having a finite minimum width (˜1.0 on a natural log scale). There is no reason to suppose that if the grain-size dispersion was narrower higher Xfd values could not be observed. Furthermore, the Xfd value is non-unique and so cannot be used quantitatively to determine the amount or grain-size distribution of SD grains in a sample  相似文献   

12.
Improved estimates of UK flood risk during a period of increased climatic variability place challenges on existing methods that rely on short instrumental records. This paper examines the value of using historical data (both documentary and epigraphic) to augment existing gauged records for the River Tay at Perth as part of a multi-method approach to assessing flood risk. Single station and pooled methods are compared with flood risk estimates based on an augmented historical series (1815–2000) using the Generalized Logistic and Generalized Pareto distributions. The value of using an even longer, but less reliable, extended historical series (1210–2000) is also examined. It is recommended that modelling flood risk for return periods >100 years should incorporate historical data, where available, and that a multi-method approach using a high threshold Generalized Pareto distribution can also add confidence in flood risk estimates for return periods <100 years based on standard methods.  相似文献   

13.
Although much work has been undertaken on urban rank-size distribution, their rural counterparts have generally been ignored. This paper considers the rank-size distributions disclosed by medieval English taxation documents, both in one county through time and in contrasting counties at one date in the 14th century. It finds that the distributions tend to be curved in a convex-upward fashion on double-logarithmic paper regardless of date, area chosen, or documents used. This appears to be the result of a right-skewed frequency distribution of settlement size.  相似文献   

14.
泥石流流域的形态特征   总被引：6，自引：0，他引：6  
泥石流是特殊的流域侵蚀作用,同其他流域过程一样,密切联系着流域的形态特征.一般说来,泥石流都发生在小流域(102km2以下),而经典的流域形态研究所涉及的流域范围却达到107km2的尺度.我们想知道泥石流小流域是否具有特殊的数量特征.通过流域特征量的统计,我们看到,与一般流域相比,泥石流小流域的特征参数之间的关系形式上相同,而在数值上不同,这从一个方面肯定了流域演化存在着普遍规律(如自组织临界性),同时也证明泥石流是流域演化历史的"特殊一幕".  相似文献   

15.
能见度与空气质量的关系研究   总被引：5，自引：2，他引：3  
利用2004年1月～5月塔克拉玛干沙漠腹地塔中气象站的PM10和能见度仪自动监测资料,分析了PM10的浓度与能见度的关系,得出它们之间符合幂函数规律,并研究了沙尘暴发生时PM10的浓度分布特征,对不同能见度条件下的PM10的频率分别用正态、Weibull、Pearson Ⅲ型分布进行拟合,经比较发现Pearson Ⅲ型分布能够比较好的拟合PM10的实测频率分布。从方差分布的角度,定义置信区间并反推其出现的概率,得出弱、中、强沙尘暴的出现频率分别为60%、25%和15%,从而给出强、中、弱沙尘暴等级的PM10浓度界限标准。  相似文献   

16.
Based on the daily precipitation data of 27 meteorological stations from 1960 to 2009 in the Huaihe River Basin, spatio-temporal trend and statistical distribution of extreme precipitation events in this area are analyzed. Annual maximum series (AM) and peak over threshold series (POT) are selected to simulate the probability distribution of extreme precipitation. The results show that positive trend of annual maximum precipitation is detected at most of used stations, only a small number of stations are found to depict a negative trend during the past five decades, and none of the positive or negative trend is significant. The maximum precipitation event almost occurred in the flooding period during the 1960s and 1970s. By the L-moments method, the parameters of three extreme distributions, i.e., Generalized extreme value distribution (GEV), Generalized Pareto distribution (GP) and Gamma distribution are estimated. From the results of goodness of fit test and Kolmogorov-Smirnov (K-S) test, AM series can be better fitted by GEV model and POT series can be better fitted by GP model. By the comparison of the precipitation amounts under different return levels, it can be found that the values obtained from POT series are a little larger than the values from AM series, and they can better simulate the observed values in the Huaihe River Basin.  相似文献   

17.
Sediments and soils often contain superparamagnetic (SP) magnetite or maghemite grains that cause a frequency dependence of low-field susceptibility X fd which does not exceed 15 per cent/decade of frequency. Present models predict very different volume distributions for samples with the largest observed frequency dependence of susceptibility. While Stephensons' (1971) power-law model predicts most grains to be smaller than the stable single domain (SSD) threshold, the phenomenological model of >Dearing et al . (1996) suggests that most grains are between 10 and 25 nm in diameter. Finally, the recent calculations of Eyre (1997) indicate very broad volume distributions. This study reviews the nature of the superparamagnetic–stable single domain (SP–SSD) transition. The change of AC susceptibilities with grain size (or temperature) at the SP–SSD boundary is more gradual than commonly assumed. When distributions of particle coercivities and volumes are also considered, X fd values are much smaller than those calculated by Eyre (1997). Nonetheless, X fd can be larger than 15 per cent, and a larger frequency dependence has indeed been measured for some samples. The question whether the observed limited X fd of soils and sediments is a result of a broad distribution or of a bimodal distribution, where SP and SSD grains are restricted to a certain relative abundance, can potentially be answered by susceptibility determinations at more than two frequencies and by measurements of the temperature dependence of susceptibility.  相似文献   

18.
Contemporary variants of the lichenometric dating technique depend upon statistical correlations between surface age and maximum lichen sizes, rather than an understanding of lichen biology. To date three terminal moraines of an Alaskan glacier, we used a new lichenometric technique in which surfaces are dated by comparing lichen population distributions with the predictions of ecological demography models with explicit rules for the biological processes that govern lichen populations: colonization, growth, and survival. These rules were inferred from size–frequency distributions of lichens on calibration surfaces, but could be taken directly from biological studies. Working with two lichen taxa, we used multinomial‐based likelihood functions to compare model predictions with measured lichen populations, using only the thalli in the largest 25% of the size distribution. Joint likelihoods that combine the results of both species estimated moraine ages of ad 1938, 1917, and 1816. Ages predicted by Rhizocarpon alone were older than those of P. pubescens. Predicted ages are geologically plausible, and reveal glacier terminus retreat after a Little Ice Age maximum advance around ad 1816, with accelerated retreat starting in the early to mid twentieth century. Importantly, our technique permits calculation of prediction and model uncertainty. We attribute large confidence intervals for some dates to the use of the biologically variable Rhizocarpon subgenus, small sample sizes, and high inferred lichen mortality. We also suggest the need for improvement in demographic models. A primary advantage of our technique is that a process‐based approach to lichenometry will allow direct incorporation of ongoing advances in lichen biology.  相似文献   

19.
本文研究了湘西亚热带地球化学景观上壤中重金属分布与土壤机械组成、矿物成分之间的关系。  相似文献   

20.
杨洋  李雅静  黄庆旭  黄聪 《地理研究》2016,35(9):1672-1686
复合1992-2012年DMSP/OLS稳定夜间灯光等多源遥感数据和统计数据,运用多种城市规模分布理论方法,对城市用地与人口规模分布时空演变特征进行系统比较。结果表明：环渤海地区城市用地规模增长速度明显快于城市人口规模增长速度,城市人口规模分布比城市用地规模分布更为均衡;城市用地规模呈首位型分布但首位优势在减弱,城市人口规模呈位次型分布但首位优势在增强;位序迅速上升的城市主要位于山东省和河北省,位序显著下降的城市主要位于辽宁省。环渤海地区城市规模空间分布呈显著的区域差异和空间极化特征,且城市用地规模分布空间特征比城市人口规模分布更为突出。研究结论可为优化区域城市空间开发格局、促进人地系统可持续发展提供参考。  相似文献   

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