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Log-normally Distributed (log-normally + distributed)

Distribution by Scientific Domains

Earth and Environmental Science	75%

Selected Abstracts

Elevation adjustments of paired natural levees during flooding of the Saskatchewan River

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 8 2009
Norman D. Smith
Abstract Natural levees control the exchange of water between an alluvial channel and its floodplain, but little is known about the spatial distribution and evolution of levee heights. The summer 2005 flood of the Saskatchewan River (Cumberland Marshes, east-central Saskatchewan) inundated large areas of floodplain for up to seven weeks, forming prominent new deposits on natural levees along main-stem channels. Measurements of flood-deposit thickness and crest heights of 61 levee pairs show that the thickest deposits occur on the lower pre-flood levee in 80% of the sites, though no clear relationship exists between deposit thickness and magnitude of height difference. Only 16% of the pairs displayed thicker deposits on the higher levee, half of which occurred at sites where relatively clear floodbasin waters re-entered turbid channels during general flooding. Difference in crest elevation (,E) between paired levees is approximately log-normally distributed, both before and after the flood, though with different mean values. Supplemental observations from tank experiments indicate that during near-bankfull flows, temporally and spatially variable deposition and erosion occur on levees due to backwater effects associated with nearby channel bars and irregular rises of the channel bed forced by channel extension. During floods, preferential deposition in lows tends to even out crest heights. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Regional-scale spatial patterns of fire in relation to rainfall gradients in sub-tropical mountains, NW Argentina

GLOBAL ECOLOGY, Issue 2 2001
Héctor Ricardo Grau
Abstract 1Spatial patterns of burns are described using Landsat TM images from the sub-tropical mountains of north-west Argentina, over a span of 6 degrees of latitude, and a precipitation range from 250 to 1300 mm/yr. Burns were discriminated easily from unburnt vegetation, mainly by using infrared spectral bands from images taken at the end of the fire season of 1986. 2Nineteen sampling units were defined on the basis of geographical proximity and relatively homogeneous rainfall as inferred from topography, and they were characterized in terms of percentage of burnt area and burn size distribution during one fire season. Regression and Correspondence Analysis were used to assess the relationship between rainfall and spatial descriptors of fire regime. 3Burnt size area was log-normally distributed with most fires in the small-size classes. Of a total of 643 burns, the five largest (more than 2000 hectares each) represented about 30% of the total burnt area. 4Percentage of burnt area, density of burns per unit area, and skewness of the burn-size frequency distribution showed a unimodal pattern along the rainfall gradient, peaking between 700 and 900 mm/yr. Mean and maximum burn size showed a negative but weak correlation with rainfall. The first axis of a Correspondence Analysis ordination of sampling units, on the basis of different descriptors of spatial patterns of fire, was significantly correlated with the rainfall of the sampling unit. 5The results suggest that climate is an important factor controlling fuel conditions and therefore fire regime at the spatial scale of this study, which includes different mountain ranges spanning , 700 km. [source]

Distribution and species richness of woody dryland legumes in Baja California, Mexico

JOURNAL OF VEGETATION SCIENCE, Issue 4 2003
Pedro P. Garcillán
Wiggins (1980); Hickman (1993); Skinner & Pavlik (1994); International Legume Database & Information Service, http://www.ildis.org/legumes.html Abstract. We analysed the biogeographic patterns of woody legumes in the Baja California peninsula, NW Mexico. From the specimen labels of eight herbaria, we digitized 4205 records from 78 species, and projected them onto a grid of 205 cartographic cells (20' longitude × 15' latitude). Most species followed distribution patterns that coincide with floristic subdivisions of the peninsula. Endemism is high, reaching 60,70% in the centre of the peninsula, where the driest deserts are found and where significant floristic changes took place during Pleistocene glacial events. The number of cartographic cells (i.e. their geographic ranges) were log-normally distributed, as has been reported for many other taxa. Floristic richness was found to be clumped around some cells where the observed richness is significantly higher than could be expected from chance variation. We tested the hypothesis that these ,hotspots' could be attributable to great collection efforts or to large land surfaces, but we still found 16 cells where richness is significantly high once these two factors are accounted for. Species richness and micro-endemism increase towards the south, conforming to Rapoport's rule that predicts that species ranges become smaller towards the equator while richness increases. The floristic hotspots for woody legumes in Baja California occur in the Cape Region and along the Sierra de la Giganta in the southern Gulf Coast, where 77% of the total peninsular legume flora can be found. These hotspots are mostly unprotected, and should be considered priority areas for future conservation efforts. [source]

A data assimilation method for log-normally distributed observational errors

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 621 2006
S. J. Fletcher
Abstract In this paper we change the standard assumption made in the Bayesian framework of variational data assimilation to allow for observational errors that are log-normally distributed. We address the question of which statistic best describes the distribution for the univariate and multivariate cases to justify our choice of the mode. From this choice we derive the associated cost function, Jacobian and Hessian with a normal background. We also find the solution to the Jacobian equal to zero in both model and observational space. Given the Hessian that we derive, we define a preconditioner to aid in the minimization of the cost function. We extend this to define a general form for the preconditioner, given a certain type of cost function. Copyright © 2006 Royal Meteorological Society [source]