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排序方式: 共有148条查询结果,搜索用时 562 毫秒
101.
Colin R. O'Farrell Arjun M. Heimsath Daniel E. Lawson Laura M. Jorgensen Edward B. Evenson Grahame Larson Jon Denner 《地球表面变化过程与地形》2009,34(15):2008-2022
Glacial erosion rates are estimated to be among the highest in the world. Few studies have attempted, however, to quantify the flux of sediment from the periglacial landscape to a glacier. Here, erosion rates from the nonglacial landscape above the Matanuska Glacier, Alaska are presented and compare with an 8‐yr record of proglacial suspended sediment yield. Non‐glacial lowering rates range from 1·8 ± 0·5 mm yr?1 to 8·5 ± 3·4 mm yr?1 from estimates of rock fall and debris‐flow fan volumes. An average erosion rate of 0·08 ± 0·04 mm yr?1 from eight convex‐up ridge crests was determined using in situ produced cosmogenic 10Be. Extrapolating these rates, based on landscape morphometry, to the Matanuska basin (58% ice‐cover), it was found that nonglacial processes account for an annual sediment flux of 2·3 ± 1·0 × 106 t. Suspended sediment data for 8 years and an assumed bedload to estimate the annual sediment yield at the Matanuska terminus to be 2·9 ± 1·0 × 106 t, corresponding to an erosion rate of 1·8 ± 0·6 mm yr?1: nonglacial sources therefore account for 80 ± 45% of the proglacial yield. A similar set of analyses were used for a small tributary sub‐basin (32% ice‐cover) to determine an erosion rate of 12·1 ± 6·9 mm yr?1, based on proglacial sediment yield, with the nonglacial sediment flux equal to 10 ± 7% of the proglacial yield. It is suggested that erosion rates by nonglacial processes are similar to inferred subglacial rates, such that the ice‐free regions of a glaciated landscape contribute significantly to the glacial sediment budget. The similar magnitude of nonglacial and glacial rates implies that partially glaciated landscapes will respond rapidly to changes in climate and base level through a rapid nonglacial response to glacially driven incision. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
102.
N. Janardhana Raju Prahlad Ram Sangita Dey 《Journal of the Geological Society of India》2009,73(2):178-192
The lower Varuna River basin in Varanasi district situated in the central Ganga plain is a highly productive agricultural
area, and is also one of the fast growing urban areas in India. The agricultural and urbanization activities have a lot of
impact on the groundwater quality of the study area. The river basin is underlain by Quaternary alluvial sediments consisting
of clay, silt, sand and gravel of various grades. The hydrogeochemical study was undertaken by randomly collecting 75 groundwater
samples from dug wells and hand pumps covering the entire basin in order to understand the sources of dissolved ions, and
to assess the chemical quality of the groundwater through analysis of major ions. Based on the total dissolved solids, two
groundwater samples are considered unsuitable for drinking purpose, but all samples are useful for irrigation. Graphical treatment
of major ion chemistry by Piper diagram helps in identifying hydro-geochemical facies of groundwaters and the dominant hydrochemical
facies is Ca-Mg-HCO3 with appreciable percentage of the water having mixed facies. As per Wilcox’s diagram and US Salinity laboratory classification,
most of the groundwater samples are suitable for irrigation except two samples (No’s 30 and 68) which are unsuitable due to
the presence of high salinity and medium sodium hazard. Irrigation waters classified based on residual sodium carbonate, have
revealed that all groundwaters are in general safe for irrigation except one sample (No. 27), which needs treatment before
use. Permeability index indicates that the groundwater samples are suitable for irrigation purpose.
Although the general quality of groundwater of the lower Varuna River basin is suitable for irrigation purpose, fifty seven
percent of the samples are found having nitrate content more than permissible limit (>45 mg/l) which is not good for human
consumption. Application of N-Fertilizers on agricultural land as crop nutrients along the Varuna River course may be responsible
for nitrate pollution in the groundwater due to leaching by applied irrigation water. The other potential sources of high
nitrate concentration in extreme northern, southern and southwestern parts of study area are poor sewerage and drainage facilities,
leakage of human excreta from very old septic tanks, and sanitary landfills. The high fluoride contamination (>1.5 mg/l) in
some of the samples may be due to the dissolution of micaceous content in the alluvium. Nitrate and fluoride contamination
of groundwater is a serious problem for its domestic use. Hence an immediate protective measure must be put into action in
the study area. 相似文献
103.
Andrew Barnes Devin J. Sapsford Mathew Dey Keith P. Williams 《Journal of Geochemical Exploration》2009,100(2-3):192-198
The determination of electrophoretic mobility and zeta potential was used as a diagnostic tool, alongside kinetic experiments, to delineate between three plausible mechanisms for the heterogeneous oxidation of Fe(II) by dissolved oxygen. One of these mechanisms is dependant on the positive surface charge that exists on Fe(III) (oxy)hydroxide surfaces at pH values below the Iso-Electric Point (IEP). However, this mechanism can be disputed as catalysis is observed on Fe(III) (oxy)hydroxide surfaces above the IEP despite a negative zeta potential. As well as an IEP shift an overall reduction of the magnitude of the zeta potential is observed in samples of field Fe(III) (oxy)hydroxide collected from the Taff Merthyr mine water treatment site in South Wales, UK. Low zeta potentials determined in mine water treatment systems will have beneficial effects for particle coagulation and settling in passive mine water treatment systems. 相似文献
104.
Raka Dona Ray Mandal Sushan Konar Mira Dey Jishnu Dey 《Monthly notices of the Royal Astronomical Society》2009,399(2):822-826
The single glitch observed in PSR B1821−24, a millisecond pulsar in M28, is unusual on two counts. First, the magnitude of this glitch is at least an order of magnitude smaller (Δν/ν∼ 10−11 ) than the smallest glitch observed to date. Secondly, all other glitching pulsars have strong magnetic fields with B ≳ 1011 G and are young, whereas PSR B1821−24 is an old recycled pulsar with a field strength of 2.25 × 109 G . We have earlier suggested that some of the recycled pulsars could actually be strange quark stars. In this work, we argue that the crustal properties of such a strange pulsar are just right to give rise to a glitch of this magnitude, explaining the scarcity of larger glitches in millisecond pulsars. 相似文献
105.
Image fusion assists in visual interpretation, mapping, change detection and many other applications. Multispectral and Panchromatic
images are fused to produce images having enhanced spatial and spectral properties. These properties are generally distorted
from original images. The aim of this paper is to identify the effectiveness of the several fusion techniques based on the
distortions and applications. This paper employs seven image fusion techniques namely, Brovey transform, intensity hue saturation,
high pass filter, principle component analysis, UNB Pansharpening, wavelet transform and multiplicative, available in various
commercial image processing software. The data for this study are panchromatic image of Cartosat-1 and multispectral image
of IRS - P6 LISS 4 sensor of study area, Bhopal Municipal Corporation area, M.P. State, India. The effectiveness of image
fusion techniques is determined by quantitative and qualitative assessments. Quantitative assessment is divided into two parts:
1) assessment of fusion techniques by statistical parameters and 2) accuracy assessment of land use maps generated from the
fused images. For part 1, three parameters namely, mean bias, correlation coefficient and Q4 quality index, have been used.
Based on the results of part 1, UNB Pansharpening and wavelet transform are the best among seven fusion techniques. For part
2, Gaussian and Artificial Neural Network classifiers have been used to generate land cover maps. However, the accuracy results
are inconclusive to identify a single best method. Nevertheless, image fusion by wavelet transform has provided best results
in both the sector. Hence, wavelet transform is concluded as the best among selected fusion techniques. 相似文献
106.
107.
Quantifying hillslope erosion rates and processes for a coastal California landscape over varying timescales 总被引:1,自引:0,他引:1
The Earth's surface erodes by processes that occur over different spatial and temporal scales. Both continuous, low‐magnitude processes as well as infrequent, high‐magnitude events drive erosion of hilly soil‐mantled landscapes. To determine the potential variability of erosion rates we applied three independent, field‐based methods to a well‐studied catchment in the Marin Headlands of northern California. We present short‐term, basin‐wide erosion rates determined by measuring pond sediment volume (40 years) and measured activities of the fallout nuclides 137Cs and 210Pb (40–50 years) for comparison with long‐term (>10 ka) rates previously determined from in situ‐produced cosmogenic 10Be and 26Al analyses. In addition to determining basin‐averaged rates, 137Cs and 210Pb enable us to calculate point‐specific erosion rates and use these rates to infer dominant erosion processes across the landscape. When examined in the context of established geomorphic transport laws, the correlations between point rates of soil loss from 137Cs and 210Pb inventories and landscape morphometry (i.e. topographic curvature and upslope drainage area) demonstrate that slope‐driven processes dominate on convex areas while overland flow processes dominate in concave hollows and channels. We show a good agreement in erosion rates determined by three independent methods: equivalent denudation rates of 143 ± 41 m Ma?1 from pond sediment volume, 136 ± 36 m Ma?1 from the combination of 137Cs and 210Pb, and 102 ± 25 m Ma?1 from 10Be and 26Al. Such agreement suggests that erosion of this landscape is not dominated by extreme events; rather, the rates and processes observed today are indicative of those operating for at least the past 10 000 years. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
108.
109.
S. T. WU T. X. Zhang E. Tandberg-Hanssen Yang Liu Xueshang Feng Arjun Tan 《Solar physics》2004,225(1):157-175
Analysis of observations from both space-borne (LASCO/SOHO, Skylab and Solar Maximum Mission) and ground-based (Mauna Loa Observatory) instruments show that there are two types of coronal mass ejections (CMEs), fast CMEs and slow CMEs. Fast CMEs start with a high initial speed, which remains more or less constant, while slow CMEs start with a low initial speed, but show a gradual acceleration. To explain the difference between the two types of CMEs, Low and Zhang (2002) proposed that it resulted from a difference in the initial topology of the magnetic fields associated with the underlying quiescent prominences, i.e., a normal prominence configuration will lead to a fast CME, while an inverse quiescent prominence results in a slow CME. In this paper we explore a different scenario to explain the existence of fast and slow CMEs. Postulating only an inverse topology for the quiescent prominences, we show that fast and slow CMEs result from different physical processes responsible for the destabilization of the coronal magnetic field and for the initiation and launching of the CME. We use a 2.5-D, time-dependent streamer and flux-rope magnetohydrodynamic (MHD) model (Wu and Guo, 1997) and investigate three initiation processes, viz. (1) injecting of magnetic flux into the flux-rope, thereby causing an additional Lorentz force that will destabilize the streamer and launch a CME (Wu et al., 1997, 1999); (2) draining of plasma from the flux-rope and triggering a magnetic buoyancy force that causes the flux-rope to lift and launch a CME; and (3) introducing additional heating into the flux-rope, thereby simulating an active-region flux-rope accompanied by a flare to launch a CME. We present 12 numerical tests using these three driving mechanisms either alone or in various combinations. The results show that both fast and slow CMEs can be obtained from an inverse prominence configuration subjected to one or more of these three different initiation processes. 相似文献
110.
Hillslope Topography from Unconstrained Photographs 总被引:1,自引:0,他引:1
Quantifications of Earth surface topography are essential for modeling the connections between physical and chemical processes of erosion and the shape of the landscape. Enormous investments are made in developing and testing process-based landscape evolution models. These models may never be applied to real topography because of the difficulties in obtaining high-resolution (1–2 m) topographic data in the form of digital elevation models (DEMs). Here we present a simple methodology to extract the high-resolution three-dimensional topographic surface from photographs taken with a hand-held camera with no constraints imposed on the camera positions or field survey. This technique requires only the selection of corresponding points in three or more photographs. From these corresponding points the unknown camera positions and surface topography are simultaneously estimated. We compare results from surface reconstructions estimated from high-resolution survey data from field sites in the Oregon Coast Range and northern California to verify our technique. Our most rigorous test of the algorithms presented here is from the soil-mantled hillslopes of the Santa Cruz marine terrace sequence. Results from three unconstrained photographs yield an estimated surface, with errors on the order of 1 m, that compares well with high-resolution GPS survey data and can be used as an input DEM in process-based landscape evolution modeling. 相似文献