ESTIMATION OF SOIL PARAMETERS FOR ASSESSING POTENTIAL WETNESS: COMPARISON OF MODEL RESPONSES THROUGH GIS     

PHILIP A. TOWNSEND  STEPHEN J. WALSH 《地球表面变化过程与地形》1996,21(4):307-326

A geographic information system (GIS) is utilized to model wetness potential for a portion of Uwharrie National Forest, North Carolina. The wetness index is derived from TOPMODEL, a hillslope-scale runoff simulation model. The wetness index is a distributed-parameter model, with the input parameters obtained from a digital elevation model (DEM) and Soil Conservation Service (SCS) soils data. The primary objectives of the research are to: (1) compare methods of estimating soil parameters for input into the wetness potential model; and (2) determine how the model outputs vary spatially as a consequence of different methods of estimating soil parameters. Three methods of estimating soil parameters are used: (a) assuming uniform soil properties; (b) using SCS data presented as ranges; and (c) using alternative literature-based estimates of soil parameters. Results indicate that the wetness model responds similarly regardless of how the soil parameters are estimated, but differences in the spatial variability of the wetness potentials occur as a result of estimating soil parameters through alternative approaches. Correlation, pair-wise regression and analysis of regression residuals are used to compare model responses within a GIS environment.  相似文献   

MODELLING THE SENSITIVITY OF CHANNEL ADJUSTMENTS IN DESTABILIZED SAND-BED RIVERS     

STEPHEN E. DARBY  COLIN R. THORNE 《地球表面变化过程与地形》1996,21(12):1109-1125

Comprehensive empirical data of the response of unstable streams over a range of environmental conditions are unavailable. In this study, as a substitute for empirical data, a physically based numerical model of channel evolution is used in a range of numerical simulation experiments designed to predict the sensitivity of channel response to changes in control variables. The scope of the study is limited by the scope of the numerical model which applies to straight, sand-bed streams with cohesive bank materials that have been destabilized by sediment starvation and evolve towards equilibrium through bed degradation followed by channel widening. Results are presented for stable and unstable channel conditions. Stable channel depths are most sensitive to channel discharge, though the critical threshold shear stress for the entrainment of cohesive bank materials and discharge are both significant in determining the width. The sediment load, channel gradient, bank material cohesion, size of failed bank material aggregates and the initial bank height have sensitivities an order of magnitude smaller than discharge for both width and depth. Variations in bed material characteristics within the sand-size range are found to have little impact on simulated stable channel morphology. For unstable channels, the relative dominance of parameter sensitivities is examined in the context of an empirical-conceptual model of channel evolution proposed by Thorne and Osman (1988), to highlight the relationships between parameter dominance, time, and the processes and forms characterizing individual stages of channel evolution. Rates of change with time of width and depth sensitivity parameters for five tested independent variables (discharge, sediment supply, channel gradient, bank material cohesion and bed material size) are found to vary as a function of time, such that different stages of channel evolution are characterized by variations in the relative dominance of tested variables. The results support the hypothesis proposed by Thorne and Osman (1988) that the critical bank height required to initiate mass-wasting and widening may be regarded as a geomorphic threshold.  相似文献   

A study on assimilating potential vorticity data     

YONG LI  RICHARD MÉNARD  LARS PETER RIISHØJGAARD  STEPHEN E. COHN  RICHARD B. ROOD 《地球，A辑:动力气象学与海洋学》1998,50(4):490-506

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A multiproxy (micro‐XRF,pollen, chironomid and stable isotope) lake sediment record for the Lateglacial to Holocene transition from Thomastown Bog,Ireland          下载免费PDF全文

JONATHAN N. TURNER  NAOMI HOLMES  STEPHEN R. DAVIS  MELANIE J. LENG  CATHERINE LANGDON  ROBERT G. SCAIFE 《第四纪科学杂志》2015,30(6):514-528

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Early and late Holocene glacial fluctuations and tephrostratigraphy,Cabin Lake,Alaska     

PAUL D. ZANDER  DARRELL S. KAUFMAN  STEPHEN C. KUEHN  KRISTI L. WALLACE  R. SCOTT ANDERSON 《第四纪科学杂志》2013,28(8):761-771

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Magma Evolution and Open-System Processes at Shiveluch Volcano: Insights from Phenocryst Zoning   总被引：7，自引：0，他引：7  

HUMPHREYS  MADELEINE C. S.; BLUNDY  JON D.; SPARKS  R. STEPHEN J. 《Journal of Petrology》2006,47(12):2303-2334

Phenocryst zoning patterns are used to identify open-systemmagmatic processes in the products of the 2001 eruption of ShiveluchVolcano, Kamchatka. The lavas and pumices studied are hornblende–plagioclaseandesites with average pre-eruptive temperatures of 840°Cand fO₂ of 1·5–2·1 log units above nickel–nickeloxide (NNO). Plagioclase zoning includes oscillatory and patchyzonation and sieve textures. Hornblendes are commonly unzoned,but some show simple, multiple or patchy zoning. Apatite microphenocrystsdisplay normal and reverse zoning of sulphur. The textural similarityof patchy hornblende and plagioclase, together with Ba–Srsystematics in patchy plagioclase, indicate that the cores ofthese crystals were derived from cumulate material. Plagioclase–liquidequilibria suggest that the patchy texture develops by resorptionduring H₂O-undersaturated decompression. When H₂O-saturatedcrystallization recommences at lower pressure, reduced pH₂Oresults in lower X_An in plagioclase, causing more Al-rich hornblendeto crystallize. Plagioclase cores with diffuse oscillatory zoning,and unzoned hornblende crystals, probably represent a populationof crystals resident in the magma chamber for long periods oftime. In contrast, oscillatory zoning in the rims of plagioclasephenocrysts may reflect eruption dynamics during decompressioncrystallization. Increasing Fe/Al in oscillatory zoned rimssuggests oxidation as a result of degassing of H₂O during decompression.A general lack of textural overlap between phenocryst typessuggests that different phenocryst populations were spatiallyor temporally isolated during crystallization. We present evidencethat the host andesite has mixed with both more felsic and moremafic magmas. Olivine and orthopyroxene xenocrysts with reactionor overgrowth rims and strong normal zoning indicate mixingwith basalt. Sieve-textured plagioclase resulted from mixingof a more felsic magma with the host andesite. The mineralogyand mineral compositions of a mafic andesite enclave are identicalto those of the host magma, which implies efficient thermalquenching, and thus small volumes of intruding magma. Mixingof this magma with the host andesite results in phenocryst zoningbecause of differences in dissolved volatile contents. We suggestthat small magma pulses differentiated at depth and ascendedintermittently into the growing magma chamber, producing incrementalvariations in whole-rock compositions. KEY WORDS: patchy zoning; magma mixing; Shiveluch  相似文献   

New discoveries of the Vedde Ash in southern Sweden and Scotland     

STEFAN WASTEGÅRD  CHRIS S. M. TURNEY  J. JOHN LOWE  STEPHEN J. ROBERTS 《Boreas: An International Journal of Quaternary Research》2000,29(1):72-78

The Vedde Ash (c. 10 300 ¹⁴ C BP) provides a key time-parallel marker horizon within the Younger Dryas chronozone or GS-1 event of the GRIP stratigraphy. Until recently, the known distribution of wind-blown Vedde Ash outside Iceland was restricted to the west coast of Norway, off-shore sequences close to the Outer Hebrides and the Greenland summit GRIP ice core. The first discoveries of the Vedde Ash in Scotland were reported in 1997, following the development of a new technique for extracting rhyolitic micro-tephra particles from minerogenic deposits. Here we report on the discovery of the Vedde Ash at additional sites in Scotland and at sites in southern Sweden. The concentration of tephra particles in sediments is highest in sites in western Norway, but is also relatively high in sites in southwestern Sweden, suggesting that the main ash cloud travelled eastwards from its volcanic source of Katla, in southern Iceland. Electron microprobe analyses do not indicate any clear geochemical evolution within the samples reported here.  相似文献   

Cordilleran Ice Sheet lobal interactions and glaciotectonic superposition through stadial maxima along a mountain front in southwestern British Columbia, Canada     

STEPHEN R. HICOCK  OLAV B. LIAN 《Boreas: An International Journal of Quaternary Research》1999,28(4):531-542

Glaciotectonic structures in subglacial till and substrate, as well as stone fabric, provenance and surface features in till, indicate that complex interactions of late Wisconsinan glacial lobes occurred along a mountain front in the western Fraser Lowland of southwestern British Columbia. Tills of this study represent subglacial deposition through the maxima of two stades in the Fraser Glaciation, the Coquitlam and the Vashon. Through each stadial maximum, temperate glacial ice was grounded and commonly overrode proglacial outwash while superimposing deformations in subglacial till during three phases: (1) pre-maximum glacier flow down valleys and into lowland piedmont ice, (2) coalescent piedmont ice during stadial maxima when flow was westward along the mountain front and across valley mouths, and (3) post-maximum glacier flow down valleys into lowland piedmont ice but prior to general deglaciation. Valley glaciers appear to have shifted flow directions during phases 1 and 3. During stadial maxima (phase 2), Fraser Lowland piedmont ice may have been part of an outlet glacier-ice stream complex that terminated in salt water over the continental shelf.  相似文献   

Low‐dimensional representation of error covariance     

MICHAEL K. TIPPETT  STEPHEN E. COHN  RICARDO TODLING  DAN MARCHESIN 《地球，A辑:动力气象学与海洋学》2000,52(5):533-553

Ensemble and reduced‐rank approaches to prediction and assimilation rely on low‐dimensional approximations of the estimation error covariances. Here stability properties of the forecast/analysis cycle for linear, time‐independent systems are used to identify factors that cause the steady‐state analysis error covariance to admit a low‐dimensional representation. A useful measure of forecast/analysis cycle stability is the bound matrix , a function of the dynamics, observation operator and assimilation method. Upper and lower estimates for the steady‐state analysis error covariance matrix eigenvalues are derived from the bound matrix. The estimates generalize to time‐dependent systems. If much of the steady‐state analysis error variance is due to a few dominant modes, the leading eigenvectors of the bound matrix approximate those of the steady‐state analysis error covariance matrix. The analytical results are illustrated in two numerical examples where the Kalman filter is carried to steady state. The first example uses the dynamics of a generalized advection equation exhibiting non‐modal transient growth. Failure to observe growing modes leads to increased steady‐state analysis error variances. Leading eigenvectors of the steady‐state analysis error covariance matrix are well approximated by leading eigenvectors of the bound matrix. The second example uses the dynamics of a damped baroclinic wave model. The leading eigenvectors of a lowest‐order approximation of the bound matrix are shown to approximate well the leading eigenvectors of the steady‐state analysis error covariance matrix.  相似文献   

Laurentide and Cordilleran fast ice flow: some sedimentological evidence from Wisconsinan subglacial till and its substrate     

OLAV B. LIAN  STEPHEN R. HICOCK  ALEKSIS DREIMANIS 《Boreas: An International Journal of Quaternary Research》2003,32(1):102-113

With the aid of three case studies we summarize sedimentological and structural evidence in Wisconsinan subglacial till and its substrate that can be used to infer the presence of former fast-moving ice. We compare sedimentological characteristics of till with its stone fabric, stone surface erosional features and orientations of fractures, faults and folds in till and substrate, in order to interpret the processes that formed the till. In the cases we studied, stones in tills that experienced ductile deformation (inferred to be associated with fast-moving ice) show variable alignment of latest surface striae. Moreover, striae are inconsistent with the orientations of lee ends and chattermarks on the same stones, and with the dip directions of shallow shear planes and (or) steeper tension fractures in the till matrix. These observations indicate that stones were rotating in a flowing till matrix prior to their final emplacement. Ductile shear of fine-textured, water-saturated till and Jeffrey-like rotation of stones are invoked where most of the following occur together: (i) delicate striae follow the curvature of stone surfaces; (ii) a-axis fabrics are multimodal to girdle-like; (iii) stone a axis eigenvectors, striae orientation and stone lee end alignments fall between the modes of stone fabrics; (iv) stone pavements are uneven with vertically oriented and inverted bullet-shaped stones; (v) soft sediment clasts and (or) fragile fossils are preserved.  相似文献