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151.
Keith M. Clayton 《地球表面变化过程与地形》1997,22(8):721-731
Developments in dating techniques applicable to the late Tertiary and Quaternary are giving us the ability to date past land surfaces. Where reasonable assumptions about the nature of such past surfaces and their partial preservation may be made, they can be reconstructed. This permits the contouring and measurement of the subsequent dissection, allowing not only calculation of the average rate of erosion over the elapsed time, but also information on the pattern of incision. Two examples where this has been attempted are present; both are dissected till surfaces in eastern England, one of Anglian and the other of Devensian age. The approach quantifies the disparity between the incision of valleys and the general denudational lowering of the surface which characterizes many landscapes. The technique is not only of academic interest, but potentially forms a useful line of approach to the assessment of the safety of the burying toxic wastes. © 1997 by John Wiley & Sons, Ltd. 相似文献
152.
153.
ABSTRACTCharacterizing, understanding and better estimating uncertainties are key concerns for drawing robust conclusions when analyzing changing socio-hydrological systems. Here we suggest developing a perceptual model of uncertainty that is complementary to the perceptual model of the socio-hydrological system and we provide an example application to flood risk change analysis. Such a perceptual model aims to make all relevant uncertainty sources – and different perceptions thereof – explicit in a structured way. It is a first step to assessing uncertainty in system outcomes that can help to prioritize research efforts and to structure dialogue and communication about uncertainty in interdisciplinary work. 相似文献
154.
Changes in monthly baseflow across the U.S. Midwest 总被引:1,自引:0,他引:1
Characterizing streamflow changes in the agricultural U.S. Midwest is critical for effective planning and management of water resources throughout the region. The objective of this study is to determine if and how baseflow has responded to land alteration and climate changes across the study area during the 50‐year study period by exploring hydrologic variations based on long‐term stream gage data. This study evaluates monthly contributions to annual baseflow along with possible trends over the 1966–2016 period for 458 U.S. Geological Survey streamflow gages within 12 different Midwestern states. It also examines the influence of climate and land use factors on the observed baseflow trends. Monthly contribution breakdowns demonstrate how the majority of baseflow is discharged into streams during the spring months (March, April, and May) and is overall more substantial throughout the spring (especially in April) and summer (June, July, and August). Baseflow has not remained constant over the study period, and the results of the trend detection from the Mann–Kendall test reveal that baseflows have increased and are the strongest from May to September. This analysis is confirmed by quantile regression, which suggests that for most of the year, the largest changes are detected in the central part of the distribution. Although increasing baseflow trends are widespread throughout the region, decreasing trends are few and limited to Kansas and Nebraska. Further analysis reveals that baseflow changes are being driven by both climate and land use change across the region. Increasing trends in baseflow are linked to increases in precipitation throughout the year and are most prominent during May and June. Changes in agricultural intensity (in terms of harvested corn and soybean acreage) are linked to increasing trends in the central and western Midwest, whereas increasing temperatures may lead to decreasing baseflow trends in spring and summer in northern Wisconsin, Kansas, and Nebraska. 相似文献
155.
Simulation of scree‐slope dynamics: investigating the distribution of debris avalanche events in an idealized two‐dimensional model 下载免费PDF全文
We present a two‐dimensional model of the development of scree slopes using the discrete‐element method. We concentrate on the dynamics of the accumulating debris at the cliff foot rather than on the failure modes of the cliff‐face or shape of the underlying rock surface. The evolution of this unconsolidated material is intermittent and systematically changing over time, with an early high disturbance regime, dominated by a characteristic event size (where 65% of particles in the debris are in motion to some extent), replaced at later times by many shallow slides interspersed with infrequent large events that involve motion through almost the full scree depth. These large slides lead to a stratigraphy in which the layers of material are stretched almost horizontal near the base of the slope. The scree surface thus shows a gradient in age with most recent rock‐fall close to the cliff and the oldest rock‐fall debris outcropping at the foot. The final surface slope tends to show little curvature, and the final mean slope is well correlated with the angle of internal friction of the particles, although the change is very small over a wide range of friction angles [final slope (in degrees relative to horizontal) ~ 0.043 × internal friction angle + 17.49, with a correlation coefficient of 0.89, p‐value 0.0001]. Some weak size‐segregation of the debris is found, but this seems to have little to do with individual particles bounding down the slope. The shape of the rock core agrees largely with the analytic forms given by Fisher–Lehmann and Bakker–Le Heux expressions, but the original simple Fisher quadratic can give the best fit. Overall the evolution shows a remarkable insensitivity to the model parameters, suggesting that the controls on dry scree‐slope evolution are primarily geometric in character. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
156.
Quantifying the rate and depth dependence of bioturbation based on optically‐stimulated luminescence (OSL) dates and meteoric 10Be 下载免费PDF全文
Michelle O. Johnson Simon M. Mudd Brad Pillans Nigel A. Spooner L. Keith Fifield Mike J. Kirkby Manuel Gloor 《地球表面变化过程与地形》2014,39(9):1188-1196
Both the rate and the vertical distribution of soil disturbance modify soil properties such as porosity, particle size, chemical composition and age structure; all of which play an important role in a soil's biogeochemical functioning. Whereas rates of mixing have been previously quantified, the nature of bioturbation's depth dependence remains poorly constrained. Here we constrain, for the first time, the relationship between mixing rate and depth in a bioturbated soil in northeast Queensland, Australia using a novel method combining OSL (optically‐stimulated luminescence) ages and meteoric beryllium‐10 (10Be) inventories. We find that the best fit mixing rate decreases non‐linearly with increasing soil depth in this soil and the characteristic length scale of 0.28 m over which the mixing coefficient decays is comparable to reported rooting depth coefficients. In addition we show that estimates of surface mixing rates from OSL data are highly dependent on erosion rate and that erosion rate must be constrained if accurate mixing rates are to be quantified. We calculate surface diffusion‐like mixing coefficients of 1.8 × 10?4 and 2.1 × 10?4 m2 yr?1 for the studied soil for two different estimates of soil erosion. © 2014 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd. 相似文献
157.
Aftershock sequence simulations using synthetic earthquakes and rate-state seismicity formulation 下载免费PDF全文
Heming Xu Yifeng Cui James H. Dieterich Keith Richards-Dinger Efecan Poyraz Dong Ju Choi 《地震科学(英文版)》2014,27(4):401-410
We use an efficient earthquake simulator that incorporates rate-state constitutive properties and uses boundary element method to discretize the fault surfaces, to generate the synthetic earthquakes in the fault system. Rate-and-state seismicity equation is subsequently employed to calculate the seismicity rate in a region of interest using the Coulomb stress transfer from the main shocks in the fault system. The Coulomb stress transfer is obtained by resolving the induced stresses due to the fault patch slips onto the optimal-oriented fault planes. The example results show that immediately after a main shock the aftershocks are concentrated in the vicinity of the rupture area due to positive stress transfers and then disperse away into the surrounding region toward the background rate distribution. The number of aftershocks near the rupture region is found to decay with time as Omori aftershock decay law predicts. The example results demonstrate that the rate-and-state fault system earthquake simulator and the seismicity equations based on the rate-state friction nucleation of earthquake are well posited to characterize the aftershock distribution in regional assessments of earthquake probabilities. 相似文献
158.
Daniel Leduc Ashley A. Rowden Conrad A. Pilditch Elizabeth W. Maas P. Keith Probert 《Marine Ecology》2013,34(3):334-344
Studying the diversity‐ecosystem function relationship in the deep sea is of primary importance in the face of biodiversity loss and for our understanding of how the deep sea functions. Results from the first study of diversity‐ecosystem function relationships in the deep sea (Danovaro et al. 2008; Current Biology, 18, 1–8) are unexpected and show an exponential relationship between deep‐sea nematode diversity and ecosystem function and efficiency, although this relationship appears largely restricted to relatively low diversities [ES(51) <25]. Here, we investigate the relationship between nematode diversity and several independent measures/proxies of ecosystem function (sediment community oxygen consumption, bacterial biomass, bacterial extracellular enzyme activity) and efficiency (ratio of bacterial/nematode carbon to organic C content of the sediment) on the New Zealand continental slope. Nematode diversity at our study sites was relatively high [ES(51) = 30–42], and there was no relationship between species/functional diversity and ecosystem function/efficiency after accounting for the effects of water depth and food availability. Our results are consistent with a breakdown of the exponential diversity‐function relationship at high levels of diversity, which may be due to increased competition or greater functional redundancy. Future studies need to take into account as many environmental factors and as wide a range of diversities as possible to provide further insights into the diversity‐ecosystem function relationship in the largest ecosystem on Earth. 相似文献
159.
Wither A Bamber R Colclough S Dyer K Elliott M Holmes P Jenner H Taylor C Turnpenny A 《Marine pollution bulletin》2012,64(8):1564-1579
The construction of a new generation of coastal power stations in the UK and other western European countries has highlighted the absence of robust standards for thermal discharges to transitional and coastal waters. The proposed discharge volumes are greater than hitherto, yet there has been little independent critical examination of their potential impact, whilst much of the existing guidance has been adapted from freshwater practice. This review considers the available knowledge on the tolerance and behaviour of fish and other marine biota to heated effluents. Appropriate ways are suggested of grouping fish species to reflect their sensitivity to thermal effects. The plethora of existing standards are considered and their validity assessed in a framework of predicted seawater temperature rise. Those species or groups of organisms most likely to be affected are identified and finally specific recommendations for thermal standards consistent with long term sustainability are proposed. 相似文献
160.
The water vapour continuum is characterised by absorption that varies smoothly with wavelength, from the visible to the microwave. It is present within the rotational and vibrational–rotational bands of water vapour, which consist of large numbers of narrow spectral lines, and in the many ‘windows’ between these bands. The continuum absorption in the window regions is of particular importance for the Earth’s radiation budget and for remote-sensing techniques that exploit these windows. Historically, most attention has focused on the 8–12?μm (mid-infrared) atmospheric window, where the continuum is relatively well-characterised, but there have been many fewer measurements within bands and in other window regions. In addition, the causes of the continuum remain a subject of controversy. This paper provides a brief historical overview of the development of understanding of the continuum and then reviews recent developments, with a focus on the near-infrared spectral region. Recent laboratory measurements in near-infrared windows, which reveal absorption typically an order of magnitude stronger than in widely used continuum models, are shown to have important consequences for remote-sensing techniques that use these windows for retrieving cloud properties. 相似文献