Analysis of the impact of conservation measures on stream flow regime in catchments of the Loess Plateau,China   总被引：3，自引：0，他引：3  

Xingmin Mu  Lu Zhang  Tim R. McVicar  Basang Chille  Peng Gau 《水文研究》2007,21(16):2124-2134

Catchments in the Loess Plateau have been under the influence of human activities for centuries. In the last four decades, soil conservation measures have accelerated and intensified. These measures were designed to reduce soil erosion, improve agricultural productivity, and enhance environmental quality. It is important to evaluate the effects of these measures on hydrology in order to develop sustainable catchment management plans in the region. This study evaluated changes in stream flow data for four selected catchments in the Loess Plateau following large‐scale soil conservation measures. The non‐parametric Mann–Kendall test was used to identify trends in annual stream flow and the results showed significant downward trends in three of the four catchments. The Pettitt test indicated that a change point occurred in 1978 in these three catchments. Annual precipitation in all the catchments showed no significant trend during the period of record. Comparison of daily flow duration curves for two 20‐year periods (1957–1978) and (1979–2003) showed significant changes in stream flow regime. Reduction in most percentile flows varied between 20 and 45%, and the reduction in low flows was greatest. Overall, the reductions in daily flow were increasing with time, with significant changes occurring in the 1990s. However, it is not clear whether these catchments have seen the full effects of the soil conservation measures, so the results of this study might underestimate the final impact of soil conservation on stream flow regime. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

The problem of underpowered rivers     

Fred Worrall  Tim P. Burt  Gregory R. Hancock  Nicholas J.K. Howden  John Wainwright 《地球表面变化过程与地形》2020,45(15):3869-3878

This study has hypothesized that for many rivers the trade-off between flow accumulation and the decrease in slope along channel length means that stream power increases downstream and, moreover, that given the low slope angles in headwater and low-order streams, they would have insufficient stream power to erode let alone transport sediment. The study considered the stream power profile, the particle travel distances and the application of the Hjulström curve based on the velocity profile of nine, large UK catchments. The study showed that:

1. Some rivers never showed a maximum in their longitudinal stream power profile, implying that some rivers never develop a deposition zone before they discharge at the tidal limit.
2. Particle travel distances during a bankfull discharge event showed that for some rivers 91% of the upper main channel would not be cleared of sediment. Furthermore, while some rivers could transport a 2 mm particle their entire length in one bankfull event, for another river it would take 89 such events.
3. The Hjulström curve shows that for three of the study rivers the upper 20 km of the river was not capable of eroding a 2 μm particle.
4. The study has shown that for all rivers studied, erosion is focused downstream and deposition upstream. Many UK rivers have a dead zone where, on time scales of the order of centuries, no erosion or transport occurs and erosion only occurs in the lower courses of the channel where discharge rather than slope dominates – we propose these as underpowered rivers.

© 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd  相似文献   

An optimal extraction algorithm for imaging photometry     

Tim Naylor 《Monthly notices of the Royal Astronomical Society》1998,296(2):339-346

This paper is primarily an investigation of whether the 'optimal extraction' techniques used in CCD spectroscopy can be applied to imaging photometry. It is found that using such techniques provides a gain of around 10 per cent in signal-to-noise ratio over normal aperture photometry. Formally, it is shown to be equivalent to profile fitting, but offers advantages of robust error estimation, freedom from bias introduced by mis-estimating the point spread function, and convenience. In addition some other techniques are presented, which can be applied to profile fitting, aperture photometry and the 'optimal' photometry. Code implementing these algorithms is available at http://www.astro.keele.ac.uk/~timn/.  相似文献   

A Discussion of IGS Solutions and Their Impact on Geodetic and Geophysical Applications   总被引：1，自引：0，他引：1  

Jan Kouba  Yves Mireault  Gerhard Beutler  Tim Springer  Gerd Gendt 《GPS Solutions》1998,2(2):3-15

The International Association of Geodesy officially established the International GPS Service (IGS) on Janaury 1, 1994. Its prime objective is to provide support and a rerefence system for a wide variety of scientific and practical applications involving GPS. To fulfill its role the IGS also generates, in addition to its fundamental products (orbital/staion positions and consistent Earth orientation parameters), additional reference-system products providing the necessary infrastructure, standards, and means of calibrations for timing and various atmospheric applications of GPS. The generation and efficient application of IGS products and their impact on a number of positioning and atmospheric applications, including low earth orbit satellites, is reviewed and discussed. @ 1998 John Wiley & Sons, Inc.  相似文献   

U-Series Chronology of Lacustrine Deposits in Death Valley, California   总被引：1，自引：0，他引：1  

Teh-Lung Ku  Shangde Luo  Tim K. Lowenstein  Jianren Li  Ronald J. Spencer 《Quaternary Research》1998,50(3):261-275

Uranium-series dating on a 186-m core (DV93-1) drilled from Badwater Basin in Death Valley, California, and on calcareous tufas from nearby strandlines shows that Lake Manly, the lake that periodically flooded Death Valley during the late Pleistocene, experienced large fluctuations in depth and chemistry over the last 200,000 yr. Death Valley has been occupied by a long-standing deep lake, perennial shallow saline lakes, and a desiccated salt pan similar to the modern valley floor. The average sedimentation rate of about 1 mm/yr for core DV93-1 was punctuated by episodes of more-rapid accumulation of halite. Arid conditions similar to the modern conditions prevailed during the entire Holocene and between 120,000 and 60,000 yr B.P. From 35,000 yr B.P. to the beginning of the Holocene, a perennial saline lake existed, over 70 m at its deepest. A much deeper and longer lasting perennial Lake Manly existed from about 185,000 to 128,000 yr B.P., with water depths reaching about 175 m, if not 330 m. This lake had two significant “dry” excursions of 10²–10³yr duration about 166,000 and 146,000 yr B.P., and it began to shrink to the point of halite precipitation between 128,000 and 120,000 yr B.P. The two perennial lake periods correspond to marine oxygen isotopic stages (OIS) 2 and 6. Based on the shoreline tufa ages, we do not rule out the possible existence 200,000 yr ago of yet a third perennial lake comparable in size to the OIS 6 lake. The²³⁴U/²³⁸U data suggest that U in tufa owes its origin mainly to Ca-rich springs fed by groundwater that emanated along lake shorelines in southern Death Valley, and that an increase of this spring-water input relative to the river-water input apparently occurred during OIS 6.  相似文献   

A Polarization Study of Radio Galaxies and Quasars selected from the Molonglo Complete Sample     

C. H. Ishwara-Chandra  D. J. Saikia  V. K. Kapahi  & P. J. McCarthy 《Monthly notices of the Royal Astronomical Society》1998,300(1):269-286

We present total intensity and linear polarization observations made with the Very Large Array at λλ20 and 6 cm of a representative sample of 42 radio galaxies and quasars selected from the Molonglo complete sample. The sources have been chosen to be of large size to probe the depolarizing medium on these scales using our present data and later with observations at lower frequencies with the Giant Metrewave Radio Telescope. The λ20 and 6 cm data are of similar resolutions and show that depolarization between these two wavelengths is seen largely only in those lobes which are within about 300 kpc of the parent galaxy. Examination of the depolarization of the lobes with arm-length asymmetry shows that depolarization is observed predominantly for the lobe which is closer to the nucleus. There is also a trend for the lobe closer to the nucleus to be brighter, consistent with the scenario that the nearer lobe is interacting with a denser environment which is responsible for the higher depolarization and greater dissipation of energy. We have also examined the depolarization asymmetry of the lobes on opposite sides of the nucleus for galaxies and quasars. This shows that the depolarization asymmetry for quasars is marginally higher than that for galaxies. The depolarization properties of our sample are possibly determined by an asymmetric environment as well as the effects of orientation.  相似文献   

The Blue Tier Batholith,Northeastern Tasmania     

T. S. McCarthy  D. I. Groves 《Contributions to Mineralogy and Petrology》1979,71(2):193-209

The Blue Tier Batholith is one of a number of high-level, essentially postkinematic, composite granitoid bodies occurring at the southern end of the Tasman orogenic belt of Eastern Australia.An integrated study of the structure, texture, and geochemistry of the batholith suggests that it has a cumulate-like character. In particular, the trace element (Ba, Rb, Sr) data, when constrained by textural features of the granitiods, indicate that the batholith formed by fractional crystallization of a single magma which underwent crystallization in situ by progressive nucleation and solidification from the roof, walls, and floor inwards. Progressive changes in liquids (cumulate) mineralogy during crystallization led to the observed sequence of early biotite and/or hornblende granodiorites followed by biotite adamellites and late muscovite biotite granites. Progressive in situ crystallization led in some instances to gradational boundaries between granitoid types whereas periodic tectonic distrubances caused the rest magma to reintrude earlier crystallizates in places: thus emplacement and crystallization sequences are parallel. The ultimate product of fractional crystallization was a water-saturated melt, enriched in incompatible elements, whose crystallization resulted in significant tin mineralization.The chemistry of the rocks comprising the batholith is in many respects analogous to that of basic cumulate rocks, although an origin by outward growth of crystals and expulsion of interstitial melt, coupled with convective mixing, rather than by crystal settling, is favoured for the granitoid suite. It is suggested that the Blue Tier Batholith is not an isolated example of a granitoid body with cumulate-like character, but that such bodies may be more common than is recognized.  相似文献   

Variations in Cr content of magnetite from the upper zone of the Bushveld Complex — evidence for heterogeneity and convection currents in magma chambers     

R. Grant Cawthorn  T.S. McCarthy 《Earth and Planetary Science Letters》1980,46(3):335-343

Mineral separates of pure magnetite from the upper zone of the Bushveld Complex have been analysed for Cr. Detailed sampling within layers of massive magnetite indicates that the Cr content frequently shows an extremely rapid but regular depletion with height (for example, by a factor of seven over 85 cm of massive magnetite), with sudden reversals of variable magnitude. Rayleigh Law fractionation, from a homogeneous liquid of the volume now seen as cumulate rocks overlying the sampled horizons, cannot satisfy these observed concentration gradients. A diffusion-controlled crystallization model is preferred. In another vertical section of massive magnetite the Cr content remains constant with increasing height, and is interpreted as indicating a steady state situation where the rate of depletion of Cr by magnetite fractionation is balanced by the rate of diffusive addition into the crystallization zone. Reversals in Cr content sometimes occur in the middle of pure magnetite layers and are attributed to convective overturn in the magma chamber. These results provide geochemical support for the model of convection cells and bottom crystallization in large magma reservoirs as proposed by Jackson [20]. Depletion of compatible elements in this bottom layer causes chemical inhomogeneity in the magma.Sections, composed of several layers of magnetite sandwiched between magnetite-bearing gabbro, have also been studied. In one case, a steady decrease in Cr content with increasing height in the separated magnetite was observed; in another, several irregular reversals were found. There appears to be no systematic relation between convection cycles, the depletion in Cr and the formation of pure magnetite layers, indicating that the chemical composition of the magma does not control the production of monomineralic layers. The fluctuating pressure model envisaged by Cameron [16] for the formation of such layers is entirely consistent with these data.  相似文献   

Pleistocene climatic change in Southern Australia and its effect on speleothem deposition in some Nullarbor caves     

Albert Coede  Russell S. Harmon  Tim C. Atkinson  Peter J. Rowe 《第四纪科学杂志》1990,5(1):29-38

Activity ratios of ²³⁴U/²³⁸U, ²³⁰Th/²³⁴U, and ²³⁰Th/²³²Th have been determined for calcite, gypsum and halite speleothems from caves of the Nullarbor Plain, mostly in the area N and NW of Mundrabilla Station, for the purpose of U-series dating. All calcite speleothems contain adequate amounts of uranium for dating, but some show an excess of ²³⁰Th. Stratigraphic relationships indicate that there were at least three phases of calcium carbonate deposition in the Nullarbor caves. The calcite samples, with one possible exception, have ages in excess of ca. 400000 yrs BP. This suggests that no significant amounts of calcium carbonate deposition have taken place during the last 400ka. At present, active deposition of speleothems is restricted almost entirely to gypsum and halite. The only gypsum speleothem dated was found to have a finite age of ca. 185 ka. Six dates on a small halite speleothem containing insect and arachnid remains indicate that it formed rapidly during Holocene time.  相似文献   

Catastrophic landslides,glacier behaviour and moraine formation – A view from an active plate margin     

James Shulmeister  Tim R. Davies  David J.A. Evans  Olivia M. Hyatt  Daniel S. Tovar 《Quaternary Science Reviews》2009,28(11-12):1085-1096

The influence of large bedrock landslides (“rock avalanches”) on the behaviour of glaciers is incompletely recognised. Here we present an example from an active tectonic margin in South Island, New Zealand where large earthquakes leave a significant imprint on glacial records. We demonstrate that terminal moraines on the western side of the Southern Alps record both ‘ordinary’ (i.e. climate-driven) and landslide-initiated glacial advances. Following consideration of the processes involved in rock avalanche-initiated moraine construction we suggest ways of determining the nature of the advance that built the terminal moraine. The implications of these observations are important in breaking the conventional linkage of individual terminal moraines with climate forcing.  相似文献