The ratio between magnetotail height and neutral point height     

Zhen-Jia Wu  Keith D. Cole 《Planetary and Space Science》1982,30(7):711-713

The ratio between the magnetotail height and the neutral point height is exactly 2 in the classical calculation of the shape of the magnetopause following Chapman and Ferraro. This ratio is useful in assessing the results of calculations. There exist possibilities for improvement of magnetopause shape calculations, especially in the tail region or in the neutral point region for “tilted” cases.  相似文献   

Continental shelf benthos off Otago Peninsula,New Zealand     

P. Keith Probert  John B. Wilson 《Estuarine, Coastal and Shelf Science》1984,19(3):373-391

Benthic macrofauna of the continental shelf off Otago Peninsula, south-eastern New Zealand (45°51′S, 170°52′E) was surveyed by dredge sampling during 1973–1975. Numerical classification (Canberra metric coefficient and flexible sorting) was used to produce site groups and species groups, and three major benthic communities were recognised: a shallow-water (14–25 m) fauna inhabiting well-sorted fine sand, a mid-shelf fauna (concentrated in the depth range 50–76 m) associated with sediments containing the greatest proportions of gravel and siltclay, and a predominantly sand-bottom fauna occurring mainly on the outer shelf (87–150 m). All station groups were dominated numerically by polychaetes (mean of 36·6–56% of individuals) with Mollusca (13·8–25%) or Crustacea (12·1–19·4%) the next most abundant group.The inshore sand fauna was the most distinct, characteristic elements being the trochid gastropod Antisolarium egenum, an amphipod of the genus Hippomedon and dense patches of the spionid polychaete Spiophanes bombyx. Diagnostic species of the mid-shelf mixed sediments were Lepidonotus jacksoni, Psammolyce antipoda, Lumbrineris brevicirra and Phyllamphicteis foliata (Polychaeta), Terenochiton otagoensis, Micrelenchus caelatus caelatus, Maoricolpus roseus roseus and Zegalerus tenuis (Mollusca), Ampelisca chiltoni (Amphipoda) and Amphipholis squamata (Ophiuroidea). Outer shelf sand stations were faunally less distinct, but among the more characteristic species were Euthalenessa fimbriata, Sigalion sp. and Euchone sp. (Polychaeta) and Gari stangeri (Bivalvia). Several abundant species were widely distributed among station groups, notably Nephtys macroura, Lumbrineris magalhaensis, Phyllochaetopterus socialis and Owenia fusiformis (Polychaeta) and Nucula nitidula and Tawera spissa (Bivalvia).Free-living lunulitiform Bryozoa of the genus Otionella were a characteristic component of inner and outer shelf sand faunas, and their inshore penetration probably marks the outer limit of a turbulent zone.Species diversity (Margalef's d) varied markedly across the shelf and appeared to be related primarily to the degree of sediment heterogeneity. Mixed sediments of the middle shelf were particularly rich in species.  相似文献   

Aboveground and belowground productivity ofSpartina alterniflora (Smooth Cordgrass) in natural and created Louisiana salt marshes     

Keith R. Edwards  Kaili P. Mills 《Estuaries and Coasts》2005,28(2):252-265

In Louisiana, salt marshes are being created in an effort to offset the large loss of such habitat that has occurred over the last 50 yr. Primary productivity is an important function and indicator of success for salt marsh creation and restoration projects. The aim of this study was to determine whether the aboveground and belowground productivity of the dominant salt marsh grassSpartina alterniflora in created marshes in southwest Louisiana began to approximate productivity levels in natural marshes, over time. Net annual aboveground primary productivity (NAPP) was measured by a harvest technique, while the ingrowth core method was used to estimate net annual belowground primary productivity (NBPP). NAPP levels were similar to those found in other, Louisiana salt marshes, while NBPP levels were similar to or higher than the reported range forS. alterniflora studied along the Atlantic and Gulf of Mexico coasts. NAPP tended to decrease as the created marshes aged, but the levels in the oldest, 19 year old, created marsh were still well above values measured in the, natural marshes. It was estimated that it would take 35 yr after marsh creation for NAPP in the created marshes to become equivalent to that in natural marshes. NBPP in the created marshes became equivalent to levels found in the natural marshes after 6–8 yr, but then belowground production increased with marsh age, reaching an asymptote that surpassed natural marsh levels. Equivalency in primary productivity has not been reached in these marshes. Elevation also affected productivity, as higher elevational sites with greater topographic heterogeneity had significantly lower aboveground and belowground biomass levels than those with elevations closer to mean sea level. This underscores the need to construct marshes so that their mean elevation and degree of topographic heterogeneity are similar to natural marshes.  相似文献   

A comparison of a GCM response to historical anthropogenic land cover change and model sensitivity to uncertainty in present-day land cover representations   总被引：5，自引：1，他引：5  

Johannes Feddema  Keith Oleson  Gordon Bonan  Linda Mearns  Warren Washington  Gerald Meehl  Douglas Nychka 《Climate Dynamics》2005,25(6):581-609

This study assesses the sensitivity of the fully coupled NCAR-DOE PCM to three different representations of present-day land cover, based on IPCC SRES land cover information. We conclude that there is significant model sensitivity to current land cover characterization, with an observed average global temperature range of 0.21 K between the simulations. Much larger contrasts (up to 5 K) are found on the regional scale; however, these changes are largely offsetting on the global scale. These results show that significant biases can be introduced when outside data sources are used to conduct anthropogenic land cover change experiments in GCMs that have been calibrated to their own representation of present-day land cover. We conclude that hybrid systems that combine the natural vegetation from the native GCM datasets combined with human land cover information from other sources are best for simulating such impacts. We also performed a prehuman simulation, which had a 0.39 K ~higher average global temperature and, perhaps of greater importance, temperature changes regionally of about 2 K. In this study, the larger regional changes coincide with large-scale agricultural areas. The initial cooling from energy balance changes appear to create feedbacks that intensify mid-latitude circulation features and weaken the summer monsoon circulation over Asia, leading to further cooling. From these results, we conclude that land cover change plays a significant role in anthropogenically forced climate change. Because these changes coincide with regions of the highest human population this climate impact could have a disproportionate impact on human systems. Therefore, it is important that land cover change be included in past and future climate change simulations.  相似文献   

Toward Optimal Calibration of the SLEUTH Land Use Change Model   总被引：3，自引：0，他引：3  

Charles Dietzel  Keith C Clarke 《Transactions in GIS》2007,11(1):29-45

SLEUTH is a computational simulation model that uses adaptive cellular automata to simulate the way cities grow and change their surrounding land uses. It has long been known that models are of most value when calibrated, and that using back‐casting (testing against known prior data) is an effective calibration method. SLEUTH's calibration uses the brute force method: every possible combination and permutation of its control parameters is tried, and the outcomes tested for their success at replicating prior data. Of the SLEUTH calibration approaches tried so far, there have been several suggested rules to follow during the brute force procedure to deal with problems of tractability, most of which leave out many of the possible parameter combinations. In this research, we instead attempt to create the complete set of possible outcomes with the goal of examining them to select the optimum from among the millions of possibilities. The self‐organizing map (SOM) was used as a data reduction method to pursue the isolation of the best parameter sets, and to indicate which of the existing 13 calibration metrics used in SLEUTH are necessary to arrive at the optimum. As a result, a new metric is proposed that will be of value in future SLEUTH applications. The new measure combines seven of the current measures, eight if land use is modeled, and is recommended as a way to make SLEUTH applications more directly comparable, and to give superior modeling and forecasting results.  相似文献   

Women’s pay in English rural districts     

Keith Hoggart  Shen Cheng 《Geoforum》2006,37(2):287-306

This paper examines hourly wage payments for women who live in the rural districts of England, both in terms of variability within such districts and in terms of comparisons with non-rural districts. Drawing on the Labour Force Survey’s Local Annual Database, the paper argues that, once education, industry and occupation are taken into account, there is little to distinguish wage rates in rural and urban districts. Regionally there are important differences in wage rates, but these apply to rural as well as urban districts and are largely confined to contrasts between South East England and elsewhere. Rural-urban contrasts in wage levels are apparent in pay differences for men and women, although even here distinctions along a rural-urban divide are restricted to managerial workers, with the key distinction for professionals and technicians being between South East England and the rest of the country. For other occupations there is little to distinguish the relative magnitude of difference in men’s and women’s pay along rural-urban or regional lines. It is suggested that more attention to gender relations in wage determination practices within the workplace and within professions are worth exploring to develop a stronger understanding of division lines in wage inequalities.  相似文献   

Multimode surface waveform tomography of the Pacific Ocean: a closer look at the lithospheric cooling signature   总被引：2，自引：0，他引：2  

Alessia Maggi  Eric Debayle  Keith Priestley  Guilhem Barruol 《Geophysical Journal International》2006,166(3):1384-1397

We present a regional surface waveform tomography of the Pacific upper mantle, obtained using an automated multimode surface waveform inversion technique on fundamental and higher mode Rayleigh waves, to constrain the   V_SV   structure down to ∼400 km depth. We have improved on previous implementations of this technique by robustly accounting for the effects of uncertainties in earthquake source parameters in the tomographic inversion. We have furthermore improved path coverage in the South Pacific region by including Rayleigh wave observations from the French Polynesian Pacific Lithosphere and Upper Mantle Experiment deployment. This improvement has led to imaging of vertical low-velocity structures associated with hotspots within the South Pacific Super-Swell region. We have produced an age-dependent average cross-section for the Pacific Ocean lithosphere and found that the increase in   V_SV   with age is broadly compatible with a half-space cooling model of oceanic lithosphere formation. We cannot confirm evidence for a Pacific-wide reheating event. Our synthetic tests show that detailed interpretation of average   V_SV   trends across the Pacific Ocean may be misleading unless lateral resolution and amplitude recovery are uniform across the region, a condition that is difficult to achieve in such a large oceanic basin with current seismic stations.  相似文献   

Inadvertent alkalization of a Florida lake caused by increased ionic and nutrient loading to its watershed     

Thomas J. Whitmore  Mark Brenner  Keith V. Kolasa  William F. Kenney  Melanie A. Riedinger-Whitmore  Jason H. Curtis  Joseph M. Smoak 《Journal of Paleolimnology》2006,36(4):353-370

We evaluate the effects of land-use change since c.1890 on Little Lake Jackson in south-central Florida, USA. The lake currently is alkaline despite the prevalence of acidic lakes in the region. Watershed soils are acidic and poorly drained, but are underlain by limestone bedrock. Limnetic pH inferences, based on weighted-averaging tolerance regression of sedimented diatoms, indicate that Little Lake Jackson became significantly alkalized during the 1900s. Two driving forces that appear to be responsible for water-quality change are increased ionic loading and increased nutrient loading. Golf courses and residential lawns in the watershed receive substantial applications of lime, fertilizer, and irrigation with alkaline waters from deep wells, some of which reaches the lake in channelized runoff. Stormwater runoff and septic leachate also contribute to nutrient and solute loading. Sedimentary total P accumulation increased 5-fold and total N accumulation increased 3-fold since c. 1890. δ¹⁵N values suggest agricultural and septic sources for N loading. Sedimented pigments, inferred limnetic chlorophyll a values, and δ¹³C values of organic matter indicate that increased primary productivity occurred. Surface and subsurface inflow is nutrient-rich but low in hardness. Increased cation deposition in sediments indicates that ionic input might have reduced the lake’s natural resistance to alkalization. Lake waters remain low in ionic content, which suggests that the addition of base from carbonate sources is not responsible for all of the observed alkalization. Acid neutralization might have been facilitated by phosphate loading that led to increased base generation through greater nitrate assimilation. Inadvertent alkalization might occur commonly in regions where poorly buffered lakes are subject to significant ionic and nutrient loading from agriculture, turfgrass, and septic sources in their watersheds.  相似文献   

The use of short‐lived radionuclides to quantify transitional bed material transport in a regulated river     

Nira L. Salant  Carl E. Renshaw  Francis J. Magilligan  James M. Kaste  Keith H. Nislow  Arjun M. Heimsath 《地球表面变化过程与地形》2007,32(4):509-524

We investigate the use of the short‐lived fallout radionuclide beryllium‐7 (⁷Be; t_1/2 = 53·4 days) as a tracer of medium and coarse sand (0·25–2 mm), which transitions between transport in suspension and as bed load, and evaluate the effects of impoundment on seasonal and spatial variations in bed sedimentation. We measure ⁷Be activities in approximately monthly samples from point bar and streambed sediments in one unregulated and one regulated stream. In the regulated stream our sampling spanned an array of flow and management conditions during the annual transition from flood control in the winter and early spring to run‐of‐the‐river operation from late spring to autumn. Sediment stored behind the dam during the winter quickly became depleted in ⁷Be activity. This resulted in a pulse of ‘dead’ sediment released when the dam gates were opened in the spring which could be tracked as it moved downstream. Measured average sediment transport velocities (30–80 metres per day (m d^?1)) exceed those typically reported for bulk bed load transport and are remarkably constant across varied flow regimes, possibly due to corresponding changes in bed sand fraction. Results also show that the length scale of the downstream impact of dam management on sediment transport is short (c. 1 km); beyond this distance the sediment trapped by the dam is replaced by new sediment from tributaries and other downstream sources. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

An Alternative Function For The Wind And Temperature Gradients In Unstable Surface Layers     

D. Keith Wilson 《Boundary-Layer Meteorology》2001,99(1):151-158

The function ()=(1+|z/L|^2/3)^1/2,where z is the height, L the Obukhov length, and a constant,is proposed for the nondimensional wind speed and temperaturegradients (flux-profile relationships) in anunstable surface layer. This function agrees quite well withboth wind speed and temperature data,has the theoretically correct behaviour in convective conditions,and leads to simple results when integrated to produce the mean profiles.  相似文献