A SPATIAL ANALYSIS OF AGRICULTURAL INTENSITY IN A BASOTHO VILLAGE OF SOUTHERN AFRICA     

Richard Stevens  Yuk Lee 《The Professional geographer》1979,31(2):177-183

The effect of distance upon agricultural production should be as apparent among primitive subsistence agriculturalists as in market-oriented societies. This analysis of yield variations among the Basotho of southern Africa attempts to determine the frictional effect of distance. The results show that distance does not explain variations in yield even when other physical factors are held constant. Perhaps such variables as household characteristics, not included in the analysis, would provide a better explanation.  相似文献   

Modelling of diffusion boundary-layers in subtidal macroalgal canopies: The response to waves and currents     

Craig L. Stevens  Catriona L. Hurd  Pål E. Isachsen 《Aquatic Sciences - Research Across Boundaries》2003,65(1):81-91

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A ROSAT survey of Wolf–Rayet galaxies — II. The extended sample     

Ian R. Stevens  & David K. Strickland 《Monthly notices of the Royal Astronomical Society》1998,301(1):215-230

We present results from an ongoing X-ray survey of Wolf–Rayet (WR) galaxies, a class of objects believed to be very young starbursts. This paper extends the first X-ray survey of WR galaxies by Stevens &38; Strickland by studying WR galaxies identified subsequent to the original WR galaxy catalogue of Conti.   Out of a sample of 40 new WR galaxies a total of 10 have been observed with the ROSAT PSPC, and of these seven have been detected (NGC 1365, NGC 1569, I Zw 18, NGC 3353, NGC 4449, NGC 5408 and a marginal detection of NGC 2366). Of these, all are dwarf starbursts except for NGC 1365, which is a barred spiral galaxy possibly with an active nucleus. We also report on observations of the related emission-line galaxy IRAS 0833+6517.   The X-ray properties of these galaxies are broadly in line with those found for the original sample; they are X-ray overluminous compared with their blue luminosity and have thermal spectra with typically kT  ∼ 0.4 − 1.0 keV. There are some oddities: NGC 5408 is very overluminous in X-rays, even compared with other WR galaxies; I Zw 18 has a harder X-ray spectrum; NGC 1365, although thought to contain an active nucleus, has X-ray properties that are broadly similar to other WR galaxies, and we suggest that the X-ray emission from NGC 1365 is due to starburst activity.   A good correlation between X-ray and blue luminosity is found for the WR galaxy sample as a whole. However, when just dwarf galaxies are considered there is little evidence of correlation. We discuss the implications of these results on our understanding of the X-ray emission from WR galaxies and suggest that the best explanation for the X-ray activity is starburst activity from a young starburst region.  相似文献   

Alpine tectonics and diapiric structures in the Pre-Betic zone of southeast Spain   总被引：1，自引：0，他引：1  

F. Moseley  J.C. Cuttell  E.W. Lange  D. Stevens  J.R. Warbrick 《Journal of Structural Geology》1981,3(3):237-251

The Pre-Betic is the most northerly of the Alpine zones forming the Betic Cordilleras of southern Spain. It consists of strongly folded and faulted Mesozoic and Tertiary rocks, the oldest of which are ferruginous and gypsiferous Triassic mudstones, followed by a predominantly carbonate facies of Cretaceous, Palaeogene and Miocene age. Although this sequence is interrupted by a number of minor unconformities, the major structures were formed during the middle or late Miocene. The highly incompetent Triassic rocks are the most strongly deformed, and form diapiric intrusions discordant to regional structural trends in the younger rocks. The latter are essentially of two facies: massive competent limestones which are deformed by relatively simple folds of large wavelength, and highly incompetent marl-limestone interbeds with complex disharmonic folds and crush belts. Faults include low-angle and high-angle thrusts, gravity slides and wrench faults. The regional tectonic strike is ENE to NE, but the diapiric intrusions mostly follow WNW and N directions. These intrusions have pushed the younger rocks aside, the result being polyphase structures of several trends.Less intense post-Miocene tectonics are mostly associated with continued diapirism and have resulted in the folding and tilting of the late Miocene to Quaternary elastic sediments.  相似文献   

In Situ Time-Series Measurements of Subseafloor Sediment Properties     

Wheatcroft  R.A. Stevens  A.W. Johnson  R.V. 《Oceanic Engineering, IEEE Journal of》2007,32(4):862-871

The capabilities and diversity of subsurface sediment sensors lags significantly from what is available for the water column, thereby limiting progress in understanding time-dependent seabed exchange and high-frequency acoustics. To help redress this imbalance, a new instrument, the autonomous sediment profiler (ASP), is described herein. ASP consists of a four-electrode, Wenner-type resistivity probe and a thermistor that log data at 0.1-cm vertical intervals over a 58-cm vertical profile. To avoid resampling the same spot on the seafloor, the probes are moved horizontally within a 20 times 100-cm^-2 area in one of three preselected patterns. Memory and power capacities permit sampling at hourly intervals for up to 3-mo duration. The system was tested in a laboratory tank and shown to be able to resolve high-frequency sediment consolidation, as well as changes in sediment roughness. In a field test off the southern coast of France, the system collected resistivity and temperature data at hourly intervals for 16 d. Coupled with environmental data collected on waves, currents, and suspended sediment, the ASP is shown to be useful for understanding temporal evolution of subsurface sediment porosity, although no large depositional or erosional events occurred during the deployment. Following a rapid decrease in bottom-water temperature, the evolution of the subsurface temperature field was consistent with the 1-D thermal diffusion equation coupled with advection in the upper 3-4 cm. Collectively, the laboratory and field tests yielded promising results on time-dependent seabed change.  相似文献   

Data collection methodology for dynamic temperature model testing and corroboration     

B. T. Neilson  D. K. Stevens  S. C. Chapra  C. Bandaragoda 《水文研究》2009,23(20):2902-2914

This article describes a data collection approach for determining the significance of individual heat fluxes within streams with an emphasis on testing (i.e. identification of possible missing heat fluxes), development, calibration and corroboration of a dynamic temperature model. The basis for developing this approach was a preliminary temperature modelling effort on the Virgin River in southwestern Utah during a low‐flow period that suggested important components of the energy balance might be missing in the original standard surface‐flux temperature model. Possible missing heat fluxes were identified as bed conduction, hyporheic exchange, dead zone warming and exchange and poor representation of the amount of solar radiation entering the water column. To identify and estimate the relative importance of the missing components, a comprehensive data collection effort was developed and implemented. In particular, a method for measuring shortwave radiation behaviour in the water column and an in situ method for separating out bed conduction and hyporheic influences were established. The resulting data and subsequent modelling effort indicate that hyporheic and dead zone heat fluxes are important, whereas solar radiation reflection at the water surface was found to be insignificant. Although bed conduction can be significant in certain rivers, it was found to have little effect on the overall heat budget for this section of the Virgin River. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

New Urbanist developments in flood-prone areas: safe development,or safe development paradox?     

Mark R. Stevens  Yan Song  Philip R. Berke 《Natural Hazards》2010,53(3):605-629

Government policies intended to reduce flood losses can increase the potential for catastrophe by stimulating development inside the floodplain, a phenomenon referred to as the “safe development paradox.” New Urbanist design has the potential to both exacerbate and alleviate flood risks. Because they are built at relatively high densities, New Urbanist developments can exacerbate risk by placing more people and property in harm’s way. Conversely, New Urbanist design features theoretically better enable designers of New Urbanist developments to avoid floodplain portions of project sites than designers of conventional subdivisions. Using a sample of New Urbanist developments in the US that contain floodplain portions within their boundaries, this paper focuses on whether and why these developments locate built structures inside the floodplain. The authors find that roughly 30% of the developments locate structures inside the floodplain, and that the odds of locating structures inside the floodplain increase with the proportion of the project site located inside the floodplain and decrease with the presence of government policies that prohibit residential development in the floodplain. The authors also identify confusion among government planners regarding the distinction between pre and postconstruction floodplain boundaries. A subset of New Urbanist developments is found to have built structures located outside the postconstruction floodplain boundary, but inside the preconstruction floodplain boundary. This finding is cited as an example of the “safe development paradox” in action. The authors recommend changes in New Urbanist design codes and local government floodplain management to increasingly direct new development away from the floodplain.  相似文献   

An ultraluminous X-ray microquasar in NGC 5408?     

R. Soria  R. P. Fender  D. C. Hannikainen  A. M. Read  I. R. Stevens 《Monthly notices of the Royal Astronomical Society》2006,368(4):1527-1539

We studied the radio source associated with the ultraluminous X-ray source in NGC 5408  ( L _X≈ 10⁴⁰ erg s⁻¹)  . The radio spectrum is steep (index  ≈−1  ), consistent with optically thin synchrotron emission, not with flat-spectrum core emission. Its flux density (≈0.28 mJy at 4.8 GHz, at a distance of 4.8 Mpc) was the same in the March 2000 and December 2004 observations, suggesting steady emission rather than a transient outburst. However, it is orders of magnitude higher than expected from steady jets in stellar-mass microquasar. Based on its radio flux and spectral index, we suggest that the radio source is either an unusually bright supernova remnant, or, more likely, a radio lobe powered by a jet from the black hole (BH). Moreover, there is speculative evidence that the source is marginally resolved with a radius ∼30 pc. A faint H  ii region of similar size appears to coincide with the radio and X-ray sources, but its ionization mechanism remains unclear. Using a self-similar solution for the expansion of a jet-powered electron–positron plasma bubble, in the minimum-energy approximation, we show that the observed flux and (speculative) size are consistent with an average jet power  ≈ 7 × 10³⁸ erg s⁻¹∼ 0.1 L _X∼ 0.1 L _Edd  , an age ≈10⁵ yr, a current velocity of expansion ≈80 km s⁻¹. We briefly discuss the importance of this source as a key to understand the balance between luminosity and jet power in accreting BHs.  相似文献   

Dust in spiral galaxies: global properties     

J. A. Stevens  M. Amure  W. K. Gear 《Monthly notices of the Royal Astronomical Society》2005,357(1):361-380

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A phase-resolved XMM–Newton campaign on the colliding-wind binary HD 152248     

H. Sana  I. R. Stevens  E. Gosset  G. Rauw  J.-M. Vreux 《Monthly notices of the Royal Astronomical Society》2004,350(3):809-828

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