首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   577篇
  免费   25篇
  国内免费   11篇
测绘学   15篇
大气科学   89篇
地球物理   119篇
地质学   237篇
海洋学   51篇
天文学   70篇
自然地理   32篇
  2024年   1篇
  2023年   2篇
  2022年   7篇
  2021年   9篇
  2020年   12篇
  2019年   14篇
  2018年   15篇
  2017年   13篇
  2016年   20篇
  2015年   15篇
  2014年   28篇
  2013年   44篇
  2012年   34篇
  2011年   45篇
  2010年   28篇
  2009年   61篇
  2008年   37篇
  2007年   34篇
  2006年   33篇
  2005年   33篇
  2004年   20篇
  2003年   8篇
  2002年   31篇
  2001年   9篇
  2000年   5篇
  1999年   7篇
  1998年   2篇
  1997年   9篇
  1996年   1篇
  1995年   1篇
  1994年   3篇
  1992年   2篇
  1991年   2篇
  1990年   6篇
  1989年   2篇
  1988年   5篇
  1987年   4篇
  1986年   1篇
  1985年   2篇
  1982年   2篇
  1981年   1篇
  1979年   1篇
  1978年   1篇
  1976年   1篇
  1975年   1篇
  1971年   1篇
排序方式: 共有613条查询结果,搜索用时 31 毫秒
561.
The forcing of a hydrologic model (ABC) by both observed and simulated precipitation from a regional climate model (MAR) has been performed over the Sirba watershed (39,000 km2) located in the Sahelian region. Two aspects have been more specifically examined: the spatial and temporal representations of precipitation. The comparison between simulated and observed discharges—using observed rainfall datasets as forcing of the hydrologic model—has shown that the representation of daily precipitation (which is mainly convective in the Sahelian region) was not sufficiently accurate to correctly simulate the hydrologic response of the watershed. Since this response drives the soil water budget and consequently the amount of evaporation in forthcoming coupling experiments, it is thus necessary to develop more realistic infra-daily precipitation associated with convective events. A new temporal disaggregation scheme has been then developed. Considering observed as well as simulated precipitation fields, this method has significantly improved the simulated discharge at the catchment outlet. The major role played by the temporal component compared to spatial component of the precipitation has been then underlined. In addition, the present study shows the unsuitability of the simulated precipitation from the RCM to directly force a hydrologic model at infra daily timescale even if the cumulative amount and the main features of the precipitation seasonal cycle are well simulated.  相似文献   
562.
Piton des Neiges (PN) Volcano on Reunion Island offers a rare opportunity to study deposits related to degradation processes in a deeply eroded oceanic shield volcano. Both the inner parts and flanks reveal a large amount of resedimented volcaniclastic material, including extensive debris avalanche deposits. PN litho–structural units, first studied by Upton and Wadsworth [1965, Philos. Trans. R. Soc. Lond., A 271, pp. 105–130], are re-examined. This review highlights the importance of long volcanic repose periods and erosion processes during PN history. volcaniclastic deposits have been studied in the field in order to evaluate the spatial and temporal distribution of the three main types of PN degradation processes. The deposits of these processes have been classified into: (1) talus, (2) mudflow and debris flow, and (3) debris avalanche. Lithology, frequency and estimated volumes of each deposit type imply that the structural evolution of PN can be considered in terms of the competition between the volcanic productivity and the degradation and erosion processes. The occurrence of huge catastrophic avalanches produced by flank failure is convincingly linked to the basaltic activity of PN, which implies a very low risk at present. On the contrary, mudflows and debris flows pose an important risk due to the high population density focussed around the basin outlets. Moreover, if smaller debris avalanches can occur in the cirques of PN, another major risk must be evaluated.  相似文献   
563.
A simple model which describes the soil–pipe interaction and accounts for the longitudinal soil variation has been developed. It is used for the analysis of the static response of a section of a buried sewer. A probabilistic analysis (Monte-Carlo method) enabling to quantify the influence of spatial variability of the geomechanical characteristics of the soil makes possible to study the parameters which can influence and drive the longitudinal response of a section of sewer. The system response is complex, soil–structure interaction depending on three different stiffnesses: soil stiffness, pipe components stiffness and joints stiffness. Various analyses have been performed to identify the parameters whose influence is the larger. A specific attention has been devoted to the fluctuation scale of the soil properties and to the stiffness of joints.  相似文献   
564.
565.
Hydrous ferric oxide (HFO) colloids formed, in strictly anoxic conditions upon oxidation of Fe2+ ions adsorbed on mineral surface, were investigated under in situ conditions by contact mode atomic force microscopy (AFM). Freshly cleaved and acid-etched large single crystals of near endmember phlogopite were pre-equilibrated with dissolved Fe(II) and then reacted with Hg(II), As(V) and trichlorethene (TCE)-bearing solutions at 25 °C and 1 atm. HFO structures are found to be of nanometer scale. The As(V)–Fe(II) and Hg(II)–Fe(II) reaction products are round (25 nm) microcrystallites located predominantly on the layer edges and are indicative of an accelerated Fe(II) oxidation rate upon formation of Fe(II) inner sphere surface complexes with the phyllosilicate edge surface sites. On the other hand, TCE–Fe(II)–phlogopite reaction products are needle-shaped (45 nm long) particles located on the basal plane along the Periodic Bond Chains (PCBs) directions. Experiments with additions of sodium chloride confirm the importance of the Fe(II) adsorption step in the control of the overall heterogeneous Fe(II)–TCE electron transfer reaction.

Kinetic measurements at the nanomolar level of Hg° formed upon reduction of Hg(II) by Fe(II) in presence of phlogopite particles provide further convincing evidence for reduction of Hg(II)aq coupled to the oxidation of Fe(II) adsorbed at the phlogopite–fluid interface, and indicate that sorption of Fe(II) to mineral surfaces enhances the reduction rate of Hg(II) species. The Hg(II) reduction reaction follows a first-order kinetic law. Under our experimental conditions, which were representative of many natural systems, 80% of the mercury is transferred to the atmosphere as Hg° in less than 2 h.

The reduction of a heavy metal (Hg), a toxic oxyanion (arsenate ion) and a chlorinated solvent (TCE) thus appear to be driven by the high reactivity of adsorbed Fe(II). This is of environmental relevance since these three priority pollutants are that way reductively transformed to a volatile, an immobilizable and a biodegradable species, respectively. Such kinetic data and reaction pathways are important in the evaluation of natural evaluation scenarios, in the optimization of Fe(II)/mineral mixtures as reductants in technical systems, and in general, in predicting the fate and transport of pollutants in natural systems.  相似文献   

566.
This paper presents an approach for GIS integrating local participatory land management information used for regional planning, and contributing to a bottom-up approach to land use planning. In participatory planning, the integration between local and regional levels should facilitate the communication and co-operation among the parties at both levels, for an efficient use of available resources. For coherently linking these two levels it is necessary to transform the data produced at one level, in order to be usable by the other. This transformation consists of a spatial procedure, which allows scaling-up the local participatory rural appraisal (PRA) information for regional purposes and scaling down the regional information for local use, using a GIS. Such an integration procedure is presented and discussed using data from a case study in south-western Burkina Faso.  相似文献   
567.
Strain analysis of a shear zone in a granodiorite   总被引:1,自引:0,他引:1  
A ductile shear zone in a late Precambrian granodiorite, from the Rouergue (southwest part of the French Massif Central) has been studied.A single episode of deformation is responsible for the formation of a foliation and a well-defined lineation which are localized into an elongated zone, a few decimeters wide.The strain features can be attributed to a simple-shear mechanism (Ramsay and Graham, 1970), so that the main parameters of the deformation are defined.At stages of increasing deformation, the quartz isotropic sub-fabric of the undeformed host rock is progressively transformed into an anisotropic fabric composed of a single oblique girdle while the subgrain size progressively decrease and the dislocation density remains constant. It is suggested that the gliding planes of quartz are the basal plane (0001) and a predominant prismatic plane 101̄0 the slip directions may be a for both glide-planes.The results obtained in this investigation provide a basis for a high voltage electron microscope (H.V.E.M.) study which shows that the fabrics development may be related to dislocation processes. The difference of strain rates in the host rock and in the shear zone is calculated from the dislocation microstructures.  相似文献   
568.
To help improve the safety of its population faced with natural disasters, the Cameroon Government, with the support of the French Government, initiated a programme of geological risk analysis and mapping on Mount Cameroon. This active volcano is subject to a variety of hazards: volcanic eruptions, slope instability and earthquakes. Approximately 450,000 people live or work around this volcano, in an area which includes one of Cameroon’s main economic resources. An original methodology was used for obtaining the information to reply to questions raised by the authorities. It involves several stages: identifying the different geological hazard components, defining each phenomenon’s threat matrix by crossing intensity and frequency indices, mapping the hazards, listing and mapping the exposed elements, analysing their respective values in economic, functional and strategic terms, establishing typologies for the different element-at-risk groups and assessing their vulnerability to the various physical pressures produced by the hazard phenomena, and establishing risk maps for each of the major element-at-risk groups (population, infrastructures, vegetation, atmosphere). At the end of the study we were able (a) to identify the main critical points within the area, and (b) provide quantified orders of magnitude concerning the dimensions of the risk by producing a plausible eruption scenario. The results allowed us to put forward a number of recommendations to the Cameroon Government concerning risk prevention and management. The adopted approach corresponds to a first level of response to the authorities. Later developments should make it possible to refine the quality of the methodology.  相似文献   
569.
570.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号