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   27199篇
  免费   695篇
  国内免费   1777篇
测绘学   1604篇
大气科学   2700篇
地球物理   5174篇
地质学   13065篇
海洋学   1407篇
天文学   1731篇
综合类   2389篇
自然地理   1601篇
  2024年   18篇
  2023年   69篇
  2022年   125篇
  2021年   195篇
  2020年   158篇
  2019年   167篇
  2018年   4905篇
  2017年   4155篇
  2016年   2737篇
  2015年   391篇
  2014年   247篇
  2013年   206篇
  2012年   1156篇
  2011年   2908篇
  2010年   2190篇
  2009年   2449篇
  2008年   2052篇
  2007年   2491篇
  2006年   186篇
  2005年   308篇
  2004年   492篇
  2003年   504篇
  2002年   348篇
  2001年   121篇
  2000年   141篇
  1999年   155篇
  1998年   113篇
  1997年   106篇
  1996年   91篇
  1995年   73篇
  1994年   72篇
  1993年   49篇
  1992年   34篇
  1991年   37篇
  1990年   30篇
  1989年   25篇
  1988年   21篇
  1987年   11篇
  1986年   19篇
  1985年   9篇
  1984年   11篇
  1983年   10篇
  1982年   5篇
  1981年   25篇
  1980年   23篇
  1979年   5篇
  1978年   3篇
  1976年   10篇
  1966年   3篇
  1958年   3篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
This article deals with women's use of public space and sense of place in El Raval, a neighbourhood located in the historical center of Barcelona. Attention will be paid to discover to what degree the existence of a quality public space fosters the creation of socially meaningful places, thus contributing to the construction of womens sense of place and urban identities. A qualitative approach, based on direct observation and in-depth interviews with women living in the neighbourhood, has allowed us to capture the main aspects of the building of a sense of place and belonging, that is the use of public space and facilities, the attitude towards living in the neighbourhood, etc. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
992.
993.
The absolute amounts and relative distributions of neutral nitrogen compounds in the Tabei oilfield (e. G. Blocks Ln1-Ln1 1) showed remarkable migration fractionation in the vertical direction. From Ordovician reservoirs (O) to oil legs TⅢ and T Ⅰ of Triassic reservoirs in blocks LN1-LN11, the concentrations of [a] [c] decreased from 1. 59μg/g, 0.49μg/g to 0.17 μg/g (oil). The ratios of various alkylcarbazole isomers, such as 1,8-dimethylcarbazole/nitrogenpartially shielded isomers and 1,8-dimethylcarbazole/nitrogen-exposed isomers, were adopted as the indicators of petroleum migration. The ratios increased from 0.13, 0.20 to 0.67 and from going from the south to the north of the Tabei oilfield, the absolute concentrations of neutral nitrogen compounds decreased drastically, and the nitrogen-shielded isomers were enriched relative to nitrogen-exposed isomers and nitrogen-partially shielded isomers. Crude oils in the Tabei oilfield migrated laterally from the Jilake structure to the Sangtamu fault uplift and Lunnan fault uplift, and crude oils in the same fault uplift migrated and remigrated vertically from Ordovician reservoirs, to oil legs TⅢ to TⅠ of Triassic reservoirs.  相似文献   
994.
The Hanjiang River, the largest tributaries of the Changjiang (Yangtze) River, is the water source area of the Middle Route of China’s South-to-North Water Transfer Project. The chemical and strontium isotopic compositions of the river waters are determined with the main purpose of understanding the contribution of chemical weathering processes and anthropogenic inputs on river solutes, as well as the associated CO2 consumption in the carbonate-dominated basin. The major ion compositions of the Hanjiang River waters are characterized by the dominance of Ca2+ and HCO3 , followed by Mg2+ and SO4 2−. The increase in TDS and major anions (Cl, NO3 , and SO4 2−) concentrations from upstream to downstream is ascribed to both extensive influences from agriculture and domestic activities over the Hanjiang basin. The chemical and Sr isotopic analyses indicate that three major weathering sources (dolomite, limestone, and silicates) contribute to the total dissolved loads. The contributions of the different end-members to the dissolved load are calculated with the mass balance approach. The calculated results show that the dissolved load is dominated by carbonates weathering, the contribution of which accounts for about 79.4% for the Hanjiang River. The silicate weathering and anthropogenic contributions are approximately 12.3 and 6.87%, respectively. The total TDS fluxes from chemical weathering calculated for the water source area (the upper Hanjiang basin) and the whole Hanjiang basin are approximately 3.8 × 106 and 6.1 × 106 ton/year, respectively. The total chemical weathering (carbonate and silicate) rate for the Hanjiang basin is approximately 38.5 ton/km2/year or 18.6 mm/k year, which is higher than global mean values. The fluxes of CO2 consumption by carbonate and silicate weathering are estimated to be 56.4 × 109 and 12.9 × 109 mol/year, respectively.  相似文献   
995.
This study analyzes the response of glacier to climate change during the past 49 years in Urumqi River source region, the Tianshan Mountains of China. The temporal and spatial variations of winter mass balance (bn-w) at different time scales were analyzed to identify their response to climate change during 1988–2006 (The observation of winter mass balance observation began in 1988) on the Glacier No.1 at the headwaters of the Urumqi River, Tianshan Mountains, China. The winter accumulation shows a significantly decreasing trend. The results show that the cumulative values on Glacier No.1 is 2,202 mm water equivalent during 1988–2006 and the mean values is 116 mm a−1. Furthermore, the trend analysis of the winter mass balance indicates a rapid decrease since 1990, and the mean mass balance is only 79 mm a−1 during 1997–2006. Winter mass balance correlates well negatively with the total evaporation from September to April (r = −0.68, α = 0.01), and positively with the total precipitation from September to April (r = 0.74, α = 0.01). However, winter mass balance shows a weak correlation with mean minimum air temperature during September to April (r = −0.35), and runoff on September (r = −0.13).  相似文献   
996.
Water balance in the playa-lakes of an arid environment,Monegros, NE Spain   总被引:1,自引:0,他引:1  
The playa-lakes of the Monegros desert in north-east Spain are saline wetlands in an arid environment, a rare phenomenon in Europe. These extremely valuable habitats are threatened by changes associated with agricultural expansion and incorporation of new irrigated areas. An understanding of the present hydrologic regime will enable changes to be identified, particularly those brought about by flooding and pollution caused by irrigation surplus. This study sets out to show the results of applying a daily water balance in three selected playa-lakes. The balance was in two parts and consisted of: (1) the average balance for all the endorheic basin using the BALAN_11 program, and (2) the water balance in some playa-lakes, applying discharge flows obtained from the previous balance. The resulting volumes of water were converted to water depths and contrasted with reference volumes taken from field and Landsat images. The model was calibrated by applying various hypotheses of function which enabled the results to be adjusted. The proposed balance is an acceptable reproduction of field water measurements during this period, and underlines the consistency of the conceptual model. The methodology used is appropriate for understanding the playa-lakes function and for monitoring them for conservation purposes.  相似文献   
997.
In this article the implementation and potential of the Seismotectonic Information System of the Campania Region (SISCam) are described, in particular an application of this Web-based GIS system to the seismotectonic analysis of the Sannio area (Southern Apennines) is performed. WEB-GIS technologies greatly contribute to both the environmental monitoring and the disaster management of areas affected by high natural risks. Specifically the SISCam system has been developed with the aim of providing easy access and fast diffusion, through Internet technology, of the most significant geological, geophysical, and territorial data relative to the Campania Region. The Sannio area has been selected as our application example because it is among the most active seismic regions in Italy. This portion of the Southern Apennines which was hit by the June 5, 1688 strong earthquake (M W = 6.7, CPTI 1999) and by some low- and moderate-energy seismic sequences (1990–1992, 1997), is characterized by a complex inherited tectonic setting and low-tectonic deformation rates that hide the seismogenic sources position. Since this case study turned out to be complicated, the use of the SIScam WEB-GIS has become indispensable because it allowed us to visualize, integrate and analyze all the data available, in order to obtain an accurate and direct picture of the seismotectonic setting of the area. Moreover, a different approach of data analysis was necessary, due to the lack of up-to-date neotectonic and structural data; therefore, the operation of this GIS system enabled us to process and generate some original informative layers, through image analysis, such as new structural lineaments represented on a map of the potential active faults of the area, which has been the final result of our application, as a contribution to new knowledge about the local seismic risk parameters.  相似文献   
998.
A compilation of B–Be–Li data on rocks that cover the entire eruptive history of Somma-Vesuvius is presented and interpreted in the light of evolution models for the Somma-Vesuvius rocks. Using major and trace element data, fractional crystalllization models are presented for different geochemical units. These data were used to constrain the source mineralogy of the Somma-Vesuvius rocks (ol-opx-cpx-gar-amp of 0.4-0.3-0.1-0.1-0.1), the amount of sediment added (5–10%) and the melt fraction from batch partial melting computations (0.05–0.1). From the B–Li data it is inferred that the main process responsible for the B isotopic signature is sediment recycling. However, the B–Li data show a major variation in Li abundances respect to B which is explained with Li dehydration before the fluid enriched the mantle wedge that produced the arc magmas. The Somma-Vesuvius B isotope composition is intermediate between that of the Campi Flegrei and the broad field of the Eolian Island arc. A low Be isotopes in the recent volcanic rocks can be explained as: (a) the top 1–22 m of the incoming sediment is accreted, (b) large amounts of sediment erosion, (c) a slow rate of subduction which have provoked a long magmatic history for the Vesuvius magma, (d) the sediment component takes several Myr longer than the subducting plate to reach the magma source region beneath Italy.  相似文献   
999.
Retrospective understanding of the magnitude and pace of urban expansion is necessary for effective growth management in metropolitan regions. The objective of this paper is to quantify the spatial–temporal patterns of urban expansion in the Greater Kumasi Sub-Region (GKSR)—a functional region comprising eight administrative districts in Ghana, West Africa. The analysis is based on Landsat remote sensing images from 1986, 2001 and 2014 which were classified using supervised maximum likelihood algorithm in ERDAS IMAGINE. We computed three complementary growth indexes namely; Average Annual Urban Expansion Rate, Urban Expansion Intensity Index (UEII) and Urban Expansion Differentiation Index to estimate the amount and intensity of expansion over the 28-year period. Overall, urban expansion in the GKSR has been occurring at an average annual rate of 5.6 %. Consequently, the sub-region’s built-up land increased by 313 km2 from 88 km2 in 1986 to 400 km2 in 2014. The analysis further show that about 72 % of the total built-up land increase occurred in the last 13 years alone, with UEII value of 0.605 indicating a moderate intensity of urban expansion. Moreover, the metropolitan-core of the sub-region, being the focal point of urban development and the historical origins of expansion, accounted for more than half of the total built-up land increase over the 28-year period. Over the last decade and half however, urban expansion has spilled into the neighbouring peripheral districts, with the highest intensity and fastest rate of expansion occurring in districts located north and north east of the sub-regional core. We recommend a comprehensive regional growth management strategy grounded in effective strategic partnerships among the respective administrative districts to curb unsustainable urban expansion.  相似文献   
1000.
This study presents the correlations between quantified shape parameters and geotechnical properties for nine sand specimens. Four shape parameters, sphericity, convexity, elongation and slenderness, were quantified with two-dimensional microscopic images with the aid of image processing techniques. An instrumented oedometer cell is used to measure compressibility, thermal conductivity and shear wave velocity during loading, unloading and reloading stages. As the particle shape inherently determines the initial loose packing condition, initial void ratio and shape parameters are well correlated with compressibility. The applied stress in soils increases the interparticle contact area and contact quality; round particles tend to achieve higher thermal conductivity and shear wave velocity during stress-induced volume change. Multiple linear regression is implemented to capture the relative contributions of involved variables, revealing that the thermal evolution is governed by the initial packing density and particle shape. The experimental observations underscore the predominant effect that particle shape has on the geomechanical and physical properties upon stress-induced soil behavior.  相似文献   
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