全文获取类型
收费全文 | 29015篇 |
免费 | 1109篇 |
国内免费 | 1967篇 |
专业分类
测绘学 | 1581篇 |
大气科学 | 3079篇 |
地球物理 | 5814篇 |
地质学 | 13912篇 |
海洋学 | 1451篇 |
天文学 | 1897篇 |
综合类 | 2562篇 |
自然地理 | 1795篇 |
出版年
2024年 | 30篇 |
2023年 | 102篇 |
2022年 | 236篇 |
2021年 | 281篇 |
2020年 | 229篇 |
2019年 | 249篇 |
2018年 | 4960篇 |
2017年 | 4235篇 |
2016年 | 2810篇 |
2015年 | 466篇 |
2014年 | 389篇 |
2013年 | 293篇 |
2012年 | 1261篇 |
2011年 | 2983篇 |
2010年 | 2271篇 |
2009年 | 2570篇 |
2008年 | 2110篇 |
2007年 | 2574篇 |
2006年 | 220篇 |
2005年 | 367篇 |
2004年 | 531篇 |
2003年 | 514篇 |
2002年 | 369篇 |
2001年 | 171篇 |
2000年 | 211篇 |
1999年 | 254篇 |
1998年 | 200篇 |
1997年 | 188篇 |
1996年 | 171篇 |
1995年 | 156篇 |
1994年 | 108篇 |
1993年 | 127篇 |
1992年 | 87篇 |
1991年 | 58篇 |
1990年 | 33篇 |
1989年 | 56篇 |
1988年 | 43篇 |
1987年 | 24篇 |
1986年 | 16篇 |
1985年 | 14篇 |
1984年 | 18篇 |
1983年 | 7篇 |
1982年 | 12篇 |
1981年 | 29篇 |
1980年 | 27篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1976年 | 7篇 |
1974年 | 2篇 |
1958年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
952.
Exploring spatiotemporal relationships among meteorological,agricultural, and hydrological droughts in Southwest China 总被引:2,自引:1,他引:2
Zhiyong Wu Yun Mao Xiaoyan Li Guihua Lu Qingxia Lin Huating Xu 《Stochastic Environmental Research and Risk Assessment (SERRA)》2016,30(3):1033-1044
Each type of drought has different characteristics in different regions. It is important to distinguish different types of droughts and their correlations. Based on gauged precipitation, temperature, simulated soil moisture, and runoff data during the period 1951–2012, the relationships among meteorological, agricultural, and hydrological droughts were analyzed at different time scales in Southwest China. The standardized precipitation evapotranspiration index (SPEI), soil moisture anomaly percentage index (SMAPI), and standardized runoff index (SRI) were used to describe meteorological, agricultural, and hydrological droughts, respectively. The results show that there was a good correlation among the three indices. SMAPI had the best correlation with the 3 month SPEI and SRI values. It indicates that agricultural drought was characterized by a 3-month scale. The three drought indices displayed the similar special features such as drought scope, drought level, and drought center during the extreme drought of 2009–2010. However, the scope and level of SPEI were bigger than those of SMAPI and SRI. The propagation characteristics of the three types of droughts were significantly different. The temporal drought process in typical grids reflect that the meteorological drought occurred ahead of agricultural and hydrological droughts by about 1 and 3 months, respectively. Agricultural drought showed a stable drought process and reasonable time periods for the drought beginning and end. These results showed the quantitative relationships among three types of drought and thus provided an important supporting evidence for regional drought monitoring and strategic decisions. 相似文献
953.
编 制 《中国 北 方岩 溶充 水 煤矿 区矿 床 水文 地质 图 集》的目 的主 要 是为 了合 理 开发 煤炭 资 源和 利用 煤 系 基 盘中奥 陶 统 灰岩 地 下 水 ;图 中 内 容应 力 求 充 分反 映 矿 区 不同 充 水 类 型的 充 水 特 征、充 水程 度 、水质 好 坏 及 可利 用 的 水 量 ,为 矿山 综 合防 治水 和 排供 结合 宏 观决 策提 供 所需 的水 文 地质 依据 。 相似文献
954.
YANG ChuPeng GENG AnSong LIAO ZeWen SUN YongGe & ZHANG LüHui State Key Laboratory of Organic Geochemistry Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou China Graduate University of Chinese Academy of Sciences Beijing 《中国科学D辑(英文版)》2009,52(1):12-21
Gas washing has been known in the Tazhong area of the Tarim Basin, but its quantitative assessment has not yet been reported.
Here the influence of gas washing fractionation in the area was discussed based on the gas chromatogram data of 68 oils and
the results of the mixing experiments of a black oil and a condensate. The results show that the intensity of gas washing
fractionation decreased generally from northern to southern part and vertically from deep reservoirs to shallow reservoirs.
The gas washing fractionation was mainly controlled by fault systems in this area, with the increase of n-alkane mass depletion positively correlated to the number and scale of faults. Gas washing fractionation appears to have
affected the hydrocarbon property, and as a result the diversity of the crude oils is markedly controlled by gas washing.
In addition, the occurrence of waxy oil in this area may be resulted from multiple factors including gas washing, mixed filling
and migration fractionation.
Supported by National Basic Research Program of China (Grant No. 2006CB202303) and the National Natural Science Foundation
of China (Grant No. 40672091) 相似文献
955.
István Major Brigitta Gyökös Marianna Túri István Futó Ágnes Filep András Hoffer Enikő Furu A. J. Timothy Jull Mihály Molnár 《Journal of Atmospheric Chemistry》2018,75(1):85-96
Comprehensive atmospheric studies have demonstrated that carbonaceous particles are one of the main components of atmospheric aerosols over Europe. The aim of our study was to establish an automated elemental analyser interfaced to a stable isotope mass spectrometer (EA-IRMS) method at the Hertelendi Laboratory of Environmental Studies (HEKAL), as a suitable method of quantification of total carbon mass in individual PM2.5 aerosol samples. Total carbon (TC) mass and simultaneous stable isotopic ratios were determined for both test standard and genuine aerosol samples. Finally, the results were compared to the ones obtained independently by an alternative sealed tube combustion method developed previously at HEKAL. The TC recovery tests of standard material prepared by the sealed tube method confirmed at least a carbon recovery yield of 92% for a broad range of carbon mass (100–2000 μg). The stable isotopic results confirmed that sealed tube method is reproducible and suitable to be used as a reference to verify our new EA-IRMS method. The EA-IRMS TC measurements of genuine aerosols gave on average 3% higher carbon recovery yield, relative to the uncorrected results of the sealed tube method. The comparison of the stable isotopic results by the two methods for aerosols also showed minimal differences. Consequently, the possibility of simultaneous TC and stable isotopic analyses makes the EA-IRMS method a very attractive alternative for continuous measurement of aerosols, with an accuracy and reliability similar to other commercial devices. 相似文献
956.
The distribution and geochemical composition of suspended-particulate matter (SPM) in the East China Sea (ECS) were investigated
during the summer period of high continental runoff to elucidate SPM sources, distribution and cross-shelf transport. The
spatial variability of SPM distribution (0.3–6.5 mg l−1) and geochemical composition (POC, Al, Si, Fe, Mn, Ca, Mg and K) in the ECS was pronounced during summer when the continental
fluxes of freshwater and terrestrial materials were highest during the year. Under the influences of Changjiang runoff, Kuroshio
intrusion, surface production and bottom resuspension, the distribution generally showed strong gradients decreasing seaward
for both biogenic and lithogenic materials. Particulate organic carbon was enriched in surface water (mean ∼18%) due to the
influence of biological productivity, and was diluted by resuspended and/or laterally-transported materials in bottom water
(mean 9.4%). The abundance of lithogenic elements (Al, Si, Fe, Mn) increased toward the bottom, and the distribution correlations
were highly significant. Particulate CaCO3 distribution provided evidence that the SPM of the bottom water in the northern part of the study area was likely mixed with
sediments originally derived from Huanghe. A distinct benthic nepheloid layer (BNL) was present in all seaward transects of
the ECS shelf. Sediment resuspension may be caused by tidal fluctuation and other forcing and be regarded as the principal
agent in the formation of BNL. This BNL was likely responsible for the transport of biogenic and lithogenic particles across
or along the ECS shelf. Total inventories of SPM, POC and PN are 46, 2.8 and 0.4 Tg, respectively, measured over the total
area of 0.45 × 106 km2 of the ECS shelf. Their mean residence times are about 27, 13 and 11 days, respectively. The inventory of SPM in the water
column was higher in the northernmost and southernmost transects and lower in the middle transects, reflecting the influences
of terrestrial inputs from Changjiang and/or resuspended materials from Huanghe deposits in the north and perhaps from Minjiang
and/or Taiwan’s rivers in the south. The distribution and transport patterns of SPM and geochemical elements strongly indicate
that continental sources and cross-shelf transport modulate ECS particulate matter in summer. 相似文献
957.
利用全国(港澳台地区除外)1985—2013年的面板数据,分析各省份城镇化水平的演进态势及耕地数量的动态变化,并分不同时段测算各省份城镇化发展水平与耕地资源变动的耦合协调程度。结果表明:(1)1985年以来,全国大部分地区的城镇化水平整体上都呈现上升的变化趋势,东三省城镇化水平稳定上升但活力不足,西北多省份上升中波动较大,西南地区水平偏低。(2)全国耕地资源在20世纪80年代普遍减少,后期多省份有所增加,但京、津、沪、苏、粤等地的耕地面积一直在以一定的速率减少。(3)全国城镇化与耕地资源关系的协调度呈现西高东低、北高南低的空间格局。较多地区在快速城镇化进程中城镇化发展与耕地资源的协调度呈恶化趋势。 相似文献
958.
Rb-Sr isochron age of fluid inclusions in quartz from the Fuwan super-large silver deposit is 68 - 6 Ma, the silver deposit is characterized by high μ values (10.67 - 10.95 ), which are much higher than those of the ore-hosted Paleozoic strata and are close to those of ores hosted in the Proterozoic metamorphic basement in western Guangdong Province. Based on the Pb isotopic characteristics, coupled with much high background silver contents (200-1000ng/g) in the Proterozoic basement and relatively low silver contents in the Paleozoic strata in the region of the Sanshui Basin, it is concluded that the ore-forming material of the super-large silver deposit came mainly from the old basement. The super-large silver deposit related genetically to the intense volcanic activities during the Upper Cretaceous to Eogene. The formation of the Fuwan super-large silver deposit is controlled by the following favorable geological conditions : ( 1 ) The intersection of deep faults and contemporaneous faults at the margin of the Sanshui Basin led to the formation of an excellent structure as passageway for ore fluids; (2) The special ore-hosted rock association forms a ore gathering-trap structure that favors the precipitation of ore; (3) The silver-rich old basement, multi-stage mineralization and multi-episode volcanic activities which constitute a geothermal convection system. 相似文献
959.
中国上地幔角闪石及其成因意义 总被引:12,自引:2,他引:12
我国土地幔成因角闪石以高铝(一般>2.0)为特征,以TiO_2、Cr_2O_3含量差异相区别,划分为韭闪石和钛角闪石两大类。韭闪石又有四种不同的成因类型:贫铬、钛型(Cr_2O_3=0.15%,TiO_2=0.07%),富铬型(Cr_2O_3=2.18%),富钛型(TiO_2=3.14%)和富铬、钛型(Cr_2O_3=2.59%,TiO_2=3.16%)。除贫铬、钛型韭闪石发现于苏北-胶南高压变质岩石中外,其它三种韭闪石(产于大陆区)和钛角闪石(产于台湾西部)均来自各种地幔橄榄岩、辉石岩及新生代玄武岩中。不管它们的产出方式如何(填隙、嵌晶、巨晶和包裹体子矿物),都直接或间接与地幔流体的交代作用有关,但各具成因之特点。 相似文献
960.