全文获取类型
收费全文 | 15612篇 |
免费 | 3911篇 |
国内免费 | 2612篇 |
专业分类
测绘学 | 880篇 |
大气科学 | 3071篇 |
地球物理 | 4272篇 |
地质学 | 7748篇 |
海洋学 | 2191篇 |
天文学 | 861篇 |
综合类 | 1394篇 |
自然地理 | 1718篇 |
出版年
2025年 | 16篇 |
2024年 | 253篇 |
2023年 | 379篇 |
2022年 | 679篇 |
2021年 | 812篇 |
2020年 | 748篇 |
2019年 | 668篇 |
2018年 | 775篇 |
2017年 | 742篇 |
2016年 | 861篇 |
2015年 | 691篇 |
2014年 | 831篇 |
2013年 | 931篇 |
2012年 | 805篇 |
2011年 | 839篇 |
2010年 | 884篇 |
2009年 | 839篇 |
2008年 | 688篇 |
2007年 | 760篇 |
2006年 | 571篇 |
2005年 | 525篇 |
2004年 | 384篇 |
2003年 | 446篇 |
2002年 | 426篇 |
2001年 | 398篇 |
2000年 | 515篇 |
1999年 | 752篇 |
1998年 | 592篇 |
1997年 | 633篇 |
1996年 | 607篇 |
1995年 | 539篇 |
1994年 | 459篇 |
1993年 | 379篇 |
1992年 | 331篇 |
1991年 | 249篇 |
1990年 | 207篇 |
1989年 | 160篇 |
1988年 | 159篇 |
1987年 | 98篇 |
1986年 | 103篇 |
1985年 | 60篇 |
1984年 | 57篇 |
1983年 | 52篇 |
1982年 | 49篇 |
1981年 | 34篇 |
1980年 | 38篇 |
1979年 | 30篇 |
1978年 | 11篇 |
1974年 | 12篇 |
1958年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
Integrative fuzzy set pair model for land ecological security assessment: a case study of Xiaolangdi Reservoir Region, China 总被引:5,自引:1,他引:4
Shiliang Su Xia Chen Stephen D. DeGloria Jiaping Wu 《Stochastic Environmental Research and Risk Assessment (SERRA)》2010,24(5):639-647
Due to the increasingly serious ecological degradation of land systems, the land ecological security issues have attracted
more and more attention of policy makers, researchers and citizens. Aiming at overcoming the disadvantages in subjectivity
and complexity of the currently used assessment methods, an integrative fuzzy set pair model for assessing the land ecological
security was developed by integrating fuzzy assessment and set pair analysis (SPA). The approximate degree of land ecological
security to the optimal standard set was calculated to describe the secure level by combining multiple indices. The indices
and weights were determined by a pressure-state-response model and the fuzzy analytic hierarchy process (AHP), respectively.
Aided by a geographic information system, this model was applied to evaluate comprehensively the status of land ecological
security in Xiaolangdi Reservoir Region, China, taking the administrative division as the assessment unit. The results showed
that 20% of the total area maintained a slightly secure status, while 50% of the study area was of a middle or seriously low
grade of land ecological security. The remaining portion (30%) was the most ecologically insecure. From the spatial prospective,
obvious variations were observed. The land eco-security gradually decreased from the Xiaolangdi Dam to its surrounding regions.
It was concluded that the status of the integral land ecological security of Xiaolangdi Reservoir Region was in the middle
level, and increasingly intense human activities speeded up the degradation of regional land ecosystem in recent years and
thus induced the crisis of land ecological security. 相似文献
32.
Chinese central government made a commitment to achieve a 40–45% reduction in carbon dioxide (CO2) per unit of GDP by 2020 compared with 2005. This targeted reduction was allocated averagely among all the provinces rather than individually according to different situations of each province. Though some research has been done regarding this rough allocation, two shortcomings in previous studies exist: Firstly, CO2 marginal abatement cost (MAC) has been ignored as one of the CO2 emission reduction allocation indexes. Secondly, either subjective or objective method has been used rather than comprehensively of both subjective and objective method to calculate the weight of each index in the previous studies. In order to fill the gaps, this paper builds a two-stage Shapley information entropy model to allocate CO2 emission reduction quota among the Chinese provinces based on the equity and efficiency principles. Afterward, three CO2 emission reduction quota allocation scenarios have been proposed. The results show that the CO2 MAC is an indispensable index in CO2 emission reduction quota allocation, because its value of CO2 Shapley information entropy is the highest among five indexes. CO2 emission reduction quota of lower-MAC provinces should be allocated larger, while the quota of higher-MAC provinces should be allocated smaller. Therefore, two suggested policies have been proposed: First, differential CO2 emission reduction quota allocation should be proposed. Second, synergetic development should be promoted. 相似文献
33.
Because of the increasing public awareness of the greenhouse effect, geological emissions of methane (GEM) have gained more
attention. The Intergovernmental Panel on Climate Change included GEM in the natural sources of methane category in the Fourth
Assessment Report in 2007. The methane flux from mud volcanoes (MVs) is the second leading source within GEM. However, given
that gas flux from MVs has been measured directly only in Sicily and Azerbaijan, the global methane estimation is still a
conservative assumption based on limited data. The behavior of MV activities is seldom reported. This study attempts to estimate
annual methane emissions through MVs by using a video recorder to record MV activities along the Chishan Fault and determining
the bubble size and flux rate of each MV. Observation results, after a 1 year observation period, indicate that the annual
emissions of the 19 MVs along the Chishan Fault are 0.1–0.2% of the global methane flux from MVs, highlighting that the methane
emission from these MVs should not be neglected. In addition, the continuous monitoring of a particular MV disclosed periodic
characteristics of MV activities. These results manifest the spatial and temporal variations of MV activities along the Chishan
Fault. The spatial variations are related to the extensional kinematics of the Chishan fault, while the temporal variation
relates to the earth-tide-related MV activities. The estimated annual methane emissions from the study are far more than those
of previous studies and are still considered conservative. Results of this study may provide valuable information for researchers
attempting to estimate the quantity of GEM. 相似文献
34.
35.
GuoLiang Zhang ZhiGang Zeng XueBo Yin DaiGeng Chen XiaoYuan Wang XiaoMei Wang 《中国科学D辑(英文版)》2008,51(12):1786-1801
Thirty-six basalt samples from near East Pacific Rise 13°N are analyzed for major and trace elements. Different types of zoned plagioclase phenocrysts in basalts are also backscatter imaged, and major element profiles scanned and analyzed for microprobe. Basalts dredged from a restricted area have evolved to different extents (MgO=9.38wt%—6.76wt%). High MgO basalts are modeled for crystalliza-tion to MgO of about 7wt%, and resulted in the Ni contents (≈28 ppm) that are generally lower than that in observed basalts (>60 ppm). It suggests that low MgO basalts may have experienced more intensive magma mixing. High MgO (9.38wt%) basalt is modeled for self-"mixing-crystallization", and the high Ni contents in low MgO basalts can be generated in small scale and periodical self-mixing of new magma (high MgO). "Mixing-crystallization" processes that low MgO magmas experienced accord with recent 226Ra/230Th disequilibria studies for magma residence time, in which low MgO magmas have experi-enced more circles of "mixing-crystallization" in relatively longer residence time. Magma mixing is not homogeneous in magma chamber, however, low MgO magmas are closer to stable composition pro-duced by periodical "mixing-crystallization", which is also an important reason for magma diversity in East Pacific Rise. Zoned plagioclase phenocrysts can be divided into two types: with and without high An# cores, both of which have multiple reversed An# zones, suggesting periodical mixing of their host magmas. Cores of zoned plagioclase in low MgO (7.45wt%) basalt differ significantly with their mantle in An#, but are similar in An# with microlite cores (products of equilibrium crystallization) in high MgO (9.38wt%) basalt, which further shows that plagioclase phenocryst cores in low MgO basalts may have formed in their parental magmas before entering into the magma chamber. 相似文献
36.
Xuwei Chen 《The Professional geographer》2013,65(2):160-173
This article investigates the effectiveness of simultaneous and staged evacuation strategies for hurricane evacuations of Galveston Island using agent-based microsimulation techniques. In the simultaneous strategy the entire population is informed to evacuate simultaneously, whereas in a staged evacuation strategy, people are informed to evacuate in a sequence. The results suggest that (1) the most efficient staged evacuation strategy can help reduce the evacuation time for Galveston Island by approximately one hour, (2) previous studies might have underestimated the evacuation time of Galveston, and (3) an evacuation under the rapid response assumption does not necessarily lead to an effective evacuation. 相似文献
37.
高精度扫描是数字摄影测量生产工作流程的一个重要部分。评定性能优良扫描仪的一个重要的条件是:在一定要求下连续高速的进行高光学分辨率的胶卷扫描。目前徕卡公司在试图改善图像的质量和扫描平台控制的同时,将在DSW彩色扫描仪上提供这种快速的扫描功能。徕卡公司为了这些新的性能,已经进行了许多针对性的革新,这些新的革新技术构成了一个新的型号DSW700。 相似文献
38.
Trace metals in seawater and copepods in the ocean outfall area off the northern Taiwan coast 总被引:3,自引:0,他引:3
The distribution, partitioning and concentrations of trace metals (Cd, Cr, Cu, Fe, Mn, Pb and Zn) in seawater, including dissolved and particulate phases, and in copepods in the ocean outfall area off the northern coast of Taiwan were investigated. Normalization of metal concentrations to the background metal concentration to yield relative enrichment factors (EF), which were used to evaluate the contamination of dissolved and particulate trace metals in seawater around the ocean outfall. The EF results indicated that the outfall area was significantly contaminated by dissolved Fe and Zn, and by particulate Fe, Cr, Cu, Pb and Zn. In addition, most trace metals were chiefly in the particulate phase. The average percentage of total metal concentrations (dissolved plus particulate phases) bound by suspended particulate matter followed the sequence Al(95%) = Mn(95%) > Pb(88%) > Cu(86%) > Fe(72%) > Zn(32%) > Cr(17.5%) > Cd(3.4%). Therefore, metal contamination is better evaluated in solid phase than in the dissolved phase. The concentration ranges of trace metals in the copepods, Temora turbinata, Oncaea venusta and Euchaeta rimana, near the outfall were: Cd, 0.23-1.81 microg g(-1); Cr, 16.5-195 microg g(-1); Cu, 14-160 microg g(-1); Fe, 256-7255 microg g(-1); Mn, 5.5-80.8 microg g(-1); Pb, 2.6-56.2 microg g(-1); Zn, 132-3891 microg g(-1); and Al, 0.21-1.13%. Aluminum, and probably Fe, seemed to be the major elements in copepods. The concentrations of trace metals in copepods, especially Temora turbinata, near the outfall were generally higher than those obtained in the background station. The mean increase in bioconcentration factor of metals in copepods ranged from 4 to 7 and followed the sequence Al(6.4) > Cu(6.2) > Fe(6.0) > Zn(5.7) > Pb(5.6) > Cr(5.5) > Cd(5.1) > Mn(4.7). Therefore, marine copepods in the waters of northern Taiwan can accumulate trace metals over background concentrations and act as contamination indicators. 相似文献
39.
Li Chen Basmah Alabbadi Chih-Hung Tan Tai-Sheng Wang Kuo-Chang Li 《Journal of the Indian Society of Remote Sensing》2017,45(4):699-707
The purpose of this study is to demonstrate the use of an improved genetic algorithm combining operation tree method (IGAOT) and apply it to monitor the salinity of the Taiwan Strait by using remote-sensing data. The genetic algorithm combining operation tree (GAOT) is a data mining method used to automatically discover relationships among nonlinear systems. Based on genetic algorithms (GAs), the relationships between input and output can be expressed as parse trees. The GAOT method typically has the disadvantages of premature convergence, which means it cannot produce satisfying solutions and performs satisfactorily when applied to only low-dimensional problems. Therefore, the GAOT method is enhanced using an automatic incremental procedure to improve the search ability of the method and avoid trapping in a local optimum. In this case study, an IGAOT is used to determine the relationship between the in situ data on the salinity of the Taiwan Strait and the data on the spectral parameters, seven wavebands, of a Moderate-Resolution Imaging Spectroradiometer (MODIS) sensor. The results indicate that the IGAOT model performs more favorably than do the GAOT and linear regression (LR1 and LR2) models, exhibits higher correlation coefficients, and involves fewer estimating errors. The results of this study indicate that the proposed technique is useful for estimating the Taiwan Strait salinity. 相似文献
40.