全文获取类型
收费全文 | 24864篇 |
免费 | 4427篇 |
国内免费 | 6551篇 |
专业分类
测绘学 | 4538篇 |
大气科学 | 4808篇 |
地球物理 | 6031篇 |
地质学 | 11109篇 |
海洋学 | 3609篇 |
天文学 | 256篇 |
综合类 | 1957篇 |
自然地理 | 3534篇 |
出版年
2024年 | 162篇 |
2023年 | 437篇 |
2022年 | 904篇 |
2021年 | 1071篇 |
2020年 | 1231篇 |
2019年 | 1362篇 |
2018年 | 1154篇 |
2017年 | 1326篇 |
2016年 | 1452篇 |
2015年 | 1549篇 |
2014年 | 1634篇 |
2013年 | 1858篇 |
2012年 | 1694篇 |
2011年 | 1720篇 |
2010年 | 1394篇 |
2009年 | 1570篇 |
2008年 | 1566篇 |
2007年 | 1658篇 |
2006年 | 1552篇 |
2005年 | 1373篇 |
2004年 | 1207篇 |
2003年 | 1026篇 |
2002年 | 977篇 |
2001年 | 816篇 |
2000年 | 746篇 |
1999年 | 681篇 |
1998年 | 633篇 |
1997年 | 548篇 |
1996年 | 484篇 |
1995年 | 436篇 |
1994年 | 393篇 |
1993年 | 332篇 |
1992年 | 200篇 |
1991年 | 178篇 |
1990年 | 122篇 |
1989年 | 91篇 |
1988年 | 91篇 |
1987年 | 61篇 |
1986年 | 30篇 |
1985年 | 30篇 |
1984年 | 17篇 |
1982年 | 10篇 |
1981年 | 9篇 |
1980年 | 13篇 |
1979年 | 10篇 |
1978年 | 10篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1972年 | 3篇 |
1954年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
An overview of the Oyashio ecosystem 总被引:3,自引:0,他引:3
The Oyashio shelf region and the seasonally ice-covered areas north of Hokkaido are highly productive, supporting a wide range of species including marine mammals, seabirds and commercially important species in the western subarctic Pacific. The fishes include gadids, such as walleye pollock and Pacific cod, and subarctic migratory pelagic fishes such as chum salmon and pink salmon. It is also an important summer feeding ground for subtropical migrants such as the Japanese sardine, Japanese anchovy, Pacific saury, mackerels, Japanese common squid, whales and seabirds. In recent decades, some components of the Oyashio ecosystem (i.e., phytoplankton, mesozooplankton, gadid fish, and subtropical migrants) have shown changes in species abundance or distribution that are correlated with environmental changes such as the 1976/1977 and 1988/1989 regime shifts. The First Oyashio Intrusion moved northward from the mid-1960s until the late 1970s, when it moved southward until the 1980s, after which it returned to the north again after the mid-1990s. The sea-surface temperature in spring decreased after the late 1970s, increased after the late 1980s, and remained high during the 1990s. The extent of ice cover in the Sea of Okhostk also decreased during the latest warming in the 1980–1990s but has increased again since the late 1990s. This and other variabilities affect the Oyashio ecosystem and the surrounding region. 相似文献
92.
93.
The method has been established to calculate the environmental capacity (ECo),surplus environment capacity (SECo) of water with respect to marine petroleum hydrocarbons associated with oil (PHAOs) and the self-purification capacity (SPCo) of main self-purification process to PHAOs in the Jiaozhou Bay, China, according to the dynamic model for distribution of marine PHAOs among multiphase environments. The variation of concentration of PHAOs in the Jiaozhou Bay is well simulated by the dynamic model. Based on the model.the ECo, SECo of water with respect to PHAOs in the Jiaozhou Bay were calculated during the last 10 years under the first-class and second-class quality standard requirement, according to SPCoof main self-purification process to PHAOs. The results show that about 200 tons of PHAOs could be discharged into the Jiaozhou Bay for maintaining the first class seawater quality standard, and about 600 tons of PHAOs for the second class seawater quality standard later. 相似文献
94.
95.
96.
Estimation of Land Subsidence Based on Groundwater Flow Model 总被引:8,自引:0,他引:8
This article presents an approach for estimating land subsidence due to withdrawal of groundwater. The proposed method calculates the groundwater seepage in 3-D-condition and calculates the land subsidence one-dimensionally. The governing equation on groundwater seepage is based on the three-dimensional mass conservation law and the principle of effective stress. The land subsidence calculation method is derived based on the following assumptions: (1) displacements occur only in the vertical direction, and (2) in vertical direction the total stresses do not change. The governing equation is solved by numerical method, i.e., finite element method (FEM) in spatial discretization and finite difference method (FDM) in time series discretization. In FEM Galerkin method is adopted and in FDM, lumped matrix method is employed. The proposed method is calibrated via analyzing 1-D consolidation problem and the results are compared with those from Terzaghi's one-dimensional consolidation theory and oedometer test. The proposed method is employed to analyze the consolidation of a soft layer due to withdrawal of groundwater from an aquifer under it. Moreover, this method is also applied to a field case of land subsidence due to groundwater pumping in a gas production field in Japan. The analytical results are compared with the field observed data. The results show that this approach simulates the field case well. 相似文献
97.
Katsumi Matsumoto 《Journal of Oceanography》2006,62(6):887-902
Iron fertilization of nutrient-rich surface waters of the ocean is one possible way to help slow the rising levels of atmospheric
CO2 by sequestering it in the oceans via biological carbon export. Here, I use an ocean general circulation model to simulate
a patch of nutrient depletion in the subpolar northwest Pacific under various scenarios. Model results confirm that surface
fertilization is an inefficient way to sequester carbon from the atmosphere (Gnanadesikan et al., 2003), since only about 20% of the exported carbon comes initially from the atmosphere. Fertilization reduces future production
and thus CO2 uptake by utilizing nutrients that would otherwise be available later. Effectively, this can be considered as leakage when
compared to a control run. This “effective” leakage and the actual leakage of sequestered CO2 cause a significant, rapid decrease in carbon retention (only 30–45% retained after 10 years and less than 20% after 50 years).
This contrasts markedly with the almost 100% retention efficiency for the same duration using the same model, when carbon
is disposed directly into the northwest Pacific (Matsumoto and Mignone, 2005). As a consequence, the economic effectiveness
of patch fertilization is poor in two limiting cases of the future price path of carbon. Sequestered carbon in patch fertilization
is lost to the atmosphere at increasingly remote places as time passes, which would make monitoring exceedingly difficult.
If all organic carbon from one-time fertilization reached the ocean bottom and remineralized there, acidification would be
about −0.05 pH unit with O2 depletion about −20 μmol kg−1. These anomalies are probably too small to seriously threaten deep sea biota, but they are underestimated in the model because
of its large grid size. The results from this study offer little to advocate purposeful surface fertilization as a serious
means to address the anthropogenic carbon problem. 相似文献
98.
99.
100.
A multidisciplinary study in the Gulf of Cadiz is revisited, using additional diagnostic modelling tools. The dissolved trace metal (Cu, Ni, Zn, Co) distributions in the Gulf of Cadiz are analysed using modelled tracer evolutions, field observations and the concept of tracer ages. This study shows that a significant part of the observed metal distributions can be explained by the metal inputs of three river systems (Guadiana, Rio Tinto and Odiel, Guadalquivir) discharging into the Gulf of Cadiz, while the remainder of the signal is most likely associated with the benthic metal remobilisation along the shelf of this coastal region. 相似文献