全文获取类型
收费全文 | 26931篇 |
免费 | 4775篇 |
国内免费 | 7434篇 |
专业分类
测绘学 | 4527篇 |
大气科学 | 4729篇 |
地球物理 | 5855篇 |
地质学 | 14474篇 |
海洋学 | 3677篇 |
天文学 | 243篇 |
综合类 | 2069篇 |
自然地理 | 3566篇 |
出版年
2024年 | 178篇 |
2023年 | 484篇 |
2022年 | 1010篇 |
2021年 | 1192篇 |
2020年 | 1341篇 |
2019年 | 1499篇 |
2018年 | 1263篇 |
2017年 | 1399篇 |
2016年 | 1599篇 |
2015年 | 1657篇 |
2014年 | 1835篇 |
2013年 | 1953篇 |
2012年 | 1867篇 |
2011年 | 1844篇 |
2010年 | 1506篇 |
2009年 | 1662篇 |
2008年 | 1697篇 |
2007年 | 1754篇 |
2006年 | 1665篇 |
2005年 | 1521篇 |
2004年 | 1325篇 |
2003年 | 1144篇 |
2002年 | 1074篇 |
2001年 | 897篇 |
2000年 | 848篇 |
1999年 | 768篇 |
1998年 | 712篇 |
1997年 | 622篇 |
1996年 | 553篇 |
1995年 | 484篇 |
1994年 | 435篇 |
1993年 | 362篇 |
1992年 | 237篇 |
1991年 | 194篇 |
1990年 | 135篇 |
1989年 | 95篇 |
1988年 | 103篇 |
1987年 | 66篇 |
1986年 | 39篇 |
1985年 | 31篇 |
1984年 | 18篇 |
1983年 | 2篇 |
1982年 | 10篇 |
1981年 | 8篇 |
1980年 | 13篇 |
1979年 | 10篇 |
1978年 | 10篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1954年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
91.
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
92.
From the phase-resolving improved Boussinesq equations (Beji and Nadaoka, Ocean Engineering 23 (1996) 691), a phase-averaged Boussinesq model for water waves is derived by more effectively describing carrier wave groups and accompanying long wave evolution with less CPU time. Linear shoaling characteristics of carrier wave equations are investigated and found to agree exactly with the analytical expression obtained from the constancy of energy flux for the improved Boussinesq equations themselves, showing that the present model equations are the results of a consistent derivation procedure regarding energy considerations. Numerical simulations of the derived equations for the single wave group and narrow-banded random waves show the validity of the present model and its high performance, especially on the CPU time. 相似文献
93.
94.
95.
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
96.
97.
98.
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
99.
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
100.