全文获取类型
收费全文 | 25990篇 |
免费 | 297篇 |
国内免费 | 1085篇 |
专业分类
测绘学 | 1514篇 |
大气科学 | 2083篇 |
地球物理 | 4886篇 |
地质学 | 12205篇 |
海洋学 | 1237篇 |
天文学 | 1847篇 |
综合类 | 2220篇 |
自然地理 | 1380篇 |
出版年
2024年 | 8篇 |
2023年 | 29篇 |
2022年 | 60篇 |
2021年 | 76篇 |
2020年 | 66篇 |
2019年 | 69篇 |
2018年 | 4820篇 |
2017年 | 4091篇 |
2016年 | 2658篇 |
2015年 | 312篇 |
2014年 | 163篇 |
2013年 | 108篇 |
2012年 | 1073篇 |
2011年 | 2828篇 |
2010年 | 2125篇 |
2009年 | 2384篇 |
2008年 | 1959篇 |
2007年 | 2406篇 |
2006年 | 119篇 |
2005年 | 228篇 |
2004年 | 454篇 |
2003年 | 443篇 |
2002年 | 302篇 |
2001年 | 75篇 |
2000年 | 88篇 |
1999年 | 39篇 |
1998年 | 45篇 |
1997年 | 19篇 |
1996年 | 23篇 |
1995年 | 19篇 |
1994年 | 20篇 |
1993年 | 11篇 |
1992年 | 16篇 |
1991年 | 8篇 |
1990年 | 10篇 |
1989年 | 9篇 |
1988年 | 5篇 |
1987年 | 21篇 |
1986年 | 8篇 |
1985年 | 6篇 |
1984年 | 10篇 |
1983年 | 11篇 |
1982年 | 20篇 |
1981年 | 33篇 |
1980年 | 36篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1976年 | 15篇 |
1974年 | 5篇 |
1973年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
Rainer Schneck Arne Micheels Volker Mosbrugger 《International Journal of Earth Sciences》2012,101(1):323-338
The Late Miocene belongs to the late phase of the Cenozoic. Climate at that time was still warmer and more humid as compared
to today, especially in the high latitudes. Corresponding to the climate situation, palaeobotanical evidences support that
vegetation in the high northern latitudes changed significantly from the Late Miocene until today. To quantify the climate
impact of this vegetation change, we analyse how vegetation in the high northern latitudes contribute to climate evolution.
For that, we perform climate modelling sensitivity experiments for the present and for the Late Miocene (Tortonian, 11–7 Ma).
For our present-day sensitivity experiment, we introduce the Tortonian vegetation in the high northern latitudes. For our
Tortonian sensitivity experiment, we introduce the modern vegetation on the same grid cells. In the Tortonian and in the present,
the modern vegetation leads to a strong cooling of the northern extratropics (up to −4°C). Nevertheless, the meridional heat
transports remain nearly unchanged in both cases. In general, the vegetation impact on climate is similar in the Tortonian
and in the present. However, some exceptions occur. Due to the Tethys Ocean in the Tortonian, temperatures decline only weakly
in eastern Europe and western Asia. In the Tortonian climate, temperatures on the Sahara realm rise (up to +1.5°C), while
the temperatures do not change remarkably in the present-day climate. This different behaviour is caused by a stronger and
more sensitive hydrological cycle on the Sahara region during the Tortonian. 相似文献
942.
A growing body of field evidence indicates that hypersolidus fabrics preserved in syntectonic plutons are likely to have formed
in highly crystallized ‘rigid sponge’ magma. This paper demonstrates that such magma could be idealized as a rheological solid
and that the development of non-coaxial fabrics in plutonic rocks can very conveniently be modeled in the framework of solid
mechanics. Using the finite element method (FEM), we modeled two strain regimes of small magnitudes (plane-strain horizontal
simple shear with the shear strain γ of up to 0.30 and plane-strain pure shear of up to 15% shortening) superposed onto vertically
oriented and variously spaced elastic phenocrysts set in the viscoelastic matrix. In the simple shear regime, the phenocrysts
slightly rotate toward the shear plane, while the principal strain directions in the matrix are instantaneously oriented at
an angle of about 45° or less to the phenocryst fabric. Simple shear thus can only lead to the formation of oblique phenocryst
and matrix fabrics. By contrast, the vertical phenocryst fabric is maintained in the pure shear regime, and a new horizontal
fabric can develop almost instantaneously in the matrix even for small amounts of superposed shortening (5% shortening after
10 ky in our model). We conclude that such a mechanism can easily produce perpendicular hypersolidus fabrics in plutonic rocks
and that only a very short time span (first thousands of years) is required to develop magmatic fabric in a pluton for ‘normal’
rates (10−15 to 10−13 s−1) of tectonic deformation. 相似文献
943.
C K CHANG H MD AZAMATHULLA N A ZAKARIA A AB GHANI 《Journal of Earth System Science》2012,121(1):125-133
This paper evaluates the performance of three soft computing techniques, namely Gene-Expression Programming (GEP) (Zakaria
et al 2010), Feed Forward Neural Networks (FFNN) (Ab Ghani et al 2011), and Adaptive Neuro-Fuzzy Inference System (ANFIS) in the prediction of total bed material load for three Malaysian
rivers namely Kurau, Langat and Muda. The results of present study are very promising: FFNN (R
2 = 0.958, RMSE = 0.0698), ANFIS (R
2 = 0.648, RMSE = 6.654), and GEP (R
2 = 0.97, RMSE = 0.057), which support the use of these intelligent techniques in the prediction of sediment loads in tropical
rivers. 相似文献
944.
945.
Gábor Dobosi Pamela D. Kempton Hilary Downes Antal Embey-Isztin Matthew Thirlwall Peter Greenwood 《Contributions to Mineralogy and Petrology》2003,144(6):671-683
Mafic granulite xenoliths from the lower crust of the Pannonian Basin are dominated by LREE-depleted bulk-rock compositions. Many of these have MORB-like 143Nd/144Nd but 87Sr/86Sr is elevated relative to most MORBs. Their '18O values cover a wide range from +3.8 to +9.5. A group of LREE-enriched mafic granulites have higher 87Sr/86Sr (0.704-0.708) and lower 143Nd/144Nd (0.5128-0.5124), with higher '18O values on average (+7.8 to +10.6) than the LREE-depleted granulites. The LREE-enriched granulites are, however, isotopically similar to newly discovered metasedimentary granulite xenoliths. A sublinear correlation in )Hf-)Nd isotope space has a shallower slope than the crust-mantle array, with the metasedimentary rocks forming the low )Hf end member; the radiogenic end is restricted to the LREE-depleted granulites and these overlap the field of MORB. Pb isotopes for the LREE-depleted samples are less radiogenic on average than those of the LREE-enriched and metasedimentary xenoliths, and metasedimentary granulites have consistently higher 208Pb/204Pb. The wide range in '18O over a restricted range in Nd and Sr isotope values, in combination with the predominance of LREE-depleted trace-element compositions, is consistent with an origin as a package of hydrothermally altered oceanic crust. The existence of '18O values lower than average MORB and/or mantle peridotite requires that at least some of these rocks were hydrothermally altered at high temperature, presumably in the oceanic lower crust. The low 143Nd/144Nd of the LREE-enriched mafic granulites cannot be explained by simple mixing between a LREE-depleted melt and an enriched component, represented by the recovered metasediments. Instead, we interpret these rocks as the metamorphic equivalent of the shallowest levels of the ocean crust where pillow basalts are intimately intercalated with oceanic sediments. A possible model is accretion of oceanic crustal slices during subduction and convergence, followed by high-grade metamorphism during the Alpine orogeny. 相似文献
946.
947.
948.
Mujib watershed is an important groundwater basin which is considered a major source for drinking and irrigation water in
Jordan. Increased dependence on groundwater needs improved aquifer management with respect to understanding deeply recharge
and discharge issues, planning rates withdrawal, and facing water quality problems arising from industrial and agricultural
contamination. The efficient management of this source depends on reliable estimates of the recharge to groundwater and is
needed in order to protect Mujib basin from depletion. Artificial groundwater recharge was investigated in this study as one
of the important options to face water scarcity and to improve groundwater storage in the aquifer. A groundwater model based
on the MODFLOW program, calibrated under both steady- and unsteady-state conditions, was used to investigate different groundwater
management scenarios that aim at protecting the Mujib basin. The scenarios include variations of abstraction levels combined
with different artificial groundwater recharge quantities. The possibilities of artificial groundwater recharge from existing
and proposed dams as well as reclaimed municipal wastewater were investigated. Artificial recharge options considered in this
study are mainly through injecting water directly to the aquifer and through infiltration from reservoir. Three scenarios
were performed to predict the aquifer system response under different artificial recharge options (low, moderate, and high)
which then compared with no action (recharge) scenario. The best scenario that provides a good recovery for the groundwater
table and that can be feasible is founded to be by reducing current abstraction rates by 20% and implementing the moderate
artificial recharge rates of 26 million(M)m3/year. The model constructed in this study helps decision makers and planners in selecting optimum management schemes suitable
for such arid and semi-arid regions. 相似文献
949.
In this study subsidence due to groundwater withdrawal was investigated. Kerman Province in Iran is struggling with land subsidence
problem due to extensive groundwater withdrawal mainly for farming. The rate and type of groundwater withdrawal has very important
impact on settlement rate. In this research, effective parameters on land subsidence caused by groundwater withdrawal were
determined by laboratory tests. Sampling had done up to depth of 300 m mainly with remolded specimens from Shams-abad, Nouq
plain in Kerman province. Similar to the field preconsolidation pressure was applied on specimens in the laboratory. Rate
of applied stress on prepared specimens was similar to effect of oscillation of groundwater level. In order to model the actual
soil behavior in the laboratory, one-dimensional consolidation device (odometer) was adopted for testing. In these tests,
the effect of loading caused by seasonal oscillation of groundwater table is considered by means of cyclic loading in the
testing which has great effect on rate of settlements. The results of tests show that when the water table level periodically
increases and decreases the amount of settlement decrease, comparing with the case when the groundwater table drop to a constant
level. In order to predict the further effects of groundwater level oscillation and actual field condition on land subsidence,
a finite element model based on Biots’ three-dimensional consolidation theory was developed. After calibration of finite element
model with laboratory tests, this model was used for prediction the effect of groundwater level oscillation on actual field
conditions. 相似文献
950.
Haimanti Biswas Mitali Dey Dipnarayan Ganguly Tarun K. De Sandip Ghosh Tapan K. Jana 《Estuaries and Coasts》2010,33(2):395-398
Inter-annual variations of phytoplankton abundance and community organization were observed over a two-decade period along
with the ancillary parameters at the land–ocean boundary associated with the Sundarban mangrove forest (21°32′ and 22°40′
N and 88°05′ and 89° E), along the NE Coast of the Bay of Bengal. The number of definable Bacillariophyceae species exceeded Dinophyceae taxa, and the total number of bloom-forming species declined from a maximum of ten in 2000 and a minimum of two in 2007.
Blooms of the diatom Coscinodiscus radiatus were common in 2000 and 2007. Tide cycles and the onset of the monsoon season played important roles in diurnal and seasonal
variability of phytoplankton. Phytoplankton biovolume showed seasonality, with the highest levels during post-monsoon periods
and lowest levels during the monsoon period. Phytoplankton abundance was correlated to rainfall patterns, which may be altered
by long-term changes in climate. 相似文献