全文获取类型
收费全文 | 43535篇 |
免费 | 2057篇 |
国内免费 | 3339篇 |
专业分类
测绘学 | 2100篇 |
大气科学 | 4619篇 |
地球物理 | 9042篇 |
地质学 | 19882篇 |
海洋学 | 2706篇 |
天文学 | 4667篇 |
综合类 | 2981篇 |
自然地理 | 2934篇 |
出版年
2023年 | 182篇 |
2022年 | 473篇 |
2021年 | 525篇 |
2020年 | 462篇 |
2019年 | 593篇 |
2018年 | 5387篇 |
2017年 | 4618篇 |
2016年 | 3406篇 |
2015年 | 890篇 |
2014年 | 1025篇 |
2013年 | 1122篇 |
2012年 | 1800篇 |
2011年 | 3737篇 |
2010年 | 2938篇 |
2009年 | 3316篇 |
2008年 | 2775篇 |
2007年 | 3202篇 |
2006年 | 903篇 |
2005年 | 882篇 |
2004年 | 1051篇 |
2003年 | 1023篇 |
2002年 | 824篇 |
2001年 | 579篇 |
2000年 | 617篇 |
1999年 | 637篇 |
1998年 | 580篇 |
1997年 | 567篇 |
1996年 | 492篇 |
1995年 | 413篇 |
1994年 | 429篇 |
1993年 | 357篇 |
1992年 | 274篇 |
1991年 | 253篇 |
1990年 | 204篇 |
1989年 | 193篇 |
1988年 | 192篇 |
1987年 | 188篇 |
1986年 | 154篇 |
1985年 | 140篇 |
1984年 | 147篇 |
1983年 | 141篇 |
1982年 | 151篇 |
1981年 | 148篇 |
1980年 | 109篇 |
1979年 | 96篇 |
1978年 | 50篇 |
1977年 | 69篇 |
1976年 | 74篇 |
1975年 | 60篇 |
1973年 | 64篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
M. U. Shirazi M. Y. Ashraf M. A. Khan M. H. Naqvi 《International Journal of Environmental Science and Technology》2005,2(3):233-236
Water culture experiments were conducted to study the response of ten wheat genotypes to external K application (10 mmol KCI dm?3) at seedling stage under saline condition (0 and 100 mmol NaCl dm?3). The data showed that there was an increase in the shoot and root length with the application of external K. The increase was more pronounced under control than under saline conditions. The better performing genotypes under two treatments were Bhitai, NIAB-41, NIAB-I076 and Khirman. The enhanced growth of these genotypes under saline condition might be due to the quick response to external K application, resulting in high K/Na ratio. The results indicated that the genotypes, which have the ability of enhanced K/Na discrimination, might perform better under saline conditions when sufficient potassium is applied in the rooting medium. 相似文献
992.
993.
994.
The major obstacles for modelling flood processes in karst areas are a lack of understanding and model representations of
the distinctive features and processes associated with runoff generation and often a paucity of field data. In this study,
a distributed flood-modelling approach, WetSpa, is modified and applied to simulate the hydrological features and processes
in the karst Suoimuoi catchment in northwest Vietnam. With input of topography, land use and soil types in a GIS format, the
model is calibrated based on 15 months of hourly meteorological and hydrological data, and is used to simulate both fast surface
and conduit flows, and groundwater discharges from karst and non-karst aquifers. Considerable variability in the simulation
accuracy is found among storm events and within the catchment. The simulation results show that the model is able to represent
reasonably well the stormflows generated by rainfall events in the study catchment. 相似文献
995.
996.
Landscape change and sandy desertification in arid areas: a case study in the Zhangye Region of Gansu Province, China 总被引:3,自引:0,他引:3
The Zhangye Region of Gansu Province is an important agricultural base in arid northwestern China. During the twentieth century, especially in the last five decades, the region has experienced sandy desertification. To document the status and causes of this deterioration, satellite images, meteorological and socioeconomic data to assess landscape change from 1993 to 2002 were interpreted and analyzed. The results show that during the intervening 9-year period the area of sandy lands has increased by 642.2 km2, which consist of aeolian sand dune (357.1 km2) and potential sandy land (216.3 km2). Although the development and reversion of sandy desertification co-exist, the sandy desertification in this area seems serious and is attributable to the irrational use of water and land. 相似文献
997.
L. G. Tham H. Liu C. A. Tang P. K. K. Lee Y. Tsui 《Rock Mechanics and Rock Engineering》2005,38(1):1-19
Summary To understand the failure mechanism of quasi-brittle materials like rock under tensile stress, observations on the failure process of granite and marble plate specimens under tension are summarized and presented. Micro- and macro-failure properties of rock plates under uniaxial tension have been characterized by using an acoustic emission technique. Acoustic emission signals associated with micro-fractures are captured to locate the sources. An algorithm based on arrival time difference is developed for this purpose. The results reveal clearly the failure processes of rock which include initiation, nucleation and propagation of micro-fractures when the axial stress is close to the peak strength of rock. It is believed that the difference in heterogeneity between granite and marble specimens leads to different fracture shapes and different behaviors of associated acoustic emissions. Numerical simulation of acoustic emissions for two-dimensional tensile test is also carried out. The simulated characteristics are in good agreement with the experimental results. 相似文献
998.
Hydrochemical characteristics and salinity of groundwater in the Ejina Basin, Northwestern China 总被引:8,自引:0,他引:8
A hydrochemical investigation was conducted in the Ejina Basin to identify the hydrochemical characteristics and the salinity
of groundwater. The results indicate that groundwater in the area is brackish and are significantly zonation in salinity and
water types from the recharge area to the discharge area. The ionic ration plot and saturation index (SI) calculation suggest
that the silicate rock weathering and evaporation deposition are the dominant processes that determine the major ionic composition
in the study area. Most of the stable isotope δ18O and δD compositions in the groundwater is a meteoric water feature, indicating that the groundwater mainly sources from
meteoric water and most groundwater undergoes a long history of evaporation. Based on radioactive isotope tritium (3H) analysis, the groundwater ages were approximately estimated in different aquifers. The groundwater age ranges from less
than 5 years, between 5 years and 50 years, and more than 50 years. Within 1 km of the river water influence zone, the groundwater
recharges from recent Heihe river water and the groundwater age is about less than 5 years in shallow aquifer. From 1 km to
10 km of the river water influence zone, the groundwater sources from the mixture waters and the groundwater age is between
5 years and 50 years in shallow aquifer. The groundwater age is more than 50 years in deep confined aquifer. 相似文献
999.
M. Koch-Müller P. Dera Y. Fei H. Hellwig Z. Liu J. Van Orman R. Wirth 《Physics and Chemistry of Minerals》2005,32(5-6):349-361
We synthesized superhydrous phase B (shy-B) at 22 GPa and two different temperatures: 1200°C (LT) and 1400°C (HT) using a
multi-anvil apparatus. The samples were investigated by transmission electron microscopy (TEM), single crystal X-ray diffraction,
Raman and IR spectroscopy. The IR spectra were collected on polycrystalline thin-films and single crystals using synchrotron
radiation, as well as a conventional IR source at ambient conditions and in situ at various pressures (up to 15 GPa) and temperatures
(down to −180°C). Our studies show that shy-B exists in two polymorphic forms. As expected from crystal chemistry, the LT
polymorph crystallizes in a lower symmetry space group (Pnn2), whereas the HT polymorph assumes a higher symmetry space group (Pnnm). TEM shows that both modifications consist of nearly perfect crystals with almost no lattice defects or inclusions of additional
phases. IR spectra taken on polycrystalline thin films exhibit just one symmetric OH band and 29 lattice modes for the HT
polymorph in contrast to two intense but asymmetric OH stretching bands and at least 48 lattice modes for the LT sample. The
IR spectra differ not only in the number of bands, but also in the response of the bands to changes in pressure. The pressure
derivatives for the IR bands are higher for the HT polymorph indicating that the high symmetry form is more compressible than
the low symmetry form. Polarized, low-temperature single-crystal IR spectra indicate that in the LT-polymorph extensive ordering
occurs not only at the Mg sites but also at the hydrogen sites. 相似文献
1000.