全文获取类型
收费全文 | 3796篇 |
免费 | 765篇 |
国内免费 | 988篇 |
专业分类
测绘学 | 178篇 |
大气科学 | 869篇 |
地球物理 | 917篇 |
地质学 | 2086篇 |
海洋学 | 384篇 |
天文学 | 262篇 |
综合类 | 402篇 |
自然地理 | 451篇 |
出版年
2024年 | 12篇 |
2023年 | 61篇 |
2022年 | 168篇 |
2021年 | 213篇 |
2020年 | 129篇 |
2019年 | 176篇 |
2018年 | 208篇 |
2017年 | 173篇 |
2016年 | 187篇 |
2015年 | 184篇 |
2014年 | 231篇 |
2013年 | 225篇 |
2012年 | 203篇 |
2011年 | 184篇 |
2010年 | 178篇 |
2009年 | 203篇 |
2008年 | 172篇 |
2007年 | 173篇 |
2006年 | 116篇 |
2005年 | 137篇 |
2004年 | 111篇 |
2003年 | 125篇 |
2002年 | 118篇 |
2001年 | 140篇 |
2000年 | 168篇 |
1999年 | 226篇 |
1998年 | 203篇 |
1997年 | 181篇 |
1996年 | 145篇 |
1995年 | 141篇 |
1994年 | 122篇 |
1993年 | 114篇 |
1992年 | 81篇 |
1991年 | 69篇 |
1990年 | 62篇 |
1989年 | 40篇 |
1988年 | 28篇 |
1987年 | 30篇 |
1986年 | 34篇 |
1985年 | 21篇 |
1984年 | 9篇 |
1983年 | 10篇 |
1982年 | 12篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 2篇 |
1978年 | 5篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1958年 | 4篇 |
排序方式: 共有5549条查询结果,搜索用时 15 毫秒
991.
992.
Field observations of suspended particulate matter (SPM) in the Bohai Bay, China have not been widely reported. The aim of this paper is to describe the horizontal and vertical distribution of mass and volume concentrations of SPM, respectively, based on observed data at 312 stations in the northern Bohai Bay during summer of 2006. A numerical model ECOMSED coupled with a sediment transport module was also established to further discuss the mechanism of the thermocline effect on the vertical distribution of SPM. The mass concentrations of SPM exhibited high inshore values and low offshore values in the horizontal distribution; while in the vertical direction, characteristics of the volume concentration of SPM can be divided into two types: one with a sharp peak at depth of 10–15 m and another without. The peak value at the depth of the thermocline was resulted from concentrated phytoplankton. A numerical experiment further displayed that the thermocline can also prevent particles from being resuspended upward. 相似文献
993.
Sediment transport in the Yellow Sea and East China Sea 总被引:2,自引:0,他引:2
Eight survey cruises in different seasons have been conducted in the Yellow Sea (YS) and East China Sea (ECS) during the period from 2000 to 2008. Suspended sediment concentration (SSC) and hydrological data were collected during each cruise. Data analysis showed that total suspended sediment mass was approximately 0.18 × 109 tons in the surveyed area during spring and autumn seasons. Highly turbid waters were found in the shallow waters between the Subei coast, the Changjiang estuary and the Zhejiang coast with seasonal variations. 相似文献
994.
Food safety and its related influencing factors in China are the hot research topics currently, and cultivated land conversion
is one of the significant factors influencing food safety in China. Taking the North China Plain as the study area, this paper
examines the changes of cultivated land area using satellite images, estimates land productivity from 1985 to 2005 using the
model of Estimation System for Land Productivity (ESLP), and analyzes the impact of cultivated land conversion on the land
production. Compared with the grain yield data from statistical yearbooks, the results indicate that ESLP model is an effective
tool for estimating land productivity. Land productivity in the North China Plain showed a slight decreasing trend from 1985
to 2005, spatially, increased from the north to the south gradually, and the net changes varied in different areas. Cultivated
land area recorded a marginal decrease of 8.0 × 105 ha, mainly converted to other land uses. Cultivated land conversion had more significant negative impacts on land production
than land productivity did. Land production decreased by about 6.48 × 106 t caused by cultivated land conversion between 1985 and 2005, accounting for 91.9% of the total land production reduction.
Although the land productivity increased in Anhui and Jiangsu provinces, it can not offset the overall adverse effects caused
by cultivated land conversion. Therefore, there are significant meanings to control the cultivated land conversion and improve
the land productivity for ensuring the land production in the North China Plain. 相似文献
995.
滠水河流域经济环境协调发展系统动力学模拟 总被引:5,自引:0,他引:5
从系统分析的视角出发,根据滠水河流域的自然与社会经济特点,构建经济环境协调发展系统动力学模型,模型主要参数包括人口、GDP、城市化水平、产业结构、水资源可利用量、水环境容量等。通过模型正负反馈环进行因果关系分析,考虑人口、资源、环境与经济之间的互动与制约关系,进行子系统划分,并确定人口平均增长率、服务业废水率、工业废水率以及水土流失率为敏感性因素,以敏感因素为核心,设计3种不同情景,得出经济、环境协调发展情景为最优方案。通过对不同发展情景的分析与比较,提出滠水河流域有必要采取措施控制人口增长,促进经济发展,保护生态环境,实现该流域经济与环境协调发展。 相似文献
996.
2022年4月下旬至5月上旬,贵州西部共发生了6次冰雹天气过程,有4次冰雹灾害天气严重影响贵州西部红心猕猴桃生产,其主要是发生在4月22日、5月6日、5月8日、5月9日的冰雹天气过程。为了探明2022年4月下旬至5月上旬4次冰雹天气过程对红心猕猴桃果实的影响,通过6次冰雹天气过程影响区的调查,获取冰雹特征数据和红心猕猴桃在4次冰雹灾害过程中的受灾情况,分析诱发冰雹天气发生的大气环流特征,并采用相关统计方法对4次冰雹灾害对红心猕猴桃造成的损失进行评估。结果表明:6次冰雹天气过程中,冰雹最大直径达20mm,但最大直径大多在10mm以下,其中水城区的冰雹最大直径较其他地区大,遭遇冰雹天气次数也最多。影响红心猕猴桃生产的4次冰雹天气过程的天气背景具有贵阳站的CAPE均远远大于威宁站、中低层相对湿度均在90%以上、中高层相对湿度均在80%左右的共同特点。4月22日、5月6日和5月9日冰雹天气发生前,500hPa均有浅槽、700hPa均有低涡切变线入侵贵州西部;5月8日冰雹天气发生前,500hPa和700hPa贵州西部均处于西太副高外围。4次冰雹灾害造成红心猕猴桃885.4hm2绝收,占其基地种植面积的9.04%;因冰雹灾害造成产量损失约1.33万t,直接经济损失19921.5万元。 相似文献
997.
998.
Stokes polarization profiles of the Mg?b2 5172.68 Å spectral line on two simple sunspots are obtained with the Multi-Channel Solar Telescope (MCST) at the Huairou Solar Observing Station (HSOS). This is done by means of scanning this line over the wavelength interval from 200 mÅ redward of the line center to 200 mÅ blueward, in steps of 10 mÅ. A generalized analytic solution to the transfer equation for polarized radiation is presented. With a nonlinear least-square fitting technique, the linear calibration coefficients for the low-chromospheric longitudinal magnetic field is obtained in the weak-field case. We also discuss the problems in calibrating the transverse field with this line. It is shown that the weak-field approximation is not applicable to the chromospheric Mg?b2 line for the transverse component of the magnetic field. 相似文献
999.
西藏色布塔铜钼矿床中辉钼矿Re-Os定年及其成矿意义 总被引:4,自引:0,他引:4
色布塔铜钼矿床位于冈底斯成矿带北部,具有细脉浸染状矿化,围岩蚀变发育等斑岩型铜钼矿床特征.本文使用电感耦合等离子体质谱仪TJA X-series ICP-MS对色布塔铜钼矿床的花岗斑岩中的辉钼矿进行ReOs同位素定年研究,获得了6件样品的模式年龄为87.52±1.35~88.8±1.26Ma,等时线年龄为88.8±1.5Ma.本文研究成果表明,色布塔铜钼矿成矿时代为晚白垩世早期,属燕山期构造-岩浆活动的产物. 相似文献
1000.
Geological and thermochronological studies of the Dashui gold deposit, West Qinling Orogen, Central China 总被引:3,自引:0,他引:3
Qingtao Zeng Noreen J. Evans Brent I. A. McInnes Geoffrey E. Batt Campbell T. McCuaig Leon Bagas Eric Tohver 《Mineralium Deposita》2013,48(3):397-412
The Dashui gold deposit is a structurally controlled, Carlin-type gold deposit hosted by recrystallised limestone in the West Qinling Orogen of Central China. The major, structurally late east-trending Dashui Fault forms the hanging wall to the gold mineralisation at the Dashui mine and defines the contact between Middle Triassic limestone and a steeply dipping overlying succession of Middle Triassic argillaceous limestone, dolomite, and sandstone. Multiple carbonate veins and large-scale supergene enrichment, represented by hematite, goethite, limonite and jarosite, characterise the deposit. Detailed geochronological investigation using zircon SHRIMP U-Pb dating reveals that volcanic rocks closely associated with the Dashui gold deposit were synchronous with the Ge’erkuohe Granite and pre-date mineralisation. The igneous dyke sample from the hanging wall has the same U-Pb zircon age as the footwall, ca. 213 Ma. (U-Th)/He thermochronology on dykes in the hanging wall and footwall of the Dashui Fault yields identical (U-Th)/He zircon ages of ca. 210 Ma but distinct (U-Th)/He apatite ages of ca. 136 and 211 Ma, respectively. Therefore, the hanging wall and footwall are interpreted as having distinct post-mineralisation exhumation histories. Reverse fault movement exhumed the hanging wall ~2 to 4 km since the Late Triassic with the main component of faulting taking place between the Late Triassic and Early Cretaceous. These relationships suggest a Late Triassic to Early Cretaceous age for the primary gold mineralisation at the Dashui gold deposit, with the corollary that any ‘missing portion’ of the deposit, previously hypothesised to exist in the hanging wall of the Dashui Fault, has been eroded away. The mineralisation in the footwall may have been supergene enriched soon after the primary mineralisation was emplaced, because it has been located at shallow depth since the Late Triassic. Semi-quantitative results obtained in this study also constrain the maximum depth of formation of the Dashui gold at no more than 2 km. 相似文献