全文获取类型
收费全文 | 17051篇 |
免费 | 2971篇 |
国内免费 | 4080篇 |
专业分类
测绘学 | 866篇 |
大气科学 | 3764篇 |
地球物理 | 4482篇 |
地质学 | 8638篇 |
海洋学 | 1760篇 |
天文学 | 888篇 |
综合类 | 1677篇 |
自然地理 | 2027篇 |
出版年
2024年 | 83篇 |
2023年 | 315篇 |
2022年 | 762篇 |
2021年 | 855篇 |
2020年 | 725篇 |
2019年 | 800篇 |
2018年 | 980篇 |
2017年 | 854篇 |
2016年 | 989篇 |
2015年 | 775篇 |
2014年 | 1029篇 |
2013年 | 948篇 |
2012年 | 821篇 |
2011年 | 917篇 |
2010年 | 939篇 |
2009年 | 885篇 |
2008年 | 834篇 |
2007年 | 825篇 |
2006年 | 672篇 |
2005年 | 586篇 |
2004年 | 490篇 |
2003年 | 532篇 |
2002年 | 545篇 |
2001年 | 469篇 |
2000年 | 575篇 |
1999年 | 781篇 |
1998年 | 690篇 |
1997年 | 709篇 |
1996年 | 607篇 |
1995年 | 516篇 |
1994年 | 535篇 |
1993年 | 401篇 |
1992年 | 326篇 |
1991年 | 250篇 |
1990年 | 187篇 |
1989年 | 168篇 |
1988年 | 180篇 |
1987年 | 105篇 |
1986年 | 85篇 |
1985年 | 76篇 |
1984年 | 54篇 |
1983年 | 58篇 |
1982年 | 44篇 |
1981年 | 41篇 |
1980年 | 28篇 |
1979年 | 20篇 |
1978年 | 10篇 |
1977年 | 5篇 |
1958年 | 16篇 |
1954年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Zhao Kang Zhou Yun Yu Xiang Yan Yajing Song Yufeng Wang Junqiang Suo Tianyuan Guo Xiao 《Natural Resources Research》2021,30(5):3839-3852
Natural Resources Research - Depletion of shallow mineral resources caused by deep mining has become an inevitable trend, and deep mining can increase safety accidents and geological hazards.... 相似文献
52.
本文以澜沧江下游流域边坡稳定性评价为实例,对模糊评判模型的应用问题和处理技术进行了论述。认为该模型简便实用、效果稳健、适应性强,是边坡稳定性评价工作中适宜推广的一个实用模型。此外,文中还提出了用于模糊评判模型的信息检验法,讨论了这一方法在遴选有效评价因子和优化模型效能中的作用。 相似文献
53.
新疆阿拉套山花岗岩类的岩石化学 总被引:6,自引:0,他引:6
新疆阿拉套山南坡东西向展布的花岗岩石的岩石化学研究表明,本区同时存在I型和S型两种类型的花岗岩,其分布受构造环境控制,靠近古板块缝合线为I型花岗岩分布区、远离古板块缝合线为S型花岗岩分布区。源岩性质和作用强弱等因素可能是造成岩石化学成分差异的原因。 相似文献
54.
To ensure the safety of structures, high-speed railway and highway founded on liquefiable soils, liquefaction evaluation is a primary task. Herein, a dynamic set pair analysis method based on variable weights is introduced to assess liquefaction. A concept of connection degree of set pair consisting of the evaluation sample and the classification standard was described to interpret the uncertainty of liquefaction and relationship. Moreover, based on reward and penalty mechanism, state variable vectors were presented to assign composite weights of evaluation indicators in order to take into account impacts of indicator variation. And the integrated connection degree to each grade was calculated to evaluate liquefaction of the evaluated cases. Results from practical example and comparison with field investigation and extension method indicated that this proposed model used to evaluate liquefaction is feasible and effective. In addition, it can quantitatively describe liquefaction hierarchy of evaluated samples. 相似文献
55.
56.
TingSheng Xiang XiaoBo Liu Min Zhang Fang Liu BaiGuang Li HuaE Fu Li Zhao WangShui Hu 《中国科学D辑(英文版)》2009,52(1):128-134
The distribution of indigenous microorganisms was surveyed in Block 1 of Daqing Oilfield. Based on this survey, the indigenous microorganisms in the formation water were activated with different activator systems at the simulated stratum ecological environment. The changes of the number of bacteria of various physiological groups were determined during the process of activation. Also changes of pH value and composition of gas productions were analyzed at the end of culturing. The results showed that the selected block formation water contained a great number of saprophytic bacteria, hydrocarbon-oxidizing bacteria, fermentative bacteria, methane-producing bacteria and sulfate-reducing bacteria. Under the conditions that the growth of sulfate-reducing bacteria was controlled the block had the potential to enhance oil recovery by activating beneficial bacteria. The growth of sulfate-reducing bacteria can be inhibited through the activation of nitrate-reducing bacteria. The number of nitrate-reducing bacteria reached 106–107 cells/mL, but sulfate-reducing bacteria reached only 0–45 cells/mL in A system. Methane-producing bacteria can be activated by C, D activators. The relative content of biological methane in the light hydrocarbon gas reached 80% in C, D systems. B activator was conducive to the propagation of acid-producing bacteria, so that the pH value of the culture medium decreased from 7.5 to around 5.0. Hydrocarbon-oxidizing bacteria can be activated by various activator systems. There was low molecular light hydrocarbon in gas production according to the analysis of gas chromatograph. According to the content of methane and the number of methane-producing bacteria, methane only can be generated through activating methane-producing bacteria. By choosing different activator systems, various populations of indigenous microorganisms can be activated accordingly. 相似文献
57.
Xin Zhao 《地理学报(英文版)》2014,24(6):1199-1200
The 2014 Regional Conference of the International Geographical Union(IGU)was held on August 18-22 in Krakow,Poland.The total number of participants was 1374 from 64 countries and regions,including 357 from Poland.Germany,Japan,Czech,UK,China,Russia,USA,France and Hungary are the top countries of attendees in this conference. 相似文献
58.
为探明气候变化下干旱半干旱地区湿草甸参考作物蒸散发(ET0)影响因子,使用FAO 56 P-M模型对科尔沁湿草甸ET0进行模拟,利用涡度相关系统对模型的适用性进行评价,并通过通径分析及指标敏感性分析对ET0的影响因子进行辨识。结果表明:(1)小时尺度模拟精度最高,日尺度次之,月尺度较差,小时尺度上晴、阴、雨3种天气条件下模拟效果不同,晴天最优,阴雨天较差。(2)ET0年内变化呈单峰曲线状,生长季明显高于非生长季,集中在3—10月,占全年89.79%。生长季典型晴天ET0逐小时分布特征遵循倒“U”单峰型变化规律。(3)通径分析结果显示,对ET0的通径系数以及对回归方程估测可靠程度E的总贡献均表现为VPD(饱和水汽压差) > Tmin(最低气温) > Rn(冠层表面净辐射)>u2(2 m高度风速),即VPD为影响ET0最重要的因子;指标敏感性分析中,在去除VPD后引起的E变化最大,说明ET0对VPD的变化最为敏感,其次为u2、Tmin和Rn。 相似文献
59.
60.
Soil erosion has become a major global environmental problem and is particularly acute on the Loess Plateau (LP), China. It is therefore highly important to control this process in order to improve ecosystems, protect ecological security, and maintain the harmonious relationship between humans and nature. We compared the effects of rainfall and land use (LU) patterns on soil erosion in different LP watersheds in this study in order to augment and improve soil erosion models. As most research on this theme has so far been focused on individual study areas, limited analyses of rainfall and LU patterns on soil erosion within different- scale watersheds has so far been performed, a discrepancy which might influence the simulation accuracies of soil erosion models. We therefore developed rainfall and LU pattern indices in this study using the soil erosion evaluation index as a reference and applied them to predict the extent of this process in different-scale watersheds, an approach which is likely to play a crucial role in enabling the comprehensive management of this phenomenon as well as the optimized design of LU patterns. The areas considered in this study included the Qingjian, Fenchuan, Yanhe, and Dali river watersheds. Results showed that the rainfall erosivity factor (R) tended to increase in these areas from 2006 to 2012, while the vegetation cover and management factor (C) tended to decrease. Results showed that as watershed area increased, the effect of rainfall pattern on soil erosion gradually decreased while patterns in LU trended in the opposite direction, as the relative proportion of woodland decreased and the different forms of steep slope vegetation cover became more homogenous. As watershed area increased, loose soil and craggy terrain properties led to additional gravitational erosion and enhanced the effects of both soil and topography. 相似文献