全文获取类型
收费全文 | 12310篇 |
免费 | 2730篇 |
国内免费 | 3024篇 |
专业分类
测绘学 | 966篇 |
大气科学 | 2619篇 |
地球物理 | 3226篇 |
地质学 | 6491篇 |
海洋学 | 1700篇 |
天文学 | 521篇 |
综合类 | 1205篇 |
自然地理 | 1336篇 |
出版年
2024年 | 28篇 |
2023年 | 212篇 |
2022年 | 529篇 |
2021年 | 610篇 |
2020年 | 515篇 |
2019年 | 557篇 |
2018年 | 661篇 |
2017年 | 591篇 |
2016年 | 715篇 |
2015年 | 579篇 |
2014年 | 746篇 |
2013年 | 740篇 |
2012年 | 680篇 |
2011年 | 704篇 |
2010年 | 717篇 |
2009年 | 710篇 |
2008年 | 625篇 |
2007年 | 590篇 |
2006年 | 512篇 |
2005年 | 500篇 |
2004年 | 392篇 |
2003年 | 430篇 |
2002年 | 565篇 |
2001年 | 561篇 |
2000年 | 429篇 |
1999年 | 580篇 |
1998年 | 470篇 |
1997年 | 468篇 |
1996年 | 446篇 |
1995年 | 335篇 |
1994年 | 337篇 |
1993年 | 290篇 |
1992年 | 252篇 |
1991年 | 211篇 |
1990年 | 164篇 |
1989年 | 136篇 |
1988年 | 123篇 |
1987年 | 57篇 |
1986年 | 70篇 |
1985年 | 41篇 |
1984年 | 37篇 |
1983年 | 22篇 |
1982年 | 38篇 |
1981年 | 13篇 |
1980年 | 20篇 |
1979年 | 13篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1962年 | 5篇 |
1958年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
41.
明确干旱区产水量的驱动因素,能为区域水资源优化和可持续发展提供科学依据。基于 MODIS 植被指数、HWSD 的土壤数据集以及气象要素数据,采用 InVEST 模型和地理探测器探究疏 勒河流域多年平均产水量的空间分布,揭示不同空间尺度上产水量的单因子及双因子交互驱动机 制。结果表明:疏勒河流域多年平均产水量呈现南部>北部>中部。流域尺度上,产水量空间格局 的主导驱动力为降水,坡度与降水交互驱动作用最为显著。区域尺度上,南部山区、北部马鬃山地 区和中部平原区的主导驱动力各不相同,分别为日照时数、人为干扰强度、降水,双因子交互作用 显示人为干扰强度与其它因子的交互最为显著。不同土地利用类型中,耕地产水量的主导驱动力 为坡度,而其它地类产水量的主要影响因子为降水。各地类中降水与其他因子的交互均大大增强 了单因子驱动力。因此,干旱区产水量多尺度驱动机制研究对区域水资源可持续管理至关重要。 相似文献
42.
新疆S214(考干-米兰,考米线)公路位于库鲁克塔格沙漠东南缘,穿越台特玛湖干涸湖盆。由于该区气候干旱,风力强劲,沙源丰富,流动沙丘广布且快速移动,风沙危害对公路运输构成严重威胁。通过对风沙环境和风沙危害定位观测和土壤水盐特征系统调查,确定了风沙危害极为严重的公路区段,提出了合理的风沙防护措施及防沙体系结构。结果表明:该区具有明显的单风向风况,主要盛行ENE和NE风,偶有反向风沙活动,起沙风频率、输沙势、输沙率极高,属于高能-大比率风能环境;受土壤水分、盐分含量影响,地表紧实度差异较大,部分地段为极疏松的沙层,部分地段为紧实的盐壳;S214公路风沙危害防护区段为K4+900~K18+200,设计建造的阻-固-输相结合的机械-植物复合防沙体系,防沙效果明显,保障了道路安全运营。这一强风沙、高盐区公路防沙体系建设模式可为类似环境地区工程防沙提供借鉴。 相似文献
43.
Journal of Geographical Sciences - Since 2007, the Chinese government has initiated the building of national eco-cultural protection areas (NECPAs), thereby embarking on a significant... 相似文献
44.
Natural Resources Research - Coal and gas outbursts often result in a large number of casualties and economic losses. Its complex processes involve the multi-field coupling of gas flow, coal... 相似文献
45.
卡里马塔海峡水体交换的季节变化 总被引:2,自引:0,他引:2
Four trawl-resistant bottom mounts, with acoustic Doppler current profilers(ADCPs) embedded, were deployed in the Karimata Strait from November 2008 to June 2015 as part of the South China Sea-Indonesian Seas Transport/Exchange and Impact on Seasonal Fish Migration(SITE) Program, to estimate the volume and property transport between the South China Sea and Indonesian seas via the strait. The observed current data reveal that the volume transport through the Karimata Strait exhibits significant seasonal variation. The winteraveraged(from December to February) transport is –1.99 Sv(1 Sv=1×10~6 m~3/s), while in the boreal summer(from June to August), the average transport is 0.69 Sv. Moreover, the average transport from January 2009 to December2014 is –0.74 Sv(the positive/negative value indicates northward/southward transport). May and September are the transition period. In May, the currents in the Karimata Strait turn northward, consistent with the local monsoon. In September, the southeasterly trade wind is still present over the strait, driving surface water northward, whereas the bottom flow reverses direction, possibly because of the pressure gradient across the strait from north to south. 相似文献
46.
Lin He Song Wang Xiao-Jie Xu Roberto Soria Ji-Feng Liu Xiang-Dong Li Yu Bai Zhong-Rui Bai Jin-Cheng Guo Yan-Li Qiu Yong Zhang Ruo-Chuan Xu Ke-Cheng Qian 《天文和天体物理学研究(英文版)》2019,(7)
X-ray emission is an important indicator of stellar activity. In this paper, we study stellar Xray activity using the XMM-Newton and LAMOST data for different types of stars. We provide a sample including 1259 X-ray-emitting stars, of which 1090 have accurate stellar parameter estimations. Our sample size is much larger than those used in previous works. We find a bimodal distribution of the X-ray to optical flux ratio(log(fX/fV)) for G and K stars. We interpret that this bimodality is due to two subpopulations with different coronal heating rates. Furthermore, using the full widths at half maxima calculated from Hα and Hβ lines, we show that these stars in the inactive peaks have smaller rotational velocities. This is consistent with the magnetic dynamo theory that presumes stars with low rotational velocities have low levels of stellar activity. We also examine the correlation between log(fX/fV) and luminosity of the excess emission in the Hα line, and find a tight relation between the coronal and chromospheric activity indicators. 相似文献
47.
利用曲靖非相干散射雷达2017-2018年春夏季观测数据首次分析了电离层日间150~450km电子温度的地方时与高度变化特征及其与电子密度的相关性.发现hmF2及以上的电子温度在日出日落时具有两个峰值,在11∶00-16∶00LT之间变化较小,高度越高午后上升的时间越早;从150km开始迅速增加,在约220km达到最大值,然后开始降低,在约300~350km达到最小值,最后单调上升;200km以下电子温度与电子密度成正相关(主要由热传导控制),200~450km之间存在明显的反相关(光电离过程占主导),电子-离子温度差与电子密度对数之间存在近似线性关系,电子温度逐日变化与光电离因子的变化趋势相似,这种相关性在中午与午后更明显;以上结果与其他非相干散射雷达观测和电离层模型计算结果基本一致,但也存在一些差别,需要结合更多数据深入分析. 相似文献
48.
49.
Bedrock erosion due to hoeing as tillage technique in a hilly agricultural landscape,southwest China
Tillage on hillslopes may not only induce severe soil erosion, but may also cause bedrock erosion under certain conditions. Yet, little is known about bedrock erosion by tillage in a hilly agricultural landscape, southwest China. The aim of this study is to quantify the translocation of rock fragments derived from bedrock fragmentation by hoeing under different conditions, including slope gradient, hoeing depth and soil-covered thickness using a gravel tracing method. The reliability of the gravel tracing method was confirmed by the bedrock dyeing tracing method. Hoeing depth is a significant factor affecting the translocation rate of rock fragments (Qr ). Meanwhile, under the condition of overlying soil layers (0.06−0.10 m thick), the values of Qr were significantly smaller with a reduction of 20.7−25.6%, compared with rock fragmentation by hoeing for bare bedrock. However, slope gradient was found to have insignificant effects on Qr . Fractured bedrock moved as individual small fragments, which was mainly controlled by the hitting force of the hoe, while soil moved in the shape of lumps, which was dominated by both drag force of the hoe and gravity. This study suggests that hoeing into soil-covered bedrock can diminish bedrock erosion while providing soil matrix for shallow soil layers. Our work presents a quantitative assessment of bedrock erosion by hoeing and an underlying insight into characteristics of bedrock erosion by tillage operations in hilly agricultural regions with mudstone and shale, southwest China. © 2020 John Wiley & Sons, Ltd. 相似文献
50.
Ocean Dynamics - Based on a non-hydrostatic two-dimensional and high-resolution model, evolution of internal solitary waves (ISWs) on the typical slope-shelf topography in the northern South China... 相似文献