全文获取类型
收费全文 | 4262篇 |
免费 | 833篇 |
国内免费 | 1033篇 |
专业分类
测绘学 | 295篇 |
大气科学 | 844篇 |
地球物理 | 1222篇 |
地质学 | 2158篇 |
海洋学 | 524篇 |
天文学 | 152篇 |
综合类 | 428篇 |
自然地理 | 505篇 |
出版年
2024年 | 23篇 |
2023年 | 83篇 |
2022年 | 185篇 |
2021年 | 248篇 |
2020年 | 160篇 |
2019年 | 194篇 |
2018年 | 206篇 |
2017年 | 222篇 |
2016年 | 244篇 |
2015年 | 207篇 |
2014年 | 263篇 |
2013年 | 241篇 |
2012年 | 224篇 |
2011年 | 214篇 |
2010年 | 204篇 |
2009年 | 234篇 |
2008年 | 229篇 |
2007年 | 200篇 |
2006年 | 139篇 |
2005年 | 121篇 |
2004年 | 140篇 |
2003年 | 191篇 |
2002年 | 203篇 |
2001年 | 175篇 |
2000年 | 186篇 |
1999年 | 208篇 |
1998年 | 138篇 |
1997年 | 161篇 |
1996年 | 163篇 |
1995年 | 134篇 |
1994年 | 102篇 |
1993年 | 98篇 |
1992年 | 78篇 |
1991年 | 57篇 |
1990年 | 59篇 |
1989年 | 41篇 |
1988年 | 34篇 |
1987年 | 24篇 |
1986年 | 18篇 |
1985年 | 15篇 |
1984年 | 11篇 |
1983年 | 9篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1977年 | 2篇 |
1962年 | 4篇 |
1958年 | 4篇 |
1957年 | 2篇 |
排序方式: 共有6128条查询结果,搜索用时 0 毫秒
101.
梯级小水电的开发一定程度上破坏了河流连通性并阻碍了鱼类洄游通道。黑水河流域松新坝址为了满足鱼类上行产卵修建了松新鱼道,然而作为洄游物种生命周期中的关键行为,鱼类下行洄游的需求也同等重要。为了寻求一种可行、高效、低损伤的下行过鱼方式,了解河流中不同环境因素对鱼类下行效率的影响,本研究在金沙江下游支流黑水河松新水电站补水通道开展鱼类下行过坝效果评估研究。针对两种鲤科鱼类(棒花鱼Abbottina rivularis和短须裂腹鱼Schizothorax wangchiachii)幼鱼和两种鳅科鱼类(短体副鳅Paracobitis potanini和红尾副鳅Paracobitis variegatus)幼鱼各放鱼90尾,定量分析鱼种、水体透明度、放鱼位置、流量、水温、水位等因素对鱼类下行效率的影响。结果表明:棒花鱼、短须裂腹鱼、短体副鳅、红尾副鳅的下行率依次为22.22%(20尾)、28.89%(26尾)、8.89%(8尾)、15.56%(14尾);幼鱼下行成功率与上游河道水温呈正相关趋势,且放鱼位置、鱼种、上游河道水温和流量均对鱼类下行成功率有显著影响。通过构建二元Logistic回归模型识... 相似文献
102.
Spatiotemporal variation and driving forces of reference evapotranspiration in Jing River Basin,northwest China 总被引:1,自引:0,他引:1 下载免费PDF全文
Lihong Xu Zhongjie Shi Shulan Zhang Xinzheng Chu Pengtao Yu Wei Xiong Haijun Zuo Yunni Wang 《水文研究》2015,29(23):4846-4862
Evapotranspiration is an important component of the hydrological cycle, which integrates atmospheric demands and surface conditions. Research on spatial and temporal variations of reference evapotranspiration (ETo) enables understanding of climate change and its effects on hydrological processes and water resources. In this study, ETo was estimated by the FAO‐56 Penman–Monteith method in the Jing River Basin in China, based on daily data from 37 meteorological stations from 1960 to 2005. ETo trends were detected by the Mann–Kendall test in annual, seasonal, and monthly timescales. Sensitivity coefficients were used to examine the contribution of important meteorological variables to ETo. The influence of agricultural activities, especially irrigation on ETo was also analyzed. We found that ETo showed a decreasing trend in most of the basin in all seasons, except for autumn, which showed an increasing trend. Mean maximum temperature was generally the most sensitive parameter for ETo, followed by relative humidity, solar radiation, mean minimum temperature, and wind speed. Wind speed was the most dominant factor for the declining trend in ETo. The more significant decrease in ETo for agricultural and irrigation stations was mainly because of the more significant decrease in wind speed and sunshine hours, a mitigation in climate warming, and more significant increase in relative humidity compared with natural stations and non‐irrigation stations. Changes in ETo and the sensitivity coefficient of meteorological variables in relation to ETo were also affected by topography. Better understanding of ETo response to climate change will enable efficient use of agricultural production and water resources, which could improve the ecological environment in Jing River Basin. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
103.
对拟建设的新疆且末地磁台进行场址勘选,采取十字剖面测量和密跨度测量方法进行地磁场总强度F测量,对测量结果进行初步分析.分析结果表明,且末地磁台的地质条件、磁场梯度分布和背景噪声环境等地磁观测环境条件,符合国家地磁台建设规范的要求,适宜地磁基准台站建设. 相似文献
104.
Sedimentary records of natural and artificial Huanghe (Yellow River) channel shifts during the Holocene in the southern Bohai Sea 总被引:4,自引:0,他引:4
Shuqing QiaoXuefa Shi Yoshiki SaitoXiaoyan Li Yonggui YuYazhi Bai Yanguang LiuKunshan Wang Gang Yang 《Continental Shelf Research》2011,31(13):1336-1342
Two gravity cores collected off the modern Huanghe (Yellow River) delta in the southern Bohai Sea were analyzed for grain size, the total organic carbon (TOC)/total nitrogen (TN) ratio, color diffraction, magnetic susceptibility, 14C dating and 137Cs and 210Pb isotope contents to clarify changes in the sedimentary environment during the Holocene. In particular, the effect of natural and artificial river-course shifts of the Huanghe on the Bohai Sea sediment was investigated. A peat layer, scouring surface and sharp changes in the grain size, TOC/TN ratio, sediment color (L?, a?) and magnetic susceptibility were identified and are likely to be due to the early-Holocene sea-level rise resulting in environmental changes from coastal to shelf environments in the Bohai Sea. After the sea level reached its maximum at 6-7 ka BP, the lateral shifts in the river course of the Huanghe formed 10 superlobes, and superlobe 7 (11-1048 AD) and superlobe 10 (1855-present) of the Huanghe delta affected the core sites. The northern site of BH-239 has been more affected by the Huanghe since the middle Holocene. Notably, in the superlobe 10 period, the reshaping of the northern Huanghe delta due to an artificial river-course shift from northward to eastward in 1976 (e.g., a∼10 km shoreline retreat due to coastal erosion) was recorded in the core sediments, particularly in terms of the TOC/TN ratio, sediment color (L? and a?) and magnetic susceptibility, owing to the huge sediment supply from coastal erosion of the former river mouth area. 相似文献
105.
Characteristics of the Seismic Waves from a New Active Source Based on Methane Gaseous Detonation 总被引:1,自引:0,他引:1
WANG Weitao WANG Xiang MENG Chuanmin DONG Shi WANG Zhigang XIE Junju WANG Baoshan YANG Wei XU Shanhui WANG Tao 《中国地震研究》2019,33(2):354-366
Active seismic sources are critical for obtaining high resolution images of the subsurface. For active imaging in urban areas, environment friendly and green seismic sources are required. In present work, we introduce a new type of green active source based on the gaseous detonation of methane and oxygen. When fired in a closed container, the chemical reaction, i.e. gaseous detonation, will produce high pressure air over 150MPa. Seismic waves are produced when high pressure air is quickly released to impact the surroundings. The first field experiment of this active source was carried out in December, 2017 in Jingdezhen, Jiangxi Province, where a series of active sources were excited to explore their potential in mine exploration. In current work, we analyzed the seismic waves recorded by near-field accelerators and a dense short-period seismic array and compared them with those from a mobile airgun source, another kind of active source by releasing high pressure air into water. The results demonstrate that it can be used for high resolution near surface imaging. Firstly, the gaseous detonation productions are harmless CO2 and water, making it a green explosive source. Secondly, the dominant seismic frequencies are 10-80Hz and a single shot can be recorded up to 15km, making it suitable for local structure investigations. Thirdly, it can be excited in vertical wells, similar to traditional powder explosive sources. It can also act as an additional on-land active source to airgun sources, which requires a suitable water body as intermediate media to generate repeating signals. Moreover, the short duration and high frequency signature of the source signals make it safe with no damage to nearby buildings. These make it convenient to excite in urban areas. As a new explosive source, the excitation equipment and conditions, such as gas ratio, sink depth and air-releasing directions, need further investigation to improve seismic wave generation efficiency. 相似文献
106.
107.
Groundwater inflow estimation is essential for the design and construction of tunnel and the assessment of the environmental impacts. Analytical solutions used in current engineering practice do not adequately account for the effect of the excavation‐induced drawdown, which leads to significant change in pore water pressure distribution and reductions of the water level beyond tunnel. Based on the numerical analysis results, this article proposes semianalytical method to predict the height of lowered water level and groundwater tunnel inflow. The tunnel problem is conceptualized as two‐dimensional flow in a plane perpendicular to the tunnel axis. The analytical formula, considering the effect of the excavation‐induced drawdown, provides a better prediction of the tunnel inflow compared to the existing analytical formulas, even for the cases with inclined groundwater level. 相似文献
108.
109.
滑动速率是研究断裂运动学特征、地震活动性和区域应变分配的重要参数和依据。前人关于甘孜-玉树断裂带滑动速率的研究结果存在较大差异,因此,其晚第四纪滑动速率有待进一步调查研究。本文基于卫星影像解译和野外实地考察,对甘孜-玉树断裂带西段(玉树断裂)上典型断错地貌点进行测量分析,得到玉树断裂晚第四纪走滑速率为6.6±0.1-7.4±1.2mm/a。通过与前人对甘孜-玉树断裂带东段(甘孜断裂)滑动速率的研究结果进行对比,发现甘孜-玉树断裂带东、西段滑动速率不一致,其原因是甘孜断裂的左旋滑移在向西传递的过程中,一部分应变被分配到了巴塘盆地南缘断裂上。巴塘盆地南缘断裂的存在很好地解释了玉树断裂的走滑速率比甘孜断裂偏低的原因。但是,从区域变形来看,巴塘盆地南缘断裂分配的滑动速率恰好说明了甘孜-玉树断裂带东、西段及鲜水河断裂带的水平构造变形是协调一致的。 相似文献
110.
CSAMT法在四台矿402盘区陷落柱构造探测中的应用 总被引:10,自引:7,他引:10
可控源音频大地电磁法应用于寻找煤田落陷落柱构造,乃是个新课题。通过对大同矿务局四台矿402盘区的野外实际应用,发现了两个陷柱落构造,与实际资料基本吻合。 相似文献