全文获取类型
收费全文 | 3088篇 |
免费 | 629篇 |
国内免费 | 803篇 |
专业分类
测绘学 | 237篇 |
大气科学 | 675篇 |
地球物理 | 838篇 |
地质学 | 1527篇 |
海洋学 | 398篇 |
天文学 | 107篇 |
综合类 | 314篇 |
自然地理 | 424篇 |
出版年
2024年 | 18篇 |
2023年 | 60篇 |
2022年 | 193篇 |
2021年 | 214篇 |
2020年 | 178篇 |
2019年 | 201篇 |
2018年 | 232篇 |
2017年 | 189篇 |
2016年 | 225篇 |
2015年 | 165篇 |
2014年 | 205篇 |
2013年 | 194篇 |
2012年 | 194篇 |
2011年 | 161篇 |
2010年 | 169篇 |
2009年 | 151篇 |
2008年 | 149篇 |
2007年 | 158篇 |
2006年 | 96篇 |
2005年 | 94篇 |
2004年 | 99篇 |
2003年 | 76篇 |
2002年 | 92篇 |
2001年 | 100篇 |
2000年 | 99篇 |
1999年 | 138篇 |
1998年 | 77篇 |
1997年 | 99篇 |
1996年 | 81篇 |
1995年 | 76篇 |
1994年 | 57篇 |
1993年 | 67篇 |
1992年 | 39篇 |
1991年 | 39篇 |
1990年 | 26篇 |
1989年 | 19篇 |
1988年 | 10篇 |
1987年 | 8篇 |
1986年 | 18篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 7篇 |
1968年 | 1篇 |
1958年 | 1篇 |
1951年 | 1篇 |
1938年 | 1篇 |
排序方式: 共有4520条查询结果,搜索用时 156 毫秒
81.
Nancy England April L. James Krystopher J. Chutko Richard S. Pyrce Huaxia Yao 《水文研究》2019,33(6):905-919
Stable water isotope surveys have increasingly been integrated into river basins studies, but fewer have used them to evaluate impact of hydropower regulation. This study applies hydrologic and water isotope survey approaches to a Canadian Shield river basin with both regulated and natural flows. Historical streamflow records were used to evaluate the influence of three hydroelectric reservoirs and unregulated portions of the basin on downstream flows and changes in water level management implemented after an extreme flood year (1979). In 2013, water isotope surveys of surface and source waters (e.g., rainfall, groundwater, snowmelt) were conducted to examine spatial and temporal variation in contributions to river flow. Seasonal changes in relative groundwater contribution were assessed using a water‐isotope mass balance approach. Within the basin, two regulated reservoirs exhibited inverted hydrographs with augmented winter flows, whereas a third exhibited a hydrograph dominated by spring snowmelt. In 2013, spatial variation in rain‐on‐snow and air temperatures resulted in a critical lag in snowmelt initiation in the southern and northern portions of the basin resulting in a dispersed, double peak spring hydrograph, contrasting with 1979 when a combination of rain‐on‐snow and coincident snowmelt led to the highest flood on record. Although eastern basin reservoirs become seasonally enriched in δ18O and δ2H values, unregulated western basin flows remain less variable due to groundwater driven baseflow with increasing influence downstream. Combined analysis of historical streamflow (e.g., flood of 1979, drought of 2010) and the 2013 water isotope surveys illustrate extreme meteorological conditions that current management activities are unable to prevent. In this study, the influence of evaporative fractionation on large surface water reservoirs provides important evidence of streamflow partitioning, illustrating the value of stable water isotope tracers for study of larger catchments. 相似文献
82.
83.
In‐situ analysis of trace elements and Sr–Pb isotopes of K‐feldspars from Tongshankou Cu–Mo deposit,SE Hubei Province,China: Insights into early potassic alteration of the porphyry mineralization system 下载免费PDF全文
In the Tongshankou porphyry deposit (SE Hubei Province, South China), three types of K‐feldspars are recognized: (I) the phenocryst type in the porphyry that crystalized during the magmatic stage, (II) the megacryst type and (III) the vein type in the altered porphyry and orebody that was produced by hydrothermal fluids. A detailed in‐situ analysis of trace elements and Sr–Pb isotopes was carried out on K‐feldspars in an attempt to unravel their formation processes and to trace the element sources during potassic alteration. The Type III K‐feldspars show lower Sr contents and Sr‐isotope ratios but higher Pb contents and Pb‐isotope ratios than the Type I and II K‐feldspars, possibly reflecting a contribution from the country carbonate rocks with less radiogenic Sr but more radiogenic Pb sources, and indicate that the ore‐forming fluids and materials may have been partially derived from external sources such as the host sedimentary rocks during the early potassic alteration stage. 相似文献
84.
85.
86.
87.
88.
89.
90.
利用1960—2012年5—6月NCEP/NCAR逐日再分析资料,基于冷涡经典定义,采取客观识别方法检索东北冷涡活动过程,根据东北冷涡活动时空变化特征给出东北冷涡持续活动过程标准, 通过冷涡强度指数进行定量化分析,该指数对冷涡持续活动过程具有较好表征意义。冷涡活动强对应5月乌拉尔山阻塞高压、贝加尔湖阻塞高压和6月鄂霍次克海阻塞高压活动频繁。通过强弱指数年合成,得到6月强指数年冷涡系统较深厚,集中于对流层中高层,冷心结构明显,具有一定大气斜压特征; 高层存在冷中心,低层有冷空气活动,中高层西风带呈明显的上游分流和下游汇合特征,分汇流之间呈东北高、西南低的偶极子阻塞形势;弱指数年冷涡系统较浅薄,主要集中在对流层中低层,冷心结构不明显,不存在阻塞形势。 相似文献