全文获取类型
收费全文 | 3356篇 |
免费 | 583篇 |
国内免费 | 922篇 |
专业分类
测绘学 | 653篇 |
大气科学 | 631篇 |
地球物理 | 527篇 |
地质学 | 1633篇 |
海洋学 | 422篇 |
天文学 | 53篇 |
综合类 | 336篇 |
自然地理 | 606篇 |
出版年
2024年 | 22篇 |
2023年 | 67篇 |
2022年 | 176篇 |
2021年 | 201篇 |
2020年 | 162篇 |
2019年 | 187篇 |
2018年 | 197篇 |
2017年 | 177篇 |
2016年 | 184篇 |
2015年 | 205篇 |
2014年 | 222篇 |
2013年 | 270篇 |
2012年 | 223篇 |
2011年 | 217篇 |
2010年 | 232篇 |
2009年 | 246篇 |
2008年 | 263篇 |
2007年 | 243篇 |
2006年 | 182篇 |
2005年 | 162篇 |
2004年 | 146篇 |
2003年 | 125篇 |
2002年 | 166篇 |
2001年 | 136篇 |
2000年 | 101篇 |
1999年 | 90篇 |
1998年 | 53篇 |
1997年 | 29篇 |
1996年 | 37篇 |
1995年 | 40篇 |
1994年 | 27篇 |
1993年 | 22篇 |
1992年 | 19篇 |
1991年 | 9篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1982年 | 1篇 |
1979年 | 2篇 |
1958年 | 2篇 |
1957年 | 1篇 |
排序方式: 共有4861条查询结果,搜索用时 46 毫秒
101.
102.
This paper presents the detailed results and analyses on the ecological footprints and bio-capacities of the individual cities and the province as a whole for the year 2001, providing a clear picture of sustainability for the province. Results show that the ecological footprints of most cities in Liaoning exceeded their respective bio-capacities, incurring high ecological deficits. The ecological deficit of the province as a whole was 1.31 ha/cap. Those cities with resources extraction and/or primary material-making as their major industries constitute the "ecologically black band", whose ecological deficits ranged from 2.45 to 5.23 ha/cap, the highest of all cities in the province. Fossil energy consumption was the major source of footprint amounting to 1.63 ha/cap at the provincial level, taking up 67.3% of the total. For cropland, modest ecological surpluses occurred in Jinzhou, Tieling, Huludao, and Panjin while modest ecological deficits in Dalian, Benxi, Fushun, and Dandong, resulting in an overall surplus for the province. Liaoning had a certain level of surplus in fishing ground (water area), mainly distributed in the coastal cities of Dalian, Panjin, Huludao, Yingkou, Jinzhou, and Dandong. Most cities had a small ecological deficit in pasture and all had a small ecological surplus in forest. The eco-efficiency, expressed as GDP value per hectare of footprint, exhibits high variations among the cities, with the highest (Shenyang) more than 10 times the lowest (Fuxin). Cities with manufacture, high-tech, and better developed service industries had high eco-efficiency, while those with resources extraction, primary material-making, and less developed service industries had low eco-efficiency. Based on the components and geographical distribution of ecological footprint, strategic policy implications are outlined for Liaoning's development toward a sustainable future. 相似文献
103.
2004年齐河县小麦全生育期气象条件分析 总被引:1,自引:0,他引:1
通过对2003-2004年齐河县冬小麦全生育期的气象条件分析,发现有利的气象条件和合理灌溉、科学施肥是小麦高产的主要因素,评价了气象条件对小麦生长和产量的影响. 相似文献
104.
The chemical forms, spatial distribution and sources of As, Hg, Cd, Pb and Zn in sediments of the Miyun reservoir were studied. The results of sequential extraction demonstrate that most of As, Pb and Zn were bound to the residual fraction, Hg was associated with the sulfide fraction while Cd was associated with the carbonate fraction and the residual fraction. On the vertical profiles the concentrations of the heavy metals in total and each fractions mostly decreased with increasing depths in sediments, suggesting that the heavy metals input from the upstream watershed increases yearly. Summation of the residual fraction, the sulfide fraction and the carbonate fraction accounts for 60.03%―85.60% of the total heavy metal contents in the sediments, which represent the geochemical background values of the elements and relate closely to soil erosion. Results of the main factor analysis show that most sediments of the reservoir come from the upstream soil erosion, the point source pollution and domestic waste. Moreover, the microbial activities taking place on the sediment-water interface are also one of the major factors to cause the increasing content of the organic matter fraction and the iron-manganese oxide fraction. Environmental change of the reservoir water could make the removability of the heavy metals increase, leading to the increase of their concentrations in pore water in sediments, and imperiling water quality of the reservoir. 相似文献
105.
对东北四海龙湾玛珥湖SHLF6孔纹层沉积物的137Cs放射性测量表明:137Cs比活度的最大值出现在55cm处,对应于1963年世界原子弹试爆高峰期。纹层计年表明0~6cm共有35个纹层层偶。从75cm到65cm,137Cs比活度从256±009dpm/g急剧增加到1868±017dpm/g,可能65cm对应于1954年。通过测量226Ra子核214Pb和214Bi(能量为295keV,352keV和609keV)放射的光子数获得226Ra比活度数据,然后求得过剩210Pb比活度(210Pbuns)。210Pbuns比活度随深度增加而呈指数衰减,其异常波动可能与人类活动以及沉积速率变化有关,例如55cm处210Pb比活度较高,与137Cs的峰值对应,这可能与1963年前后人工核实验的高峰有关,因为核试验不仅产生137Cs,而且可以产生208Pb和210Pb;45cm处210Pb比活度较低,而226Ra较高,可能与人类活动加剧,导致沉积速率增加有关。根据210PbunsCRS模式,SHLF6孔0~19cm的平均沉积速率为20mg/cm2·a,或约为011cm/a。210Pb测年数据与137Cs时标及纹层计年均有很好的一致性。四海龙湾玛珥湖发育的纹层为年纹层,可以建立高分辨率时间序列。 相似文献
106.
地理新课程改革是基础教育课程改革的重要内容之一.随着地理课程的全方位变革,新教师培训也是一个不容忽视的问题.地理新课程教师培训模式从不同的方面划分有不同的类型,每种模式既有优越性,也有局限性.要更好地推进地理新课程改革,必须对地理新课程教师培训模式进行不断的修正、补充和完善. 相似文献
107.
Biostratigraphic and detrital zircon age constraints on the basement of the Himalayan Foreland Basin: Implications for a Proterozoic link to the Lesser Himalaya and cratonic India 下载免费PDF全文
The Himalayan Foreland Basin in the Ganga Valley is key to assessing the pre‐collision relationship between cratonic India and the Himalaya – the world's largest mountain chain. The subsurface Ganga Supergroup, representing the sedimentary basement of the Ganga Valley, has been interpreted as a northern extension of the Proterozoic Vindhyan Supergroup in cratonic India. This interpretation is contentious because the depositional age of the Ganga Supergroup is not resolved: whereas the lower Ganga Supergroup is widely regarded as Proterozoic, the upper Ganga Supergroup has been variously inferred to include Neoproterozoic, lower Palaeozoic, or Cretaceous strata. Here, we integrate biostratigraphic and detrital zircon data from drill cores to show that the entire Ganga Supergroup is likely Proterozoic and can be correlated with Proterozoic successions on the northern Indian craton and in the Lesser Himalaya. This helps redefine the first‐order stratigraphic architecture and indicates broad depositional continuity along the northern Indian margin during the Proterozoic. 相似文献
108.
109.
Composition and flux of suspended organic matter in the middle and lower reaches of the Changjiang (Yangtze River) — impact of the Three Gorges Dam and the role of tributaries and channel erosion 下载免费PDF全文
To investigate the sources of particulate organic matter (POM) and the impact of Three Gorges Dam (TGD), two large lakes and erosion processes on determining the composition and flux of POM in low water discharge periods along the middle and lower Changjiang, suspended particulate samples were collected along the middle and lower reaches of the Changjiang (Yangtze River) in January 2008. Organic geochemistry of bulk sediment (particulate organic carbon, organic carbon to nitrogen molar ratio (C/N), stable carbon isotope (δ13C) and grain size) and biomarker of bulk sediment (lignin phenols) were measured to trace the sources of POM. The range of C/N ratios (6.4–8.9), δ13C (?24.3‰ – ?26.2‰) and lignin phenols concentration Λ8 (0.45 mg/100 mg OC‐2.00 mg/100 mg OC) of POM suggested that POM originated from the mixture of soil, plant tissue and autochthonous organic matter (OM) during the dry season. POM from lakes contained a higher portion of terrestrial OM than the mainstream, which was related to sand mining and hydropower erosion processes. A three end‐member model based on δ13C and Λ8 was performed. The results indicated that soil contributed approximately 50% of OM to the POM, which is the dominant OM source in most stations. POM composition was affected by total suspended matter (TSM) and grain size composition, and the direct OM input from two lakes and channel erosion induced OM. The lower TSM concentration in January 2008 was mainly caused by seasonal variations; the impact from the TGD in the dry season was relatively small. A box model indicated that more than 90% of the terrestrial OM transported by the Changjiang in January 2008 was from the middle and lower drainage basins. Channel erosion induced OM, and contributions from Poyang Lake were the major terrestrial OM sources in the dry season. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
110.