全文获取类型
收费全文 | 1581篇 |
免费 | 297篇 |
国内免费 | 412篇 |
专业分类
测绘学 | 192篇 |
大气科学 | 299篇 |
地球物理 | 414篇 |
地质学 | 808篇 |
海洋学 | 201篇 |
天文学 | 151篇 |
综合类 | 94篇 |
自然地理 | 131篇 |
出版年
2024年 | 5篇 |
2023年 | 32篇 |
2022年 | 64篇 |
2021年 | 53篇 |
2020年 | 58篇 |
2019年 | 74篇 |
2018年 | 54篇 |
2017年 | 62篇 |
2016年 | 83篇 |
2015年 | 77篇 |
2014年 | 80篇 |
2013年 | 89篇 |
2012年 | 100篇 |
2011年 | 112篇 |
2010年 | 95篇 |
2009年 | 127篇 |
2008年 | 126篇 |
2007年 | 122篇 |
2006年 | 104篇 |
2005年 | 101篇 |
2004年 | 103篇 |
2003年 | 76篇 |
2002年 | 67篇 |
2001年 | 71篇 |
2000年 | 78篇 |
1999年 | 69篇 |
1998年 | 56篇 |
1997年 | 29篇 |
1996年 | 15篇 |
1995年 | 15篇 |
1994年 | 23篇 |
1993年 | 19篇 |
1992年 | 24篇 |
1991年 | 2篇 |
1990年 | 7篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1986年 | 5篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有2290条查询结果,搜索用时 765 毫秒
101.
石海燕 《测绘与空间地理信息》2009,32(4):94-95
地形图等高线是4D产品的基本要素,等高线的采集精度直接决定着4D产品的质量.根据作业实践,提出等高线立体采集的方法与采集时常见问题及其对策.本文对类似的4D产品的等高线采集与编辑有一定的借鉴和参考价值. 相似文献
102.
103.
对南海北部C4站柱状沉积剖面进行了高分辨率孢粉、藻类成分的分析和研究,同时测定了部分样品的AMS14C年代,划分了该站柱状沉积物地层时代,相应恢复了南海北部9 800 a B P以来植被、气候、环境的3个演替阶段.南海北部C4站从下至上可划分成3个孢粉组合带,依次为:1带(258~194 cm):以Pinus-Quercus(常绿)-Polypodiaceae-Pterdium-Pteris-Dicranopteris-Adiantum为主孢粉带;2带(194~94 cm):以Pinus-Quercus(常绿)-Pteridium-Polypodiaceae-Microlepia-Dicranopteris为主孢粉带;3带(94~4 cm):以Pinus-Pteridium-Polypodiaceae-Quercus(常绿)-Microlepia-Dicranopteris为主孢粉带.与3个孢粉带相对应的古植被阶段依次为:伴有针叶树的常绿阔叶、落叶阔叶混交林,热带半常绿季雨林,热带常绿阔叶、落叶阔叶混交林,与周边地区的植被演替相一致.由此可见,研究区经历了全新世早期气候转暖,中期气候炎热、干湿季明显、海平面上升以及晚期气候暖热、湿润3个古环境演变阶段. 相似文献
104.
合作原则对大学英语四六级听力测试对话的指导 总被引:1,自引:0,他引:1
黄舒 《成都信息工程学院学报》2008,23(3):349-353
听力理解在英语四六级考试中占有重要的比例,提高听力水平既是教学的要求也是通过大学英语四六级考试的关键。因此在分析近年来大学英语四六级听力理解对话部分的难点的基础上,提出合作原则及相关理论对解答听力理解对话部分具有指导作用。 相似文献
105.
以2006年7月至2007年10月期间,4个季节的4个航次对37°N断面的调查资料为依据,分析并讨论了该断面四季生源要素的分布特征以及变化趋势。结果表明:该断面受黄海冷水团和陆源输入的影响,东西两侧的生源要素及温盐特征明显不同。夏季断面东侧由于受到冷水团的控制,底层出现明显的低温、高盐、高营养盐和高溶解氧的特征,并且跃层明显;西侧浅水区的温盐及生源要素的变化则相对均匀。冬季由于温跃层的消失,上下水体混合均匀,故而温盐及生源要素的垂直分布趋于均匀,断面西侧由于受到陆源输入的影响营养盐的浓度明显较东侧高。春季是增温降盐,营养盐逐渐消耗,东侧温跃层逐渐形成的过程;而秋季则恰恰相反。 相似文献
106.
利用硫酸镁和造纸草浆黑液制备含镁氧化合物的吸附剂,通过化学分析、表面性质分析、X射线衍射和红外光谱等对吸附剂进行了表征.考察了吸附剂焙烧温度、投加量、吸附时间以及pH值等因素对活性黄ST-4RFN吸附性能的影响,吸附过程可用Freundlich模型来描述. 相似文献
107.
Food web structure of two Mediterranean lagoons under varying degree of eutrophication 总被引:2,自引:0,他引:2
Antoine Carlier Pascal Riera Jean-Michel Amouroux Jean-Yves Bodiou Martin Desmalades Antoine Grmare 《Journal of Sea Research》2008,60(4):264-275
The food web structure and functioning of two north-western Mediterranean lagoons exhibiting contrasting degrees of eutrophication and marine influences were compared through δ13C and δ15N analysis of major potential food sources and consumers. The Lapalme Lagoon is well preserved and has kept a natural and temporary connection with the open sea. Conversely, the Canet Lagoon is heavily eutrophicated and its water exchange with the open sea has been artificially reduced. In Lapalme, all potential food sources and consumers exhibited δ15N values indicative of pristine coastal areas. Suspended particulate organic matter (POM) and sediment organic matter (SOM) pools seemed to constitute the main food sources of most primary consumers. Both primary producers and all consumers were much more 15N-enriched (by 10‰) and more 13C-depleted in Canet than in Lapalme. This reflected: (1) the assimilation of important amounts of anthropogenic nitrogen in the food web, and (2) a marked and uniform influence of 13C-depleted allochtonous sources of carbon. Based on the mean δ15N of primary consumers, we found rather similar food web lengths in both lagoons with top consumers at trophic levels 3.6 and 4.0 in Canet and Lapalme, respectively. However, the eutrophication of the Canet Lagoon resulted in a simplification of the food web structure (i.e., a single trophic pathway from a 15N-enriched fraction of the SOM pool to top predators) compared to what was observed in Lapalme Lagoon where additional 13C-enriched food sources played a significant trophic role. Moreover, some consumers of Canet tended to exploit primary producers to a larger extent (and thus to exhibit lower trophic levels) than in Lapalme. 相似文献
108.
通过象山港水域的潮波运动数值模拟,分析了象山港M4分潮的生成和增长机制.结果说明,潮波传播中的非线性底摩擦效应是M4分潮生成和增长的主要控制因子,M4分潮在湾内的共振现象也起着重要的放大作用二平流效应在绝大部分区域中抑制了M4分潮的发展,只有在佛渡水道中一些岛屿周围极小区域内对M4分潮有增强作用.潮滩在湾内对M4分潮的影响极其微弱. 相似文献
109.
研究了TRMM/TMI海表降水率资料的四维变分同化在热带气旋(TC)数值模拟中的作用.使用中尺度气象模式MM5设计了若干数值试验模拟了TC Danas(2001)由热带低压初生到台风生成的发展过程.在满足MM5模式动力约束的前提下,将TRMM海表降水率资料直接同化进入较高分辨率(18 km)的模式初始场.结果表明,使用MM5模式的4D-VAR同化系统直接同化TRMM/TMI海表降水率资料是可行的.这种做法提高了TRMM资料的利用率,不仅在模式初始场中加入了更多实测信息,而且避免了两次同化(1DVAR+4DVAR)可能引起的误差.直接同化TRMM资料通过调整气压、温度、湿度等要素初始场,改善了模式对热带气旋结构(如暖心、涡度、散度)的描述和降水的模拟.在此基础上,同化不仅改进了对Danas强度的模拟,而且成功地模拟了热带气旋环境场的演变过程,因而改进了路径的模拟. 相似文献
110.
Impact of 4DVAR assimilation of rainfall data on the simulation of mesoscale precipitation systems in a mei-yu heavy rainfall event 总被引:1,自引:0,他引:1
The multi-scale weather systems associated with a mei-yu front and the corresponding heavy precipitation during a particular heavy rainfall event that occurred on 4 5 July 2003 in east China were successfully simulated through rainfall assimilation using the PSU/NCAR non-hydrostatic, mesoscale, numerical model (MM5) and its four-dimensional, variational, data assimilation (4DVAR) system. For this case, the improvement of the process via the 4DVAR rainfall assimilation into the simulation of mesoscale precipitation systems is investigated. With the rainfall assimilation, the convection is triggered at the right location and time, and the evolution and spatial distribution of the mesoscale convective systems (MCSs) are also more correctly simulated. Through the interactions between MCSs and the weather systems at different scales, including the low-level jet and mei-yu front, the simulation of the entire mei-yu weather system is significantly improved, both during the data assimilation window and the subsequent 12-h period. The results suggest that the rainfall assimilation first provides positive impact at the convective scale and the influences are then propagated upscale to the meso- and sub-synoptic scales.
Through a set of sensitive experiments designed to evaluate the impact of different initial variables on the simulation of mei-yu heavy rainfall, it was found that the moisture field and meridional wind had the strongest effect during the convection initialization stage, however, after the convection was fully triggered, all of the variables at the initial condition seemed to have comparable importance. 相似文献
Through a set of sensitive experiments designed to evaluate the impact of different initial variables on the simulation of mei-yu heavy rainfall, it was found that the moisture field and meridional wind had the strongest effect during the convection initialization stage, however, after the convection was fully triggered, all of the variables at the initial condition seemed to have comparable importance. 相似文献