全文获取类型
收费全文 | 16864篇 |
免费 | 1230篇 |
国内免费 | 1255篇 |
专业分类
测绘学 | 5184篇 |
大气科学 | 1147篇 |
地球物理 | 1525篇 |
地质学 | 3039篇 |
海洋学 | 1051篇 |
天文学 | 32篇 |
综合类 | 1737篇 |
自然地理 | 5634篇 |
出版年
2024年 | 57篇 |
2023年 | 162篇 |
2022年 | 652篇 |
2021年 | 757篇 |
2020年 | 705篇 |
2019年 | 790篇 |
2018年 | 582篇 |
2017年 | 877篇 |
2016年 | 815篇 |
2015年 | 864篇 |
2014年 | 929篇 |
2013年 | 1310篇 |
2012年 | 1064篇 |
2011年 | 1038篇 |
2010年 | 756篇 |
2009年 | 793篇 |
2008年 | 805篇 |
2007年 | 943篇 |
2006年 | 877篇 |
2005年 | 739篇 |
2004年 | 741篇 |
2003年 | 570篇 |
2002年 | 516篇 |
2001年 | 393篇 |
2000年 | 329篇 |
1999年 | 243篇 |
1998年 | 210篇 |
1997年 | 169篇 |
1996年 | 144篇 |
1995年 | 83篇 |
1994年 | 95篇 |
1993年 | 74篇 |
1992年 | 75篇 |
1991年 | 27篇 |
1990年 | 38篇 |
1989年 | 27篇 |
1988年 | 23篇 |
1987年 | 14篇 |
1986年 | 11篇 |
1985年 | 22篇 |
1984年 | 12篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1973年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
1.
The structure, functioning and hydrodynamic properties of aquifers can be determined from an analysis of the spatial variability of baseflow in the streams with which they are associated. Such analyses are based on simple low‐cost measurements. Through interpreting the hydrological profiles (Q = f(A)) it is possible to locate the aquifer(s) linked to the stream network and to determine the type of interrelated flow, i.e. whether the stream drains or feeds the aquifer. Using an analytical solution developed for situations with a positive linear relationship, i.e. where the baseflow increases linearly with increasing catchment size, it is also possible to estimate the permeability of the aquifer(s) concerned at catchment scale. Applied to the hard‐rock aquifers of the Oman ophiolite, this method shows that the ‘gabbro’ aquifer is more permeable than the ‘peridotite’ aquifer. As a consequence the streams drain the peridotites and ‘leak’ into the gabbro. The hydrological profiles within the peridotite are linear and positive, and indicate homogeneity in the hydrodynamic properties of these formations at the kilometre scale. The permeability of the peridotite is estimated at 5 · 10?7 to 5 · 10?8 m/s. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
2.
Abstract. Boonea (= Odostomia) impressa is a common ectoparasite of oysters. In the laboratory, small oysters (Crassostrea virginica) parasitized by natural densities of B. impressa produced 75 % less new shell than unparasitized oysters. Shell deposition rates of previously parasitized oysters increased significantly after all B. impressa were removed. Thus, the decrease in growth rate, although significant, apparently was not permanent. B. impressa preferentially parasitized small, living oysters (≤2.5cm) in the field, even though a higher percentage of large, living oysters (>2.5cm) was available. The snails maintained an aggregated distribution on the oyster reef. The number of B. impressa per oyster clump was positively correlated with the number of living oysters per clump, however some clumps with few or no living oysters had many B. impressa. Thus, food availability only partially explained the pattern of distribution. B. impressa was very mobile. About 50 % of the population moved in one week. Reproduction occurred throughout the year with a peak period in May. Recruitment was greatest in July, however new recruits were observed throughout the year. The reduction in growth rate of parasitized oysters, the snaiľs propensity towards parasitizing small oysters and the snail's tendency to be contagiously distributed suggests that B. impressa potentially exerts a significant influence on the population structure and health of oyster populations. 相似文献
3.
公用微机硬盘信息保护的一种有效方法 总被引:1,自引:0,他引:1
张玉彪 《中国海洋大学学报(自然科学版)》1994,(Z2)
介绍一种保护公用微机硬盘信息的有效方法.不仅可以预防各种计算机病毒,更重要的是可以防止人为操作对硬盘信息的破坏。适用于学校等开放性微机实验室的信息安全与保护. 相似文献
4.
5.
长江口深水航道数学模型信息系统研究 总被引:3,自引:0,他引:3
利用GIS二次开发软件ArcView,完成数学模拟所需的计算网格生成及线边界法中地形数据提取的优化。集成ArcView、Visual Basic及长江口潮流数学模型,实现了数值模拟计算成果的三维可视化及实时互动的动态演示。所开发的长江口数学模型信息系统能根据整治工程需要,提交各类工程数据库。 相似文献
6.
7.
8.
9.
10.
Abstract. Four sites were sampled in kelp (Macrocysiis pyrifera) forests occupying rocky bottom habitats along a wave exposure gradient in central California. Consistent betwecn-site differences were found in the three major structural elements - the surface canopy, the undcrstory assemblage, and the ground cover/turf assemblage - of kelp forest communities. Macrocysiis pyrifera was found at all four sites. Nereucyslis tuelkeana only at the most exposed site. The understory kelps Laminaria setchellii and Pterygopltora californica were also characteristic of exposed sites. Articulated coralline algae were more abundant at exposed sites than protected, while fleshy red algae showed the opposite pattern. All four study sites were located along 8.5 km of coastline, and thus were assumed to have available to them the same species pool for colonization. The substrate composition was the same and the amount of unconsolidated substrate was similar at all four sites. We suggest that exposure to wave-generated water motion, through its influence on the surface canopy and therefore on the amount of light reaching the bottom, is responsible for these between-site differences. 相似文献