全文获取类型
收费全文 | 12260篇 |
免费 | 1063篇 |
国内免费 | 1221篇 |
专业分类
测绘学 | 2919篇 |
大气科学 | 817篇 |
地球物理 | 1527篇 |
地质学 | 2368篇 |
海洋学 | 856篇 |
天文学 | 23篇 |
综合类 | 1062篇 |
自然地理 | 4972篇 |
出版年
2024年 | 53篇 |
2023年 | 127篇 |
2022年 | 561篇 |
2021年 | 649篇 |
2020年 | 620篇 |
2019年 | 705篇 |
2018年 | 531篇 |
2017年 | 671篇 |
2016年 | 690篇 |
2015年 | 681篇 |
2014年 | 697篇 |
2013年 | 986篇 |
2012年 | 788篇 |
2011年 | 756篇 |
2010年 | 555篇 |
2009年 | 629篇 |
2008年 | 606篇 |
2007年 | 678篇 |
2006年 | 602篇 |
2005年 | 515篇 |
2004年 | 497篇 |
2003年 | 368篇 |
2002年 | 330篇 |
2001年 | 225篇 |
2000年 | 208篇 |
1999年 | 143篇 |
1998年 | 112篇 |
1997年 | 111篇 |
1996年 | 82篇 |
1995年 | 66篇 |
1994年 | 57篇 |
1993年 | 38篇 |
1992年 | 51篇 |
1991年 | 17篇 |
1990年 | 24篇 |
1989年 | 19篇 |
1988年 | 16篇 |
1987年 | 15篇 |
1986年 | 10篇 |
1985年 | 22篇 |
1984年 | 13篇 |
1983年 | 7篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 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.
This paper proposes a multi‐level parallelized substructuring–frontal combined algorithm for the analysis of the problem of thermo/hydraulic/mechanical behaviour of unsaturated soil. Temperature, displacement, pore water pressure and pore air pressure are treated as the primary variables in a non‐linear analysis. Details are given firstly of the substructuring–frontal combined approach. The incorporation of the algorithm in a multi‐level parallel strategy is then discussed. The parallel processing can thus be carried out at different substructural levels. The method thus developed impacts, in a positive way, on both computer storage requirement and execution time. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
3.
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
4.
5.
6.
7.
8.
9.
AbstractWith the large-scale development and utilization of ocean resources and space, it is inevitable to encounter existing submarine facilities in pile driving areas, which necessitates a safety assessment. In this article, by referring to a wharf renovation project as a reference, the surrounding soil response and buried pipe deformation during pile driving in a near-shore submarine environment are investigated by three-dimensional (3D) numerical models that consider the pore water effect. Numerical studies are carried out in two different series: one is a case of a single pile focusing on the effect of the minimum plane distance of the pile–pipe, and the other is a case of double piles focusing on the effect of the pile spacing. 相似文献
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