全文获取类型
收费全文 | 14430篇 |
免费 | 2037篇 |
国内免费 | 2788篇 |
专业分类
测绘学 | 2577篇 |
大气科学 | 2289篇 |
地球物理 | 2144篇 |
地质学 | 4285篇 |
海洋学 | 1720篇 |
天文学 | 365篇 |
综合类 | 1397篇 |
自然地理 | 4478篇 |
出版年
2024年 | 54篇 |
2023年 | 145篇 |
2022年 | 480篇 |
2021年 | 620篇 |
2020年 | 618篇 |
2019年 | 748篇 |
2018年 | 596篇 |
2017年 | 726篇 |
2016年 | 715篇 |
2015年 | 737篇 |
2014年 | 888篇 |
2013年 | 1109篇 |
2012年 | 977篇 |
2011年 | 976篇 |
2010年 | 839篇 |
2009年 | 898篇 |
2008年 | 916篇 |
2007年 | 941篇 |
2006年 | 890篇 |
2005年 | 759篇 |
2004年 | 722篇 |
2003年 | 581篇 |
2002年 | 550篇 |
2001年 | 466篇 |
2000年 | 398篇 |
1999年 | 332篇 |
1998年 | 276篇 |
1997年 | 259篇 |
1996年 | 214篇 |
1995年 | 149篇 |
1994年 | 133篇 |
1993年 | 128篇 |
1992年 | 99篇 |
1991年 | 67篇 |
1990年 | 65篇 |
1989年 | 36篇 |
1988年 | 34篇 |
1987年 | 30篇 |
1986年 | 20篇 |
1985年 | 18篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 8篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1954年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 375 毫秒
961.
André Robert 《Mathematical Geology》1991,23(3):367-382
Bed roughness characteristics in coarse-grained channels are fairly complex. A hierarchy of roughness elements can be observed, ranging from variable particle sizes and shapes and small-scale sedimentary structures, to large-scale bedforms such as riffle-pool sequences. The effects of these scales of roughness on the flow geometry still remain to be thoroughly investigated. The semivariogram has been suggested in the past as a means of quantifying bed roughness effects on streamflow, as well as for distinguishing between scales of roughness. However, field measurements are rather time-consuming. The low number of bed profiles measured in the field precludes the identification of generally applicable relationships between the statistical properties derived from the semivariograms (such as the Hausdorff dimensions and the scale of autocorrelation corresponding to each fractal band) and the bed configuration itself (geometrical and sedimentological properties). Simulation results of gravel-bed profiles are, therefore, presented in order to complement the original investigation of Robert (1988a). The simulation experiments, based on grain characteristics of sizes and shapes and on morphological properties of small-scale bedforms, yield very significant information on boundary roughness at the microscale and give insight into the interpretation of empirical semivariograms (derived from field measurements). Bed-material sorting, variable grain shapes, and height and spacing of cluster bedforms control the fractal dimensions obtained from the semivariograms, as well as the location of the break of slope and the range of the process. 相似文献
962.
The warm pool in the Indian Ocean 总被引:2,自引:0,他引:2
The structure of the warm pool (region with temperature greater than 28°C) in the equatorial Indian Ocean is examined and
compared with its counterpart in the Pacific Ocean using the climatology of Levitus. Though the Pacific warm pool is larger
and warmer, a peculiarity of the pool in the Indian Ocean is its seasonal variation. The surface area of the pool changes
from 24 × 106 km2 in April to 8 × 106 km2 in September due to interaction with the southwest monsoon. The annual cycles of sea surface temperature at locations covered
by the pool during at least a part of the year show the following modes: (i) a cycle with no significant variation (observed
in the western equatorial Pacific and central and eastern equatorial Indian Ocean), (ii) a single maximum/minimum (northern
and southern part of the Pacific warm pool and the south Indian Ocean), (iii) two maxima/minima (Arabian Sea, western equatorial
Indian Ocean and southern Bay of Bengal), and (iv) a rapid rise, a steady phase and a rapid fall (northern Bay of Bengal). 相似文献
963.
964.
965.
NUMERICAL EXPERIMENTS ON THE EFFECT OF QINGHAIXIZANG PLATEAU SNOW COVER ON SUMMER MONSOON FORMATION
下载免费PDF全文
![点击此处可从《Acta Meteorologica Sinica》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The calculating schemes of underlying surface processes in the model described by Li et al.(1989)are modified with inclusion of simple land surface processes and oceanic mixed layer processes,then a simulation on the zonal wind along 90°E from the Northern to the Southern Hemisphere with moun-tains is performed.Comparisons of the results and the observations show that the modified model not onlyhas an excellent stability in calculation but also can better display the seasonal change of the wind field,theability of the present model is improved as compared with that of the previous one.Based on the simulations,the authors investigate the effects of Qinghai-Xizang Plateau snow cover on theformation of South Asian monsoon by thickcning the snow depth and by increasing the snow albedo.Themain results arc as follows:The summer meridional circulation over the south of the Plateau and its vicinityis weakeued,and the precipitation reduced.However,over the northern tropics,the circulation is enhanced,and the ecipitation is increased,and the land and the air above it become warmer,the tropical easterly jetis weakened. 相似文献
966.
北京城郊近地层湍流实验观测 总被引:13,自引:2,他引:13
本文利用两台FA-11超声风速温度仪于1992年3月到4月间在北京325 m气象观测塔47 m和120 m两个高度观测到的风速三个正交分量以及声虚温的湍流脉动资料,计算和分析了北京城郊粗糙下垫面近地层湍流特征量及其日变化,无量纲垂直速度和声虚温的方差随稳定度的变化,风速分量和温度的归一化湍流能谱以及动量和热通量的互谱及其随稳定度的变化。并且与平坦均一下垫面近地层湍流观测的结果进行了对比。 相似文献
967.
试用极轨气象卫星遥感监测土壤墒情 总被引:2,自引:0,他引:2
讨论了用极轨气象卫星资料监测表层土壤湿度的方法。在建立土壤湿度与卫星遥感资料的关系时,为了消除土壤条件的影响,把全省分为4个区域来研究。考虑了气候条件,植被状况不同的影响,并给出了简单的订正方法,同时也给出了卫星资料的重新地理校正,资料的输出及存储方法等。 相似文献
968.
The surface parameters are being evaluated using a methodology which considers the vertical temperature structure of MONTBLEX
’90. For this 30 metre micrometeorological tower data are utilised. In this process, a concept of isolated layers has been
introduced. The parameters have been evaluated following two successive iterative processes to give a consistent value both
in the flux-profile relation and the related similarity relation.
The heat flux obtained using the present methodology, is then utilised to examine its interplay with the synoptic as well
as mesoscale features. The existence of such an interplay is observed. 相似文献
969.
Summary Laboratory model test results are presented that determine the effectiveness of using layers of geogrids as reinforcement in sand to reduce the settlement of square surface foundations subjected to transient loading. The model tests were conducted with only one type of geogrid at one relative density of compaction of sand. The maximum intensity of the transient load applied always exceeded the static ultimate bearing capacity of the foundation when supported by unreinforced sand. The settlement reduction factors for various depths of reinforcement have been determined. 相似文献
970.