全文获取类型
收费全文 | 3648篇 |
免费 | 815篇 |
国内免费 | 1175篇 |
专业分类
测绘学 | 259篇 |
大气科学 | 699篇 |
地球物理 | 1006篇 |
地质学 | 2398篇 |
海洋学 | 359篇 |
天文学 | 31篇 |
综合类 | 260篇 |
自然地理 | 626篇 |
出版年
2024年 | 23篇 |
2023年 | 77篇 |
2022年 | 145篇 |
2021年 | 160篇 |
2020年 | 174篇 |
2019年 | 202篇 |
2018年 | 164篇 |
2017年 | 163篇 |
2016年 | 159篇 |
2015年 | 197篇 |
2014年 | 228篇 |
2013年 | 229篇 |
2012年 | 257篇 |
2011年 | 239篇 |
2010年 | 220篇 |
2009年 | 282篇 |
2008年 | 231篇 |
2007年 | 238篇 |
2006年 | 268篇 |
2005年 | 221篇 |
2004年 | 219篇 |
2003年 | 191篇 |
2002年 | 168篇 |
2001年 | 178篇 |
2000年 | 141篇 |
1999年 | 108篇 |
1998年 | 129篇 |
1997年 | 104篇 |
1996年 | 114篇 |
1995年 | 85篇 |
1994年 | 71篇 |
1993年 | 58篇 |
1992年 | 55篇 |
1991年 | 24篇 |
1990年 | 27篇 |
1989年 | 18篇 |
1988年 | 15篇 |
1987年 | 13篇 |
1986年 | 13篇 |
1985年 | 8篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1954年 | 2篇 |
排序方式: 共有5638条查询结果,搜索用时 15 毫秒
231.
《水文科学杂志》2013,58(5)
Abstract Some unique coupled wind–water erosion processes exist in the desert-loess transitional zone in the middle Yellow River basin. Based on data from 40 stations on 29 rivers, a study was made on the influence of such processes on suspended sediment grain-size characteristics of the tributaries of the Yellow River. Results show that the percentage of >0.05-mm grain size decreases with the increased annual mean precipitation, but increases with the increase in the annual mean number of sand-dust storm days. The percentage of <0.01-mm grain size increases with the increase in the annual mean precipitation, but decreases with the increase in the annual number of sand-dust storm days. Based on annual mean data from 40 stations, multiple regression equations were established between the percentages of >0.05-mm grain size (r >0.05) and <0.01-mm grain size (r <0.01), annual mean precipitation (P m) and annual mean number of sand-dust storm days (D ss). On this basis, the relative contributions of the variations in D ss and P m to the variation in r >0.05 and r <0.01 were estimated. The results indicate that the variation in sand-dust storm frequency exerts greater influences on the variation in grain-size characteristics of suspended load than does the variation in annual mean precipitation. With the increase in the coupled wind–water processes index, expressed by P m/D ss, the percentage of >0.05-mm grain size in suspended sediment decreases and the percentage of <0.01-mm grain size increases. With the variation in P m/D ss, different combinations of r >0.05 with r <0.01 appear, which have some influence on the formation of hyperconcentrated flows. There exist some optimal ratios of coarse to fine fractions in suspended sediment that make sediment concentrations of hyperconcentrated flows the highest. The optimal r >0.05/r <0.01 value is related to some range of the index P m/D ss. When the P m/D ss index falls in this range, the optimum combination of relative coarse with fine sediments in the suspended load appears, and thus results in the peak values of sediment concentration. 相似文献
232.
This study shows the usefulness of the semivariogram for modelling sand ripples created by water flows of varied flow intensity. A combination of two mathematical functions is fitted to each sample semivariogram, that is an exponential (or stochastic) component and a periodic component. The parameters of each of these components have direct physical meaning. A non-dimensional ratio combining the two parameters of the exponential model is interpreted as a regularity index (which increases with the degree of regularity of bedform arrangement). This regularity index is inversely related to the Froude number of the flow. The non-dimensional wavelength, estimated from the dominant periodic function, is also inversely and closely related to the Froude number. The wave height, accurately estimated from properties of the two fitted components, is a direct function of flow velocity and is also proportional to the standard deviation of bed elevations. The bedform shape introduces a considerable discrepancy between the generally assumed normal frequency distribution and the empirical distributions of bed height. The series of bed elevations are generally characterized by a mixture of normal distributions having the same variance but different means. The calculation of a covariance assuming a constant and single mean (as in spectral analysis) can therefore be misleading and the problem may be avoided by using the semivariogram. 相似文献
233.
234.
The growth process, palaeoecological features and the function of organisms in reef-building of the Jukoupu bioherm in Hunan are discussed in detail. The bioherm is divided into 10 microfacies. Organisms are very abundant in the reef. Thirty-two genera Belonging to 7 phyla have been recognized. They make up 5 communities composed mainly of stromatopoids, Corals and algae, each of which has its own salient ecological features. An analysis of all communities shows that the organic functions are very complex. The same organisms may possess diverse functions simultaneously as the reef growth enters the late stage. The growth process of the reef may involve five stages' (l)biostrome stage, (2) stabilization stage, (3) frame-type reef-building stage, (4) binding and covering-type reef-building stage, and (5) baffle- type reef -building stage. 相似文献
235.
北京北山泥石流的分布受构造控制,呈北东方向展布。其分布在时间上和空间上均有一定的规律可寻。一条泥石流沟谷的形成从开始到结束,概括分为形成、位移和堆积。区内泥石流的形成主要有三种形式:以水力侵蚀为主,以坡面侵蚀为主和以沟源崩、滑塌形式触发沟床物质活动而形成的泥石流。 相似文献
236.
237.
江苏北部开敞淤泥质海岸的侵蚀过程及防护 总被引:9,自引:0,他引:9
开敞淤泥质海岸是淤泥质海岸的主要类型,其自然演变过程,主要受沿海河流泥沙供应条件的制约,一旦泥沙供应消失,海岸往往进入不可逆转的侵蚀过程。江苏北部淤泥质海岸的侵蚀过程,起自1855年黄河入海口北归利津入海以后,主要是在近岸破波对岸滩物质的冲刷和潮流的输移扩散作用下进行的。通过对淤泥质海岸动力和沉积物性质的认识和概化,对海岸侵蚀过程作出定量模拟,并对该地区海岸防护工程作出评价和讨论。 相似文献
238.
239.
240.