全文获取类型
收费全文 | 46990篇 |
免费 | 20295篇 |
国内免费 | 44949篇 |
专业分类
测绘学 | 3940篇 |
大气科学 | 23020篇 |
地球物理 | 15404篇 |
地质学 | 41366篇 |
海洋学 | 17373篇 |
天文学 | 1442篇 |
综合类 | 5678篇 |
自然地理 | 4011篇 |
出版年
2024年 | 290篇 |
2023年 | 711篇 |
2022年 | 1376篇 |
2021年 | 1779篇 |
2020年 | 2782篇 |
2019年 | 6078篇 |
2018年 | 6359篇 |
2017年 | 6006篇 |
2016年 | 6094篇 |
2015年 | 5385篇 |
2014年 | 5066篇 |
2013年 | 5560篇 |
2012年 | 5302篇 |
2011年 | 5137篇 |
2010年 | 5007篇 |
2009年 | 4253篇 |
2008年 | 3493篇 |
2007年 | 3437篇 |
2006年 | 3016篇 |
2005年 | 2878篇 |
2004年 | 3063篇 |
2003年 | 2761篇 |
2002年 | 2476篇 |
2001年 | 2316篇 |
2000年 | 1966篇 |
1999年 | 2151篇 |
1998年 | 2152篇 |
1997年 | 2136篇 |
1996年 | 1785篇 |
1995年 | 1620篇 |
1994年 | 1482篇 |
1993年 | 1413篇 |
1992年 | 1195篇 |
1991年 | 890篇 |
1990年 | 824篇 |
1989年 | 681篇 |
1988年 | 579篇 |
1987年 | 448篇 |
1986年 | 360篇 |
1985年 | 280篇 |
1984年 | 304篇 |
1983年 | 181篇 |
1982年 | 220篇 |
1981年 | 147篇 |
1980年 | 114篇 |
1979年 | 115篇 |
1978年 | 54篇 |
1977年 | 45篇 |
1971年 | 54篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
91.
(姚建衢)(杨焕宗)COORDINATINGDEVELOPMENTOFAGRICULTURALRESOURCESANDENVIRONMENTOFTHETROPICREGIONINYUNNANPROVINCE¥YaoJianqu(Instituteof... 相似文献
92.
93.
Simulation of double cold cores of the 35°N section in the Yellow Sea with a wave-tide-circulation coupled model 总被引:1,自引:0,他引:1
Based on the MASNUM wave-tide-circulation coupled numerical model, the temperature structure along 35°N in the Yellow Sea
was simulated and compared with the observations. One of the notable features of the temperature structure along 35°N section
is the double cold cores phenomena during spring and summer. The double cold cores refer to the two cold water centers located
near 122°E and 125°E from the depth of 30m to bottom. The formation, maintenance and disappearance of the double cold cores
are discussed. At least two reasons make the temperature in the center (near 123°E) of the section higher than that near the
west and east shores in winter. One reason is that the water there is deeper than the west and east sides so its heat content
is higher. The other is invasion of the warm water brought by the Yellow Sea Warm Current (YSWC) during winter. This temperature
pattern of the lower layer (from 30m to bottom) is maintained through spring and summer when the upper layer (0 to 30m) is
heated and strong thermocline is formed. Large zonal span of the 35°N section (about 600 km) makes the cold cores have more
opportunity to survive. The double cold cores phenomena disappears in early autumn when the west cold core vanishes first
with the dropping of the thermocline position.
Supported by the National Basic Research Program of China (No. G1999043809) and the National Science Foundation of China (No.
49736190). 相似文献
94.
95.
Universal cokriging is used to obtain predictions when dealing with multivariate random functions. An important type of nonstationarity is defined in terms of multivariate random functions with increments which are stationary of orderk. The covariance between increments of different variables is modeled by means of the pseudo-cross-covariance function. Criteria are formulated to which the parameters of pseudo-cross-covariance functions must comply so as to ensure positive-definiteness. Cokriging equations and the induced cokriging equations are given. The study is illustrated by an example from soil science. 相似文献
96.
97.
98.
The purpose of this work was to reinvestigate the existing hydrogeological conceptual model of the basin of Madrid, Spain. A cumulative chemical isotopic diagram which enabled the distinction between different groups of water as well as calculation of the mode of their blending was used for this investigation. It was found that the groups of discharge were lighter in their isotopic composition than that of recharge. The previous explanation of this fact, backed by carbon-14 dating, was the long residence time due to flow lines going down to depths of more than 1000 m. This flow model assumes homogenous conditions to these depths. This assumption can not be supported by evidence from deep wells. Thus a modified model is suggested which maintains homogenous conditions only to about 300 m and a deep confined aquifer below containing paleowater. The higher degree of depletion of this water has been explained by a colder climate on top of an altitude effect. Another interesting observation was the correlation between the isotopic composition of the rains, the month of the rain event and the composition of the recharge group groundwater. It could be seen that the winter rains resemble the groundwater composition, which shows that practically all the spring and summer rains were evapotranspirated. 相似文献
99.
IMPACTS OF FUTURE SEA LEVEL RISE ON SALT WATER INTRUSION IN THE CHANGJIANG RIVER ESTUARY 总被引:1,自引:0,他引:1
杨桂山 《中国地理科学(英文版)》1992,2(1):30-41
Sea level rise could increase the salinity of an estuary by altering the balance between fresh water and salt water. The implications of sea level rise for increasing salinity have been examined in the Changjiang (Yangtze) River estuary. By correlative analysis of chlorinity, discharge and tidal level and calculation of two-dimensional chlorinity, distribution of the Changjiang River estuary, the changes of the intensity and lasting hours of salt water intrusion at Wusong Station and the changes of chlorinity distribution in the South Branch of the Changjiang River estuary have been estimated when future sea level rises 50-100 cm. The intensity of salt water intrusion in the future will be far more serious than current trend. 相似文献
100.
介绍一种自行设计和加工的直接雾化石墨炉进样装置的结构、工作原理和操作方法。该装置由微机控制与石墨炉加热程序同步工作,可以实现自动控制和自动测量。对雾化进样石墨炉分析的仪器参数和实验条件进行了研究和选择。结果表明,该装置与石墨炉结合具有自动化程度高、重现性好、样品利用率高和分析速度快等特点。 相似文献