首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   17186篇
  免费   1922篇
  国内免费   2647篇
测绘学   2020篇
大气科学   1889篇
地球物理   2328篇
地质学   5889篇
海洋学   2044篇
天文学   185篇
综合类   2341篇
自然地理   5059篇
  2024年   67篇
  2023年   234篇
  2022年   348篇
  2021年   361篇
  2020年   309篇
  2019年   476篇
  2018年   254篇
  2017年   211篇
  2016年   285篇
  2015年   369篇
  2014年   1599篇
  2013年   990篇
  2012年   1229篇
  2011年   1269篇
  2010年   1142篇
  2009年   1294篇
  2008年   1099篇
  2007年   950篇
  2006年   833篇
  2005年   1156篇
  2004年   905篇
  2003年   1042篇
  2002年   846篇
  2001年   722篇
  2000年   540篇
  1999年   526篇
  1998年   404篇
  1997年   363篇
  1996年   324篇
  1995年   257篇
  1994年   264篇
  1993年   170篇
  1992年   194篇
  1991年   192篇
  1990年   166篇
  1989年   163篇
  1988年   34篇
  1987年   24篇
  1986年   13篇
  1985年   15篇
  1984年   10篇
  1983年   7篇
  1982年   13篇
  1980年   12篇
  1979年   5篇
  1965年   4篇
  1959年   4篇
  1943年   5篇
  1941年   5篇
  1930年   4篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
夏月。温润如玉,村口那棵大树下,早早站着不少人。有的手拿蒲扇,有的蹲在树旁叨着烟袋,季风轻轻地吹,脊梁沟里好像有幼童用手挨。  相似文献   
962.
About forty productive oil/gas fields hosted in volcanic reservoirs have been found since 1957 in fourteen basins of China. They can be simply subdivided into two groups, the east and the west. Reservoir volcanic rocks of the east group are predominantly composed of Late Jurassic to Early Cretaceous rhyolite and Tertiary basalt, preferred being considered as rift type volcanics developed in the circum-Pacific tectonic regime. Those of the west are Permo-Carboniferous intermediate/basic volcanic rocks, being island-arc type ones developed in paleo-Asian Ocean tectonic regime.  相似文献   
963.
日前,中国国家能源局副局长孙勤透露,中国今年内将安排5个新的核电项目开工建设。据悉,当前中国在建项目有24个机组,2500多万千瓦,到2020年,中国核电装机容量有望超过7500万千瓦。  相似文献   
964.
工业和信息化部部长李毅中近日表示,支持钢铁协会的意见,内部要统一、联合起来,在铁矿石谈判中要维护国家的利益,不要互相竞价,把价格抬高,否则最后损失的是中国钢铁业。这次的教训太深了,中国不能再吃第二次亏了。  相似文献   
965.
2008年12月26日,汝阳县被授予“中国恐龙之乡”。为保护地质遗迹,进一步扩大汝阳恐龙独特的自然资源及知名度,今年伊始,汝阳县委、县政府再次决定自筹资金1500万元筹建“中国汝阳恐龙遗址馆”,目前各项工作进展顺利。  相似文献   
966.
张亚明,笔名萧枫,哑呜,安徽人。中国报告文学学会会员、中国国土资源作家协会会员,现为中国矿业报首席记者。  相似文献   
967.
在不同温度下,对中国对虾(Fenneropenaeus chinensis)越冬亲虾进行人工感染白斑综合症病毒(WSSV)实验,利用腹部采血和剪取游泳足进行活体取样.统计感染对虾发病及死亡情况,利用巢式PCR技术检测WSSV,统计实对虾受WSSV感染情况.结果显示,在对虾累计死亡率上游泳足取样的高温组(18~21 ℃)和低温组(9~12 ℃)有显著差异(P=0.041<0.05),高温组明显高于低温组;在对虾受WSSV感染程度上,高温组主要呈严重感染(91.67%),中温组(13~16 ℃)和低温组则主要呈轻度感染,且温度越低受感染程度越轻;在跟踪WSSV感染对虾进程研究上,高温组在48 h检测出WSSV,中温组和低温组则分别在72 h和96 h检测出WSSV,明显滞后于高温组.表明温度对WSSV感染对虾有一定的影响,低温(9~12 ℃)条件下WSSV也能感染对虾且导致死亡,但低温可抑制WSSV的增殖,减缓感染进程,延迟白斑综合症(WSS)的发生.  相似文献   
968.
The landscape pattern of Da'an County, Northeast China has undergone significant changes since the 1950s as a result of climatic change and human activities. The aim of this paper is to quantitatively study landscape pattern and its spatial dynamics of Da'an County at the landscape level over the nearly 50-year span. Patch dynamics were examined according to land use and land cover change processes built from a series of images, as well as topographic maps, and temporal patterns built from landscape pattern metrics. The transition matrix of landscape patch types and changes of various landscape metrics were applied. The results showed that, from 1956 to 2000, the landscape within the study area had undertaken a complicated evolution in landscape structure and composition. The outstanding characteristic is that saline-alkali land increased and grassland decreased. As some smaller patches amalgamated, the heterogeneity of patch decreased. All those changes were the synthetic result of both climatic and anthropogenic influences, but the predominant factor was different in different parts. In the southern part of the study area, the landscape pattern changes resulted from the modification of climate obviously, while in the northern part, the landscape pattern changes were mainly caused by human activities, such as the conversion between farmland and saline-alkali land. This phenomenon showed that human activities played more important role in the north than in the south of Da'an County.  相似文献   
969.
A non-dimensional relative sensitivity coefficient was employed to predict the responses of reference crop evapotranspiration (ET0) to perturbation of four climate variables in Tao'er River Basin of the northeastern China. Mean monthly ET0 and yearly ET0 from 1961 to 2005 were estimated with the FAO-56 Penman-Monteith Equation. A 45-year historical dataset of average monthly maximum/minimum air temperature, mean air temperature, wind speed, sunshine hours and relative humidity from 15 meteorological stations was used in the analysis. Results show that: 1) Sensitivity coefficients of wind speed, air temperature and sunshine hours were positive except for those of air temperature of Arxan Meteorological Station, while those of relative humidity were all negative. Relative humidity was the most sensitive variable in general for the Tao'er River Basin, followed by sunshine hours, wind speed and air temperature. 2) Similar to climate variable, monthly sensitivity coefficients exhibit large annual fluctuations. 3) Sensitivity coefficients for four climate variables all showed significant trends in seasonal/yearly series. Also, sensitivity coefficients of air temperature, sunshine hours and wind speed all showed significant trends in spring. 4) Among all sensitivity coefficients, the average yearly sensitivity coefficient of relative humidity was highest throughout the basin and showed largest spatial variability. Longitudinal distribution of sensitivity coefficients for air temperature, relative humidity and sunshine hours was also found, which was similar to the distribution of the three climate variables.  相似文献   
970.
The goal of land consolidation in China is still to develop agricultural production. The study of landscape effects of land consolidation projects (LCPs) faces many difficulties because of the lack of government's interest and data. This paper, taking Tianmen City of Hubei Province in Central China as an example, presents a methodology for analyzing landscape effects of LCPs by GIS and Fragstats3.3. It describes landscape effects with indexes of Patch Density (PD), Largest Patch Index (LPI), Landscape Shape Index (LSI), Interspersion and Juxtaposition Index (IJI), Aggregation Index (AI), and Shannon's Diversity Index (SHDI), showing more regular shape, simpler structure and less habitat diversity after LCPs. It computes ten landscape indexes of four categories of patches including Cultivated Land, Road, Water Channel, and River and Pond. The indexes show that 1) cultivated land becomes more fragmental in patch area, less irregular in patch shape and more concentrated in block; 2) the transport capacity of roads and irrigation and drainage capacity of water channels have been improved; 3) the landscape change of river and pond can be summarized as decreasing scale, more regular shape, reducing connectivity and diversity of the class. LCPs can facilitate agricultural production as well as protect cultivated land and food security. However, it is doubted that the increase of cultivated land from LCPs results from the reducing in landscape diversity of water area.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号