全文获取类型
收费全文 | 61856篇 |
免费 | 14761篇 |
国内免费 | 22970篇 |
专业分类
测绘学 | 11033篇 |
大气科学 | 8603篇 |
地球物理 | 11704篇 |
地质学 | 41354篇 |
海洋学 | 12170篇 |
天文学 | 718篇 |
综合类 | 5150篇 |
自然地理 | 8855篇 |
出版年
2024年 | 743篇 |
2023年 | 2019篇 |
2022年 | 4013篇 |
2021年 | 4792篇 |
2020年 | 3953篇 |
2019年 | 4493篇 |
2018年 | 4022篇 |
2017年 | 3830篇 |
2016年 | 3948篇 |
2015年 | 4503篇 |
2014年 | 4290篇 |
2013年 | 5139篇 |
2012年 | 5669篇 |
2011年 | 5609篇 |
2010年 | 5603篇 |
2009年 | 5136篇 |
2008年 | 5169篇 |
2007年 | 4895篇 |
2006年 | 4622篇 |
2005年 | 3932篇 |
2004年 | 3010篇 |
2003年 | 2032篇 |
2002年 | 2060篇 |
2001年 | 1895篇 |
2000年 | 1550篇 |
1999年 | 761篇 |
1998年 | 305篇 |
1997年 | 215篇 |
1996年 | 177篇 |
1995年 | 131篇 |
1994年 | 106篇 |
1993年 | 120篇 |
1992年 | 102篇 |
1991年 | 80篇 |
1990年 | 93篇 |
1989年 | 53篇 |
1988年 | 38篇 |
1987年 | 38篇 |
1986年 | 45篇 |
1985年 | 41篇 |
1984年 | 43篇 |
1983年 | 34篇 |
1982年 | 30篇 |
1981年 | 24篇 |
1979年 | 30篇 |
1978年 | 21篇 |
1977年 | 15篇 |
1976年 | 13篇 |
1957年 | 21篇 |
1954年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
212.
The concepts of regional resources and environmental carrying capacity are important aspects of both academic inquiry and government policy. Although notable results have been achieved in terms of evaluating both these variables, most researchers have utilized a traditional analytical method that incorporates the "pressure-state-response" model. A new approach is proposed in this study for the comprehensive evaluation of regional resources and environmental carrying capacity; applying a "pressure-support", "destructiveness-resilience", and "degradation-promotion"("PS-DR-DP") hexagon interaction theoretical model, we divided carrying capacity into these three pairs of interactive forces which correspond with resource supporting ability, environmental capacity, and risk-disaster resisting ability, respectively. Negative carrying capacity load in this context was defined to include pressure, destructiveness, and degradation, while support, resilience, and promotion comprised positive attributes. The status of regional carrying capacity was then determined via the ratio between positive and negative contribution values, expressed in terms of changes in both hexagonal shape and area that result from interactive forces. In order to test our "PS-DR-DP" theory-based model, we carried out a further empirical study on Beijing over the period between 2010 and 2015. Analytical results also revealed that the city is now close to attaining a perfect state for both resources and environmental carrying capacity; the latter state in Beijing increased from 1.0143 to 1.1411 between 2010 and 2015, an improved carrying capacity despite the fact that population increased by two million. The average contribution value also reached 0.7025 in 2015, indicating that the city approached an optimal loading threshold at this time but still had space for additional carrying capacity. The findings of our analysis provide theoretical support to enable the city of Beijing to control population levels below 23 million by 2020. 相似文献
213.
内蒙西乌旗白音布拉格蛇绿岩地球化学特征 总被引:4,自引:5,他引:4
内蒙古西乌旗白音布拉格蛇绿岩带是新发现的内蒙古西乌旗迪彦庙蛇绿岩带的北带,主要由蛇纹石化方辉橄榄岩、层状-块状辉长岩、斜长岩、枕状玄武岩、角斑岩-石英角斑岩及硅质岩等构造单元组成.白音布拉格蛇绿岩中的熔岩按照地球化学特征可以分为三组:第1组属于玻安岩系,以富Si(SiO2=52.71%、61.22%)、Mg(MgO=6.81%和10.88%)和贫Ti(TiO2 =0.49%、0.51%)、HREE及HFSE为特征;第2组具有低Ti(TiO2 =0.62%~0.78%)、高Mg(MgO =5.20% ~11.30%)的特征,LREE弱亏损、类似N-MORB的稀土配分模式,但相对N-MORB,又具有富集LILE,亏损Nb、Ta等高场强元素的特征,类似岛弧拉斑玄武岩(IAT);第3组表现为:岩石具有高Ti(TiO2=1.86%、1.91%)、高Mg(MgO=5.25%和5.46%)及高P(P2O5=0.23%、0.27%),LREE和HREE分异较为明显((La/Yb)N=2.32、2.53)等特征,类似OIB.根据玻安岩与IAT的存在,推测白音布拉格蛇绿岩产于岛弧和弧前环境. 相似文献
214.
215.
1 INTRODUCTION The typhoon is a destructive weather phenomenon that stands at the top of ten major natural disasters[1]. Typhoon-related damages are the immediate consequences of weather it brings forth, which include heavy rain, strong winds and storm surges (water gain). They cause flash floods, bring down houses and break through dams[1]. Of the research on typhoons for the recent 10 years[2], new advances have been reported on abrupt changes in the motion, structure and intensity of… 相似文献
216.
217.
传统的网络RTK中数据中心和流动站的数据传输一般采用RTCM SC‐104格式,而数据中心和参考站之间一般采用接收机厂商自定义的实时数据格式。为了满足新一代网络RTK多系统,多信息类型的实时数据传输,RTCM 委员会专门推出了最新的 RTCM 10403.2数据格式。本文详细介绍了最新版的 RTCM3.2电文特点,新增的MSM 电文,编、解码方式以及对BDS系统的支持。给出了M SM电文组的解码流程,通过对实时数据进行解码实验,证明了算法的正确性和可靠性。 相似文献
218.
钢结构在长期荷载及不均匀受力的作用下会产生空间变形,其中扰度是其重要的衡量指标。通常采用全站仪采集钢结构轴线上若干特征点进行分析、计算,由于钢结构特征点难以捕捉,测量存在误差,并且有限的空间离散点难以全面反应钢结构空间变形。本文采用徕卡RTC360三维激光扫描进行钢结构扰度测量;介绍了其作业流程及数据处理方法;利用标靶将各个测站的三维点云拼接成一个整体;采用拟合的方法提取空间特征点及轴线;利用三维点云构建空间模型,并与设计模型进行碰撞分析;可全面地反映钢结构的空间变形情况。 相似文献
219.
Relationships between Interannual and Intraseasonal Variations of the Asian-Western Pacific Summer Monsoon Hindcasted by BCC_CSM1.1(m) 总被引:4,自引:0,他引:4
LIU Xiangwen ;WU Tongwen ;YANG Song ;LI Qiaoping ;CHENG Yanjie ;LIANG Xiaoyun ;FANG Yongjie ;JIE Weihua ;NIE Suping 《大气科学进展》2014,31(5):1051-1064
Using hindcasts of the Beijing Climate Center Climate System Model, the relationships between interannual variability (IAV) and intraseasonal variability (ISV) of the Asian-western Pacific summer monsoon are diagnosed. Predictions show reasonable skill with respect to some basic characteristics of the ISV and IAV of the western North Pacific summer monsoon (WNPSM) and the Indian summer monsoon (ISM). However, the links between the seasonally averaged ISV (SAISV) and seasonal mean of ISM are overestimated by the model. This deficiency may be partially attributable to the overestimated frequency of long breaks and underestimated frequency of long active spells of ISV in normal ISM years, although the model is capable of capturing the impact of ISV on the seasonal mean by its shift in the probability of phases. Furthermore, the interannual relationships of seasonal mean, SAISV, and seasonally averaged long-wave variability (SALWV; i.e., the part with periods longer than the intraseasonal scale) of the WNPSM and ISM with SST and low-level circulation are examined. The observed seasonal mean, SAISV, and SALWV show similar correlation patterns with SST and atmospheric circulation, but with different details. However, the model presents these correlation distributions with unrealistically small differences among different scales, and it somewhat overestimates the teleconnection between monsoon and tropical central-eastern Pacific SST for the ISM, but underestimates it for the WNPSM, the latter of which is partially related to the too-rapid decrease in the impact of E1 Nifio-Southern Oscillation with forecast time in the model. 相似文献
220.