全文获取类型
收费全文 | 4692篇 |
免费 | 969篇 |
国内免费 | 1392篇 |
专业分类
测绘学 | 376篇 |
大气科学 | 958篇 |
地球物理 | 995篇 |
地质学 | 2624篇 |
海洋学 | 782篇 |
天文学 | 170篇 |
综合类 | 465篇 |
自然地理 | 683篇 |
出版年
2024年 | 44篇 |
2023年 | 123篇 |
2022年 | 276篇 |
2021年 | 277篇 |
2020年 | 270篇 |
2019年 | 281篇 |
2018年 | 320篇 |
2017年 | 262篇 |
2016年 | 324篇 |
2015年 | 288篇 |
2014年 | 331篇 |
2013年 | 290篇 |
2012年 | 299篇 |
2011年 | 313篇 |
2010年 | 325篇 |
2009年 | 337篇 |
2008年 | 293篇 |
2007年 | 241篇 |
2006年 | 194篇 |
2005年 | 179篇 |
2004年 | 146篇 |
2003年 | 138篇 |
2002年 | 154篇 |
2001年 | 147篇 |
2000年 | 149篇 |
1999年 | 171篇 |
1998年 | 131篇 |
1997年 | 128篇 |
1996年 | 99篇 |
1995年 | 95篇 |
1994年 | 70篇 |
1993年 | 76篇 |
1992年 | 53篇 |
1991年 | 41篇 |
1990年 | 30篇 |
1989年 | 35篇 |
1988年 | 21篇 |
1987年 | 24篇 |
1986年 | 20篇 |
1985年 | 7篇 |
1984年 | 10篇 |
1983年 | 13篇 |
1982年 | 10篇 |
1981年 | 2篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1958年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有7053条查询结果,搜索用时 15 毫秒
401.
Xianfeng Jin Suocheng Dong Lingjun Luo Yu Li Chao Yuan Kesheng Shu Jiaquan Chen 《中国地理科学(英文版)》2012,22(5):626-636
The studies on environmental effects of foreign trade and its spatial variations are helpful to design and implement environmental protection countermeasures.In order to eliminate the adverse effects of insufficient observation values on the accuracy of regression results and dynamic information quantity of fitting equation during empirical study,panel data of the mid-eastern provinces and cities of China from 1985 to 2007 were selected based on the adjustment of classical regression model in this paper.Panel unit root test and panel cointegration analysis method were applied to investigating the environmental effects of foreign trade and its spatial variations in the mid-eastern provinces and cities of China and its three groups divided by foreign trade dependence.The results show that all scale effects are positive,while all technical effects are negative and unable to counteract positive scale effects.Foreign trade development is regarded as an important cause for outstanding eco-environmental problems in the mid-eastern provinces and cities of China.Total effects and structural effects are significantly different among different groups because of spatial variations in environmental policies,export destinations,source of FDI,etc.Following the principle of′coordinating generality and considering differences comprehensively′,it is essential to issue a series of policies and countermeasures corresponding to differences in regional environmental effect of foreign trade,in order to coordinate the relationship between foreign trade development and eco-environment in each region. 相似文献
402.
国土空间生态保护修复是生态文明建设的重要抓手。省域国土空间生态修复需要以维持区域生态保护格局为基础,聚焦核心生态区、重要生态带和重点生态绿心,构建国土空间生态修复格局,实现整体保护、系统修复、综合治理。以山东为例,在生态问题诊断基础上,基于省域生态保护格局,构建了“两核、三带”的生态修复总体格局,提出了“六心、六原、多城”的生态修复关键区域,提出了筑牢生态保护基础框架,锚定重点生态绿心保护修复,因地制宜开展平原土地整治修复,差异化推进城镇生态修复,以期为省域生态修复提供支撑。 相似文献
403.
全球及中国周边地区资源环境科学数据是支撑包括全球变化在内的资源环境前沿科学研究,以及我国宏观决策制定重要的基础性支撑数据,对于保障国家安全具有重要意义。针对现阶段尚无一套完整的、标准化的全球及中国周边地区资源环境科学数据库的现状,结合中国科学院 "资源环境科学数据库建设与决策支持研究"实践工作,本文构建了包括3大体系在内的全球及中国周边地区资源环境科学数据库的基本框架,即资源环境科学数据库标准规范体系结构、全球资源环境科学数据库系统、中国周边资源环境科学数据库系统。其中,资源环境科学数据库标准规范体系结构,包括3个大类共计22类标准规范;全球资源环境科学数据库系统包括2大类、13亚类、35个小类的数据库;中国周边地区资源环境科学数据库由东北亚、东南亚、南亚和中亚4个区域共计23小类数据库构成,以期为后续相关大尺度资源环境科学数据库的构建提供探索性的借鉴。今后将进一步完善全球及中国周边地区资源环境科学数据库构建工作的深化与拓展研究。 相似文献
404.
The effects of various salinity fluctuation amplitudes (2, 4, 6 and 8) on the growth, osmolarity, Na+-K+-ATPase activity and Hsp70 of juvenile Fenneropenaeus chinensis cultured in seawater with a salinity of 20 were studied. The results show that weight gain in the salinity fluctuation treatments was better than that in control; in particular, the weight gain of treatments S4 and S0, at 231.8% and 196.3%, respectively, was significantly different (P<0.05). The hemolymph osmolarity of treatments S0, S2, S4, S6 and S8 was 635.4, 630.8, 623.6, 614.4 and 600.3 mOsm/kg, respectively, and decreased with increasing salinity fluctuation amplitude. The level of Na+-K+-ATPase activity in gills of F. chinensis was higher than that in hepatopancreas, but there were no significant differences among all treatments, either in gills or hepatopancreas (P>0.05). The relative level of Hsp70 in treatment S4 was 48.4% and 40.4% higher than control in muscle and eyestalks, respectively, with the highest values being recorded under a salinity fluctuation amplitude of 4. 相似文献
405.
Using wavelet analysis, regression analysis and the Mann-Kendall test, this paper analyzed time-series (1959–2006) weather
data from 23 meteorological stations in an attempt to characterize the climate change in the Tarim River Basin of Xinjiang
Uygur Autonomous Region, China. Major findings are as follows: 1) In the 48-year study period, average annual temperature,
annual precipitation and average annual relative humidity all presented nonlinear trends. 2) At the 16-year time scale, all
three climate indices unanimously showed a rather flat before 1964 and a detectable pickup thereafter. At the 8-year time
scale, an S-shaped nonlinear and uprising trend was revealed with slight fluctuations in the entire process for all three
indices. Incidentally, they all showed similar pattern of a slight increase before 1980 and a noticeable up-swing afterwards.
The 4-year time scale provided a highly fluctuating pattern of periodical oscillations and spiral increases. 3) Average annual
relative humidity presented a negative correlation with average annual temperature and a positive correlation with annual
precipitation at each time scale, which revealed a close dynamic relationship among them at the confidence level of 0.001.
4) The Mann-Kendall test at the 0.05 confidence level demonstrated that the climate warming trend, as represented by the rising
average annual temperature, was remarkable, but the climate wetting trend, as indicated by the rising annual precipitation
and average annual relative humidity, was not obvious. 相似文献
406.
研究东日本地震、汶川地震和玉树地震震中及其附近区域在地震前后的水汽时间序列变化。首先分析震中MODIS水汽序列和震中附近探空站点水汽序列在地震前后的变化;然后基于GNSS ZTD与水汽之间的高相关性,以GNSS ZTD代替GNSS水汽,讨论震源区周围IGS站点的ZTD序列变化。研究发现,震后震中及其附近区域水汽值变化出现异常,且距离越近所受影响越大;水汽不断聚积,达到峰值后发生降水。 相似文献
407.
MIN Jianxiong LI Guangxue DING Dong QIAO Lulu MA Yanyan YANG Gang ZHANG Yaqi ZHANG Lei LI Shuhao 《中国海洋大学学报(英文版)》2019,(1):9-19
Based on the observed surface suspended matter in the East China Sea in February 2007 and June 2015, an empirical model was established using L1 b's band 4 data to retrieve surface suspended matter from the Moderate Resolution Imagine Spectroradiometer Terra imagery. The squared correlation coefficient is 0.8358, and the root mean square error is 0.4285 mg L-1. The model reflects the distribution characteristics of surface suspended matter in the inner shelf of the East China Sea. In this paper, the satellite images of the study area were retrieved in January from 2001 to 2015, and the monthly distribution of surface suspended matter were obtained. The inter-annual distribution of the study area is similar, and the concentration of surface suspended matter is higher near the shore than offshore. A large amount of surface suspended matter is transported southeast under the influence of Zhejiang and Fujian coastal current and Taiwan warm current. Only a small amount of surface suspension can reach the Kuroshio area. The surface suspended matter concentration changes obviously near the estuary because of the effect of differences in the flux of the Yangtze River. Meanwhile, winter monsoon, temperature front, El Ni?o events, and other factors affect the distribution of surface suspended matter in 100 m isobath to coastal water but minimally influence the distribution in 100 m isobath to deep sea. 相似文献
408.
本文给出利用中国、意大利、南非射电望远镜对一个近赤道源样本在5GHz频率上进行MK2VLBI普测中10颗弱源的相关流量和其中部分源用HartRAO望远镜观测的总流量值,作为进一步的MK3或VLBA记录系统的VLBI观测依据. 相似文献
409.
A GIS-based DRASTIC model for assessing groundwater vulnerability in the Ordos Plateau, China 总被引:4,自引:2,他引:4
Lihe Yin Eryong Zhang Xiaoyong Wang Jochen Wenninger Jiaqiu Dong Li Guo Jinting Huang 《Environmental Earth Sciences》2013,69(1):171-185
Groundwater plays a key role in arid regions as the majority of water is supplied by it. Groundwater pollution is a major issue, because it is susceptible to contamination from land use and other anthropogenic impacts. A study was carried out to build a vulnerability map for the Ordos Plateau using the DRASTIC model in a GIS environment. The map was designed to show the areas of the highest potential for groundwater pollution based on hydrogeological conditions. Seven environmental parameters, such as depth to water table, net recharge, aquifer media, soil media, topography, impact of the vadose zone media, and hydraulic conductivity of the aquifer, were incorporated into the DRASTIC model and GIS was used to create a groundwater vulnerability map by overlaying the available data. The results of this study show that 24.8 % of the study area has high pollution potential, 24.2 % has moderate pollution potential, 19.7 % has low pollution potential, and the remaining 31.3 % of the area has no risk of groundwater pollution. The regional distribution of nitrate is well correlated with the DRASTIC vulnerability index. In contrast to this, although the DRASTIC model indicated that the western part had no risk, nitrate concentrations were higher in some of these areas. In particular, higher nitrate concentrations were recorded along river valleys and around lakes, such as the Mulin River valley. This is mainly caused by the intensive agricultural development and favorable conditions for recharge along river valleys. 相似文献
410.
Study of the feeding capacity of silver carp and bighead carp by means of experimental ecologyshowed that the filtering frequency of silver carp was slightly greater than that of bighead carp,but thatthe latter's suction volume was much greater than the former's so the filtering rate(filtering frequency mul-tiplied by the suction volume)of silver carp was smaller than that of bighead carp.The filtering efficien-cy of silver carp for phytoplankton was greater than that of bighead carp for them.The removal ratesof silver carp for phytoplankton were greater than those of bighead carp,but for zooplankton the formerwere smaller than the latter.For food particles about 70 μm both removal rates were almost equal.The feeding habits of the two species in natural waters is also discussed in the paper. 相似文献