全文获取类型
收费全文 | 18229篇 |
免费 | 2999篇 |
国内免费 | 3855篇 |
专业分类
测绘学 | 1483篇 |
大气科学 | 2296篇 |
地球物理 | 2743篇 |
地质学 | 9288篇 |
海洋学 | 3100篇 |
天文学 | 576篇 |
综合类 | 1296篇 |
自然地理 | 4301篇 |
出版年
2024年 | 89篇 |
2023年 | 302篇 |
2022年 | 635篇 |
2021年 | 744篇 |
2020年 | 695篇 |
2019年 | 806篇 |
2018年 | 684篇 |
2017年 | 753篇 |
2016年 | 796篇 |
2015年 | 857篇 |
2014年 | 1045篇 |
2013年 | 936篇 |
2012年 | 1134篇 |
2011年 | 1107篇 |
2010年 | 957篇 |
2009年 | 1185篇 |
2008年 | 1053篇 |
2007年 | 1275篇 |
2006年 | 1270篇 |
2005年 | 1170篇 |
2004年 | 1046篇 |
2003年 | 993篇 |
2002年 | 899篇 |
2001年 | 691篇 |
2000年 | 566篇 |
1999年 | 520篇 |
1998年 | 428篇 |
1997年 | 388篇 |
1996年 | 363篇 |
1995年 | 302篇 |
1994年 | 269篇 |
1993年 | 230篇 |
1992年 | 221篇 |
1991年 | 174篇 |
1990年 | 109篇 |
1989年 | 162篇 |
1988年 | 65篇 |
1987年 | 50篇 |
1986年 | 34篇 |
1985年 | 21篇 |
1984年 | 12篇 |
1983年 | 9篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1954年 | 3篇 |
1877年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
111.
112.
青藏高原腹地湖泊沉积对第四纪晚期古季风变化的响应 总被引:2,自引:0,他引:2
通过对青藏高原腹地的综合科学考察和对中心钻孔岩芯剖面的最新研究,用层序地层学与年代地层学和气候地层学相结合的方法,分辨出可可西里地区湖泊沉积记录(孔深7.25 m)的第四纪晚期距今3万余年以来的古气候变化,沉积物磁化率等因子综合表征的高原古季风变化是波动发展的,发生在仙女木期地质环境事件中的季风活动具有强烈暴发的特点,是高原季风发展中的突变事件,而且地表热点效应对其起到了激发作用。综合分析的研究成果表明,它的变化频谱与激变因子及其运行机制是伴随着青藏高原地质效应的演变而发展的,为研究第四纪冰消期以来的气候变化提供了新的信息。 相似文献
113.
A series of patterns in the chemical and isotopic composition of the groundwater of the Cerro Prieto area have been recognized. These patterns were used to define four hydrochemical and isotopical zones in the area. One of these areas located at the east side was found to be the source of recharge, while another in the central part is where an older aquifer exist in lacustrine deposits. The low permeability of the area causes the aquifer to act as an aquitard, which at least is partly surrounded by an interface zone where mixing of waters occurs. Tritium and radiocarbon method revealed that the aquitard is an isolate system whose residence time is of 32 ka. 相似文献
114.
Yunchao HAO Zhigang GUO Dejiang FAN 《中国地球化学学报》2006,25(B08):13-13
In the East China Sea (ECS), there are some mud areas, including the south coastal mud area, the north coastal mud area, and the mud area to the southwest of Cheju Island. X-ray fluorescence (XRF) techniques and Thermal Ionization Mass Spectrometry (TIMS) were used to study the high-resolution sedimentary record of Pb concentrations and Pb stable isotopic compositions in the past one hundred and fifty years in the coastal mud of the ECS. Pb concentrations of a ^210Pb dating S5 core in the study area have increased rapidly since 1980, and reached the maximal value with 65.08 μg/g in 2000, corresponding to the fast economic development of China since the implementation of the "Reform and Open Policy" in 1978; ^206Pb/^207Pb ratios generally had stabilized at 1.195 from 1860 to 1966, and decreased gradually from 1966 to 2000, indicating that the anthropogenic source Pb contribution to the ECS has increased gradually since 1966, especially since 1980. Pb concentrations decreased distinctly from 2000 to 2003 and ^206Pb/^207Pb ratios increased from 2001 to 2003, corresponding closely to the ban of lead gasoline from 2000 in China. From 1950 to 2003, there occurred four distinct decrease events of ^206Pb/^207Pb, possibly responding to the Changjiang River (Yangtze River) catastrophic floods in 1998, 1991, 1981 and 1954; from 1860 to 1966, there were two decrease periods of ^206Pb/^207Pb, which may respond to the catastrophic floods of Changjiang River in 1931 and 1935, and 1870. As a result of the erosion and drowning by the catastrophic floods, the anthropogenic lead accumulated in soil and water environments over a long period of time was brought into the Changjiang River, then part of them was finally transported into the ECS, which leads to changes in Pb stable isotopic composition. 相似文献
115.
WANG Lei ZHANG Guang-Xin 《湿地科学》2006,4(3):193-197
1 INTRODUCTIONNitrogen and phosphorus in the water are the nu-trients limited in natural wetlands,which seriously in-fluence on the ecosystem production and the biodiver-sity(Mitsch,Grosselin,2000).Nitrogen and phos-phorus recycles have been interrupted b… 相似文献
116.
Xinghui XIA 《中国地球化学学报》2006,25(B08):263-263
Photodegradation of chrysene, benzo (a) pyrene and benzo (g, h, i) perylene in natural water of the Yellow River was studied using simulation sunlight. The effects of particulates on the photodegradation were explored. Several results arose from this study. (1) The photodegradation of PAHs can be fitted with first-order kinetics when no particulate exists in water system, and the first-order constant increases with decreasing of initial concentration of PAHs. The photodegradation rates of the three PAHs are related to their molecule absorption spectrum. (2) The existence of loess exerts two kinds of effects on the photodegradation of PAHs, including the inner filter effects and the photosensitizing effects of humic substance in loess. These two contrary effects lead to the difference of net effects among different contents of loess. When the loess contents are 0.1 and 5.0 g/L, the existence of loess stimulates the photodegradation of chrysene, benzo (a) pyrene. When the loess content is 5.0 g/L, the existence of loess stimulates the photodegradation of benzo (g, h, i) perylene. In addition, the photodegradation of PAHs can be fitted with the second-order kinetics when there is loess in the water system. (3) The soluble humic substances in loess can accelerate the photodegradation of PAHs while the in soluble part cannot. (4) Since the soluble humic substances in the suspended solids of the river have been dissolved in water, 相似文献
117.
Substantial damage to water supply systems, including water delivery pipelines, water treatment plants, reservoirs, and water
storage tanks, was reported after the 1999 Chi–Chi Taiwan Earthquake. This paper first summarizes the damage survey and then
presents the results of seismic fragility analysis for underground pipelines. Construction blueprints of the water delivery
pipelines and repair work orders of 11 townships and cities in the disastrous area were digitized into a Geographical Information
System (GIS) for analysis and assessment. With the aid of the GIS system, we found that PVC pipes made up 86% of water delivery
pipelines while steel, cast iron, ductile iron, PE and others took the rest. Therefore, this paper focuses on the fragility
analysis of PVC pipes. Three different methods were applied to derive the fragility relations between the PVC water pipes
having nominal diameters (approximately inner diameters) greater than or equal to 65 mm and earthquake intensity parameters
such as peak ground acceleration and peak ground velocity. The results were then examined with those of other countries. The
discrepancy between our results and the empirical equation used by HAZUS, an earthquake loss estimation software developed
by the Federal Emergency Management Agency was not significant. 相似文献
118.
R&D投入的区位模式与区域经济发展 总被引:2,自引:1,他引:2
基于对中国R&D投入的区位模式的实证检验,从投入一产出绩效的角度认为集中投入的区位模式具有较强的适宜性。利用聚类分析技术,按照R&D的投入强度和及其对经济的带动效应、科技化效应、产业化效应将中国各省区划分为6类地区,具有代表性的是科技化模式和产业化模式。其中前者适合于技术存置较高的少数地区,而后者具有较强的借鉴意义。 相似文献
119.
长江口水域多光谱遥感水深反演模型研究 总被引:3,自引:0,他引:3
利用Landsat-7 ETM 遥感影像反射率和实测水深值之间的相关性可以探测水深。该文介绍单波段、双波段比值和多波段3种线性回归模型以及动量BP人工神经网络水深反演模型。选择长江口北港河道上段作为研究区,利用上述模型,分两种情况进行水深反演:一是以河道全部历史样本建模;二是将河道按自然水深划分为浅水区和深水区分别建模。结果表明:神经网络模型预测精度高于线性回归模型;水深分区后线性回归和神经网络模型预测误差均有所减小。 相似文献
120.
MODIS数据在广西生态环境监测及评价中的应用方法 总被引:8,自引:0,他引:8
利用MODIS数据广西遥感影像,用监督分类法做出了地物覆盖分类图,用主成分分析法做出了土壤侵蚀强度图和生态环境质量综合评价指数图。 相似文献