全文获取类型
收费全文 | 4970篇 |
免费 | 929篇 |
国内免费 | 1071篇 |
专业分类
测绘学 | 191篇 |
大气科学 | 1134篇 |
地球物理 | 1427篇 |
地质学 | 2416篇 |
海洋学 | 445篇 |
天文学 | 275篇 |
综合类 | 476篇 |
自然地理 | 606篇 |
出版年
2024年 | 31篇 |
2023年 | 101篇 |
2022年 | 234篇 |
2021年 | 281篇 |
2020年 | 223篇 |
2019年 | 247篇 |
2018年 | 293篇 |
2017年 | 271篇 |
2016年 | 278篇 |
2015年 | 233篇 |
2014年 | 295篇 |
2013年 | 268篇 |
2012年 | 286篇 |
2011年 | 292篇 |
2010年 | 284篇 |
2009年 | 289篇 |
2008年 | 247篇 |
2007年 | 242篇 |
2006年 | 171篇 |
2005年 | 179篇 |
2004年 | 140篇 |
2003年 | 115篇 |
2002年 | 127篇 |
2001年 | 126篇 |
2000年 | 158篇 |
1999年 | 241篇 |
1998年 | 181篇 |
1997年 | 186篇 |
1996年 | 168篇 |
1995年 | 155篇 |
1994年 | 107篇 |
1993年 | 124篇 |
1992年 | 81篇 |
1991年 | 58篇 |
1990年 | 30篇 |
1989年 | 55篇 |
1988年 | 43篇 |
1987年 | 24篇 |
1986年 | 15篇 |
1985年 | 15篇 |
1984年 | 16篇 |
1983年 | 7篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1980年 | 8篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1958年 | 9篇 |
排序方式: 共有6970条查询结果,搜索用时 12 毫秒
991.
基于等高线的DEM生成算法研究和实现 总被引:5,自引:0,他引:5
数字高程模型是近年来发展起来的为地理信息系统提供空间分析和辅助决策的数据基础。随着科学技术特别是计算机技术的迅速发展,DEM在数据获取方法、数据存储和数据处理速度等方面已经取得突破性进展。通过系统介绍生成DTM过程中等高线的离散化方法,DELAUNAY三角网的特性和基于等高线数据的TIN的建立,以及对产生的问题的一些解决方法。 相似文献
992.
阿尔金断裂花海段新生代变形特征及时间 总被引:6,自引:0,他引:6
在地表地质调查并结合钻井资料分析的基础上,主要通过地震剖面的精细地质解释,对阿尔金断裂东段的构造变形特征进行了系统分析。认为阿尔金断裂经阿克塞至肃北向北东东方向延伸,既没有止于玉门市西北的宽台山,也没有延伸至金塔盆地,而是止于其间的花海盆地花探7井东侧。阿尔金断裂花海段的变形时间主要为渐新世晚期、上新世末期和晚更新世末,以后者最为强烈。与红柳峡段的对比说明变形强度自西向东逐渐减弱,变形方式主要为走滑兼逆冲,主要构造样式为半花状构造,并派生有一系列向南西西方向收敛的弧形断裂。 相似文献
993.
Transformation of ammonium nitrogen and response characteristics of nitrifying functional genes in tannery sludge contaminated soil 下载免费PDF全文
Kong Xiang-ke Zhang Zi-xuan Wang Ping Wang Yan-yan Zhang Zhao-ji Han Zhan-tao Ma Li-sha 《地下水科学与工程》2022,10(3):223-232
High concentrations of ammonium nitrogen released from tannery sludge during storage in open air may cause nitrogen pollution to soil and groundwater. To study the transformation mechanism of NH4+-N by nitrifying functional bacteria in tannery sludge contaminated soils, a series of contaminated soil culture experiments were conducted in this study. The contents of ammonium nitrogen (as NH4+-N), nitrite nitrogen (as NO2?-N) and nitrate nitrogen (as NO3?-N) were analyzed during the culture period under different conditions of pollution load, soil particle and redox environment. Sigmodial equation was used to interpret the change of NO3?-N with time in contaminated soils. The abundance variations of nitrifying functional genes (amoA and nxrA) were also detected using the real-time quantitative fluorescence PCR method. The results show that the nitrification of NH4+-N was aggravated in the contaminated silt soil and fine sand under the condition of lower pollution load, finer particle size and more oxidizing environment. The sigmodial equation well fitted the dynamic accumulation curve of the NO3?-N content in the tannery sludge contaminated soils. The Cr(III) content increased with increasing pollution load, which inhibited the reproduction and activity of nitrifying bacteria in the soils, especially in coarse-grained soil. The accumulation of NO2?-N contents became more obvious with the increase of pollution load in the fine sand, and only 41.5% of the NH4+-N was transformed to NO3?-N. The redox environment was the main factor affecting nitrification process in the soil. Compared to the aerobic soil environment, the transformation of NH4+-N was significantly inhibited under anaerobic incubation condition, and the NO3?-N contents decreased by 37.2%, 61.9% and 91.9% under low, medium and high pollution loads, respectively. Nitrification was stronger in the silt soil since its copy number of amoA and nxrA genes was two times larger than that of fine sand. Moreover, the copy numbers of amoA and nxrA genes in the silt soil under the aerobic environment were 2.7 times and 2.2 times larger than those in the anaerobic environment. The abundance changes of the amoA and nxrA functional genes have a positive correlation with the nitrification intensity in the tannery sludge-contaminated soil. 相似文献
994.
Nan Ma Zhaoliang Song Baoli Wang Fushun Wang Xiaomin Yang Xiaodong Zhang Qian Hao Yuntao Wu 《中国地球化学学报》2017,36(4):626-637
Rivers link terrestrial ecosystems and marine ecosystems, and they transport large amounts of substances into oceans each year, including several forms of silicon (Si), carbon (C), and other nutrients. However, river damming affects the water flow and biogeochemical cycles of Si, C, and other nutrients through biogeochemical interacting processes. In this review, we first summarize the current understanding of the effects of river damming on the processes of biogeochemical Si cycle, especially the source, composition, and recycling process of biogenic silica (BSi). Then, we introduce dam impacts on the cycles of C and some other nutrients. Dissolved silicon in rivers is mainly released from phytolith dissolution and silicate weathering. BSi in suspended matter or sediments in most rivers mainly consists of phytoliths and mainly originates from soil erosion. However, diatom growth and deposition in many reservoirs formed by river interception may significantly increase the contribution of diatom Si to total BSi, and thus significantly influence the biogeochemical Si, C, and nutrient cycles. Yet the turnover of phytoliths and diatoms in different rivers formed by river damming is still poorly quantified. Thus, they should be further investigated to enhance our understanding about the effects of river damming on global biogeochemical Si, C and nutrient cycles. 相似文献
995.
巴丹吉林沙漠湖泊与地下水的补给来源及化学组成(英文) 总被引:1,自引:0,他引:1
Based on the analysis of ion chemical composition of lake water and shallow groundwater in the Badain Jaran Desert, this paper discussed the characteristics of ion chemical composition, spatial variation of lake water, and possible supply sources of lake water and groundwater in the desert areas. The results show that the pH, salinity, TDS and electrical conductivity of the lake water are greater than those of groundwater. The ion con-tents of water samples are dominated by Na+ and Cl?. Most of the higher salinity lakes are Na (K)-Cl-(SO4) type, and a few of low salinity lakes belong to the Na-(Mg)-(Ca)-Cl-(SO4)-(HCO3) type. Most of the groundwater is Na-(Ca)-(Mg)-Cl-(SO4)-(HCO3) type, attributed to subsaline lake, and only a few present the Na-Cl-SO4 type, flowing under saline lake. The pH, salinity, TDS and electrical conductivity in the southeastern lakes are relatively low, and there are slightly alkaline lakes. The pH, salinity, TDS and electrical conductivity in the northern lakes are much greater than those of the southeastern lakes, and the northern lakes are moderately alkaline and saline ones. In the southeastern part of the Badain Jaran Desert, the ion chemical characteristics of the lake water from south to north show a changing trend of sub-saline →saline→hypersaline. The changing trend of chemical compositions of ions in recent 9 years indicates that most of the ion contents have a shade of reduction in Boritaolegai, Badain, Nuoertu and Huhejilin lakes, which state clearly that the amount of fresh water supply is increasing in the 9-year period. The ion chemical composition of the lake water reveals that the flow direction of lake water is from southeast to northwest in the Badain Jaran Desert. The ion chemical composition, moisture content of sand layer water level height and hierarchical cluster analysis of the lake water and groundwater demonstrate that the lake water is mainly supplied by local rainfall and infiltration of precipitation in Yabulai Mountains and Heishantou Mountain, and the supply from the Qilian Mountains is almost impossible. 相似文献
996.
中国短期气候变化的一个重要原因 总被引:10,自引:0,他引:10
通过数值试验对高原地表反射率变化的气候效应进行了敏感性研究,同时与观测的近40年中国区域气候变化趋势作了对比分析。结果表明,高原主体地表反射率增加是我国短期气候变化的重要控制因子之一,它能造成东亚夏季风和高原夏季风的显著减弱,使夏季我国东部季风区北方变暖,南方变冷,季风降水普遍减少 相似文献
997.
探讨地电直流供电对GM4分钟值数据干扰的解决方案,及施工改造效果.结果表明,引入地埋铠装电缆布设外线,可以屏蔽直流供电产生的部分感应磁场,较好的解决台站场地条件的限制,提高GM4分钟值数据的观测质量. 相似文献
998.
本文收集整理了距2010年4月14日玉树7.1级地震震中700km范围内的全部13个地下流体观测井孔的温度观测资料,统计分析了2008年5月以来各井温度当月超过3倍均方差的数据个数(以下简称月超差个数).结果发现,在玉树地震前6个月,13个观测井中有10个井观测数据出现月超差个数升高现象,有4个观测井月超差个数出现了同震升高和震后升高现象.2008、2009年大柴旦发生的2次6级以上地震前,也有类似现象.此异常现象可能反映了玉树地震前场兆和源兆的共同作用过程. 相似文献
999.
结合联合国2030年可持续发展目标(SDGs)构建城镇化可持续发展评价指标体系,从省内、外两个角度研究吉林省城镇化可持续发展状况。结果表明:吉林省城镇化可持续发展水平波动上升,发展趋势良好,大致可分为3个阶段;研究期内各子系统可持续发展水平均呈上升趋势,其中社会子系统上升幅度最大,经济子系统上升幅度最小。城镇化可持续发展水平空间格局呈“核心-边缘”结构,即长春市水平高、其它地区水平低;经济、社会、资源和环境子系统可持续发展水平总体上表现为“中东部高,西部低”。研究期内吉林省城镇化可持续发展水平低于黑龙江省,2015年后高于辽宁省;相较辽黑两省,吉林省社会子系统可持续发展水平较低。 相似文献
1000.
利用四川地区2010 ~ 2013 年流动重力观测数据作出的重力场动态变化图像和重力段差时序变化图像,对芦山7. 0 级地震前后的重力场动态演化特征进行了研究,结果表明:震前重力场经历了约3 年的重力增大和约1 年的减小变化,重力变化呈“上升→加速上升→减速上升→加速下降→减速下降”特征; 芦山地震发生在重力场变化由下降转为上升的回调过程中;震后重力场变化以较快的速度恢复到接近震前状况,并出现重力正值变化异常区域;临震前震中区域重力场变化数值较小,构造带活动速度变慢。 相似文献