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   7110篇
  免费   1487篇
  国内免费   1995篇
测绘学   812篇
大气科学   1058篇
地球物理   1499篇
地质学   4131篇
海洋学   1110篇
天文学   336篇
综合类   603篇
自然地理   1043篇
  2024年   39篇
  2023年   124篇
  2022年   364篇
  2021年   455篇
  2020年   352篇
  2019年   457篇
  2018年   431篇
  2017年   428篇
  2016年   394篇
  2015年   411篇
  2014年   395篇
  2013年   488篇
  2012年   467篇
  2011年   524篇
  2010年   486篇
  2009年   484篇
  2008年   472篇
  2007年   463篇
  2006年   375篇
  2005年   349篇
  2004年   320篇
  2003年   251篇
  2002年   267篇
  2001年   321篇
  2000年   226篇
  1999年   198篇
  1998年   137篇
  1997年   160篇
  1996年   118篇
  1995年   103篇
  1994年   75篇
  1993年   72篇
  1992年   82篇
  1991年   39篇
  1990年   46篇
  1989年   36篇
  1988年   41篇
  1987年   17篇
  1986年   21篇
  1985年   17篇
  1984年   13篇
  1983年   7篇
  1982年   9篇
  1981年   8篇
  1980年   6篇
  1979年   4篇
  1978年   8篇
  1977年   7篇
  1976年   5篇
  1958年   9篇
排序方式: 共有10000条查询结果,搜索用时 762 毫秒
991.
由于地铁隧道结构变形监测控制网的特殊性,在变形数据处理过程中基准点稳定性判断是不容忽视的环节,通常稳定性判断是在室内数据处理时进行,为了优化变形数据处理过程,本文采用已有组合后验方差检验法进行基准点稳定性检验,并验证该方法的可靠性,在此基础上提出灵敏度的概念,即检验观测值出现多大变动时能被该方法检测出来,并测试出组合后验方差检验法的灵敏度。通过灵敏度的研究和应用可以现场进行稳定性分析,指导外业变形监测工作,提高观测结果利用率和数据处理效率。  相似文献   
992.
为了验证程家村航磁异常圈闭,对该异常圈闭进行了查证工作,针对该区域铁矿体均呈深部隐伏的赋存状态,以"物探先行,钻探验证"的工作方法开展工作,利用地面高精度磁测扫面确定磁异常圈闭平面形态及位置;对高精度磁测剖面数据进行切线法剖面反演,确定强磁性地质体位置和埋深,再结合重磁剖面联合反演方法,推断强磁性地质体的赋存状态,从而给钻探验证提供依据。通过上述工作,取得了良好的找矿效果,表明重磁方法是深部隐伏磁铁矿勘查中的重要工作手段。  相似文献   
993.
Dating data, altitude of Neolithic sites, climatic changes from sedimentary records and previous research results were collected and analyzed to detect possible connections between climatic changes and human activities in the Changjiang River Delta in the Neolithic Age. The results indicated that hydrological changes greatly impacted the human activities in the study region. Low-lying geomorphology made the floods and sea level changes become the important factors affecting human activities, especially the altitude change of human settlements. People usually moved to higher places during the periods characterized by high sea level and frequent floods to escape the negative influences from water body expansion, which resulted in cultural hiatus in certain profiles. However, some higher-altitude settlements were not the results of climatic changes but the results of social factors, such as religious ceremony and social status. Therefore, further research will be necessary for the degree and types of impacts of climatic changes on human activities in the study area at that time.  相似文献   
994.
Yu  Xia  Zhou  Weijian  Wang  Yunqiang  Cheng  Peng  Hou  Yaoyao  Xiong  Xiaohu  Du  Hua  Yang  Ling  Wang  Ya 《地理学报(英文版)》2020,30(6):921-934
The vertical distribution and exchange mechanisms of soil organic and inorganic carbon(SOC, SIC) play an important role in assessing carbon(C) cycling and budgets. However, the impact of land use through time for deep soil C(below 100 cm) is not well known. To investigate deep C storage under different land uses and evaluate how it changes with time, we collected soil samples to a depth of 500 cm in a soil profile in the Gutun watershed on the Chinese Loess Plateau(CLP); and determined SOC, SIC, and bulk density. The magnitude of SOC stocks in the 0–500 cm depth range fell into the following ranking: shrubland(17.2 kg m~(-2)) grassland(16.3 kg m~(-2)) forestland(15.2 kg m~(-2)) cropland(14.1 kg m~(-2)) gully land(6.4 kg m~(-2)). The ranking for SIC stocks were: grassland(104.1 kg m~(-2)) forestland(96.2 kg m~(-2)) shrubland(90.6 kg m~(-2)) cropland(82.4 kg m~(-2)) gully land(50.3 kg m~(-2)). Respective SOC and SIC stocks were at least 1.6-and 2.1-fold higher within the 100–500 cm depth range, as compared to the 0–100 cm depth range. Overall SOC and SIC stocks decreased significantly from the 5 th to the 15 th year of cultivation in croplands, and generally increased up to the 70 th year. Both SOC and SIC stocks showed a turning point at 15 years cultivation, which should be considered when evaluating soil C sequestration. Estimates of C stocks greatly depends on soil sampling depth, and understanding the influences of land use and time will improve soil productivity and conservation in regions with deep soils.  相似文献   
995.
An increasing impervious area is quickly extending over the Wu‐Tu watershed due to the endless demands of the people. Generally, impervious paving is a major result of urbanization and more recently has had the potential to produce more enormous flood disasters than those of the past. In this study, 40 available rainfall–runoff events were chosen to calibrate the applicable parameters of the models and to determine the relationships between the impervious surfaces and the calibrated parameters. Model inputs came from the outcomes of the block kriging method and the non‐linear programming method. In the optimal process, the shuffled complex evolution method and three criteria were applied to compare the observed and simulated hydrographs. The tendencies of the variations of the parameters with their corresponding imperviousness were established through regression analysis. Ten cases were used to examine the established equations of the parameters and impervious covers. Finally, the design flood routines of various return periods were furnished through use of approaches containing a design storm, block kriging, the SCS model, and a rainfall‐runoff model with established functional relationships. These simulated flood hydrographs were used to compare and understand the past, present, and future hydrological conditions of the watershed studied. In the research results, the time to peak of flood hydrographs for various storms was diminished approximately from 11 h to 6 h in different decrements, whereas peak flow increased respectively from 127 m3 s?1 to 629 m3 s?1 for different storm intensities. In addition, this study provides a design diagram for the peak flow ratio to help engineers and designers to construct hydraulic structures efficiently and prevent possible damage to human life and property. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   
996.
Understanding the influence of complex interactions among hydrological factors, soil characteristics and biogeochemical functions on nutrient dynamics in overland flow is important for efficiently managing agricultural nonpoint pollution. Experiments were conducted to assess nutrient export from Ultisol soils in the Sunjia catchment, Jiangxi province, southern China, between 2003 and 2005. Four plots were divided into two groups: two peanut plots and two agroforestry (peanut intercropped with citrus) plots. During the study period, we collected water samples for chemical analyses after each rainfall event that generated overland flow to assess nutrient export dynamics. The concentrations of potassium (K) and nitrate‐N (NO3–N) in overland flow were higher during the wetting season (winter and early spring). This reflects the solubility of K and NO3–N, the accumulation of NO3–N during the dry season and an increase in desorption processes and mixing with pre‐event water caused by prolonged contact with soil in areas with long‐duration, low‐intensity rainfall. In contrast, concentrations of total nitrogen (TN) and total phosphorus (TP) were higher during the wet season (late March to early July) and during the dry season (mid‐July to the end of September or early October). This was due to the interaction between specific hydrological regimes, the properties of the Ultisol and particulate transport processes. Variations in nutrient concentrations during storm events further identified that event water was the dominant source of total nitrogen and total phosphorus, and pre‐event water was the dominant source of NO3–N. In addition, the results obtained for the different land uses suggest that agroforestry practices reduce nutrient loss via overland flow. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
997.
Random variable simulation has been applied to many applications in hydrological modelling, flood risk analysis, environmental impact assessment, etc. However, computer codes for simulation of distributions commonly used in hydrological frequency analysis are not available in most software libraries. This paper presents a frequency‐factor‐based method for random number generation of five distributions (normal, log–normal, extreme‐value type I, Pearson type III and log‐Pearson type III) commonly used in hydrological frequency analysis. The proposed method is shown to produce random numbers of desired distributions through three means of validation: (1) graphical comparison of cumulative distribution functions (CDFs) and empirical CDFs derived from generated data; (2) properties of estimated parameters; (3) type I error of goodness‐of‐fit test. An advantage of the method is that it does not require CDF inversion, and frequency factors of the five commonly used distributions involves only the standard normal deviate. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
998.
999.
Wang  ZhenQi  Zhi  DongMing  Zhang  ChangMin  Xue  XinKe  Zhang  ShangFeng  Li  TianMing  Yang  Fei  Liu  LouJun  Cheng  Liang  Lu  Dong  Zhou  FengJuan  Chen  YuanYong 《中国科学:地球科学(英文版)》2010,52(1):106-114

Well che89, located in the Chepaizi area in the northwest margin of Junggar basin, acquires high production industrial oil flow, which is an important breakthrough in the exploration of the south foreland slope area of Junggar basin. The Chepaizi area is near two hydrocarbon generation depressions of Sikeshu and Shawan, which have sets of hydrocarbon source rock of Carboniferous to Jurassic as well as Upper Tertiary. Geological and geochemical parameters are proper for the accumulation of mixed source crude oil. Carbon isotope, group composition and biomarkers of crude oil in Upper Tertiary of well Che89 show that the features of crude oil in Upper Tertiary Shawan Formation are between that of Permian and Jurassic, some of them are similar to these two, and some are of difference, they should be the mixed source of Permian and Jurassic. Geochemical analysis and geological study show that sand extract of Lower Tertiary Wulunguhe Formation has the same source as the crude oil and sand extract of Upper Tertiary Shawan Formation, but they are not charged in the same period. Oil/gas of Wulunguhe Formation is charged before Upper Tertiary sedimentation, and suffered serious biodegradation and oxidation and rinsing, which provide a proof in another aspect that the crude oil of Upper Tertiary Shawan Formation of well Che89 is not from hydrocarbon source rock of Lower Tertiary.

  相似文献   
1000.
河西地区重力动态变化的二维离散小波多尺度分解   总被引:7,自引:5,他引:2  
采用二维离散小波的多尺度分解技术,对1994~1995年和2002~2003年的河西地区重力动态变化进行分解,提取该地区不同层次的重力动态变化异常,结果反映了不同深度在一定尺度意义上的介质密度的动态变化分布。  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