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   1613篇
  免费   345篇
  国内免费   435篇
测绘学   76篇
大气科学   459篇
地球物理   441篇
地质学   737篇
海洋学   242篇
天文学   121篇
综合类   171篇
自然地理   146篇
  2024年   7篇
  2023年   38篇
  2022年   65篇
  2021年   72篇
  2020年   65篇
  2019年   70篇
  2018年   64篇
  2017年   65篇
  2016年   79篇
  2015年   64篇
  2014年   60篇
  2013年   79篇
  2012年   82篇
  2011年   94篇
  2010年   87篇
  2009年   86篇
  2008年   81篇
  2007年   110篇
  2006年   85篇
  2005年   53篇
  2004年   45篇
  2003年   62篇
  2002年   45篇
  2001年   47篇
  2000年   75篇
  1999年   100篇
  1998年   91篇
  1997年   73篇
  1996年   74篇
  1995年   54篇
  1994年   52篇
  1993年   53篇
  1992年   29篇
  1991年   26篇
  1990年   26篇
  1989年   20篇
  1988年   31篇
  1987年   11篇
  1986年   13篇
  1985年   9篇
  1984年   12篇
  1983年   10篇
  1982年   8篇
  1981年   5篇
  1980年   5篇
  1978年   3篇
  1976年   3篇
  1973年   1篇
  1972年   1篇
  1958年   1篇
排序方式: 共有2393条查询结果,搜索用时 31 毫秒
891.
????XPL????Ч?????????HPL??VPL??????????????????м?????????????????????HPL??VPL??????????????HPL??VPL?????????С??9 m??12 m?????????12 m??20 m?????????????????????????????????????  相似文献   
892.
根据欧亚大陆及西太平洋地区102个数字化台站记录的近万次地震事件的长周期垂直向瑞雷波资料,利用时频分析方法测量并筛选后共得到11213条质量较高的基阶瑞雷波群速度频散资料.纯路径频散的反演中同时计算方位各向异性,反演获得了欧亚大陆及西太平洋边缘海地区(10°E—150°E,10°S—80°N)8—200s共28个周期的瑞雷波群速度及各向异性空间分布图象.瑞雷波高分辨率层析成像表明,30—60s周期,以青藏高原为中心呈极低速分布;100—120s周期,速度差异幅度较大,在东亚东部及西太平洋边缘海,自北向南显示出一条宽2500—4000km,长约8000km的巨型低速异常带.相对海洋来说,欧亚大陆各向异性强度较弱且快波方向较复杂.由于受到印度板块与欧亚板块的碰撞,中国大陆西部的各向异性强度明显大于东部.  相似文献   
893.
Temperature‐index models are widely favoured as a pragmatic means of simulating glacier melt because of their generally good performance, computational simplicity and limited demands for in situ data. However, their coefficients are normally treated as temporally stationary, unrealistically assuming a constancy of the prevailing weather. We address this simplification by prescribing model coefficients as a function of synoptic weather type, in a procedure that utilizes reanalysis data and preserves the minimal data requirements of temperature‐index models. Using a cross‐validation procedure at Vestari Hagafellsjökull, Iceland, and Storglaciären, Sweden, we demonstrate that applying transient model coefficients, for three temperature‐index models, results in statistically significant increases in the skill with which melt is modelled: Median simulation improvements in the Nash–Sutcliffe efficiency coefficient of 7.3 and 23.6% are achieved when hourly and daily melt totals are evaluated respectively. Our weather‐type modelling approach also yields insight to processes driving parameter variability, revealing dependence that is consistent with a priori considerations of the surface energy balance. We conclude that incorporating weather types into temperature‐index models holds promise for improving their performance, as well as enhancing understanding variability in coefficient values. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   
894.
The recent (1970–1999) and future (2070–2099) climates under the SRES A1B scenario, simulated by the regional climate model RegCM4.0 driven with lateral boundary conditions from the ECHAM5 general circulation model, are utilized to force a large-scale hydrological model for assessing the hydrological response to climate changes in the Yangtze River Basin, China. The variable infiltration capacity model (VIC) is utilized to simulate various hydrological components for examining the changes in streamflow at various locations throughout the Yangtze River Basin. In the end of the twenty-first century, most of the Yangtze River Basin stands out as “hotspots” of climate change in China, with an annual temperature increase of approximately 3.5 °C, an increase of annual precipitation in North and a decrease in South. Runoff in the upper reach of Yangtze River is projected to increase throughout the year in the future, especially in spring when the increase will be approximately 30 %. Runoff from the catchments in the northern part of Yangtze River will increase by approximately 10 %, whereas that in the southern part will decrease, especially in the dry season, following precipitation changes. The frequency of extreme floods at three mainstream stations (Cuntan, Yichang, and Datong) is projected to increase significantly. The original extreme floods with return periods of 50, 20, and 10 years will change into floods with return periods of no more than 20, 10, and 5 years. The projected increase in extreme floods will have significant impacts on water resources management and flood control systems in the Yangtze River Basin.  相似文献   
895.
我国目前在碳酸盐岩油气勘探阶段常用的储层分类评价标准已难以满足溶蚀孔洞缝广泛存在及构造裂缝发育的古岩溶储层评价的实际需要。选择塔中西部上奥陶统良里塔格组古岩溶储层, 在系统分析储层特征的基础上, 通过表征残余岩溶和开展构造裂缝预测, 引入残余岩溶强度(R)和岩体破坏接近度系数(η)2个参数, 作为古岩溶储层评价的重要定量指标;并与多项储层地质参数综合, 进一步完善了碳酸盐岩古岩溶储层的分类评价标准, 并用于对该地区良里塔格组叠加型古岩溶储层的评价预测。将本区良里塔格组古岩溶储层划分为Ⅰ、Ⅱ、Ⅲ、Ⅳ4类, 并应用多因素综合分析叠合成图法在平面上进行储层评价预测, 认为Ⅰ、Ⅱ类储层主要分布于S2井TZ45井TZ12井一带和 Z11井TZ10井TZ11井一带。  相似文献   
896.
The idea of urban fringe belts has been recognised as a powerful means of understanding the physical form of urban areas and the process of urban outward growth and internal change. Surrounding Auckland's city core, an inner fringe belt can be recognised. Farther out there are middle and outer fringe belts. Fluctuations in socio-economic development, the adoption of new transport modes, land reclamation and topography are major factors influencing the formation of these fringe belts. The examination of changes to fringe belts highlights important issues of urban transformation, which are pertinent to planning and design policies for urban landscape management.  相似文献   
897.
Elastic fundamental frequency is a key-parameter of simplified seismic design and vulnerability assessment methods. Empirical relationships exist in codes to estimate this frequency but they miss experimental data to validate them accounting for national feature of building design and, above all, corresponding uncertainties. Even if resonance frequency extracted from ambient vibrations may be larger than the elastic frequency (at yield) generally used in earthquake engineering, ambient vibration recordings may provide a large set of data for statistical analysis of periods versus building characteristics relationships. We recorded ambient vibrations and estimated the fundamental frequency of about 60 buildings of various types (RC and masonry) in Grenoble City (France). These data complete the set existing yet, made of 26 RC-buildings of Grenoble (Farsi and Bard 2004) and 28 buildings in Nice (France) (Dunand 2005). Statistical analysis of these experimental data was performed for fundamental frequencies of RC shear wall structures and the results are compared with existing relationships. Only building height or number of stories has a statistical relevancy to estimate the resonance frequency but the variability associated to the proposed relationships is large. Moreover, we compared the elastic part of capacity curves of RC and masonry buildings used in the European Risk-UE method for vulnerability assessment with the experimental frequencies. The variability is also large and the curves may not be consistent with French existing buildings.  相似文献   
898.
顾悦婷  孙波  陈丹红  姜梅 《极地研究》2010,22(4):431-440
南极特别管理区(ASMAs)是南极环境保护体系的重要组成部分,目前其覆盖面积已超过5万平方公里。通过整理和分析南极条约秘书处(ATS)公布的南极特别管理区的相关资料,简要概述南极特别管理区的现状、管理计划(草案)的编写及其申报程序。目前,已设立的南极特别管理区共有7个,分别由15个南极条约协商国(ATCPs)和1个南极条约缔约国(ATSs)通过单独或联合的方式提出,其中以美国和澳大利亚为主。我国作为提出国之一,开展了南极特别管理区的设置工作。另外,本文针对目前南极特别管理区存在的问题提出初步建议,并简要分析其未来的发展动向。  相似文献   
899.
黄河源区径流对气候变化的响应及敏感性分析(英文)   总被引:4,自引:1,他引:3  
Response of the runoff in the headwater region of the Yellow River to climate change and its sensibility are analyzed based on the measured data at the four hydrological stations and ten weather stations during the period 1959–2008. The result indicates that change of temperature in the region has an obvious corresponding relationship with global warming and the changes of annual average temperature in each subregion in the region have been presenting a fluctuant and rising state in the past 50 years. However the change of precipitation is more intricate than the change of temperature in the region because of the influences of the different geographical positions and environments in various areas, and the change of annual precipitation in the main runoff-producing area has been presenting a fluctuant and decreasing state in the past 50 years. And there is a remarkable nonlinear correlativity between runoff and precipitation and temperature in the region. The runoff in the region has been decreasing continuously since 1990 because the precipitation in the main runoff-producing area obviously decreases and the annual average temperature continuously rises. As a whole, the runoff in each subregion of the headwater region of the Yellow River is quite sensitive to precipitation change, while the runoff in the subregion above Jimai is more sensitive to temperature change than that in the others in the region, correspondingly.  相似文献   
900.
在综合考虑沉积微相和成岩作用对储集层孔隙度影响的基础上,建立了辽河坳陷双清地区不同沉积微相成岩指数ID与储集层平均孔隙度的相关关系,预测了该地区沙四段储集层在各沉积时期的平均孔隙度,恢复了孔隙演化史,确定了不同沉积时期有效油气储集层的分布范围。孔隙度预测结果表明,有效油、气储集层分布于研究区北部孔隙度大于58%的地区。孔隙演化史的模拟结果表明,储集层孔隙度在埋藏早期主要受沉积相的影响,而在晚期则主要受成岩作用的控制。储集层预测孔隙度与实测孔隙度的绝对误差为2.7%.,而研究区储集层的填隙物含量在1.0%~40.0%之间,平均为21.7%.,由此可见,所建预测模型可用于填隙物含量较高的储集层钻前孔隙度预测和孔隙演化史模拟。  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