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   539篇
  免费   86篇
  国内免费   87篇
测绘学   105篇
大气科学   34篇
地球物理   66篇
地质学   258篇
海洋学   37篇
天文学   87篇
综合类   34篇
自然地理   91篇
  2023年   6篇
  2022年   10篇
  2021年   9篇
  2020年   12篇
  2019年   20篇
  2018年   22篇
  2017年   21篇
  2016年   22篇
  2015年   18篇
  2014年   33篇
  2013年   34篇
  2012年   25篇
  2011年   31篇
  2010年   20篇
  2009年   31篇
  2008年   32篇
  2007年   30篇
  2006年   20篇
  2005年   23篇
  2004年   40篇
  2003年   26篇
  2002年   28篇
  2001年   17篇
  2000年   21篇
  1999年   16篇
  1998年   25篇
  1997年   20篇
  1996年   10篇
  1995年   10篇
  1994年   9篇
  1993年   18篇
  1992年   4篇
  1991年   10篇
  1990年   6篇
  1989年   7篇
  1988年   9篇
  1987年   6篇
  1986年   4篇
  1985年   4篇
  1984年   1篇
  1983年   1篇
  1981年   1篇
排序方式: 共有712条查询结果,搜索用时 453 毫秒
681.
Abstract

Estimating water resources is important for adequate water management in the future, but suitable data are often scarce. We estimated water resources in the Vilcanota basin (Peru) for the 1998–2009 period with the semi-distributed hydrological model PREVAH using: (a) raingauge measurements; (b) satellite rainfall estimates from the TRMM Multi-satellite Precipitation Analysis (TMPA); and (c) ERA-Interim re-analysis data. Multiplicative shift and quantile mapping were applied to post-process the TMPA estimates and ERA-Interim data. This resulted in improved low-flow simulations. High-flow simulations could only be improved with quantile mapping. Furthermore, we adopted temperature and rainfall anomalies obtained from three GCMs for three future periods to make estimations of climate change impacts (Delta-change approach) on water resources. Our results show more total runoff during the rainy season from January to March, and temporary storages indicate that less water will be available in this Andean region, which has an effect on water supply, especially during dry season.

Editor Z.W. Kundzewicz; Associate editor D. Gerten  相似文献   
682.
极摩尔圆法计算二维平均运动学涡度   总被引:2,自引:0,他引:2       下载免费PDF全文
刘江  张进江  张波 《地质科学》2012,47(1):13-21
通过有限应变椭圆长短轴比(Rs)和长轴与剪切面夹角(α)构建极摩尔圆,本文计算得出二维平面应变平均运动学涡度(Wm)计算公式为Wm=cos ,并以此绘制了Wm关于Rs和α的等值线投影图。公式计算和有限应变数值投影都是计算平均运动学涡度的简捷有效的方法。极摩尔圆计算提供了判定剪切类型(简单剪切,纯剪切,减薄一般剪切,增厚一般剪切)的两种方法:α值判定瞬时应变剪切类型; Rs×tan2α值判定有限应变剪切类型。  相似文献   
683.
岱河矿区地下水系统包含三个主要子系统:第四系全新统孔隙含水层、煤系砂岩裂隙含水层组及煤系下伏灰岩岩溶裂隙含水层组。通过对矿区三个子系统地下水化学特征分析,确定各子系统涌出水来源判别依据的水溶组分并建立各子系统所特有的水质模型,进而提出矿井涌水水源判别模式。实际应用表明,该判别模式具有快速准确的特点,可以为了解矿井涌水来源,预测矿井涌水量,防治矿井突水提供理论依据。  相似文献   
684.
The fatigue assessment plays an increasing role for the offshore structural safety. Many fatigue assessment methods have been developed for this purpose. Among those methods, the time domain method is regarded as the most accurate method but less adopted in practice due to time consuming. In order to improve the efficiency of the time domain method, an innovative block partition and equivalence method of the wave scatter diagram is developed for offshore structural fatigue assessment. After the wave scatter diagram is partitioned into several blocks, the newly developed method, involves determination of the equivalent wave height, wave period and occurrence probability of the representative sea states based on modified energy equivalent principle. The equivalent wave period of the representative sea state is calculated via the spectral moment formula in which the equivalent spectral moments of zero and second order are obtained based on the weighted averaging principle. Combining with the determined wave period, the equivalent significant wave height can be determined by reversing the wave spectrum integral formula, where the equivalent wave energy of a divided block of the wave scatter diagram is modified by introducing a factor to compensate the effect of low- and high-amplitude cycles fatigue damage. The equivalent occurrence probability is equal to the summation of the original sea states’ occurrence probability within the divided block. The developed method has the advantage of preserving the stochastic characteristics of the short term sea states within the divided block during determining the representative sea state. At the same time the newly developed method has no limitation on block partition and can be applied on different offshore structure. Two structural models, a fixed mono-pile platform and a floating semi-submersible platform, are demonstrated in the numerical examples. Results indicate that the newly developed method is robust, computationally affordable, and accurate within engineering expectations.  相似文献   
685.
Since early Pliocene, a counterclockwise surface gyre transported Nile derived silt and clay northeastwards along the Levant coast, where a basinward prograding plastered drift emerged. Based on high-resolution seismic reflection data we develop a middle to late Pleistocene sequence stratigraphic scheme for this plastered drift. For creating stacked sections of the seismic data we used the common reflection surface (CRS) stack technology which enhanced lateral reflection continuity and visibility of deep reflections. The shelf comprises stratigraphic sequences which show classical, systems tract like stacking patterns of sea level controlled sequences such as offlapping forced regression deposits or diachronous ravinement surfaces which formed during base level rise. On the slope, base level was periodically located well below the wave base and thus rather controlled by hydrodynamics, presumably by high-velocity contour currents. Hence, the term ‘deep base level’ is introduced. The deep base level controlled especially down and backstepping slope deposits. This example shows that care has to be taken when interpreting subsurface data containing typical systems tract like seismic sequences, since such geometries do not necessarily imply shallow water deposition of the sediments. A chronostratigraphic analysis based on the seismic stratigraphy indicates that base level fluctuations were related to eccentricity driven glacio-eustatic fluctuations. Periodic mass wasting, facilitated by foreset over-steepening and possibly triggered by salt tectonics or erosion by the contour current occurred during late base level fall or early base level rise.  相似文献   
686.
Al_2O_3和Ti在风化和热液蚀变等地球化学过程中通常被认为是不活动元素,两者的比值Al_2O_3/Ti常被用来指示地球化学作用过程。通过对中国157件火成岩样品元素含量平均值的统计发现,火成岩样品中Al_2O_3/Ti与SiO_2含量值之间存在着较好的幂函数关系:ln(Al_2O_3/Ti)=0.073×SiO_2-0.89,式中Al_2O_3和SiO_2和Ti含量单位均为%。本文基于得到的经验方程和TAS图解构建了一个新的判别岩石类型的图解——Al_2O_3-Ti图解。该图解可以区分酸性岩、中酸性岩、中性岩、中基性岩-基性岩四类岩性。通过对三个火成岩风化剖面的研究发现,花岗岩风化剖面从新鲜基岩到风化形成的土壤在Al_2O_3-Ti图解中均落在酸性岩区,花岗闪长岩风化剖面从新鲜基岩到风化形成的土壤样品均落在中酸性岩区,玄武质安山岩风化剖面从新鲜基岩到风化形成的土壤样品均落在中基性岩-基性岩区。不同风化程度的风化产物与其母岩在Al_2O_3-Ti图解中所在的区域一致,即Al_2O_3-Ti图解可以用来追溯火成岩风化产物的母岩岩性。通过对胶东焦家金矿和豫西牛头沟金矿两个矿区岩石的研究发现,黑云母花岗岩从新鲜岩石到其蚀变岩及其形成矿石的样品在Al_2O_3-Ti图解中均落在酸性岩区。玄武质安山岩从新鲜岩石到其蚀变岩及其形成矿石的样品在Al_2O_3-Ti图解中均落在中基性岩-基性岩区。即不同蚀变程度的蚀变产物与其原岩在Al_2O_3-Ti图解中所在的区域一致,这表明新构建的Al_2O_3-Ti图解可以用来示踪蚀变岩的原岩性质。  相似文献   
687.
正1 Introduction Salt lakes are widely distributed in the world,and salt lakes in China are mainly located in the area of the Qinghai-Xizang(Tibet),and the Autonomous Regions of Xinjiang and Inner Mongolia.There are more than 700salt lakes,each with an area larger than 1 km2,in the  相似文献   
688.
正As a natural mineral,Potash Magnesium Sulphate fertilizer has a quite high nutrient utilization rate without affecting soil p H in long time running related to Potassium Sulphate.And to displace Potassium Sulphate with Potash  相似文献   
689.
岩土工程勘察报告是工程地质勘察的最终成果。报告是否能正确反映场地的工程地质条件和岩土工程特点,关系到工程设计和建筑施工是否安全可靠、措施得当、经济合理。工程项目在不同的工作阶段,报告反映的内容和侧重点也有所不同,工业与民用建筑的岩土工程勘察报告侧重于详细勘察阶段。以成都平原地区报告编写为例,说明了勘察报告的编制程序、对工作及勘察成果的论述方法及图表的说明技巧等。  相似文献   
690.
在电厂和输电线路的可研和初设阶段中,需要测绘人员向设计专业人员快速地提供小比例尺地形图和线路平(断)面图作为设计依据和参考,而传统的作业模式无法满足上述要求.为此提出运用GoodyGIS软件提供的免费卫星航空影像资源数据来快速获取地形图、平(断)面图的方法和线路路径优化方法以满足设计的需求.结合具体工程实例进行数据比较分析,确认方法在电力工程应用中的可行性和实用性.  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