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   478篇
  免费   51篇
  国内免费   112篇
测绘学   72篇
大气科学   56篇
地球物理   69篇
地质学   234篇
海洋学   64篇
天文学   5篇
综合类   25篇
自然地理   116篇
  2024年   1篇
  2023年   6篇
  2022年   20篇
  2021年   12篇
  2020年   16篇
  2019年   29篇
  2018年   15篇
  2017年   30篇
  2016年   16篇
  2015年   26篇
  2014年   28篇
  2013年   47篇
  2012年   23篇
  2011年   24篇
  2010年   18篇
  2009年   28篇
  2008年   46篇
  2007年   33篇
  2006年   24篇
  2005年   30篇
  2004年   23篇
  2003年   31篇
  2002年   21篇
  2001年   9篇
  2000年   16篇
  1999年   8篇
  1998年   9篇
  1997年   4篇
  1996年   8篇
  1995年   8篇
  1994年   5篇
  1993年   5篇
  1992年   4篇
  1991年   4篇
  1988年   2篇
  1987年   4篇
  1985年   1篇
  1983年   1篇
  1982年   3篇
  1978年   2篇
  1954年   1篇
排序方式: 共有641条查询结果,搜索用时 15 毫秒
621.
Based on an in‐depth case study of a rural community, this paper documents the contemporary state of Chinese smallholder agriculture and the changes that it has been experiencing in the context of dramatic socio‐economic transition through the lens of three main economic drivers: livelihood diversification, market conditions and government interventions. Results reveal that the change in Chinese smallholder agriculture has been complex and multidimensional. All three factors exert profound influence and shape the current state of Chinese agriculture. Massive rural‐urban migration has resulted in labour shortages, which in turn have led to a reduction in agricultural diversity and land use intensity and a shift from traditional labour‐intensive technologies to modern capital‐intensive technologies. However, because of well‐developed agricultural markets, input use levels are similar across farmer categories (such as income diversification), helping to maintain productivity. Furthermore, reduced profits from farming due to increasing input prices and decreasing output prices have exerted pressure on smallholders to increasingly turn to nonfarm activities and have also triggered a thriving informal land transfer market, which was previously non‐existent. Policy implications include the need to strengthen local economies, improve market conditions, invest in rural infrastructures and facilitate smallholders' mobility.  相似文献   
622.
The 1996 Irian Jaya earthquake tsunami was simulated by using the numerical model based on the linear long wave theory including Coriolis force in the spherical coordinate system. The numerical modeling result at Chichijima is in good agreement with the observed tide gauge data. The distinctive oscillation at Chichijima can be interpreted as the formation of boundary waves, so called ridge waves that are excited on the South-Honshu ridge. The mechanism of tsunami propagation trapped on an oceanic ridge is analyzed with the simple ridge model. The result explains the characteristics of ridge waves excited on theSouth-Honshu ridge.  相似文献   
623.
应用热爆裂质谱测量和高温熔融样品释气技术分别测定了山旺、栖霞和鹤壁新生代玄武岩中橄榄岩捕虏体中的橄榄石颗粒的流体和稀有气体组成.结果表明,鹤壁方辉橄榄岩具低于大气的3He/4He比值0.778Ra和大气成因的Ne、Ar、Kr、Xe组成.三个地区饱满地幔样品均以还原性流体为主,CO2/He、N2/3He、N2/Ar比值分别为(0.62~4144)×109、(2631~64482)×109、269~73467.这些还原型流体偏离典型的幔源流体组成而表现出明显的碳和氮元素过剩,具有大气-地幔-壳源组份混合的特点,反映了大气和富含有机质的壳源组份在新生岩石圈上地幔中的影响.同时,这些饱满地幔样品的Ne、Ar、Kr、Xe具有大气属性,反映了大气型稀有气体在上地幔源区的广泛混染.其中,山旺和栖霞的3He/4He比值分别为(2.91~3.07)Ra、(1.79~4.01)Ra,均高于大气低于MORB,具有交代富集地幔的特点,而鹤壁则具有类似MORB的3He/4He比值(7.03~7.05)Ra.这样的流体和稀有气体组成差别显示华北东部新生岩石圈地幔具亏损MORB型的特点并含壳源组份,且其东缘所含壳源组份比例高于中部,说明华北东部新生岩石圈地幔中有俯冲洋壳组份的记录.  相似文献   
624.
With increasing support from the international community, non-governmental organizations (NGOs) have played an important role in Bangladesh since the early 1970s in providing emergency assistance to disaster victims. After observing widespread corruption and misuse of relief aid by the Bangladesh government in 1974 and subsequent years, external sources began to channellize emergency assistance to the victims through NGOs. Realizing that NGOs were usurping its authority over external disaster assistance, the government looked for opportunities to demonstrate its ability to deliver services to victims. An opportunity came in July 1998 when Bangladesh experienced a devastating flood. Using data collected from 348 households in 11 villages, this paper compares support received by the respondents from the Bangladesh government and NGOs during and immediately after the flood. Respondent opinions regarding emergency relief distribution suggest that both sources performed satisfactorily and an overwhelming majority of them thought that the government performed better than it had previously in distributing relief assistance to flood victims. Following an analysis of the survey data, this paper discusses the policy implications for future disaster assistance efforts in Bangladesh and elsewhere.  相似文献   
625.
With the transition to democracy in 1994, South Africa was faced with an enormous challenge in redressing the highly unequal and racialized pattern of land rights inherited from the colonial and apartheid past. In Namaqualand, a history of land dispossession and racial segregation presented the new government with a complex set of problems, which led to a series of distinct policy responses within the context of the wider national land reform programme. Land reform in Namaqualand aims to impact positively on local people's access to land, improve livelihood opportunities and develop the local economy. Unique features of the land reform process in Namaqualand include the reform of tenure in the former Coloured Rural Areas, the prominent role played by local municipalities and the heavy reliance on municipal commonage as a form of landholding. This study provides and overview of the process of land reform in Namaqualand since 1994, considering the three elements of tenure reform, land redistribution and restitution of historical land rights. It concludes that, while considerable progress has been made in provision of additional land to historically disadvantaged communities, obstacles remain in the area of post-transfer support to new and emerging farmers.  相似文献   
626.
连懿  陈圣波  孟治国  张锋  张莹 《地球科学》2014,39(11):1644-1650
月壤介电常数是当前月球微波遥感探测的基础, 是月壤厚度、成分等信息提取不可或缺的参数.为了实现全月介电常数反演, 通过对嫦娥二号卫星微波辐射计亮温数据进行时角校正, 得到同一时角的全月微波亮温图.全月微波亮温表现出随月球地形、月壤成分及纬度变化的特征.基于校正后的微波辐射亮温, 结合辐射传输模型, 通过解算相关参数, 反演得到3GHz频率下全月介电常数分布.其中, 月海地区的介电常数实部高于月陆地区, 且月球极地区域介电常数实部偏低; 而介电常数虚部则在月海区域和艾肯盆地较高.通过模拟月表介电常数实验对反演结果进行温度校正, 得到22℃下全月介电常数.将反演结果和月壤真实样品的介电常数测量值进行比较评价.结果表明介电常数实部相对误差都低于11%;虚部相对误差偏大, 但其差值最大仅为0.02.因此, 基于嫦娥二号卫星微波辐射计亮温数据反演月表介电常数的方法是可行的.   相似文献   
627.
殷冠文 《地理科学》2019,39(7):1082-1092
以鄂尔多斯市康巴什新区为例,对其发展过程中的资本循环和政府作用进行分析,并提出“地方政府主导下的资本循环”这一研究框架。研究表明,新区的建设包含3个资本循环的过程,分别是煤炭的资本化,土地、基础设施、房地产的资本化,以及向先进制造业和第三产业的资本流动。通过一系列行政手段,地方政府主导了3次循环中的资本积累,以及资本在3次循环之间的转换。  相似文献   
628.
Location knowledge is typically assessed using outline maps. A new set of questions reflect spatial thinking research and helps to assess student location knowledge. A small group (145) of first-year college students helped to refine the items. Question styles include: open-response, multiple-choice, listing, labeling, and sketching. Topics include: grid knowledge, world regional references, neighboring countries, neighboring features, and shapes of borders. This instrument combines spatial thinking research and expectations in geography standards.  相似文献   
629.
 在研究天山北坡经济带2000-2009年固定资产投资和城镇集聚基本特征基础之上,采用主成分分析法和面板数据回归模型,定量分析了固定资产投资对该地区城镇集聚的影响和作用机制。结果表明:(1)天山北坡经济带全社会固定资产投资在各县市分布相对集中于区域中心城市,呈现明显的“序位-规模”分布,具有行政区经济的特点,政府主导度与投资集中度成反向关系;(2)天山北坡经济带的城镇集聚过程首先以经济集聚和土地的非农化过程为主,其次是城镇人口的集聚,最后是社会生活水平的提高;(3)以固定资产投资为代表的政府行政力是推动天山北坡经济带城镇集聚的重要力量之一,其对行政级别越高,工业化水平越高城镇的集聚作用更强;(4)不同时期天山北坡经济带固定资产投资对城镇集聚的作用效应并不相同,初期投资主要促进城镇人口和空间规模的扩大,对城镇经济集聚作用较小;末期投资对城镇经济集聚、人口与空间规模均具有明显的促进作用,对社会人均基础设施及生活水平有较小的负作用。  相似文献   
630.
西准噶尔是近年来中亚造山带的研究热点地区之一,发育多条蛇绿混杂岩带。随着研究的深入,蛇绿岩混杂岩带中不断有碱性洋岛玄武岩被识别出来。本文对克拉玛依及达尔布特蛇绿混杂岩中的晚泥盆世枕状玄武岩进行详细的岩石地球化学及Sr?Nd同位素组成研究。结果表明西准噶尔晚泥盆世枕状玄武岩属于碱性玄武岩系列,岩石具有高TiO2 (2.3%~3.8%)及P2O5 (0.38%~0.91%),低MgO (2.41%~4.97%),轻、重稀土元素分异较为明显,(La/Yb)N = 5.1~14.5,无明显Eu异常(Eu/Eu* = 0.96~1.1),相对富集Rb、Th、U,亏损Ba、K、Sr,没有明显Nb、Ta负异常,这些地球化学特征与洋岛玄武岩(OIB)极其相似。克拉玛依及达尔布特蛇绿混杂岩中枕状玄武岩具正εNd(t)值(3.3~4.4)及年轻的模式年龄(0.83~0.72 Ga),表明其来源于亏损地幔源区,通过同位素及微量元素研究认为源区可能为富辉石的橄榄岩,或不含橄榄石的辉石岩和角闪石岩,克拉玛依及达尔布特蛇绿混杂岩中的晚泥盆世枕状玄武岩形成于大洋板内与地幔柱有关的海山/大洋岛屿环境。这些明显不同于大陆板内具有富集特征的塔里木大火成岩省,它们分别属于两个明显不同的同位素地球化学省,即以富集型地幔为同位素组成的南部省及以亏损型地幔为同位素组成的北部省。  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