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   7918篇
  免费   1484篇
  国内免费   1708篇
测绘学   536篇
大气科学   348篇
地球物理   621篇
地质学   6283篇
海洋学   1026篇
天文学   15篇
综合类   767篇
自然地理   1514篇
  2024年   27篇
  2023年   112篇
  2022年   247篇
  2021年   340篇
  2020年   289篇
  2019年   374篇
  2018年   283篇
  2017年   273篇
  2016年   350篇
  2015年   363篇
  2014年   485篇
  2013年   424篇
  2012年   445篇
  2011年   525篇
  2010年   457篇
  2009年   503篇
  2008年   467篇
  2007年   518篇
  2006年   580篇
  2005年   474篇
  2004年   475篇
  2003年   406篇
  2002年   377篇
  2001年   366篇
  2000年   313篇
  1999年   285篇
  1998年   259篇
  1997年   202篇
  1996年   187篇
  1995年   136篇
  1994年   127篇
  1993年   105篇
  1992年   73篇
  1991年   67篇
  1990年   48篇
  1989年   34篇
  1988年   25篇
  1987年   20篇
  1986年   18篇
  1985年   14篇
  1984年   15篇
  1983年   7篇
  1982年   9篇
  1981年   2篇
  1980年   1篇
  1979年   2篇
  1978年   1篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
宏观生态环境遥感监测系统总体设计与关键技术   总被引:1,自引:0,他引:1  
宏观生态环境遥感监测系统是基于环境遥感监测应用技术的软件实现.本文重点介绍了宏观生态环境遥感监测系统的总体结构设计、业务流程设计和核心功能设计,以及模型转换与实现技术、实时投影转换技术与以文件和数据库相结合的数据管理技术、基于XML的数据实时交换技术、产品自动化生产技术等关键技术,并给出了系统的应用实例.系统应用结果表...  相似文献   
992.
Abstract

Gully erosion is considered to be one of the most important soil erosion processes in Mediterranean marly environments, but its actual contribution to total soil loss is still under discussion. The objectives of this paper are: (a) to acquire the distributed value of erosion rate in a permanent gully developed on a marly substratum in a Mediterranean environment; and (b) to quantify the key factors responsible for the spatial and temporal differences in erosion rates observed within the gully. A permanent gully located in Cap Bon (northeastern Tunisia) has been intensively and regularly monitored over a 7-year period with electronic survey equipment (total station) to give five field topographic surveys, as well as hydrological measurements at the gully outlet. The net soil loss for the 7-year period comprised a denudation of 51 m3 of sediment on the gully bank slopes, which corresponds to a mean soil loss of 61 m3 ha?1 year?1 or 6.1 mm year?1. Denudation was observed on bed units with a slope gradient greater than 20%, while the remainder showed deposition. By confirming the factors involved in gully evolution, and by refining the statistical link between factors and erosion rates within the gully, the results provide important information to predict gully erosion rates in Mediterranean marly environments.

Editor Z.W. Kundzewicz; Associate editor G. Mahé

Citation El Khalili, A., Raclot, D., Habaieb, H., and Lamachère, J.M., 2013. Factors and processes of permanent gully evolution in a Mediterranean marly environment (Cape Bon, Tunisia). Hydrological Sciences Journal, 58 (7), 1519–1531.  相似文献   
993.
Results from coal‐exploration drilling in the onshore part of the Port Phillip Basin, Victoria, have established stratigraphic and age correlations of the Lower Miocene Werribee Formation brown coal deposits at Bacchus Marsh to similar brown coals at Altona. The coal deposits occur in a northwest‐southeast structural depression (the Parwan Trough) that appears to be a southeast continuation of the Ballan Graben. Recent drilling for potential coal‐bed methane in the trough has provided new data on the deeper stratigraphy not penetrated by earlier drilling, including recognition of an Upper Cretaceous to Eocene Yaloak Formation coal‐bearing interval, similar to the Anglesea area, Ballan Graben and Lal Lal Basin. Up to 200 m of coal‐bearing sediment and minor volcanics underlie the Miocene coal measures. A marine facies transition takes place between the Miocene coal swamps of the Parwan Trough, through barrier sands west of Werribee, to carbonate facies near Geelong. To the south beneath Port Phillip Bay, a similar transition probably occurs between coal swamps of the Parwan Trough and fully marine carbonate environments of the contiguous Sorrento Graben. The palaeogeographical reconstructions suggest a similar coal‐to‐carbonate facies transition as in the adjacent onshore Gippsland Basin.  相似文献   
994.
建立了一种虾、鱼、贝、藻多池循环水生态养殖及水质生物调控系统 ,该系统包括对虾养殖、鱼类养殖、贝类养殖、大型海藻栽培等 4个功能不同的养殖区 ,1个水处理区及 1个应急排水渠。通过在封闭循环系统内不同池塘中放养生态位互补的经济动植物 ,对虾池水质环境进行生物调控。结果表明 ,循环系统内虾池水层悬浮物数量、COD值、氨态氮含量比对照组单养对虾池明显降低 (t检验 ,P <0 .0 1)。养殖后排放水不处于富营养化状态 (E <1)。投入每千克对虾饲料生产0 .6 6 7kg对虾 ,同时生产 0 .0 37kg罗非鱼、0 .738kg牡蛎、0 .4 37kg江蓠 ,饲料利用率和经济效益显著增加。该养殖模式还具有防病性、环保性、高效性等优点  相似文献   
995.
Lanzhou Valley Basin is composed of two smaller diamond-shaped basins striking from NW to SE and including 6 river terraces. The lower terraces remaining even and smooth are main sites for city buildings. The main function of Lanzhou urban settlement is a ferry and transport centre. Lanzhou used to be an important crossing site and post on the “silk road”, and then became a transport and trade centre in Northwest China. In the middle part of the 17th century, Lanzhou became the capital of Gansu, a new province. The development process of Lanzhou urban settlement indicates that urban settlement is a system consisting of natural environment and human society based on the former.  相似文献   
996.
This study investigates the geomicrobiological potential of Upper Pleistocene evaporite deposits of the Chott el Gharsa, a wide continental sabkha in southern Tunisia. Organic and inorganic-derived biosignatures are mostly contained in microcrystalline, laminated gypsum lithofacies consisting of light/dark alternations of concordant laminae, which have precipitated from high salt concentrated waters. These biosignatures include mineralized microbial-interpreted morphologies, such as mucilage, rods, and microfibers, and dumbbell morphologies in the hollow cores of dolomite crystals that are associated with sulfates. Mineral products that are induced by microbial activity and their organic compounds lead to the formation of lenticular-shaped gypsum crystals, with a high length/width ratio, dolomite precipitation and formation of pyrite framboids. Morphological and structural aspects of these biosignatures, and their composition, in laminated, dolomite-rich sulfate deposits could be detected through microscopic investigations and micro-analyses performed by the instrumentation that is planned for ongoing Mars sample return missions.  相似文献   
997.
Kunming Basin locates middle of Yunnan altiplano and has a particularity in geography, topographic and geological environment. With the urban dilation quickly, add the reason of the unreasonable city layout, conflicts between environment and urban resources consumption become shrill increasingly. It is human being activities that lead to vulnerability and depravation of geological environment in local. Take a few examples on geological environment to expatiate relationship between urban construction and geological environment carrying capacity, and find a way how to make a better plan for urban sustainable development to achieve new balance between man and nature in local.  相似文献   
998.
在1994年发表的试论地下水研究面临的历史转变的基础上,面对新世纪创造新辉煌的期盼,论述了新的历史时期地下水开发与经管面临的转变和任务有8项即地下水开发与经管的指导思想要从比较放任,转变到高效利用与永续经营;在三水统观统管联合运用中要把地下水作为珍贵后备资源和抗旱的救命水资源加以利用;要把保护水质不受污染作为地下水管理的重大方面,要对含水层进行脆弱性分区管理;要建立和完善包括水位、水质动态观测在内的水环境综合观测网站,加强对地下水的全面观测;要认真总结前50年的地下水开发利用及管理工作,制定未来30年的地下水事业发展规划;要完善地下水开发利用保护法规;理顺地下水专业人才的培养渠道,认真培养地下水领域的高级专门人才;加强讯息网建设,在不断提高地下水科技水平的同时,重视科普宣传工作。  相似文献   
999.
本文对塔中1井储层段进行了岩芯、薄片、阴极发光等观察和分析,划分出了角砾白云岩、残余颗粒白云岩和晶粒白云岩3种岩石类型,识别出了白云石化、硅化、方解石化、重晶石化、萤石化、黄铁矿化、自生粘土矿物化以及溶解、压溶等成岩作用类型;归纳出塔中1井储层段经历了3个成岩阶段、4种成岩环境的改造和叠加,并在物性测定的基础上给出3个岩性段的储集条件.  相似文献   
1000.
ECOLOGIC ENVIRONMENT OF TARIM RIVER AND ITS PROTECTIVE PROGRAMME ChengQichou(程其畴)XinjiangInstituteofGeography,theChineseAcade...  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