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   5313篇
  免费   1060篇
  国内免费   978篇
测绘学   199篇
大气科学   79篇
地球物理   1638篇
地质学   4159篇
海洋学   304篇
天文学   4篇
综合类   479篇
自然地理   489篇
  2024年   26篇
  2023年   23篇
  2022年   118篇
  2021年   191篇
  2020年   183篇
  2019年   197篇
  2018年   197篇
  2017年   199篇
  2016年   216篇
  2015年   243篇
  2014年   319篇
  2013年   425篇
  2012年   292篇
  2011年   287篇
  2010年   308篇
  2009年   310篇
  2008年   339篇
  2007年   389篇
  2006年   393篇
  2005年   330篇
  2004年   303篇
  2003年   272篇
  2002年   203篇
  2001年   223篇
  2000年   195篇
  1999年   201篇
  1998年   169篇
  1997年   133篇
  1996年   131篇
  1995年   142篇
  1994年   73篇
  1993年   62篇
  1992年   66篇
  1991年   43篇
  1990年   41篇
  1989年   23篇
  1988年   31篇
  1987年   15篇
  1986年   10篇
  1985年   9篇
  1984年   7篇
  1983年   3篇
  1982年   3篇
  1981年   1篇
  1979年   3篇
  1978年   2篇
  1976年   1篇
  1973年   1篇
排序方式: 共有7351条查询结果,搜索用时 15 毫秒
981.
本文详细分析了山西辛安岩溶泉的水文地质条件,并通过对泉域岩溶水的补给、径流、排泄条件的分析,将辛安泉域概化为单输入单输出线性时不变集中参数随机系统的概念模型。利用24年的水文序列资料进行基流分割,获得泉多年月平均流量过程曲线,求得泉年平均流量和年平均补给量。在此基础上,应用滑动平均模型(MA),自回归—滑动平均模型(AR-MA)及泉流量调和分析—残差自回归等三个模型,对泉域地下水进行模拟,获得了满意的结果,不仅揭示了泉水动态特征,而且对泉流量动态进行了预报,为晋东南区域经济规划提供了有益资料。  相似文献   
982.
综合物探调查评价深圳市地下水资源   总被引:2,自引:1,他引:1  
段佳松 《物探与化探》1999,23(5):373-375
在深圳市地下水资源调查评价工作中,综合物探技术所起的作用和取得的成果.  相似文献   
983.
EH4电磁成像系统在砂岩地区勘查地下水的应用研究   总被引:27,自引:2,他引:25  
伍岳 《物探与化探》1999,23(5):335-338,346
EH4电磁成像系统是1套电磁自动采集和处理系统,它是CSAMT和MT的结合体.本文介绍了EH4电磁成像系统的原理、特点和该系统在砂岩地区,如内蒙、新疆、云南等地区进行地下水勘查的应用实例和效果  相似文献   
984.
目标函数聚类法在地下水动态分类中的应用   总被引:1,自引:0,他引:1  
地下水动态类型的划分是正确认识研究区水文地质条件与地下水开发利用之间关系的一个重要环节。本文论述了目标函数聚类法的原理与计算过程,并应用该方法对某河流附近的15个观测孔的水位动态进行分类。分析结果表明该方法具有较高的可靠性与较广的适用性。  相似文献   
985.
新疆玛纳斯河流域地下水预测研究   总被引:7,自引:1,他引:6  
根据新疆玛纳斯河流域地下水年径流量的时间序列, 建立了用于描述和分析具有趋势和周期变化特性的地下径流量时间序列的组合模型应用模型对石河子地区地下水年径流量进行了预报, 为该地区水资源合理配置和优化调度提供科学依据   相似文献   
986.
试论中国古大陆中-新元古代汇聚与裂解的地质记录   总被引:1,自引:0,他引:1  
自1990年新元古Rodinia超大陆提出后,现已发展成地学研究热点之一。在全球新元古代超大陆旋回的汇聚与裂解机制影响下,中国古大陆也随之变化。中元古代末一新元古代初的汇聚和新元古代晚期的裂解是最重要的两次地质事件。塔里木、华北、华南古陆都有它的地质记录。晋宁事件应是中元古代晚期-新元古代早期,北秦岭地块与中秦岭地块俯冲一碰撞造山作用和新元古代时期Rodinia超大陆形成的主要地质事件;华南的武陵运动,使华夏古陆与扬子地块发生碰撞形成统一的“华南古陆”。在塔里木古陆与哈萨克斯坦一伊犁古陆之间的那拉提南缘碰撞带,甘肃北山南带柳园及青海柴达木盆地北部边缘的榴辉岩一花岗岩带的发现都是最新的研究成果。  相似文献   
987.
北京地下水资源与首都持续发展   总被引:4,自引:0,他引:4  
分析了北京市地下水资源与首都经济和社会持续发展的关系, 提出了科学适度地开采地下水、全面提高北京市地下水资源与地下水环境研究程度的意见和建议  相似文献   
988.
 Over-abstraction of the Dammam aquifer, the principal aquifer in Bahrain, by the agricultural and domestic sectors, has led to its salinization by adjacent brackish and saline water bodies. A hydrochemical study identified the locations of the sources of aquifer salinization and delineated their areas of influence. The investigation indicates that the aquifer water quality is significantly modified as groundwater flows from the northwestern parts of Bahrain, where the aquifer receives its water by lateral underflow from eastern Saudi Arabia, to the southern and southeastern parts. Four types of salinization of the aquifer are identified: brackish-water up-flow from the underlying brackish-water zones in north-central, western, and eastern regions; seawater intrusion in the eastern region; intrusion of sabkha water in the southwestern region; and irrigation return flow in a local area in the western region. Four alternatives for the management of groundwater quality that are available to the water authorities in Bahrain are discussed and their priority areas are proposed, based on the type and extent of each salinization source, in addition to groundwater use in that area. The effectiveness of the proposed management options in controlling the degradation of water quality in the Dammam aquifer should be evaluated using simulation modeling. Received: June 1998 Revised: November 1998 Accepted: December 1998  相似文献   
989.
 The objective of the present paper is to show that groundwater is a general geologic agent. This perception could not, and did not, evolve until the system nature of basinal groundwater flow and its properties, geometries, and controlling factors became recognized and understood through the 1960s and 1970s. The two fundamental causes for groundwater's active role in nature are its ability to interact with the ambient environment and the systematized spatial distribution of its flow. Interaction and flow occur simultaneously at all scales of space and time, although at correspondingly varying rates and intensities. Thus, effects of groundwater flow are created from the land surface to the greatest depths of the porous parts of the Earth's crust, and from a day's length through geologic times. Three main types of interaction between groundwater and environment are identified in this paper, with several special processes for each one, namely: (1) Chemical interaction, with processes of dissolution, hydration, hydrolysis, oxidation-reduction, attack by acids, chemical precipitation, base exchange, sulfate reduction, concentration, and ultrafiltration or osmosis; (2) Physical interaction, with processes of lubrication and pore-pressure modification; and (3) Kinetic interaction, with the transport processes of water, aqueous and nonaqueous matter, and heat. Owing to the transporting ability and spatial patterns of basinal flow, the effects of interaction are cumulative and distributed according to the geometries of the flow systems. The number and diversity of natural phenomena that are generated by groundwater flow are almost unlimited, due to the fact that the relatively few basic types are modified by some or all of the three components of the hydrogeologic environment: topography, geology, and climate. The six basic groups into which manifestations of groundwater flow have been divided are: (1) Hydrology and hydraulics; (2) Chemistry and mineralogy; (3) Vegetation; (4) Soil and rock mechanics; (5) Geomorphology; and (6) Transport and accumulation. Based on such a diversity of effects and manifestations, it is concluded that groundwater is a general geologic agent. Received, December 1998 · Revised, January 1999 · Accepted, January 1999  相似文献   
990.
 The Hadejia–Nguru Wetlands are annually inundated flood plains in semi-arid northeastern Nigeria. The area has a unique ecosystem that forms a natural barrier against the encroachment of the Sahara desert. Both the rich wetland vegetation and local farmers using shallow tube wells depend on a groundwater mound (with a water table less than 6 m below the surface) that is present in the unconfined aquifer under the flood-plain area. Using well records (1991–97) and a hydrogeologic profile based on piezometers that were monitored for two years, it is shown that recharge through the annually inundated flood plains is the source of the groundwater mound. Maintenance of the groundwater-recharge function of the flood plains depends on wet-season releases from two large upstream dams. On the basis of a water-budget method, the mean (1991–97) wet-season unconfined groundwater recharge in the flood-plain area between Hadejia and Nguru and in the immediate vicinity (1250 km2) is estimated to be 132 mm (range, 73–197 mm). Outflow from the unconfined flood-plain aquifer to the unconfined upland aquifer is approximately 10% of the wet-season flood-plain recharge. The unconfined groundwater outflow from the flood-plain area can provide a significant contribution to the present-day rural water supply in the surrounding uplands, but it does not offer much potential for additional groundwater abstraction. In addition to outflow to the upland aquifer (∼14 mm), the distribution of the annually recharged water volume of the shallow flood-plain aquifer is (1) domestic uses (3 mm), (2) small-scale irrigation (∼15 mm), and (3) evapotranspiration ( 1 100 mm). Along the hydrogeologic profile, the recharge in the upland (i.e., outflow from the unconfined flood-plain aquifer and possibly diffuse rain-fed recharge) is in balance with the water uses (i.e., domestic uses, groundwater outflow, and evapotranspiration). The absence of a seasonal water-level trend in the two piezometers in the upland indicates that no rain-fed recharge occurs through preferential path-way (macropore) flow. Received, June 1998 / Revised, November 1998, January 1999 / Accepted, January 1999  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