全文获取类型
收费全文 | 553篇 |
免费 | 117篇 |
国内免费 | 93篇 |
专业分类
测绘学 | 31篇 |
大气科学 | 20篇 |
地球物理 | 98篇 |
地质学 | 411篇 |
海洋学 | 42篇 |
综合类 | 39篇 |
自然地理 | 122篇 |
出版年
2024年 | 3篇 |
2023年 | 1篇 |
2022年 | 8篇 |
2021年 | 19篇 |
2020年 | 17篇 |
2019年 | 28篇 |
2018年 | 12篇 |
2017年 | 13篇 |
2016年 | 15篇 |
2015年 | 28篇 |
2014年 | 34篇 |
2013年 | 46篇 |
2012年 | 27篇 |
2011年 | 40篇 |
2010年 | 41篇 |
2009年 | 31篇 |
2008年 | 50篇 |
2007年 | 37篇 |
2006年 | 29篇 |
2005年 | 42篇 |
2004年 | 36篇 |
2003年 | 23篇 |
2002年 | 29篇 |
2001年 | 22篇 |
2000年 | 19篇 |
1999年 | 19篇 |
1998年 | 14篇 |
1997年 | 11篇 |
1996年 | 7篇 |
1995年 | 8篇 |
1994年 | 17篇 |
1993年 | 8篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1983年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有763条查询结果,搜索用时 17 毫秒
161.
苏北中部滨海平原成陆机制研究 总被引:8,自引:0,他引:8
苏北中部滨海平原位于废黄河三角洲和长江三角洲之间,其形成与全新世最大海侵之后长江、古黄河的供沙历史和供沙状况密切相关,并且现今海域南黄海辐射沙洲的辐聚点也位于这段海岸,故探讨其成陆机制具有重要意义。本文数值模拟了公元8世纪时该区的M2潮汐、潮流,在此基础上,计算了7种粒径泥沙在当时潮流场中的净输运状况,划分了海底冲淤区。计算结果表明,公元8世纪时苏北岸外存在大致以东台为顶点的辐射状古潮流场,其分布格局与现今苏北岸外大致以京港为顶点的辐射关潮流场基本一致,该辐射状潮流场的存在是由当时台东岸外的独特驻潮波波腹所决定的。进入苏北岸外古辐射状潮流场中的泥沙,在辐聚、辐散的潮流作用下,每一个潮周期均自NE、SE方向向东台附近海域净输运泥沙,在东台附近海域发生淤积。由此认为,苏北中部滨海平原是大致以东台为顶点的辐聚、辐散潮 相似文献
162.
A study investigating the contents of 13 trace elements and the correlations between these trace elements and soil parameters
of the surface soils in the eastern alluvial plains of China is summarized and discussed in this paper. The results show that
the contents of some elements studied differ from region to region. Close relations have been noted between all the trace
element contents in the soils of the eastern alluvial plains and the relevant surface materials of erosion regions, which
demonstrate the effects of parent materials, while climatic conditions and human interventions are also found to be important
factors. Close relations were found between the transition elements of the Quaternary period and the chalcophile elements
(except Cu). The chemical and geochemical properties of these elements are identified as beeing important inherent characteristics
that affect the relations of these elements in the soils.
Received: 5 June 1997 · Accepted: 2 March 1998 相似文献
163.
Assessing crop yield and crop water productivity and optimizing irrigation scheduling of winter wheat and summer maize in the Haihe plain using SWAT model 下载免费PDF全文
Haihe plain is an important food production area in China, facing an increasing water shortage. The water used for agriculture accounts for about 70% of total water resources. Thus, it is critical to optimize the irrigation scheduling for saving water and increasing crop water productivity (CWP). This study first simulated crop yield and CWP for winter wheat and summer maize in historical scenario during 1961–2005 for Haihe plain using previously well‐established Soil and Water Assessment Tool model. Then, scenarios under historical irrigation (scenario 1) and sufficient irrigation (scenario 2) were, respectively, simulated both with sufficient fertilizer. The crop yield in scenario 2 was considered as the potential crop yield. The optimal irrigation scheduling with sufficient fertilizer (scenario 3) was explored by iteratively adjusting irrigation scheduling based on the scenario 1 and previous studies related to water stress on crop growth. Results showed that net irrigation amount was, respectively, reduced 23.1% and 18.8% in scenario 3 for winter wheat and summer maize when compared with scenario 1. The CWP was 12.1% and 8.2% higher with very slight change of crop yield. Using optimal irrigation scheduling could save 8.8 × 108 m3 irrigation water and reduce about 16.3% groundwater over‐exploitation in winter wheat growth period. The corresponding yield was 18.5% and 12.9% less than potential yield for winter wheat and summer maize but using less irrigation water. Therefore, it could be considered that the optimal irrigation was reasonable, which provided beneficial suggestions for increasing efficiency of agricultural water use with sustainable crop yield in Haihe plain. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
164.
地热资源是集热、水、矿为一体的新型能源,具有污染小、易开发、投资小和环保的特点。在鲁西北平原区地热井多集中分布在城区内,且同一地区地热井的取水区段基本一致,开发利用方式单一,没有较为完善的回灌措施,且尾水处理工艺简陋甚至直接排放,造成了地热资源开采区及排放流经区域水土污染,该文在对区内地热资源开发利用现状调查的基础上,对开发利用过程中可能引起的浅层及深层地下水污染情况进行了分析,并提出了一些防治措施与建议。 相似文献
165.
时间相依模型与泊松模型计算结果的对比分析——以道孚及炉霍潜源为例 总被引:1,自引:1,他引:0
本文分析了华北平原第四纪主要构造格局和强震活动特点,划分了主要强震构造带,并对其发震能力作了评价。认为坝县-束鹿-邯郸断裂带、唐山-河间-磁县断裂带和黄骅-德州-东明断裂带3条北东向右旋走滑断裂带为华北平原裂谷系从早第三纪伸展拉张作用进入晚第三纪-第四纪走滑剪切拉张作用的产物,这3条断裂带分别位于3个北东向坳陷带中部,并构成了第四纪沉降中心,与营口-潍坊断裂带一起构成华北平原内的主要强震构造。本文最后提出了华北平原区划分潜在震源区应注意的几点问题,进而针对第四代区划图中华北平原区潜在震源区划分综合方案提出了补充修改意见。 相似文献
166.
167.
In our study,the U-Pb age spectrum of detrital zircons from terrigenous sediments is used to determine the sources.The Middle and Upper Reaches of the Yangtze River drain the western Yangtze craton and eastern Tibetan plateau,and take lots of sediments to the Jianghan (江汉) plain.Detrital zircons collected from Zhoulao (周老) core provide an ideal sample for the provenance studies.Here we report LA-ICP-MS U-Pb ages of 125 detrital zircons from one sand sample in the core sediments,which is recovered from a dep... 相似文献
168.
169.
本文通过对松嫩平原西部齐齐哈尔、大庆等地粉土及粘性土612组液限与塑限、塑性指数的对比试验数据的相关分析,建立了液限与塑限、塑性指数间的回归方程。回归方程的相关系数分别为0.897、0.931,经验关系高度显著。 相似文献
170.
Boutheina Farhat Abdallah Ben Mammou Lamia Kouzana Ismail Chenini Francesca Podda Giovanni De Giudici 《Resource Geology》2010,60(4):377-388
The present paper investigates hydrochemical processes and water quality in the Mornag aquifer in NE Tunisia. Groundwater samples were collected during a field campaign, and were analysed for major and trace elements. The collected waters have a chemical facies rich in Ca2+, Na+ and Cl-. Piper diagram shows a progressive increase in chloride ions along with increasing salinity. Saturation indexes calculated by using PHREEQC (USGS) show that the Mornag waters are slightly saturated with respect to carbonates (calcite and dolomite), while undersaturated with respect to gypsum, halite and other evaporitic minerals. The current composition of waters takes place via dissolution of halite and Ca-sulfates, where the increase in calcium is partially balanced by possible calcite precipitation. The relevant recorded pollutant is nitrate, which was likely dispersed from agricultural soils, while heavy metals were generally far below values of pollution thresholds, indicating no influence by mining activity. 相似文献