全文获取类型
| 收费全文 | 1500篇 |
| 免费 | 442篇 |
| 国内免费 | 494篇 |
专业分类
| 测绘学 | 227篇 |
| 大气科学 | 169篇 |
| 地球物理 | 382篇 |
| 地质学 | 1128篇 |
| 海洋学 | 260篇 |
| 天文学 | 79篇 |
| 综合类 | 79篇 |
| 自然地理 | 112篇 |
出版年
| 2024年 | 8篇 |
| 2023年 | 25篇 |
| 2022年 | 52篇 |
| 2021年 | 74篇 |
| 2020年 | 103篇 |
| 2019年 | 98篇 |
| 2018年 | 66篇 |
| 2017年 | 100篇 |
| 2016年 | 106篇 |
| 2015年 | 90篇 |
| 2014年 | 141篇 |
| 2013年 | 126篇 |
| 2012年 | 115篇 |
| 2011年 | 102篇 |
| 2010年 | 128篇 |
| 2009年 | 127篇 |
| 2008年 | 83篇 |
| 2007年 | 110篇 |
| 2006年 | 93篇 |
| 2005年 | 80篇 |
| 2004年 | 81篇 |
| 2003年 | 87篇 |
| 2002年 | 68篇 |
| 2001年 | 55篇 |
| 2000年 | 54篇 |
| 1999年 | 54篇 |
| 1998年 | 32篇 |
| 1997年 | 42篇 |
| 1996年 | 25篇 |
| 1995年 | 29篇 |
| 1994年 | 40篇 |
| 1993年 | 17篇 |
| 1992年 | 5篇 |
| 1991年 | 7篇 |
| 1990年 | 1篇 |
| 1989年 | 3篇 |
| 1988年 | 3篇 |
| 1986年 | 3篇 |
| 1985年 | 1篇 |
| 1984年 | 1篇 |
| 1983年 | 1篇 |
排序方式： 共有2436条查询结果，搜索用时 15 毫秒
51.
52.
根据野外水准测量与室内实验分析，本文探讨了西藏纳木错和藏北高原古大湖晚更新世以来的湖泊演化和气候变迁。在纳木错沿岸拔湖48m以下，发育了6级湖岸阶地，拔湖48-139．2m发育有高位湖相沉积。研究表明，纳木错湖泊发育与藏北高原东南部古大湖演化可划分为3个阶段：①116-37ka B．P．间的古大湖期；②37-30ka B．P．间的外流湖期；③30kaB．P．以来的纳木错期。在古大湖阶段，包括纳木错、色林错和扎日南木错、当惹雍错等藏北高原东南部的一大批现代大、中、小型湖泊，都是互相连通的一个古大湖，其范围可能超过了现代的藏北内、外流(怒江)水系的分水岭。它或许还与藏北高原南部和西部的其他古湖相连，成为统一的藏北高原“古大湖”。通过对纳木错湖相沉积形成时代与深海氧同位素对比，易溶盐、pH值、地球化学、介形类和孢粉分析等的综合研究发现，湖相沉积记录了自晚更新世以来的湖泊演化和气候变迁信息。资料显示古大湖期湖面最高，气候温和清爽；外流湖期湖面急剧下降，气温和湿度较现今略高；纳木错期以来气候经历了全新世最宜期的暖湿后日益干旱化，气温波动，湖面持续下降。表明自晚更新世以来该区气候在逐渐变干的总趋势的基础上，经历了多次明显的冷暖与干湿波动。 相似文献
53.
54.
In this study a non-hydrostatic version of Penn State University (PSU) -- NationalCenter for Atmospheric Research (NCAR) mesoscale model is used to simulate thesuper cyclonic storm that crossed Orissa coast on 29 October 1999. The model isintegrated up to 123 h for producing 5-day forecast of the storm. Several importantfields including sea level pressure, horizontal wind and rainfall are compared with theverification analysis/observation to examine the performance of the model. The modelsimulated track of the cyclone is compared with the best-fit track obtained from IndiaMeteorological Department (IMD) and the track obtained from NCEP/NCAR reanalysis. The model is found to perform reasonably well in simulating the track and in particular, the intensity of the storm. 相似文献
55.
56.
针对传统的边缘提取方法大部分不适应高光谱数据的特点，提出了基于光谱空间密度分析边缘提取的思想。在分组主分量变换提取第一主分量作为特征维的基础上，采用面向对象的二次判别边缘的方法，通过立体判决将光谱空间中低密度超椭球体集群视为真实边缘点集群。试验表明，此方法是合理可行的。 相似文献
57.
58.
Storage of large woody debris in the wide, mountain, Czarny Dunajec River, southern Poland, was investigated following two floods of June and July 2001 with a seven‐year frequency. Within a reach, to which wood was delivered only by bank erosion and transport from upstream, wood quantities were estimated for eighty‐nine, 100 m long, channel segments grouped into nine sections of similar morphology. Results from regression analysis indicated the quantity of stored wood to be directly related to the length of eroded, wooded banks and river width, and inversely related to unit stream power at the flood peak. The largest quantities of wood (up to 33 t ha?1) were stored in wide, multi‐thread river sections. Here, the relatively low transporting ability of the river facilitated deposition of transported wood while a considerable length of eroded channel and island banks resulted in a large number of trees delivered from the local riparian forest. In these sections, a few morphological and ecological situations led to the accumulation of especially large quantities of wood within a small river area. Very low amounts of wood were stored in narrow, single‐thread sections of regulated or bedrock channel. High stream power facilitated transport of wood through these sections while the high strength of the banks and low channel sinuosity prevented bank retreat and delivery of trees to the channel. Considerable differences in the character of deposited wood existed between wide, multi‐thread channel sections located at different distances below a narrow, 7 km long, channellized reach of the river. Wood deposited close to the downstream end of the channellized reach was highly disintegrated and structured into jams, whereas further downstream well preserved shrubs and trees prevailed. This apparently reflects differences in the distance of wood transport and shows that in a mountain river wider than the height of trees growing on its banks, wood can be transported long distances along relatively narrow, single‐thread reaches but is preferentially deposited in wide, multi‐thread reaches. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
59.
60.
This paper examines the transport of calcareous sand in unidirectional flow and its prediction through existing sediment transport models. A flume experiment of four sand samples collected on Oahu, Hawaii, provides 29 sets of sediment transport data in the bed-form and suspended transport stages. The measured transport data are compared with direct predictions from four energy-based transport models developed for siliceous particles. Corrections for the grain-size, fall velocity, and critical velocity of calcareous sand based on recent research are applied to the models and the results are compared with the direct calculations and measured data. The comparison illustrates the important role particle shape plays in the transport of calcareous sand. All four sediment transport models give consistent predictions and good agreement with the majority of the measured data. Two of the models respond positively to the corrections in both the bed-form and suspended transport stages indicating that such an approach may provide an interim solution for the transport of calcareous sand. 相似文献