全文获取类型
收费全文 | 6769篇 |
免费 | 1136篇 |
国内免费 | 1048篇 |
专业分类
测绘学 | 547篇 |
大气科学 | 541篇 |
地球物理 | 1485篇 |
地质学 | 3052篇 |
海洋学 | 1514篇 |
天文学 | 93篇 |
综合类 | 453篇 |
自然地理 | 1268篇 |
出版年
2024年 | 13篇 |
2023年 | 92篇 |
2022年 | 201篇 |
2021年 | 237篇 |
2020年 | 241篇 |
2019年 | 250篇 |
2018年 | 174篇 |
2017年 | 233篇 |
2016年 | 227篇 |
2015年 | 257篇 |
2014年 | 307篇 |
2013年 | 476篇 |
2012年 | 324篇 |
2011年 | 371篇 |
2010年 | 337篇 |
2009年 | 398篇 |
2008年 | 428篇 |
2007年 | 521篇 |
2006年 | 439篇 |
2005年 | 395篇 |
2004年 | 397篇 |
2003年 | 308篇 |
2002年 | 290篇 |
2001年 | 258篇 |
2000年 | 269篇 |
1999年 | 248篇 |
1998年 | 206篇 |
1997年 | 192篇 |
1996年 | 129篇 |
1995年 | 113篇 |
1994年 | 121篇 |
1993年 | 128篇 |
1992年 | 106篇 |
1991年 | 73篇 |
1990年 | 57篇 |
1989年 | 48篇 |
1988年 | 33篇 |
1987年 | 9篇 |
1986年 | 12篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1978年 | 4篇 |
1977年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有8953条查询结果,搜索用时 15 毫秒
991.
992.
Based on the practice of artificial freezing engineering in deep alluvium and simulation tests, it is found that there exists an obvious difference between frozen soil mechanics for shallow alluvium and for deep alluvium. The concept of frozen soil mechanics for deep alluvium, its research topics and methods are presented in this paper. 相似文献
993.
994.
Sediment cores from Chappice Lake, a hypersaline, groundwater-fed lake in southeastern Alberta, have been used in previous studies to reconstruct Holocene climate using lake levels as a source for proxy climate data. This assumes that the lake is fed by a shallow groundwater system sensitive to changes in climate. In this study we use the dynamics and chemistry of groundwater entering the lake to test this hypothesis.Groundwater inputs calculated from historical records using a simple water budget were highest during periods when the precipitation deficit was high. Over specific time intervals, the expected relationship between lake volumes and climate were not always found. Feedback loops between lake levels and groundwater input, and time lags within the system are the mechanisms proposed to explain these discrepancies.Field measurements suggest discharge of a local surficial groundwater system. Slug tests reveal a high conductivity system (K = 10-5 m/s) surrounding the lake. Hydraulic heads measured in standpipe, multilevel and minipiezometers installed around Chappice Lake show that the lake is situated in a closed hydraulic head contour. Hydraulic heads and water table elevations show strong annual fluctuations corresponding to seasonal changes in recharge. Horizontal hydraulic gradients measured in areas of groundwater springs indicate a strong horizontal component of flow towards the lake. Vertical hydraulic gradients are low and indicate the upward flow of water consistent with the discharge of a shallow, surfical groundwater system.Groundwater sampled from deposits surrounding Chappice Lake and springs feeding the lake have compositions similar to both shallow surficial aquifers and bedrock aquifers suggesting that the lake may be receiving inputs from both sources. However, evaporation simulations using PHRQPITZ, show that the evaporation of water typical of bedrock aquifers result in a mineral assemblage and brine composition different from that found at Chappice Lake. This suggests that discharge of a regional groundwater system can be eliminated as a dominant source over the lake's history. Evaporation simulations suggest that evaporation of groundwater from shallow surficial deposits can best explain the present mineral assemblage and brine chemistry and were likely the dominant source of water to the lake.Bedrock and shallow surficial groundwater sources have different chemistries and isotopic compositions. In hydrogeological settings such as Chappice Lake where more than one source may contribute to the lake, the relative importance of the different sources may change with changes in climate. If the source water composition to the lake changes, identifying changes in climate or hydrology based on changes in the composition of the lake preserved in sediment core will be made more difficult. This may complicate paleoclimate and paleohydrological reconstructions that rely on mineralogical and isotopic data. 相似文献
995.
Scott Lamoureaux 《Journal of Paleolimnology》1999,21(1):73-84
Multiple cores taken throughout Nicolay Lake in the Canadian High Arctic contain laminated sediments that are interpreted as varved. Annual sediment accumulation during the last 197 years reveals three major patterns that have important implications for hydroclimatic reconstruction. Widespread sediment dispersal is evident during most years and contrasts with years when anomalous localized and bifurcating patterns of deposition occur. Localized deposition is limited to the centre of the lake and is attributed to turbidity currents that originate on the delta foreslope. In contrast, a bifurcating pattern is produced when the river supplying the delta switches from one to two delta distributaries, resulting in altered proximal accumulation. Principal component analysis reveals additional accumulation anomalies that are sedimentologically indistinct, but constitute important sources of localized variance. The recognition and removal of anomalous deposition patterns is critical for accurate reconstruction of sediment flux from the catchment, particularly in long records where geomorphic conditions may have changed. This study identifies potentially misleading depositional artifacts and a methodology to recognize similar depositional controls in other lakes. 相似文献
996.
Tom Korsman 《Journal of Paleolimnology》1999,22(1):1-15
Changes in lake-water pH, alkalinity and colour were inferred from diatoms in surface sediment samples and sediment samples from pre-industrial times from 118 northern Swedish lakes. This palaeolimnological study does not support the hypothesis that there is a large-scale modern acidification in the two northernmost counties of Sweden; pH had decreased significantly in eight lakes, while five had a significant increase. Partial least-squares regression of changes in water chemistry in relation to catchment characteristics was performed to evaluate the causes of the acidity status. Furthermore, temporal trends were assessed from long sediment cores from five acidic lakes. The results suggest that the presently acid lakes have faced a long-term acidification trend over several thousand years due to soil-forming processes and vegetation development. However, due to the acid sensitivity of the region, future acidification trends in northern Swedish lakes should be carefully observed and assessed. 相似文献
997.
黄淮海平原冬小麦生长期旱情分析 总被引:7,自引:2,他引:5
黄淮海平原降水较少且年际、季际变化大,因此水分供应状况成为该区作物,特别是以冬春季为主要生育期的冬小麦的主限制因素。鉴于作物实际蒸散量与潜在蒸散量关系依赖于作物生长状况和土壤水分的事实,提出反映作物缺水状况的干旱指标———作物水分胁迫指数(CWSI)。计算黄淮海平原冬小麦生长期间的CWSI,并分析其在自然降水条件及适量灌溉条件下的时空分异规律 相似文献
998.
人工措施对川中丘陵区防护林建设的影响 总被引:2,自引:2,他引:0
人工措施对防护林建设初期的影响,主要表现在保存率,林木生长,林分结构等方面,而这些都与防护林效益密切相关,为此,提出了营建和管理防护林中应注意的问题。 相似文献
999.
1000.