全文获取类型
收费全文 | 28666篇 |
免费 | 3173篇 |
国内免费 | 4411篇 |
专业分类
测绘学 | 6389篇 |
大气科学 | 1866篇 |
地球物理 | 4368篇 |
地质学 | 10357篇 |
海洋学 | 2341篇 |
天文学 | 5483篇 |
综合类 | 1902篇 |
自然地理 | 3544篇 |
出版年
2024年 | 89篇 |
2023年 | 335篇 |
2022年 | 1007篇 |
2021年 | 1108篇 |
2020年 | 1097篇 |
2019年 | 1141篇 |
2018年 | 886篇 |
2017年 | 1068篇 |
2016年 | 1124篇 |
2015年 | 1263篇 |
2014年 | 1460篇 |
2013年 | 1712篇 |
2012年 | 1743篇 |
2011年 | 1816篇 |
2010年 | 1687篇 |
2009年 | 2018篇 |
2008年 | 2028篇 |
2007年 | 2074篇 |
2006年 | 1896篇 |
2005年 | 1630篇 |
2004年 | 1446篇 |
2003年 | 1229篇 |
2002年 | 1028篇 |
2001年 | 941篇 |
2000年 | 801篇 |
1999年 | 711篇 |
1998年 | 615篇 |
1997年 | 413篇 |
1996年 | 327篇 |
1995年 | 272篇 |
1994年 | 271篇 |
1993年 | 250篇 |
1992年 | 169篇 |
1991年 | 90篇 |
1990年 | 94篇 |
1989年 | 87篇 |
1988年 | 72篇 |
1987年 | 35篇 |
1986年 | 42篇 |
1985年 | 34篇 |
1984年 | 28篇 |
1983年 | 15篇 |
1982年 | 18篇 |
1981年 | 6篇 |
1980年 | 14篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1977年 | 18篇 |
1973年 | 7篇 |
1954年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
951.
利用多目标区域地球化学调查成果,探讨了福建沿海土壤地球化学分类的方法,提出了分类命名方案和诊断标准,将研究区土壤按SiO2含量划分为铁铝土、硅铝土、铝硅土、硅土4个大类,其中硅铝土又分为一般硅铝土、铁质硅铝土、钾质硅铝土、硫铁质硅铝土4个亚类,铝硅土分为一般铝硅土和钾质铝硅土2个亚类;提出了在各种统计量中,中值比较客观地反映元素含量背景,适合作为基准值;给出了研究区各类土壤的分布图和52种元素及pH的基准值,分析了其成因和农业利用情况。 相似文献
952.
953.
The Boao coastal system along the eastern coast of Hainan Island is a dynamic delta-tidal inlet-barrier formed during the late Holocene. The delta developed inside a shallow lagoon barred by a sandy barrier with a narrow, shallow tidal inlet opening. Two major distributary channels separated by small islands characterize the delta. The lagoon is silting up receiving and trapping sediments from both the river and, in minor measure during storms, through the tidal inlet opening and barrier washovers. The barrier at the tidal inlet is highly dynamic and changes its form, accreting (migrating spit) against the inlet during fair-weather conditions and being eroded during storms and river floods.The delta has almost completely filled the lagoon and major concerns exist on the effect that ongoing large development plans may have on the environment. These concerns include the effect on floods and rate of siltation once banks of the islands have been stabilized and floodwater and sediment load are impeded from spreading over the lowlands, and the effect of increasing pollutant loads from the new facilities on the ecosystems of the increasingly restricting lagoon water and on the seashores. 相似文献
954.
Analysis of village accessibility and its impact on land use dynamics in a mountainous province of northern Vietnam 总被引:5,自引:0,他引:5
Jean-Christophe Castella Pham Hung Manh Suan Pheng Kam Lorena Villano Nathalie Rachel Tronche 《Applied geography (Sevenoaks, England)》2005,25(4):308-326
Surveys carried out in mountainous areas of northern Vietnam at research sites selected across a gradient of market integration, revealed strong relationships between the location of the village with respect to the national road network and the nature of its land-use systems, its poverty level and more generally its potential for development. We developed and tested in Bac Kan province a method to give an objective and quantitative definition of accessibility over a large geographic area. Accessibility maps integrated in a provincial GIS showed that despite recent improvements to the road network, some remote areas do not benefit from recent development. 相似文献
955.
Sediment concentration changes in runoff pathways from a forest road network and the resultant spatial pattern of catchment connectivity 总被引:6,自引:0,他引:6
Hydrological connectivity is a term often used to describe the internal linkages between runoff and sediment generation in upper parts of catchments and the receiving waters. In this paper, we identify two types of connectivity: direct connectivity via new channels or gullies, and diffuse connectivity as surface runoff reaches the stream network via overland flow pathways. Using a forest road network as an example of a landscape element with a high runoff source strength, we demonstrate the spatial distribution of these two types of linkages in a 57 km2 catchment in southeastern Australia. Field surveys and empirical modelling indicate that direct connectivity occurs primarily due to gully development at road culverts, where the average sediment transport distance is 89 m below the road outlet. The majority of road outlets were characterised by dispersive flow pathways where the maximum potential sediment transport distance is measured as the available hillslope length below the road outlet. This length has a mean value of 120 m for this catchment. Reductions in sediment concentration in runoff plumes from both pathways are modelled using an exponential decay function and data derived from large rainfall simulator experiments in the catchment. The concept of the volume to breakthrough is used to model the potential delivery of runoff from dispersive pathways. Of the surveyed road drains (n=218), only 11 are predicted to deliver runoff to a stream and the greatest contributor of runoff occurs at a stream crossing where a road segment discharges directly into the stream. The methodology described here can be used to assess the spatial distribution and likely impact of dispersive and gullied pathways on in-stream water quality. 相似文献
956.
957.
958.
A size classification for debris flows 总被引:4,自引:0,他引:4
A 10-fold classification for debris flow size is proposed based on total volume, peak discharge and area inundated by debris. Size classes can be used for regional overview studies where detailed site investigations are either unnecessary, too costly or where the highest hazard and risk creeks need to be identified for further study. They are also useful to compare the regional impact between affected areas and the effects of rainstorms, and they allow lay-people to obtain an understanding of debris flow magnitude and consequences. Finally, different size classes allow the estimation of travel times to points of interest based on empirically derived equations. It is proposed that agencies concerned with debris flows should establish a documentation of debris flow size according to this classification, which serves as a data base for hazard and risk planning. 相似文献
959.
The Yudonghe landslide, located in western Hubei Province of China, consists of eastern and western subunits as well as a main landslide mass with upper and lower slip surfaces. As an important landslide close to Shuibuya Dam on the Qing River, its stability is crucial, as the slide might reactivate because of a change in ground-water level caused by filling of the Shuibuya Reservoir. Existing weakness zones, growth of ruptures, the downslope attitude of geologic strata, and water infiltration, which reduced the strength of rocks and soils, have been found to be the most important factors contributing to the Yudonghe landslide. With regard to the landslide processes, it can be noted that the original large-scale slide activity was due to erosion by the Qing River, the second sliding resulted from the fall of blocks from the head scarp, and the final activity was the growth of the eastern and western secondary slides. A base failure was the main type of slope movement, however, it was obvious that more than one sliding event occurred, as inferred from striations and fractures detected by microstructure analysis of soils along the failure surfaces. Slope instability was evaluated by the method of Back Propagation Neural Networks (BPNN), in which a four-layer BPNN model with five input nodes, two hidden layers, and two output nodes was constructed using a training data set of landslide samples throughout the Qing River area. The predicted results of this analysis showed that the factor of safety was 1.10, which indicates that the Yudonghe landslide is currently in a marginally stable condition. 相似文献
960.