全文获取类型
收费全文 | 16747篇 |
免费 | 2960篇 |
国内免费 | 3781篇 |
专业分类
测绘学 | 875篇 |
大气科学 | 1696篇 |
地球物理 | 4319篇 |
地质学 | 9752篇 |
海洋学 | 2902篇 |
天文学 | 64篇 |
综合类 | 1124篇 |
自然地理 | 2756篇 |
出版年
2024年 | 145篇 |
2023年 | 333篇 |
2022年 | 546篇 |
2021年 | 770篇 |
2020年 | 740篇 |
2019年 | 835篇 |
2018年 | 693篇 |
2017年 | 743篇 |
2016年 | 729篇 |
2015年 | 792篇 |
2014年 | 1037篇 |
2013年 | 1270篇 |
2012年 | 971篇 |
2011年 | 1071篇 |
2010年 | 1009篇 |
2009年 | 1007篇 |
2008年 | 1042篇 |
2007年 | 1103篇 |
2006年 | 1126篇 |
2005年 | 926篇 |
2004年 | 890篇 |
2003年 | 759篇 |
2002年 | 705篇 |
2001年 | 619篇 |
2000年 | 554篇 |
1999年 | 469篇 |
1998年 | 433篇 |
1997年 | 382篇 |
1996年 | 316篇 |
1995年 | 291篇 |
1994年 | 271篇 |
1993年 | 210篇 |
1992年 | 162篇 |
1991年 | 135篇 |
1990年 | 89篇 |
1989年 | 98篇 |
1988年 | 54篇 |
1987年 | 47篇 |
1986年 | 23篇 |
1985年 | 33篇 |
1984年 | 15篇 |
1983年 | 10篇 |
1982年 | 3篇 |
1981年 | 10篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1978年 | 9篇 |
1973年 | 1篇 |
1971年 | 3篇 |
1954年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
In irrigated agricultural landscapes, land-use conversion may have landscape-level social, hydrological, and ecological effects. We used geographic information systems (GIS) and interviews to analyze development effects on irrigation in the Henry's Fork Watershed, Idaho. Farmers developed irrigation there in the 19th century, and incidental recharge from canal seepage and flood irrigation raised groundwater levels and expanded wetlands. Cohesive culture in agricultural communities sustained irrigation systems until amenity-driven demographic shifts beginning in the 1970s led to development approved by local governments with land-use planning but no water-management authority. Although only 5% of irrigated land has been converted, development has fragmented irrigated landscapes and made canal-system operation more difficult, potentially reducing groundwater levels, wetland extent, and return flows critical to downstream irrigators and fish and wildlife. We discuss future scenarios, highlighting the need for increased communication among local and state governments regarding land use and water management in irrigated landscapes across the West. 相似文献
92.
93.
本文利用前人的成果及笔者1992/1993年的南极海冰观测和收集的资料以及水文观测资料数据阐述了南极海冰的特性,特别是南极海冰过程、冰穴以及冰川冰对南极水团(南极表层水、南极底层水、南极陆架水、南极中层水以及南极冰架水)的形成和变性所起的特殊作用。 南极海冰覆盖面积的年际变化,夏季最大年份是最小年份的2倍多,冬季年间变化较小,最大仅为20%;但其季节变化非常大,冬季平均覆盖面积通常是夏季的5倍。南极海冰对大气-海洋间相互作用有重大影响,特别是深海洋区中冬季的结冰和发育造成的垂向对流、夏季的融化是形成南极表层水(含南极冬季水和南极夏季表层水),进而形成南极中层水的主要原因;南极陆架区的的海冰兴衰过程是形成南极陆架水的直接原因,它与变性南极绕极深层水混合并受到冰川冰的进一步冷却作用,成为形成南极底层水的主要水团;南极冰架底部的冷却、融化和冰架以下水体的结冰作用形成的高盐对流过程产生的南极冰架水,亦是形成南极底层水的贡献者。 冰穴是70年代以来卫星观测的重大发现。对其形成和对大气、海洋的影响作用尚不完全清楚,初步的研究成果表明,冰穴中产生的热盐对流对南极水团的形成、变性、大洋深层的翻转以及海洋-大气间的热量传输和气体交换起有非常重要的作用。 相似文献
94.
《Marine Policy》2015
This article provides a historical account of Chinese public policy on fisheries subsidies, and a discussion of problems and limitations resulting from the World Trade Organization (WTO) Doha Round fisheries subsidies negotiation. By analyzing subsidization trends and applying a proportional assessment of subsidized areas, the article tracks the evolvement of Chinese fishery policy objective and priorities. Chinese fishery is not historically significantly subsidized in light of its relatively market-oriented structure and a sustained momentum in reducing marine capture capacity. However, due to the commitment to encouraging qualitative growth and to satisfying a rising domestic demand for fish, the government seeks for adequate policy space to employ fuel, distant water fisheries, infrastructure and various ‘green light’ subsidies. Whether or not such a claim can be accepted as a developmental right not an exception remains to be answered by the revision of current negotiation draft. 相似文献
95.
96.
腾格里沙漠地区水化学特征 总被引:6,自引:2,他引:6
水资源极为匮乏的腾格里沙漠区内所赋存的水主要有两种类型。一种是沙下潜水 ,主要是周边山区由高向低缓慢地向沙漠区运移所致 ,分布较为普遍 ;另一种是沙漠湖水 ,它是在沙漠区内之低洼地段 ,由于沙下潜水的长期溢出汇集于大小不等的长条状湖泊之中 ,以湖表水 (咸水和卤水 )的形式存在。沙下潜水矿化度在 0 .5~ 1 g/L之间 ,为本区的饮用水。湖表水咸苦 ,矿化度较高 ,就同一湖区而言 ,也有咸水和卤水之分 ,咸水矿化度为每升数十克 ,卤水则在 1 0 0~ 440 g/L之间 ,已达到了盐湖类沉积阶段 相似文献
97.
Crustal Growth by Magmatic Accretion Constrained by Metamorphic P-T Paths and Thermal Models of the Kohistan Arc, NW Himalayas 总被引:2,自引:0,他引:2
Magmatic accretion is potentially an important mechanism inthe growth of the continental crust and the formation of granulites.In this study, the thermal evolution of a magmatic arc in responseto magmatic accretion is modeled using numerical solutions ofthe one-dimensional heat conduction equation. The initial andboundary conditions used in the model are constrained by geologicalobservations made in the Kohistan area, NW Himalayas. Takingconsideration of the preferred intrusion locations for basalticmagmas, we consider two plausible modes of magmatic accretion:the first involves the repeated intrusion of basalt at mid-crustaldepths (intraplate model), and the second evaluatesthe simultaneous intrusion of basalt and picrite at mid-crustaldepths and the base of the crust respectively (double-platemodel). The results of the double-plate model accountfor both the inferred metamorphic PT paths of the Kohistanmafic granulites and the continental geotherm determined frompeak PT conditions observed for granulite terranes. Thedouble-plate model may be applicable as a key growth processfor the production of thick mafic lower crust in magmatic arcs. KEY WORDS: thermal model; magmatic underplating; PT path; granulite; lower crust 相似文献
98.
The reproductive biology of Acanthopagrus butcheri has been studied in the permanently open Swan River and intermittently open Moore River estuaries on the lower west coast of Australia (31--32 °S) and in the permanently open Nornalup Walpole and normally closed Wellstead estuaries on the southern coast of Western Australia (34--35 °S). Trends exhibited by gonadosomatic indices, gonadal maturity stages and the sizes and developmental stages of the oocytes demonstrate that A. butcher typically spawns in spring and early summer. However, spawning occurred in salinities ranging from as low as 3.5-8 g L–1 in the Moore River Estuary to as high as 41-45 g L–1 in the Wellstead Estuary. Furthermore, water temperatures during spawning were greater in the two northern estuaries (19.7--28.5 °C) than in the two southern and cooler estuaries (17.5--23.4 °C). Histological studies strongly indicate that A. butcheri spawn more than once in a breeding season and demonstrate that the development of its oocytes exhibits group synchrony sensu de Vlaming (1983). The ages and total lengths at which, on average, female and male A. butcheri both first attain maturity in the Swan River Estuary were ca 2 years and ca 215 mm. However, the age at which individual fish in that system reach maturity was influenced by body size. This suggests that the attainment of first maturity at an older age but smaller length in the Moore River and Nornalup Walpole estuaries than is the case in the Swan River Estuary is a consequence of the slower growth rates of A. butcheri in those estuaries. The combination of the young age (ca 2 years) but small length (ca 145 mm) at which maturity is first attained in the Wellstead Estuary could have resulted from selection pressures brought about by high mortality rates and/or heavy fishing pressure in this estuary. The mean fecundity of A. butcheri, based on the combined number of yolk vesicle and yolk granule oocytes found in ovaries just prior to the onset of spawning, was 1580 × 103. The significance of the sizes at first maturity, minimum legal length for capture, mesh selectivity data and closure of certain regions of estuaries to fishing for the management of the recreational and commercial fishery for A. butcheri is discussed. 相似文献
99.
Theoretical difficulties for mapping and for estimating river regime characteristics in a large-scale basin remain because of the nature of the variable under study: river flows are related to a specific area, i.e. the drainage basin, and are hierarchically organized in space through the river network with upstream-downstream dependencies. Another limitation is there are not enough gauge stations in developing countries. This presentation aims at de-veloping the hydro-stochastic approach for producing choropleth maps of average annual runoff and computing mean discharge along the main river network for a large-scale basin. The approach applied to mean annual runoff is based on geostatistical interpolation proce-dures coupled with water balance and data uncertainty analyses. It is proved by an applica-tion in the upstream at Bengbu in the Huaihe River Basin, a typical large-scale basin in China. Hydro-stochasitic approach in a first step interpolates to a regular grid net and in a second step the grid values are integrated along rivers. The interpolation scheme includes a con-straint to be able to account for the lateral water balance along the rivers. Grid runoff map with 10 km × 10 km resolution and the discharge map along the river with the 1 km basic length unit are the main results in this study. This kind of statistic approach can be widely used be-cause it avoids the complexity of hydrological models and does not depend on the meteoro-logical data. 相似文献
100.
河北沿海海侵灾害初探 总被引:4,自引:1,他引:4
文中以包含海进、海岸侵蚀和海水入侵的广义“海侵”概念,论述河北沿海地区海侵现状及其危害,探讨海侵产生的自然原因和人为活动的影响,在此基础上,评估海侵发展趋势。 相似文献