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821.
In arid lands, food resources and predation risk become major decision factors in mule deer habitat use. We examined variables related to these two factors that might determine mule deer (Odocoileus hemionus) habitat use in a warm desert at two spatial scales: the macro-habitat scale, which relates to home range decisions, and the micro-habitat scale, which relates to decisions made during foraging. Our prediction was that habitat use should primarily be explained by food variables and, to a lesser degree, by variables related to predation risk by pumas (Puma concolor). We mapped all deer pellet groups in 72, 1 ha quadrats in the Mapimí Biosphere Reserve and measured food and habitat variables over two years with different precipitation amounts. The area was used more intensively during the dry year. Under drier conditions, deer habitat use was explained primarily by food resource variables and secondarily by variables related with predation risk. During the wetter year, food resources became unimportant while cover and visibility explained deer habitat use. The data suggest that the tradeoffs deer are willing to make between food resources and predation risk in an arid environment are strongly affected by food resource levels, as influenced by precipitation. 相似文献
822.
Characteristics of Australian droughts under enhanced greenhouse conditions: Results from 14 global climate models 总被引:3,自引:0,他引:3
This paper presents characteristics of droughts simulated by global climate models (GCMs) under enhanced greenhouse gases conditions. We used a drought index called the Reconnaissance Drought Index (RDI) which takes both rainfall and potential evapotranspiration into account to investigate variations of droughts among 12 regions in Australia. The RDI was applied to simulated climate variables from 14 GCMs performed for the IPCC 4th Assessment Report.The results show a general increase in drought areal extent and/or frequency for most regions. However, the increases are not significant over the North West, North Queensland, Queensland East Coast and Central Queensland. For most regions, the change beyond 2030 is larger than that prior to 2030, but the uncertainty in the projections also increases with time. By 2030, there is a likely (>66% probability) risk of twice or more drought affected area and/or twice as often drought frequency over South West Western Australia. By 2050, this will include the Murray-Darling Basin, South Australia and Victoria, and by 2070 this will extend to New South Wales and Tasmania. For North Queensland such a risk is unlikely (<33% probability) for the next 100 years. This information can be considered indicative in long-term planning focussing on sustainability. 相似文献
823.
Although ferns are able to colonize dry regions through specific adaptations, they are suggested to be rare in arid environments. By comparing recent observations in the South-Saharan Mountains with existing data from the whole Sahara, we re-evaluated the distribution of the understudied Saharan fern diversity. Observations in the Aïr Mountains (Niger) from 2003 to 2006 identified six ferns species, among which three were new records for the Aïr and for Niger. A total of 17 species are currently known from the Sahara. The desiccation tolerant Actiniopteris radiata and Cheilanthes coriacea were located in habitats with abiotic refuges providing shade, whereas the drought avoiding Ophioglossum polyphyllum was found in a sandy highland plain. All were growing in isolated populations at elevations above 1600 m. These results, combined with data from the Hoggar and the Tibesti mountains, indicate that ferns are more frequent than initially thought in arid environments thanks to efficient dispersal, elevation refuges, physiological adaptations, and the presence of local abiotic refuges. The three newly recorded species most likely radiated from the Ethiopian highlands. Their deep isolation, their rarity, and their successful adaptation to drought suggest that their conservation status might be particularly high. 相似文献
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物态方程一直是恒星模型理论研究中的最重要的物理方程之一,对于日震学等高精度要求的研究领域来讲,只包含理想效应的物态方程并不能满足研究需要,而要更细致、包含非理想效应的物态方程对系统作更精确和完善的描述,在弱耦合及弱电子简并的系统中,为研究各种带电粒子之间的库仑耦合效应,以化学图像为基础,分别用具有硬核改正的Debye—Hueckel扩展理论来处理离子-电子作用、多体作用的经典点粒子相互作用理论描述离子-离子作用和量子统计方法处理电子-电子作用,并把此3种作用对物态方程自由能的贡献用半解析的数学公式给出,为适应日震学研究的要求,特别选取一组类太阳的恒星模型作为研究对象,从实际模型计算出发,对库仑耦合中的各种效应进行了比较分析,讨论并检验了所作的理论改进。 相似文献
827.
本文通过成都城市发展前后气候要素变化的对比分析,探索了城市化对成都市气候影响的基本规律,并用统计方法得出了城市“热岛”强度(△T,△Tmin)与城市人口总户数及建成区面积密切相关的客观事实,为改善和治理成都城市气候提供了一定的依据。 相似文献
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Han Chun-hao 《Chinese Astronomy and Astrophysics》1992,16(4):457-464
In this paper we derive the post-newtonian expressions for the VLBI time delay and gravitational delay in the barycentric coordinate system of the solar system. We discuss the effect of the various bodies and their range of action. From the transformation between the barycentric and the geocentric systems we then give the VLBI observational model in the geocentric system. Our final results are given by formula (16) for the gravitational delay and by formula (25) for the VLBI time delay. 相似文献