全文获取类型
收费全文 | 2951篇 |
免费 | 670篇 |
国内免费 | 1124篇 |
专业分类
测绘学 | 141篇 |
大气科学 | 1492篇 |
地球物理 | 907篇 |
地质学 | 795篇 |
海洋学 | 617篇 |
天文学 | 66篇 |
综合类 | 136篇 |
自然地理 | 591篇 |
出版年
2024年 | 22篇 |
2023年 | 41篇 |
2022年 | 105篇 |
2021年 | 149篇 |
2020年 | 132篇 |
2019年 | 162篇 |
2018年 | 139篇 |
2017年 | 156篇 |
2016年 | 191篇 |
2015年 | 186篇 |
2014年 | 218篇 |
2013年 | 257篇 |
2012年 | 228篇 |
2011年 | 205篇 |
2010年 | 166篇 |
2009年 | 217篇 |
2008年 | 221篇 |
2007年 | 275篇 |
2006年 | 229篇 |
2005年 | 184篇 |
2004年 | 178篇 |
2003年 | 152篇 |
2002年 | 133篇 |
2001年 | 112篇 |
2000年 | 102篇 |
1999年 | 79篇 |
1998年 | 90篇 |
1997年 | 82篇 |
1996年 | 82篇 |
1995年 | 61篇 |
1994年 | 47篇 |
1993年 | 33篇 |
1992年 | 25篇 |
1991年 | 15篇 |
1990年 | 20篇 |
1989年 | 7篇 |
1988年 | 19篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 3篇 |
排序方式: 共有4745条查询结果,搜索用时 702 毫秒
81.
Representing environments in flux: case studies from East Africa 总被引:1,自引:0,他引:1
The dominant view in the ecology and anthropology of the 1950s saw populations harmoniously interacting in self-regulating systems; climax forests and stable societies were the ruling hypotheses. Now, however, ecology and social sciences are investigating nature and culture in flux. The flux paradigms of nature and culture describe a human–ecological relationship that is non-equilibrial, historically contingent and constantly negotiated at both material and ideological levels by unequal actors. In this paper, we examine the effect of changing ecological and cultural paradigms on interpretations of environmental change in three areas of East Africa: the North Pare Mountains, Tanzania, the Mkomazi Game Reserve, Tanzania and the Tsavo National Park, Kenya. We explore how discursive and materialist approaches can complement one another, by expanding the domains of ecological inquiry and demanding that analysts cross-check their data for unquestioned assumptions regarding stability, variability and spatial and temporal scales. Rather than testing a ruling hypothesis, we suggest that ecologists and social scientists work with multiple hypotheses, with the aim of understanding the interplay between ecological, environmental and social influences. 相似文献
82.
Exploring some relationships between biological soil crusts, soil aggregation and wind erosion 总被引:5,自引:0,他引:5
A portable wind tunnel was used to test the contribution of biological and physical elements to overall soil aggregation on a soil dominated by biological soil crusts in south-eastern Australia. After moderate disturbance and simulated wind erosion, 90% of surface aggregates on the loamy soil and 76% on the sandy soil were dominated by biological elements (cryptogams). Lower levels of biological bonding were observed on the severely disturbed treatment. Linear regression indicated a significant positive relationship (r2=0·72) between biological soil crust cover and dry aggregation levels greater than 0·85mm. To maintain sediment transport below an erosion control target of 5gm−1s−1 for a 65kmh−1 wind at 10m height, a crust cover of approximately 20% is required. When a multiple regression model which sequentially fitted biological crust cover and dry aggregation greater than 0·85mm was applied to the data, dry aggregation accounted for more of the variation in sediment transport rate than biological crust cover. These data were used to develop a conceptual model which integrates crust cover and dry aggregation, and provides a useful framework within which to predict the likely impacts of changes in soil crust cover and aggregation. 相似文献
83.
84.
C Werner G Chiodini D Voigt S Caliro R Avino M Russo T Brombach J Wyngaard S Brantley 《Earth and Planetary Science Letters》2003,210(3-4):561-577
An eddy covariance (EC) station was deployed at Solfatara crater, Italy, June 8–25, 2001 to assess if EC could reliably monitor CO2 fluxes continuously at this site. Deployment at six different locations within the crater allowed areas of focused gas venting to be variably included in the measured flux. Turbulent (EC) fluxes calculated in 30-min averages varied between 950 and 4460 g CO2 m−2 d−1; the highest measurements were made downwind of degassing pools. Comparing turbulent fluxes with chamber measurements of surface fluxes using footprint models in diffuse degassing regions yielded an average difference of 0% (±4%), indicating that EC measurements are representative of surface fluxes at this volcanic site. Similar comparisons made downwind of degassing pools yielded emission rates from 12 to 27 t CO2 d−1 for these features. Reliable EC measurements (i.e. measurements with sufficient and stationary turbulence) were obtained primarily during daytime hours (08:00 and 20:00 local time) when the wind speed exceeded 2 m s−1. Daily average EC fluxes varied by ±50% and variations were likely correlated to changes in atmospheric pressure. Variations in CO2 emissions due to volcanic processes at depth would have to be on the same order of magnitude as the measured diurnal variability in order to be useful in predicting volcanic hazard. First-order models of magma emplacement suggest that emissions could exceed this rate for reasonable assumptions of magma movement. EC therefore provides a useful method of monitoring volcanic hazard at Solfatara. Further, EC can monitor significantly larger areas than can be monitored by previous methods. 相似文献
85.
青藏铁路管道通风试验路基地温变化及热状况分析 总被引:3,自引:6,他引:3
基于青藏铁路北麓河试验段管道通风路基在2个冻融循环周期内的地温监测资料,分析了路基温度的发展、温度场分布特征及多年冻土的热流量变化.结果表明:通风管埋设于路堤中部的路基温度变化和发展情况与一般路基类似,路基在施工后的2个冻融周期内仍处于整体升温的过程;通风管埋设于路堤下部的路基,虽然前2个冻融循环周期内土体温度与原始状态相比同样有所升高,但开始出现逐渐降低的趋势,同时地温场的分布在横向上的对称性也比较好,在热交换方面,一般填土路基和通风管位于路堤中部的路基在施工后的前2个冻融循环周期内一直处于吸热过程,而通风管位于路堤下部的路基在经历了第1个周期的持续吸热过程后,在第2个冻融循环周期内已经开始放热。 相似文献
86.
87.
88.
Philippe Machetel 《Comptes Rendus Geoscience》2003,335(1):91-97
The numerical models of mantle convection agree to depict avalanches behaviour according to the level of endothermicity of the spinel → perovskite phase change. Their potential effects on the global thermal and dynamical states of the mantle have been computed thanks to a numerical code, which takes into account both the 400-km exothermic and the 660-km endothermic phase changes. The cycle followed by the avalanches is: local layering, destabilization of the 660-km thermal layer, travelling and spreading on the core, and reappearing of the local layering. Therefore, mantle convection is characterized by quiet periods of partial layering embedded in catastrophic events. During the avalanche, the amplitude of the surface velocity is multiplied by two, which would imply an enhanced plate tectonic and ridge activities. The global thermal effects of the avalanche are compatible with a high mantle temperature and an acceleration of Earth's rotation during the Cretaceous. They also offer a coherent explanation to locate the origin of mantle plumes both within the CMB and just below the transition zone. 相似文献
89.
90.
1998年 11月~ 1999年 10月间在中国科学院红壤站 (江西鹰潭 )农田小气候站进行了大气二氧化硫 (SO2 )、硫酸盐粒子 (SO2 -4)浓度采样和雨水样本收集 ,利用阻力模式和全年逐时气象资料计算 SO2 和 SO2 -4的干沉降速度 ,估算干沉降通量 ,利用降水资料和雨水中硫酸根离子浓度估计大气硫的湿沉降 ,从而定量研究大气硫沉降输入农田生态系统的通量 ,结果表明 :农田下垫面上 SO2 和 SO2 -4干沉降速度的年平均值分别是 0 .373± 0 .170 cm· s-1(月均值 0 .16 1~ 0 .5 45 cm·s-1)、0 .198± 0 .12 3cm· s-1(月均值 0 .15 2~ 0 .2 6 9cm· s-1)。农田下垫面硫年总沉降量为 10 .3g· m-2 ,其中干沉降占总沉降的 83.3%。硫的干沉降又以 SO2 的干沉降为主 ,占年干沉降总量的 92 .2 %。大气硫沉降输入占农田生态系统输入总量的 90 %以上 ,是农田生态系统获取硫素的一个重要途径 相似文献