全文获取类型
收费全文 | 1060篇 |
免费 | 81篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 12篇 |
大气科学 | 112篇 |
地球物理 | 339篇 |
地质学 | 389篇 |
海洋学 | 75篇 |
天文学 | 150篇 |
综合类 | 8篇 |
自然地理 | 57篇 |
出版年
2024年 | 2篇 |
2023年 | 2篇 |
2022年 | 9篇 |
2021年 | 31篇 |
2020年 | 29篇 |
2019年 | 13篇 |
2018年 | 60篇 |
2017年 | 50篇 |
2016年 | 106篇 |
2015年 | 66篇 |
2014年 | 81篇 |
2013年 | 77篇 |
2012年 | 41篇 |
2011年 | 55篇 |
2010年 | 58篇 |
2009年 | 64篇 |
2008年 | 46篇 |
2007年 | 29篇 |
2006年 | 32篇 |
2005年 | 21篇 |
2004年 | 23篇 |
2003年 | 25篇 |
2002年 | 31篇 |
2001年 | 18篇 |
2000年 | 17篇 |
1999年 | 14篇 |
1998年 | 17篇 |
1997年 | 14篇 |
1996年 | 6篇 |
1995年 | 11篇 |
1994年 | 10篇 |
1993年 | 5篇 |
1992年 | 6篇 |
1991年 | 11篇 |
1990年 | 14篇 |
1989年 | 7篇 |
1988年 | 6篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1984年 | 3篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1978年 | 1篇 |
1975年 | 1篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1970年 | 1篇 |
1969年 | 2篇 |
1950年 | 1篇 |
排序方式: 共有1142条查询结果,搜索用时 31 毫秒
171.
Environmental factors influencing aggregation of manta rays (Manta birostris) off the northeastern coast of the Yucatan Peninsula
下载免费PDF全文
![点击此处可从《Marine Ecology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Ana Hacohen‐Domené Raúl O. Martínez‐Rincón Felipe Galván‐Magaña Natalí Cárdenas‐Palomo Jorge Herrera‐Silveira 《Marine Ecology》2017,38(3)
Manta rays inhabit tropical, subtropical and temperate waters. Aggregation sites of manta rays have been recognized worldwide, but the reasons for this behavior are still poorly understood. This study describes environmental factors influencing aggregation sites of the giant manta ray (Manta birostris) off the northeastern coast of the Yucatan Peninsula. Observations of manta rays were obtained from scientific surveys conducted during 2006–2011. Environmental data were obtained from satellite imagery. The maximum entropy (Maxent) method for habitat modeling was used to determine the effects of environmental conditions on the species and predict suitable habitat for manta rays in this region. Primary productivity and distance to the coast were the most influential variables, suggesting that aggregation occurs in highly productive coastal waters. The distribution of manta rays predicted by the Maxent model showed that the most suitable habitat within the study area is located off the northeastern coast of the Yucatán Peninsula, more precisely, northeast of Isla Holbox and northwest of Isla Contoy. Seasonal patterns of distribution suggest that the most suitable conditions are present from July through September. 相似文献
172.
I. Bejarano-Ramirez Jose M. Jurado R. Muñiz-Valencia Á. Alcázar Silvia G. Ceballos-Magaña A. Olivos-Ortiz O. Rangel 《International Journal of Environmental Science and Technology》2017,14(6):1333-1342
The contents of As, Cd, Cu, Cr, Mg, Mn, Ni, Pb and Zn have been determined in sediment and water samples from Valle de las Garzas estuary and Port Manzanillo (Colima, Mexico) using ICP-AES. The concentrations of these elements were used for a comparative study to determine the distribution of heavy metals and to evaluate which elements reflect natural or anthropogenic backgrounds. For this purpose, seven sampling points were selected: Four of them correspond to the lagoon, and three were situated in the port. Statistical analysis of the mineral content was assessed. Initially, data comparison was assessed by statistical tests for each variable. Principal component analysis was then applied considering the influence of all variables at the same time by obtaining the distribution of samples according to their scores in the principal component space. In this way, four studies were carried out: (1) study of sediments collected during the dry season; (2) study of sediments collected during the rainy season; (3) comparative study between sediments from rainy and dry season; and (4) study of water composition collected during rainy season. From the results of the performed analyses, it can be concluded that metals distribution pattern reflected natural and anthropogenic backgrounds (e.g., sediments from the lagoon, situated at the beginning of the rain channel, presented high contents of Zn and Cu, perhaps related to anthropogenic activities or the influence of igneous sediments). 相似文献
173.
174.
175.
176.
177.
178.
A new mass rig system is proposed to minimize the deficiencies in current shaking table testing setups. This is accomplished by placing the inertial mass on a convex path designed to impose P‐Delta demands on slender cantilever columns. The design and performance of the mass rig system, and the principles used in deriving the equations of motion and their analytical validation against results obtained from shaking table tests, are presented. Formulation of the governing equations of motion was based on Lagrangian mechanics and solved using an implicit linear acceleration method with an adaptive time step formulation. Friction developed in the sliding system was also incorporated in the equations of motion. Experimental results validated the accuracy in the derivation and solution of the equations of motion. Validated by analytical and experimental results, P‐Delta effects were found to increase the displacement demands on slender columns in the low‐frequency range of acceleration input, while in the high‐frequency range P‐Delta effects led to no increase and in some cases even a reduction in displacement demands. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
179.
The effect of El Niño Southern Oscillation cycles on the decadal scale suspended sediment behavior of a coastal dry‐summer subtropical catchment
下载免费PDF全文
![点击此处可从《地球表面变化过程与地形》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Andrew B. Gray Gregory B. Pasternack Elizabeth B. Watson Jonathan A. Warrick Miguel A. Goñi 《地球表面变化过程与地形》2015,40(2):272-284
Rivers display temporal dependence in suspended sediment–water discharge relationships. Although most work has focused on multi‐decadal trends, river sediment behavior often displays sub‐decadal scale fluctuations that have received little attention. The objectives of this study were to identify inter‐annual to decadal scale fluctuations in the suspended sediment–discharge relationship of a dry‐summer subtropical river, infer the mechanisms behind these fluctuations, and examine the role of El Niño Southern Oscillation climate cycles. The Salinas River (California) is a moderate sized (11 000 km2), coastal dry‐summer subtropical catchment with a mean discharge (Qmean) of 11.6 m3 s?1. This watershed is located at the northern most extent of the Pacific coastal North America region that experiences increased storm frequency during El Niño years. Event to inter‐annual scale suspended sediment behavior in this system was known to be influenced by antecedent hydrologic conditions, whereby previous hydrologic activity regulates the suspended sediment concentration–water discharge relationship. Fine and sand suspended sediment in the lower Salinas River exhibited persistent, decadal scale periods of positive and negative discharge corrected concentrations. The decadal scale variability in suspended sediment behavior was influenced by inter‐annual to decadal scale fluctuations in hydrologic characteristics, including: elapsed time since small (~0.1 × Qmean), and moderate (~10 × Qmean) threshold discharge values, the number of preceding days that low/no flow occurred, and annual water yield. El Niño climatic activity was found to have little effect on decadal‐scale fluctuations in the fine suspended sediment–discharge relationship due to low or no effect on the frequency of moderate to low discharge magnitudes, annual precipitation, and water yield. However, sand concentrations generally increased in El Niño years due to the increased frequency of moderate to high magnitude discharge events, which generally increase sand supply. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
180.
Assessment of evaporation and water fluxes in a column of dry saline soil subject to different water table levels 总被引:1,自引:0,他引:1
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
María Fernanda Hernández‐López Jorge Gironás Isabelle Braud Francisco Suárez José Francisco Muñoz 《水文研究》2014,28(10):3655-3669
Dry saline soils are common in the arid and hyper‐arid basins located in the Chilean Altiplano, where evaporation from shallow groundwater is typically the major component of the water balance. Thus, a good understanding of evaporation processes is necessary for improving water resource planning and management in these regions. In this study, we conducted laboratory experiments with a natural saline soil column to estimate evaporation rates and assess the liquid and water vapor fluxes under different water table levels. Water content, electrical conductivity and temperature at different depths were utilized to assess the liquid and water vapor fluxes in the soil column. We observed movement of water that dissolves salts from the soil and transports them to areas in the column where they accumulate. Isothermal liquid flux was predominant, while thermal and isothermal liquid and thermal water vapor fluxes were negligible, except for deep water table levels where isothermal and thermal water vapor fluxes had similar magnitude but opposite directions. Differences observed in total fluxes for all water table levels were due to different upward and downward fluxes, which depend on changes in water content and temperature within the soil profile. Both the vapor flux magnitude and direction were found to be very sensitive to the choice of empirical parameters used in flux quantification, such as tortuosity and the enhancement factor for local temperature gradients in the air phase within the column. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献