全文获取类型
收费全文 | 2340篇 |
免费 | 77篇 |
国内免费 | 31篇 |
专业分类
测绘学 | 54篇 |
大气科学 | 244篇 |
地球物理 | 531篇 |
地质学 | 749篇 |
海洋学 | 370篇 |
天文学 | 329篇 |
综合类 | 25篇 |
自然地理 | 146篇 |
出版年
2022年 | 21篇 |
2021年 | 18篇 |
2020年 | 30篇 |
2019年 | 36篇 |
2018年 | 68篇 |
2017年 | 56篇 |
2016年 | 113篇 |
2015年 | 54篇 |
2014年 | 100篇 |
2013年 | 139篇 |
2012年 | 104篇 |
2011年 | 96篇 |
2010年 | 113篇 |
2009年 | 114篇 |
2008年 | 105篇 |
2007年 | 97篇 |
2006年 | 108篇 |
2005年 | 82篇 |
2004年 | 75篇 |
2003年 | 56篇 |
2002年 | 47篇 |
2001年 | 42篇 |
2000年 | 35篇 |
1999年 | 38篇 |
1998年 | 28篇 |
1997年 | 20篇 |
1996年 | 37篇 |
1995年 | 23篇 |
1994年 | 19篇 |
1993年 | 20篇 |
1992年 | 17篇 |
1991年 | 19篇 |
1990年 | 15篇 |
1989年 | 12篇 |
1988年 | 17篇 |
1987年 | 20篇 |
1986年 | 14篇 |
1985年 | 33篇 |
1984年 | 31篇 |
1983年 | 38篇 |
1982年 | 37篇 |
1981年 | 33篇 |
1980年 | 22篇 |
1979年 | 37篇 |
1978年 | 30篇 |
1977年 | 18篇 |
1976年 | 22篇 |
1975年 | 18篇 |
1974年 | 25篇 |
1973年 | 32篇 |
排序方式: 共有2448条查询结果,搜索用时 343 毫秒
251.
Edward J. Phlips Susan Badylak Mary C. Christman Margaret A. Lasi 《Estuaries and Coasts》2010,33(2):498-512
This paper describes the results of 10 years of water quality monitoring in the Indian River Lagoon Florida, with special
emphasis on the relationships between trends in climatic conditions and the distribution, composition, and abundance of the
phytoplankton community. The Indian River Lagoon, which spans 220 km of Florida’s east coast, is a region of particular concern
because of the rapid rate of human development throughout the region and the hydrologically restricted character of the lagoon,
which heightens the potential for algal bloom. Water sampling was carried out on a monthly to twice-monthly basis at six sites
located in the northern and central lagoon. The 10-year study included both extended periods of below and above average rainfall.
A number of ecologically distinct regions exist within the lagoon, which differ considerably in water exchange properties
and watershed inputs. The northern lagoon is characterized by longer water residence times, lower phosphorus concentrations,
higher nitrogen concentrations, and more stable salinity conditions than the central lagoon. Mean phytoplankton biovolumes
were substantially higher at the sites in the northern lagoon than at the sites in the central lagoon, and algal blooms were
more common and intense in the former region. Inter-annual patterns of phytoplankton biovolume were also different in the
northern and central lagoon. In the northern lagoon, phytoplankton biovolumes were lowest during the drought period, from
the autumn of 1998 to the spring of 2001. By contrast, algal bloom events in the central lagoon were not only less frequent
but were not tied to periods of high rainfall. The most widespread and common bloom formers were the potentially toxic dinoflagellate
Pyrodinium bahamense var. bahamense and two centric diatoms, Dactyliosolen fragilissimus and Cerataulina pelagica. Many of the biovolume peaks observed over the study period were attributable to these three species. The results of time
series modeling of phytoplankton dynamics further highlighted the disparities between the two regions of the lagoon in terms
of the suite of parameters that best predict the observed trends in the biomass of phytoplankton. Overall, the outcome of
this initial modeling effort in the Indian River Lagoon suggests that time series approaches can help define the factors that
influence phytoplankton dynamics. 相似文献
252.
Gyesoon Park Samgyu Park Myeong-Jong Yi Hyoungrae Rim Seong-Jun Cho Jung-Ho Kim 《Environmental Earth Sciences》2010,60(5):965-974
Two-dimensional (2-D) electrical resistivity and three-dimensional (3-D) gravity explorations were undertaken to estimate
the 3-D distribution of karst cavities at the area of Yongweol-ri in Korea where ground subsidence has occurred. Although
the gravity method is a low-cost way of analyzing a 3-D continuous structure, its vertical resolving power is poor. In contrast,
the electrical resistivity method can provide a 2-D and/or 3-D subsurface structure with a much higher lateral and vertical
resolution than the gravity method. Accordingly, geostatistical methods and density information were used to enhance the 2-D
resistivity structure revealed by the electrical resistivity method into a 3-D structure. The assumptions are, first, that
each data set senses the same underlying geological structure in terms of different material properties and, secondly, that
two different material properties are correlated locally or globally throughout the entire target area. As a result, the distribution
of limestone cavities can be estimated under the assumption that they are mostly filled with groundwater and clayey soils
and have abnormally low levels of resistivity and density. The estimated distribution corresponds with the grouting, borehole
imaging and monitoring data. In this example, it can be seen that the integration analysis of 2-D electrical resistivity and
3-D gravity methods is a very powerful tool for 3-D subsurface imaging and that the method can provide enhanced imaging capabilities
for 3-D cavities. 相似文献
253.
Increasing recognition of the value of practice-based or experiential knowledge in natural resource management justifies the creation of a new category of articles in Society & Natural Resources that we are calling Practice-Based Knowledge (PBK). The rationale for focusing on PBK is due to its key role in the emergence of hybrid governance institutions across state, market, and civil society, understanding the complexity of dynamic socioecological systems, recognizing the challenges of multiple knowledge systems and context-specific practices, embracing the power of informal institutions and civic science, and engaging debates on the growing prevalence of market-oriented conservation. The goal is to provide a dedicated space within the published, peer-reviewed literature for scholars, government officials, nonprofit managers, and engaged citizens to share experiences informed by practical action. Relevant and timely practice-based insights may improve understanding and management of social and ecological processes and systems, while also offering the potential to contribute to theory. 相似文献
254.
The estuary can be considered as environments characterized by high productivity and uncommensurable ecological value. However, marine ecosystems are also subjected to high human pressures that increase their vulnerability. The aim of this work is to analyze grain size trend of sediment due to the action of the main hydrodynamic condition. The spatial distribution patterns of sediments were monitored according to season. The grain-size trend of surficial sediments revealed a dominant sediment transport trend towards the eastern and south eastern part of study area. This study shows that there is an agreement among sediments path ways, hydrodynamic condition and the sediment spatial distribution in the estuarine environment. The agitation by waves is an important sorting mechanism in the study area. The findings are based on the grain size trend and also corroborated by short term observations of the estuarine sediment dynamics and transport during the monsoon, postmonsoon, summer and premonson seasons in Tirumalairajan River estuary. 相似文献
255.
Hui Ding Richard J. Greatbatch Wonsun Park Mojib Latif Vladimir A. Semenov Xuguang Sun 《Climate Dynamics》2014,42(1-2):367-379
The variability of the East Asian summer monsoon (EASM) is studied using a partially coupled climate model (PCCM) in which the ocean component is driven by observed monthly mean wind stress anomalies added to the monthly mean wind stress climatology from a fully coupled control run. The thermodynamic coupling between the atmospheric and oceanic components is the same as in the fully coupled model and, in particular, sea surface temperature (SST) is a fully prognostic variable. The results show that the PCCM simulates the observed SST variability remarkably well in the tropical and North Pacific and Indian Oceans. Analysis of the rainfall-SST and rainfall-SST tendency correlation shows that the PCCM exhibits local air-sea coupling as in the fully coupled model and closer to what is seen in observations than is found in an atmospheric model driven by observed SST. An ensemble of experiments using the PCCM is analysed using a multivariate EOF analysis to identify the two major modes of variability of the EASM. The PCCM simulates the spatial pattern of the first two modes seen in the ERA40 reanalysis as well as part of the variability of the first principal component (correlation up to 0.5 for the model ensemble mean). Different from previous studies, the link between the first principal component and ENSO in the previous winter is found to be robust for the ensemble mean throughout the whole period of 1958–2001. Individual ensemble members nevertheless show the breakdown in the relationship before the 1980’s as seen in the observations. 相似文献
256.
Joshua W. Busby Kerry H. Cook Edward K. Vizy Todd G. Smith Mesfin Bekalo 《Climatic change》2014,124(4):717-731
Given its high dependence on rainfed agriculture and its comparatively low adaptive capacity, Africa is frequently invoked as especially vulnerable to climate change. Within Africa, there is likely to be considerable variation in vulnerability to climate change both between and within countries. This paper seeks to advance the agenda of identifying the hot spots of what we term “climate security” vulnerability, areas where the confluence of vulnerabilities could put large numbers of people at risk of death from climate-related hazards. This article blends the expertise of social scientists and climate scientists. It builds on a model of composite vulnerability that incorporates four “baskets” or processes that are thought to contribute to vulnerability including: (1) physical exposure, (2) population density, (3) household and community resilience, and (4) governance and political violence. Whereas previous iterations of the model relied on historical physical exposure data of natural hazards, this paper uses results from regional model simulations of African climate in the late 20th century and mid-21st century to develop measures of extreme weather events—dry days, heat wave events, and heavy rainfall days—coupled with an indicator of low-lying coastal elevation. For the late 20th century, this mapping process reveals the most vulnerable areas are concentrated in Chad, the Democratic Republic of the Congo, Niger, Somalia, Sudan, and South Sudan, with pockets in Burkina Faso, Ethiopia, Guinea, Mauritania, and Sierra Leone. The mid 21st century projection shows more extensive vulnerability throughout the Sahel, including Burkina Faso, Chad, Mali, northern Nigeria, Niger, and across Sudan. 相似文献
257.
In this study, we investigate the impact of atmospheric convection over the western tropical Pacific (100–145°E, 0–20°N) on the boreal winter North Pacific atmosphere flow by analyzing National Center for Environmental Prediction Reanalysis 1, Extended Reconstructed Sea Surface Temperature and Global Precipitation Climatology Project data. The western tropical Pacific convection is not only the main energy source driving the local Hadley and Walker circulations, but it also significantly influences North Pacific circulation, by modifying a mid-latitude Jet stream through the connection with the local Hadley circulation. On the one hand, this strong convection leads to a northward expansion of local Hadley cells simultaneous with a northward movement of the western North Pacific jet because of the close correlation between the Jet and Hadley circulation boundaries. On the other hand, this strong convection also intensifies tropical Pacific Walker circulation, which reduces the eastern Pacific sea surface temperature, resembling a La Nina state through the enhanced equatorial upwelling. The cooling of the eastern tropical Pacific has an inter-tropical convergence zone located further north; thus, the local Hadley circulation moves northward. As a result, the jet axis over the eastern North Pacific, which also corresponds to the boundary of the local Hadley circulation, moves to higher latitude. Consequently, this northward movement of the Jet axis over the North Pacific is reflected as a northwest–southeast dipole sea level pressure (SLP) pattern. The composite analysis of SLP over the North Pacific against the omega (Ω) (Pa/s) at 500 hPa over the western tropical Pacific actually reveals that this northwest-southeast dipole structure is attributed to the intensified tropical western Pacific convection, which pushes the Pacific Jet to the north. Finally we also analyzed south Pacific for the austral winter as did previously to North Pacific, and found that the results were consistent. 相似文献
258.
Corrected groundwater 14C ages from the carbonate aquifer in Yucca Flat at the former Nevada Test Site (now the Nevada National Security Site), USA, were evaluated by comparing temporal variations of groundwater 36Cl/Cl estimated with these 14C ages with published records of meteoric 36Cl/Cl variations preserved in packrat middens (piles of plant fragments, fecal matter and urine). Good agreement between these records indicates that the groundwater 14C ages are reasonable and that 14C is moving with chloride without sorbing to the carbonate rock matrix or fracture coatings, despite opposing evidence from laboratory experiments. The groundwater 14C ages are consistent with other hydrologic evidence that indicates significant basin infiltration ceased 8,000 to 10,000 years ago, and that recharge to the carbonate aquifer is from paleowater draining through overlying tuff confining units along major faults. This interpretation is supported by the relative age differences as well as hydraulic head differences between the alluvial and volcanic aquifers and the carbonate aquifer. The carbonate aquifer 14C ages suggest that groundwater velocities throughout much of Yucca Flat are about 2 m/yr, consistent with the long-held conceptual model that blocking ridges of low-permeability rock hydrologically isolate the carbonate aquifer in Yucca Flat from the outlying regional carbonate flow system. 相似文献
259.
Md. M. Rahman Z.-H. Shon C.-J. Ma R. J. C. Brown S. K. Pandey C. G. Park I. S. Bae J. R. Sohn H.-O. Yoon K.-H. Kim 《International Journal of Environmental Science and Technology》2014,11(7):1897-1910
In this work, 17-polychlorinated dibenzo-p-dioxin/furan (PCDD/Fs) isomers were measured in ambient air at four urban sites in Seoul, Korea (from February to June 2009). The concentrations of their summed values (ΣPCDD/Fs) across all four sites ranged from 1,947 (271 WHO05 TEQ) (Jong Ro) to 2,600 (349 WHO05 TEQ) fg/m3 (Yang Jae) with a mean of 2,125 (± 317) fg/m3 (292 WHO05 TEQ fg/m3). The sum values for the two isomer groups of ΣPCDD and ΣPCDF were 527 (30 WHO05 TEQ) and 1,598 (263 WHO05 TEQ) fg/m3, respectively. The concentration profile of individual species was dominated by the 2,3,4,7,8-PeCDF isomer, which contributed approximately 36 % of the ΣPCDD/Fs value. The observed temporal trends in PCDD/F concentrations were characterized by relative enhancement in the winter and spring. The relative contribution of different sources, when assessed by principal component analysis, is explained by the dominance of vehicular emissions along with coal (or gas) burning as the key source of ambient PCDD/Fs in the residential areas studied. 相似文献
260.
S. M. Kim T. I. Jang M. S. Kang S. J. Im S. W. Park 《Environmental Earth Sciences》2014,71(5):2155-2165
The objective of this study was to assess the lake sediment budget of land use changes using the Universal Soil Loss Equation (USLE), sediment delivery ratio (SDR), and trap efficiency (TE). The geographic information system was combined with the USLE to estimate the soil erosion of the Lake Asan watershed. Spatial data for each of the USLE factors were obtained from the land use, soil, and 1/25,000 scale digital contour maps. Landsat-5 TM images were selected for analyzing soil erosion changes due to land use changes. The sediment yield to Lake Asan was estimated using the SDR and TE. The estimated sediment budget was compared with observed data from the Lake Asan watershed between 1974 and 2003. The total estimated annual mean sediment budgets from Lake Asan in 1986, 1992, and 2000 were 0.267, 0.301, and 0.339 × 106 ton, respectively, with an average of 0.302 × 106 ton. The average measured sediment budget was 3.15 × 106 ton year?1. The average estimated value shows reasonable agreement with the observed sediment balance. The average estimated and measured sediment budgets contain uncertainties due to both the methods and the approach used by the observers. The simulated results indicated that soil erosion in the Lake Asan watershed increased at a rate of approximately 2 % per year from 1986 to 2000 due to land use change. This study may be useful for managers to identify reservoir rehabilitation management methods for stable irrigation water supply. 相似文献