全文获取类型
收费全文 | 6031篇 |
免费 | 180篇 |
国内免费 | 62篇 |
专业分类
测绘学 | 123篇 |
大气科学 | 536篇 |
地球物理 | 1378篇 |
地质学 | 2017篇 |
海洋学 | 596篇 |
天文学 | 961篇 |
综合类 | 14篇 |
自然地理 | 648篇 |
出版年
2021年 | 45篇 |
2020年 | 65篇 |
2019年 | 74篇 |
2018年 | 111篇 |
2017年 | 119篇 |
2016年 | 139篇 |
2015年 | 107篇 |
2014年 | 130篇 |
2013年 | 321篇 |
2012年 | 177篇 |
2011年 | 260篇 |
2010年 | 239篇 |
2009年 | 240篇 |
2008年 | 246篇 |
2007年 | 209篇 |
2006年 | 229篇 |
2005年 | 183篇 |
2004年 | 186篇 |
2003年 | 172篇 |
2002年 | 166篇 |
2001年 | 118篇 |
2000年 | 109篇 |
1999年 | 106篇 |
1998年 | 98篇 |
1997年 | 85篇 |
1996年 | 89篇 |
1995年 | 94篇 |
1994年 | 86篇 |
1993年 | 76篇 |
1992年 | 88篇 |
1991年 | 71篇 |
1990年 | 100篇 |
1989年 | 86篇 |
1988年 | 79篇 |
1987年 | 101篇 |
1986年 | 76篇 |
1985年 | 96篇 |
1984年 | 132篇 |
1983年 | 109篇 |
1982年 | 100篇 |
1981年 | 101篇 |
1980年 | 91篇 |
1979年 | 101篇 |
1978年 | 73篇 |
1977年 | 87篇 |
1976年 | 71篇 |
1975年 | 71篇 |
1974年 | 59篇 |
1973年 | 63篇 |
1972年 | 35篇 |
排序方式: 共有6273条查询结果,搜索用时 234 毫秒
981.
982.
Metals in Sediments of the Upper Laguna Madre 总被引:2,自引:0,他引:2
Virender K Sharma Kurtis B Rhudy Robert Koening Felipe G Vazquez 《Marine pollution bulletin》1999,38(12):1213-1226
The Laguna Madre system is the largest hypersaline coastal basin in the United States. Surface sediments from 22 Upper Laguna Madre (ULM) sites were analyzed for grain size and metals (Cd, Cu, Ba, Cr, Mn, Ni, Pb, V, Zn, Fe, and Al) to assess the extent of contamination in the area. Sediments were found to consist mainly of sand texture. Clay and silts were minor constituents (<10%) of the sediments. Anomalies in metal concentrations were found at some sites and were related to probable sources, i.e., recreational and industrial activities. Concentrations of metals were normalized to grain size, Al, and Fe to distinguish natural and anthropogenic sources. Most metals showed positive correlations (p < 0.001) with Fe and Al, suggesting a natural variability of metal concentrations in sediments. Concentrations of metals, except Cd and Pb, at most sites were found to be below threshold concentrations thought to produce toxic effects in marine and estuarine organisms. 相似文献
983.
Gary Bickford John Toll Judi Hansen Elaine Baker Robert Keessen 《Marine pollution bulletin》1999,39(1-12):335-345
Sydney Water has completed a risk assessment to assess the risks to human health and aquatic organisms in creeks, rivers, estuaries and ocean waters affected by wet weather sewage overflows, stormwater and sewage treatment plant discharges. The risk assessment methodology consists of a comparison of measured and predicted concentrations of chemicals with toxicity reference values. Estimates of receiving water chemical concentrations were derived using data from a 10-year period so that the variable rainfall pattern was represented. Computer models were used to simulate and predict wet weather discharges during this ten year period. Risks were validated by bioassays and bioassessments. Risks to aquatic life from wet weather discharges were attributed to 14 chemicals at one or more of the sites and stormwater was the predominate source of the chemicals. There were no risks to people engaged in water based activities. Noncarcinogenic risks from fish ingestion are predicted at three sites. Predicted cancer risks for most individual chemicals were relatively small. Carcinogenic risks were typically associated with organochlorine compounds, polycyclic aromatic hydrocarbons, dichlorobenzene, and arsenic. The predicted cancer risks also appear to be largely the result of stormwater rather than sewage overflow inputs and largely due to historical contamination by organochlorine pesticides. It is expected the concentrations of these chemicals will decrease over time. 相似文献
984.
985.
Coupling land use allocation models with raster GIS 总被引:5,自引:0,他引:5
As geographic information systems (GIS) have moved from information storage and retrieval operations towards more decision
support functions, there is a need for more integration of spatial analytical modules that can assist in locational decisions.
This paper presents a methodology for coupling land use allocation models with a raster GIS. For raster systems, the integration
of any decision module has been limited by the size of raster datasets that may contain hundreds of thousands of pixels. Therefore,
decision heuristics have been used rather than exact methods such as mathematical programming models. For the problem of land
use allocation, the special structure of the generalized assignment problem is used here to handle large scale datasets. The
advantage of the mathematical programming approach is the additional information associated with the dual variables and opportunity
costs that can be used in subsequent sensitivity analyses.
Received: 7 April 1998/Accepted: 2 October 1998 相似文献
986.
Climate Change Impacts on the Potential Productivity of Corn and Winter Wheat in Their Primary United States Growing Regions 总被引:2,自引:0,他引:2
We calculate the impacts of climate effects inferred from three atmospheric general circulation models (GCMs) at three levels of climate change severity associated with change in global mean temperature (GMT) of 1.0, 2.5 and 5.0 °C and three levels of atmospheric CO2 concentration ([CO2]) – 365 (no CO2 fertilization effect), 560 and 750 ppm – on the potential production of dryland winter wheat (Triticum aestivum L.) and corn (Zea mays L.) for the primary (current) U.S. growing regions of each crop. This analysis is a subset of the Global Change Assessment Model (GCAM) which has the goal of integrating the linkages and feedbacks among human activities and resulting greenhouse gas emissions, changes in atmospheric composition and resulting climate change, and impacts on terrestrial systems. A set of representative farms was designed for each of the primary production regions studied and the Erosion Productivity Impact Calculator (EPIC) was used to simulate crop response to climate change. The GCMs applied were the Goddard Institute of Space Studies (GISS), the United Kingdom Meteorological Transient (UKTR) and the Australian Bureau of Meteorological Research Center (BMRC), each regionalized by means of a scenario generator (SCENGEN). The GISS scenarios have the least impact on corn and wheat production, reducing national potential production for corn by 6% and wheat by 7% at a GMT of 2.5 °C and no CO2 fertilization effect; the UKTR scenario had the most severe impact on wheat, reducing production by 18% under the same conditions; BMRC had the greatest negative impact on corn, reducing production by 20%. A GMT increase of 1.0°C marginally decreased corn and wheat production. Increasing GMT had a detrimental impact on both corn and wheat production, with wheat production suffering the greatest losses. Decreases for wheat production at GMT 5.0 and [CO2] = 365 ppm range from 36% for the GISS to 76% for the UKTR scenario. Increases in atmospheric [CO2] had a positive impact on both corn and wheat production. AT GMT 1.0, an increase in [CO2] to 560 ppm resulted in a net increase in corn and wheat production above baseline levels (from 18 to 29% for wheat and 2 to 5% for corn). Increases in [CO2] help to offset yield reductions at higher GMT levels; in most cases, however, these increases are not sufficient to return crop production to baseline levels. 相似文献
987.
Changes in Climate Extremes Over the Australian Region and New Zealand During the Twentieth Century 总被引:10,自引:0,他引:10
Neil Plummer M. James Salinger Neville Nicholls Ramasamy Suppiah Kevin J. Hennessy Robert M. Leighton Blair Trewin Cher M. Page Janice M. Lough 《Climatic change》1999,42(1):183-202
Analyses of high quality data show that there have been some interesting recent changes in the incidence of some climate extremes in the Australian region and New Zealand. 相似文献
988.
Cyrille Flamant Jacques Pelon Bart Brashers Robert A. Brown 《Boundary-Layer Meteorology》1999,93(1):47-73
The internal thermal boundary layer developing over the Mediterranean during a cold-air outbreak associated with a Tramontane event has been studied by means of airborne lidar, in situ sensors, and a modelling approach that consisted of nesting the University of Washington (UW) planetary boundary-layer (PBL) model in an advective zero-order jump model. This approach bypasses some of the deficiencies associated with each model: the absence of the dynamics in the mixed layer for the zero-order jump model and the lack of an inversion at the PBL top for the UW PBL model. Particular attention is given to the parameterization of the entrainment flux at the PBL top. Values of the entrainment closure parameter derived with the model when matching PBL structure observations are much lower than those derived with standard zero-order jump models. They also are in good agreement with values measured in different meteorological situations by other studies. This improvement is a result of the introduction of turbulent kinetic energy production in the mixed layer. 相似文献
989.
990.
Stratigraphic sections are often sampled at well-defined discrete points. Because of the incompleteness of the fossil record, a particular species may not be observed even when it is extant at a sampling point. We introduce a model and Bayesian analysis for estimating the true time of disappearance of a lineage from a section in the face of the possibility that failure to find the species beyond its observed stratigraphic range may represent false negatives. We incorporate proper prior information, including an estimated longevity of the species and the probability that it will be observed if extant. Our analysis produces a posterior density for the true extinction time of the species. Summaries of this probability distribution provide a point estimate of the extinction time, a standard deviation for the uncertainty in the estimate, and confidence intervals for the time of extinction. We apply our model to stratigraphic ranges of benthic foraminifera collected from the early Late Cretaceous (Cenomanian and Turonian) from Eastbourne, England. 相似文献