全文获取类型
收费全文 | 1262篇 |
免费 | 43篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 40篇 |
大气科学 | 105篇 |
地球物理 | 265篇 |
地质学 | 403篇 |
海洋学 | 166篇 |
天文学 | 218篇 |
自然地理 | 110篇 |
出版年
2021年 | 12篇 |
2020年 | 16篇 |
2019年 | 14篇 |
2018年 | 19篇 |
2017年 | 34篇 |
2016年 | 34篇 |
2015年 | 27篇 |
2014年 | 38篇 |
2013年 | 65篇 |
2012年 | 32篇 |
2011年 | 81篇 |
2010年 | 48篇 |
2009年 | 56篇 |
2008年 | 57篇 |
2007年 | 55篇 |
2006年 | 57篇 |
2005年 | 37篇 |
2004年 | 54篇 |
2003年 | 38篇 |
2002年 | 42篇 |
2001年 | 37篇 |
2000年 | 26篇 |
1999年 | 26篇 |
1998年 | 20篇 |
1997年 | 18篇 |
1996年 | 21篇 |
1995年 | 16篇 |
1994年 | 22篇 |
1993年 | 14篇 |
1992年 | 11篇 |
1991年 | 20篇 |
1990年 | 15篇 |
1989年 | 14篇 |
1988年 | 13篇 |
1987年 | 15篇 |
1986年 | 11篇 |
1985年 | 33篇 |
1984年 | 26篇 |
1983年 | 14篇 |
1982年 | 14篇 |
1981年 | 13篇 |
1979年 | 15篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1974年 | 6篇 |
1973年 | 5篇 |
1971年 | 5篇 |
1970年 | 5篇 |
排序方式: 共有1307条查询结果,搜索用时 515 毫秒
71.
72.
Drivers of ecosystem and climate change in tropical West Africa over the past ∼540 000 years 下载免费PDF全文
73.
Yan Gao Adrian Ghilardi Jaime Paneque-Galvez Margaret Skutsch Jean François Mas 《国际地球制图》2016,31(9):1019-1031
This study assesses whether MODIS Vegetation Continuous Fields percent tree cover (PTC) data can detect deforestation and forest degradation. To assess the usefulness of PTC for detecting deforestation, we used a data set consisting of eight forest and seven non-forest categories. To evaluate forest degradation, we used data from two temperate forest types in three conservation states: primary (dense), secondary (moderately degraded) and open (heavily degraded) forest. Our results show that PTC can differentiate temperate forest from non-forest categories (p = 0.05) and thus suggests PTC can adequately detect deforestation in temperate forests. In contrast, single-date PTC data does not appear to be adequate to detect forest degradation in temperate forests. As for tropical forest, PTC can partially discriminate between forest and non-forest categories. 相似文献
74.
Land managers responsible for invasive species removal in the USA require tools to prevent the Asian longhorned beetle (Anoplophora glabripennis) (ALB) from decimating the maple-dominant hardwood forests of Massachusetts and New England. Species distribution models (SDMs) and spread models have been applied individually to predict the invasion distribution and rate of spread, but the combination of both models can increase the accuracy of predictions of species spread over time when habitat suitability is heterogeneous across landscapes. First, a SDM was fit to 2008 ALB presence-only locations. Then, a stratified spread model was generated to measure the probability of spread due to natural and human causes. Finally, the SDM and spread models were combined to evaluate the risk of ALB spread in Central Massachusetts in 2008–2009. The SDM predicted many urban locations in Central Massachusetts as having suitable environments for species establishment. The combined model shows the greatest risk of spread and establishment in suitable locations immediately surrounding the epicentre of the ALB outbreak in Northern Worcester with lower risk areas in suitable locations only accessible through long-range dispersal from access to human transportation networks. The risk map achieved an accuracy of 67% using 2009 ALB locations for model validation. This model framework can effectively provide risk managers with valuable information concerning the timing and spatial extent of spread/establishment risk of ALB and potential strategies needed for effective future risk management efforts. 相似文献
75.
76.
Sandy beach surf zones serve as alternative nursery habitats for juvenile Chinook salmon (0 age) during their early marine residency, a period considered critical due to high and variable mortality rates. Despite the importance of early marine residence, the extent of juvenile salmon surf zone use and movement along sandy beaches is not well understood. Juvenile Chinook salmon distribution and movement were studied in shallow surf zone habitats by sampling from 2006 to 2010 with a beach seine 11 beaches adjacent and distant to four estuary mouths in Oregon and Washington, USA. The estuary of origin of each juvenile was determined using genetic stock identification methods and coded wire tags. Surf zones sampled were within littoral cells, which are stretches of the coastline bordered by rocky headlands, and included estuaries with and without Chinook salmon populations. Juvenile salmonids were only collected at littoral cells with Chinook-inhabited watersheds. Most juveniles (95 %) were present at sandy beaches adjacent (<500 m from estuary mouth) to their estuary of origin. Few Chinook salmon (5 %) were collected at littoral cells that contained non-natal estuaries. These results indicate that juvenile Chinook salmon inhabiting surf zones mostly use beaches adjacent to their estuaries of origin, but some juveniles may reside in beaches distant from their point of ocean entry. 相似文献
77.
L. W. HardingJr. C. L. Gallegos E. S. Perry W. D. Miller J. E. Adolf M. E. Mallonee H. W. Paerl 《Estuaries and Coasts》2016,39(3):664-681
Climate effects on hydrology impart high variability to water-quality properties, including nutrient loadings, concentrations, and phytoplankton biomass as chlorophyll-a (chl-a), in estuarine and coastal ecosystems. Resolving long-term trends of these properties requires that we distinguish climate effects from secular changes reflecting anthropogenic eutrophication. Here, we test the hypothesis that strong climatic contrasts leading to irregular dry and wet periods contribute significantly to interannual variability of mean annual values of water-quality properties using in situ data for Chesapeake Bay. Climate effects are quantified using annual freshwater discharge from the Susquehanna River together with a synoptic climatology for the Chesapeake Bay region based on predominant sea-level pressure patterns. Time series of water-quality properties are analyzed using historical (1945–1983) and recent (1984–2012) data for the bay adjusted for climate effects on hydrology. Contemporary monitoring by the Chesapeake Bay Program (CBP) provides data for a period since mid-1984 that is significantly impacted by anthropogenic eutrophication, while historical data back to 1945 serve as historical context for a period prior to severe impairments. The generalized additive model (GAM) and the generalized additive mixed model (GAMM) are developed for nutrient loadings and concentrations (total nitrogen—TN, nitrate?+?nitrate—NO2?+?NO3) at the Susquehanna River and water-quality properties in the bay proper, including dissolved nutrients (NO2?+?NO3, orthophosphate—PO4), chl-a, diffuse light attenuation coefficient (K D (PAR)), and chl-a/TN. Each statistical model consists of a sum of nonlinear functions to generate flow-adjusted time series and compute long-term trends accounting for climate effects on hydrology. We present results identifying successive periods of (1) eutrophication ca. 1945–1980 characterized by approximately doubled TN and NO2?+?NO3 loadings, leading to increased chl-a and associated ecosystem impairments, and (2) modest decreases of TN and NO2?+?NO3 loadings from 1981 to 2012, signaling a partial reversal of nutrient over-enrichment. Comparison of our findings with long-term trends of water-quality properties for a variety of estuarine and coastal ecosystems around the world reveals that trends for Chesapeake Bay are weaker than for other systems subject to strenuous management efforts, suggesting that more aggressive actions than those undertaken to date will be required to counter anthropogenic eutrophication of this valuable resource. 相似文献
78.
Large amounts of digital data must be analyzed and integrated to generate mineral potential maps, which can be used for exploration targeting. The quality of the mineral potential maps is dependent on the quality of the data used as inputs, with higher quality inputs producing higher quality outputs. In mineral exploration, particularly in regions with little to no exploration history, datasets are often incomplete at the scale of investigation with data missing due to incomplete mapping or the unavailability of data over certain areas. It is not always clear that datasets are incomplete, and this study examines how mineral potential mapping results may differ in this context. Different methods of mineral potential mapping provide different ways of dealing with analyzing and integrating incomplete data. This study examines the weights of evidence (WofE), evidential belief function and fuzzy logic methods of mineral potential mapping using incomplete data from the Carajás mineral province, Brazil to target for orogenic gold mineralization. Results demonstrate that WofE is the best one able to predict the location of known mineralization within the study area when either complete or unacknowledged incomplete data are used. It is suggested that this is due to the use of Bayes’ rule, which can account for “missing data.” The results indicate the effectiveness of WofE for mineral potential mapping with incomplete data. 相似文献
79.
Kenneth A. Rose Sean Creekmore Peter Thomas J. Kevin Craig Md Saydur Rahman Rachael Miller Neilan 《Estuaries and Coasts》2018,41(1):233-254
We developed a spatially explicit, individual-based model to analyze how hypoxia effects on reproduction, growth, and mortality of Atlantic croaker in the northwestern Gulf of Mexico lead to population-level responses. The model follows the hourly growth, mortality, reproduction, and movement of individuals on a 300 × 800 spatial grid of 1-km2 cells for 140 years. Chlorophyll-a concentration, water temperature, and dissolved oxygen (DO) were specified daily for each grid cell and repeated for each year of the simulation. A bioenergetics model was used to represent growth, mortality was assumed stage- and age-dependent, and the movement behavior of juveniles and adults was modeled based on temperature and avoidance of low DO. Hypoxia effects were imposed using exposure effect submodels that converted time-varying exposures to low DO to reduced hourly growth, increased hourly mortality, and reduced annual fecundity. Results showed that 100 years of either mild or intermediate hypoxia produced small reductions in population abundance, while repeated severe hypoxia caused a 19% reduction in long-term population abundance. Relatively few individuals were exposed to low DO each hour, but many individuals experienced some exposure. The response was dominated by a 5% average reduction in annual fecundity of individuals. Under conditions of random sequences of mild, intermediate, and severe hypoxia years occurring in proportion to their historical frequency, the model predicted a 10% decrease in the long-term population abundance of croaker. A companion paper substitutes hourly DO values from a three-dimensional water quality model for the idealized hypoxia and results in a more realistic population reduction of about 25%. 相似文献
80.
M.?V.?LuchitskayaEmail author S.?D.?Sokolov V.?Pease E.?Miller B.?V.?Belyatsky 《Geotectonics》2018,52(3):312-330
New geochronological and isotopic geochemical data are given, which make it possible to recognize two types of granitic rocks on the eastern Chukchi Peninsula. Early Cretaceous Tkachen and Dolina granitic plutons with zircon ages (U–Pb SIMS) of 119–122 and 131–136 Ma are related to the first type. They cut through Devonian–Lower Carboniferous basement rocks and are overlain by the Aptian–Albian Etelkuyum Formation. Basal units of the latter contain fragments of granitic rocks. Late Cretaceous Provideniya and Rumilet granitic plutons, which contain zircons with ages of 94 and 85 Ma (U–Pb SIMS), respectively, belong to the second type. They cut through volcanic–sedimentary rocks of the Etelkuyum and Leurvaam formations pertaining to the Okhotsk–Chukotka Volcanic Belt. In petrographic and geochemical features, the Early Cretaceous granitic rocks of the Tkachen Pluton are commensurable with I-type granites, while Late Cretaceous granite of the Rumilet Pluton is comparable to A2-type granite. The Sr–Nd isotopic data provide evidence that from the Early Cretaceous Tkachen and Dolina plutons to the Late Cretaceous Provideniya and Rumilet plutons, the degree of crustal assimilation of suprasubduction mantle-derived melts increases up to partial melting of heterogeneous continental crust enriched in rubidium. An unconformity and various degrees of secondary alteration of volcanic–sedimentary rocks have been established in the Okhotsk–Chukotka Volcanic Belt, and this was apparently caused by transition of the tectonic setting from suprasubduction to a transform margin with local extension. 相似文献