全文获取类型
收费全文 | 3208篇 |
免费 | 177篇 |
国内免费 | 31篇 |
专业分类
测绘学 | 69篇 |
大气科学 | 232篇 |
地球物理 | 916篇 |
地质学 | 1240篇 |
海洋学 | 335篇 |
天文学 | 425篇 |
综合类 | 15篇 |
自然地理 | 184篇 |
出版年
2022年 | 22篇 |
2021年 | 60篇 |
2020年 | 54篇 |
2019年 | 55篇 |
2018年 | 129篇 |
2017年 | 133篇 |
2016年 | 195篇 |
2015年 | 128篇 |
2014年 | 174篇 |
2013年 | 263篇 |
2012年 | 163篇 |
2011年 | 195篇 |
2010年 | 169篇 |
2009年 | 217篇 |
2008年 | 162篇 |
2007年 | 154篇 |
2006年 | 96篇 |
2005年 | 94篇 |
2004年 | 107篇 |
2003年 | 83篇 |
2002年 | 55篇 |
2001年 | 51篇 |
2000年 | 51篇 |
1999年 | 39篇 |
1998年 | 33篇 |
1997年 | 42篇 |
1996年 | 31篇 |
1995年 | 22篇 |
1994年 | 39篇 |
1993年 | 23篇 |
1992年 | 22篇 |
1991年 | 17篇 |
1990年 | 18篇 |
1989年 | 21篇 |
1988年 | 14篇 |
1987年 | 15篇 |
1986年 | 16篇 |
1985年 | 12篇 |
1984年 | 22篇 |
1983年 | 19篇 |
1982年 | 16篇 |
1980年 | 12篇 |
1979年 | 13篇 |
1978年 | 11篇 |
1977年 | 9篇 |
1975年 | 16篇 |
1974年 | 16篇 |
1973年 | 12篇 |
1972年 | 10篇 |
1971年 | 9篇 |
排序方式: 共有3416条查询结果,搜索用时 343 毫秒
941.
Great Lakes Hydrology Under Transposed Climates 总被引:3,自引:0,他引:3
Thomas E. Croley II Frank H. Quinn Kenneth E. Kunkel Stanley a. Changnon 《Climatic change》1998,38(4):405-433
Historical climates, based on 43 years of daily data from areas south and southwest of the Great Lakes, were used to examine the hydrological response of the Great Lakes to warmer climates. The Great Lakes Environmental Research Laboratory used their conceptual models for simulating moisture storages in, and runoff from, the 121 watersheds draining into the Great Lakes, over-lake precipitation into each lake, and the heat storages in, and evaporation from, each lake. This transposition of actual climates incorporates natural changes in variability and timing within the existing climate; this is not true for General Circulation Model-generated corrections applied to existing historical data in many other impact studies. The transposed climates lead to higher and more variable over-land evapotranspiration and lower soil moisture and runoff with earlier runoff peaks since the snow pack is reduced up to 100%. Water temperatures increase and peak earlier. Heat resident in the deep lakes increases throughout the year. Buoyancy-driven water column turnover frequency drops and lake evaporation increases and spreads more throughout the annual cycle. The response of runoff to temperature and precipitation changes is coherent among the lakes and varies quasi-linearly over a wide range of temperature changes, some well beyond the range of current GCM predictions for doubled CO2 conditions. 相似文献
942.
Thomas a. Kursar 《Climatic change》1998,39(2-3):363-379
Predicting future changes in tropical rainforest tree communities requires a good understanding of past changes as well as a knowledge of the physiology, ecology and population biology of extant species. Climate change during the next hundred years will be more similar to climate fluctuations that have occurred in the last few thousand years and of a much smaller magnitude than the extent of climate change experienced during last glaciation or at the Pleistocene–Holocene transition. Unfortunately, the extent to which tropical rainforest tree communities have changed during the last few thousand years has been little investigated. As a consequence we lack the detailed evidence for population and range shifts of individual tropical species resulting from climate change analogous to the evidence available for temperate zone forests. Some evidence suggests that the rate of tropical forest change in the last several thousand years may have been high. If so, then CO2 increases and the likely alterations in temperature, forest turnover rate, rainfall, or severe droughts may drive substantial future forest change. How can we predict or model the effects of climate change on a highly diverse tree community? Explanations for the regulation of tropical tree populations often invoke tree physiology or processes that are subject to physiological regulation such as herbivory, pathology or seed production. In order to incorporate such considerations into climate change models, the physiology of a very diverse tree community must be understood. My work has focused on simplifying this diversity by categorizing the shade-tolerant species into functional physiological groups. Most species and most individual trees are shade-tolerant species, gap-requiring species being relatively uncommon. Additionally, in a regenerating gap most of the individuals are shade-tolerant species that established before gap formation. Despite the fact that the shade-tolerant species are of major ecological importance, their comparative physiology has received little attention. I have found that shade-tolerant species differ substantially in their responses to light flecks, treefall light gaps and drought. Furthermore, among phylogenetically unrelated species, these differences in physiology can be predicted from leaf lifetime. These results provide a general framework for understanding the mechanics of tropical rainforests from a physiological perspective that can be used to model their responses to climate change. 相似文献
943.
Streamflows have a direct dependence on precipitation and these are directly linked to the climate. Then, in this paper the temporal climatic variability in the Río de la Plata Basin is analysed through the changes in the river's discharges. These are the reflection of the climatic inputs areally integrated, and in consequence, contain more information on climate variability than that provided by the scarce punctual records of precipitation and temperature. The time series of streamflows correspond to monthly and annual means in stations selected in the basin for the period 1931–1992. However, in the present paper, the period 1901–1992 was considered in all cases whenever possible. The following changes and tendencies in the flow series were detected: 1. An important change of tendency between 1970 and 1972, and another not so significant before that date were detected in 1917–1918 and 1943–1944. 2. The jumps in the means in several sub-periods were detected using different methods. They showed jumps mainly in the period 1970–1972 in the annual streamflows series. The jumps in the annual streamflow series consist of an abrupt change in climatic variables affecting temporarily the averages of such variables during a certain period of time (years). The results are consistent with the conclusions obtained by other authors for the same region, both in precipitation and in the general circulation of the atmosphere. Keeping in mind this analysis of the series of streamflows, indicators of normal variability of tendencies relative to natural regional causes were detected, although the local causes were not anthropogenically analysed, and so no other manifestations of randomless in the zone of the Basin under study because of the lacking of data. 相似文献
944.
Milan Burša Jan Kouba Karel Raděj Scott A. True Viliam Vatrt Marie Vojtíšková 《Studia Geophysica et Geodaetica》1998,42(4):459-466
The geopotential value of W
0
= (62 636 855.611 ± 0.008) m
2
s
–2
which specifies the equipotential surface fitting the mean ocean surface best, was obtained from four years (1993 - 1996) of TOPEX/POSEIDON altimeter data (AVISO, 1995). The altimeter calibration error limits the actual accuracy of W
0
to about (0.2 - 0.5) m
2
s
–2
(2 - 5) cm. The same accuracy limits also apply to the corresponding semimajor axis of the mean Earth's level ellipsoid a = 6 378 136.72 m (mean tide system), a = 6 378 136.62 m (zero tide system), a = 6 378 136.59 m (tide-free). The variations in the yearly mean values of the geopotential did not exceed ±0.025 m
2
s
–2
(±2.5 mm). 相似文献
945.
Robert a. Berner Ji-Long Rao Soobum Chang Rachel O'brien C. Kent Keller 《Aquatic Geochemistry》1998,4(2):273-290
Chemical analyses for major ions have been conducted on waters,collected on an approximately weekly basis over the period April, 1993 toNovember, 1996, that drain three small experimental ecosystems(sandboxes) at Hubbard Brook, New Hampshire. One sandbox is planted withpine trees, another with grass, and the third is left bare (actually itis covered sporadically by bryophytes and lichens). Results show linearcorrelations, independent of discharge, between the concentrations ofdissolved Na+ and K+ on the one hand andCa++ and Mg++ on the other for all threesandboxes. No correlations between singly charged and doubly chargedcations were found. These correlations are interpreted to represent cationexchange equilibria between soil waters and clay minerals plus soil organicmatter. The correlation slope, representing the exchange constant, for Na vsK is different for the pine-covered sandbox than for the other two whereasfor Ca vs Mg the correlation is independent of the presence or absence oftrees. We interpret this as representing a shift of cation exchangeequilibria in the pine sandbox by the activities of growing trees.Concentrations of Na, K, Ca, Mg, and H4SiO4from the barren and grass-lined sandboxes were found to vary seasonally witha marked sinusoidal pattern which was independent of the discharge from eachsandbox. (The discernment of a similar pattern in the tree lined sandbox wasdifficult due to a lack of discharge over much of the year.) Concentrationmaxima occurred in August and minima in February, and there is a closeparallelism with soil temperature. We interpret this as representingtemperature induced variations in cation exchange equilibria and silicaadsorption. Independence from highly varying water discharge, e.g.,. thataccompanying severe rainstorms, indicates rapidly re-attained equilibrium.Variations in the concentrations of cations are likely due to exchange withunmeasured cations, probably H+ or dissolved Al species, as aresult of possible seasonal changes in internal acid production and externalinput of acid rain to the sandboxes. Internal production may represent aresponse to seasonal changes in respiration rate as it responds toseasonally varying temperature. Added to this is the effect of temperatureon exchange equilibrium. Seasonal variations in dissolved silica are mostlikely due to the dependence of adsorption/desorption equilibria ontemperature. The temperature dependence of a number of silica-consumingreactions are consistent with the measured values. 相似文献
946.
R. J. García López 《Astrophysics and Space Science》1998,263(1-4):365-368
ODIN is an international consortium constituted to design and build a first-light multi-purpose optical intermediate dispersion
spectrograph for the Gran Telescopio Canarias (GTC, 10-m) to be installed at the Observatorio del Roque de los Muchachos (La
Palma). This instrument should fulfill an important fraction of the needs of the Spanish astronomical community in optical
spectroscopy performed at large telescopes, and must be highly competitive in a variety of research fields. We are proposing
a spectrograph that will be capable of providing multi-object observations by means of optical fibers, integral field spectroscopy,
and also multi-integral fields. It will be mounted at the Nasmyth focus of the GTC. One of the main drivers of our proposal
is to cover the whole visible range allowed by the atmospheric cut-off with the highest efficiency, particularly at the UV
end.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
947.
948.
Milan Burša 《Studia Geophysica et Geodaetica》1996,40(4):339-352
Summary The total decrease in the mechanical energy of the Solar System due to the tidal friction during its whole history has been estimated at 1·7 × 1033 kg m2 s–2 which is about 0·85% of its present energy integral. The main contributions come from Sun-Jupiter (63·4%), Sun-Saturn (26·6%), Sun-Neptune (5·9%), Sun-Uranus (3·7%), all satellite systems (0·2%), Sun-Venus (0·1%). 相似文献
949.
Prof. Dr. María E. de Civit Furlani Prof. Dr. María J. de Manchón Gutiérrez 《GeoJournal》1996,38(1):109-118
This article outlines three modalities of relation between literature and geography: the literary work as demonstrative element, the literary work as collective image, and the literary work as analogous process. The verification of the three modalities takes place through an approachment to the local literature using a geographical concept, namely region, and a particular area: the province of Mendoza, in western Argentina. Ten literary works of different genre were selected to which content analysis was applied. First the relevance of these representations at the initiation of geographical research is verified and then the way in which they stimulate the definition of problems and the formulation of hypotheses is addressed. 相似文献
950.
本文对南极磷虾复眼的直径与磷虾体长之间的关系、在负生长状态下磷虾复眼直径的变化等进行了研究。实验结果表明:在正常情况下(磷虾身体不出现负生长),磷虾复眼的直径与体长之间存在指数函数关系。雌性磷虾的眼径(ED)与体长(BL)之间的关系为:ED=0.5898e0.0282BL(r=0.9613,n=2419);雄虾的眼径与体长之间的关系为:ED=0.544e0.0309BL(r=0.9622,n=1920)。在相同体长的情况下,雄性磷虾的复眼直径比雌虾的大。从活体培养的结果来看,当南极磷虾出现负生长时,复眼直径保持不变,即眼径不受负生长的影响。在正常情况下(不存在负生长),雌虾的体长与眼径之间的比率(BL/ED)平均为21.32±0.04(n=1347),雄虾的体长与眼径之间的比率平均为20.68±0.07(n=748),未成体虾的体长与复眼直径之间的比率平均为21.54±0.06(n=1072)。利用体长与眼径之间的比率可以准确地将在实验室内活体培养后已存在负生长的磷虾与其它处于正常状态的磷虾分开。体长与眼径之间的比率也可以作为磷虾种群是否存在负生长的监测指标 相似文献