全文获取类型
收费全文 | 33183篇 |
免费 | 694篇 |
国内免费 | 346篇 |
专业分类
测绘学 | 688篇 |
大气科学 | 2698篇 |
地球物理 | 6867篇 |
地质学 | 11399篇 |
海洋学 | 3108篇 |
天文学 | 7034篇 |
综合类 | 102篇 |
自然地理 | 2327篇 |
出版年
2021年 | 251篇 |
2020年 | 295篇 |
2019年 | 336篇 |
2018年 | 694篇 |
2017年 | 650篇 |
2016年 | 790篇 |
2015年 | 623篇 |
2014年 | 874篇 |
2013年 | 1657篇 |
2012年 | 999篇 |
2011年 | 1415篇 |
2010年 | 1230篇 |
2009年 | 1649篇 |
2008年 | 1384篇 |
2007年 | 1386篇 |
2006年 | 1265篇 |
2005年 | 1075篇 |
2004年 | 1081篇 |
2003年 | 1035篇 |
2002年 | 970篇 |
2001年 | 847篇 |
2000年 | 801篇 |
1999年 | 709篇 |
1998年 | 699篇 |
1997年 | 670篇 |
1996年 | 531篇 |
1995年 | 508篇 |
1994年 | 450篇 |
1993年 | 401篇 |
1992年 | 378篇 |
1991年 | 352篇 |
1990年 | 368篇 |
1989年 | 341篇 |
1988年 | 287篇 |
1987年 | 392篇 |
1986年 | 315篇 |
1985年 | 415篇 |
1984年 | 480篇 |
1983年 | 454篇 |
1982年 | 404篇 |
1981年 | 389篇 |
1980年 | 388篇 |
1979年 | 335篇 |
1978年 | 376篇 |
1977年 | 316篇 |
1976年 | 318篇 |
1975年 | 322篇 |
1974年 | 276篇 |
1973年 | 275篇 |
1971年 | 185篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
961.
Richard Eckersley 《Global Change & Human Health》2002,3(1):55-57
Bjorn Lomborg's critique of environmentalism has been praised by The Economist as ‘right’ on its main points and ‘just’ in
its criticism of much green activism, and damned by Scientific American as a ‘failure’ in its purpose of describing the state
of the world. Most of the debate has centred on Lomborg's treatment of environmental issues. But the book is about more than
the environment, and its flaws extend beyond his use and interpretation of the data.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
962.
A probabilistic technique is developed for assessing water level in the mouth reach of the Ural River in the course of interaction between the river runoff and irregular rises in the Caspian Sea level (positive setups) at the predicted background sea levels of –26.5 and –26.0 m abs. 相似文献
963.
Mantle Convection Modeling with Viscoelastic/Brittle Lithosphere: Numerical Methodology and Plate Tectonic Modeling 总被引:1,自引:0,他引:1
--The earth's tectonic plates are strong, viscoelastic shells which make up the outermost part of a thermally convecting, predominantly viscous layer. Brittle failure of the lithosphere occurs when stresses are high. In order to build a realistic simulation of the planet's evolution, the complete viscoelastic/brittle convection system needs to be considered. A particle-in-cell finite element method is demonstrated which can simulate very large deformation viscoelasticity with a strain-dependent yield stress. This is applied to a plate-deformation problem. Numerical accuracy is demonstrated relative to analytic benchmarks, and the characteristics of the method are discussed. 相似文献
964.
Associations Between Rural Land Uses and Ground Water Quality in the Ogallala Aquifer, Northwest Texas 总被引:1,自引:0,他引:1
Paul F. Hudak 《Ground Water Monitoring & Remediation》2002,22(4):117-120
Recent nitrate, chloride, and bromide concentrations were studied in the Ogallala Aquifer of northwest Texas. The study included 361 wells with a median depth of 92 m in a rural area dominated by agricultural activity and oil and gas production. Only five observations surpassed the 44.3 mg/L standard for nitrate (10 mg/L NO3 -N). Four other observations, and one from the preceding set, exceeded the secondary standard of 250 mg/L for chloride. Maximum concentrations were 91.2 mg/L, 1530 mg/L, and 0.70 mg/L for nitrate, chloride, and bromide, respectively. Chloride/bromide ratios covered a broad range, from 30.4 to 10930, but medians were < 160 for each of two years analyzed. There were statistically significant correlations between nitrate and chloride, and chloride and well depth. Results of this study suggest that agricultural activity has locally impacted ground water in north-west Texas. Regionally, low aquifer recharge rates have curtailed ground water contamination from potentially adverse land uses. 相似文献
965.
v--vThis second paper (Part 2) pertaining to optimized site-specific threshold monitoring addresses the application of the method to regions covered by a teleseismic or a combined regional-teleseismic network. In the first paper (Part 1) we developed the method for the general case, and demonstrated its application to an area well-covered by a regional network (the Novaya Zemlya nuclear test site). In the present paper, we apply the method to the Indian and Pakistani nuclear test sites, and show results during the periods of nuclear testing by these two countries in May 1998. Since the coverage by regional stations in these areas is poor, an optimized approach requires the use of selected, high-quality stations at teleseismic distances.¶To optimize the threshold monitoring of these test sites, we use as calibration events either one of the nuclear explosions or a nearby earthquake. From analysis of the calibration events we derive values for array beamforming steering delays, filter bands, short-term averages (STA) lengths, phase travel times (P waves), and amplitude-magnitude relationships for each station. By applying these parameters, we obtain a monitoring capability of both test sites ranging from mb 2.8-3.0 using teleseismic stations only. When including the nearby Nilore station to monitor the Indian tests, we show that the threshold can be reduced by about 0.4 magnitude units. In particular, we demonstrate that the Indian tests on 13 May, 1998, which were not detected by any known seismic station, must have corresponded to a magnitude (mb) of less than 2.4.¶We also discuss the effect of a nearby aftershock sequence on the monitoring capability for the Pakistani test sites. Such an aftershock sequence occurred in fact on the day of the last Pakistani test (30 May, 1998), following a large (mb 5.5) earthquake in Afghanistan located about 1100 km from the test site. We show that the threshold monitoring technique has sufficient resolution to suppress the signals from these interfering aftershocks without significantly affecting the true peak of the nuclear explosion on the threshold trace. 相似文献
966.
967.
B.C. Papazachos A.S. Savvaidis C.B. Papazachos G.F. Karakaisis 《Journal of Seismology》2002,6(2):237-245
On the basis of growing evidence thatstrong earthquakes are preceded by a periodof accelerating seismicity of moderatemagnitude earthquakes, an attempt is madeto search for such seismicity pattern in NWAegean area. Accelerating seismic crustaldeformation has been identified in the areaof southern Albanides mountain range(border region between Greece, formerYugoslavia and Albania). Based on certainproperties of this activity and on itssimilarity with accelerating seismicdeformation observed before a strongearthquake which occurred in the sameregion on 26 May 1960 (M = 6.5), we canconclude that a similar earthquake may begenerated in the same region during thenext few years. This conclusion is inagreement with independent results whichhave been derived on the basis of the timepredictable model. 相似文献
968.
Byron C. Shumate Claire L. Schelske Thomas L. Crisman William F. Kenney 《Journal of Paleolimnology》2002,27(1):71-77
A paleolimnological evaluation of cladoceran microfossils was initiated to study limnological changes in Lake Apopka, a large (125 km2), shallow (mean depth = 1.6 m), warm, polymictic lake in central Florida. The lake switched from macrophyte to algal dominance in the late 1940s, creating a Sediment Discontinuity Layer (SDL) that can be visually used to separate sediments derived from macrophytes and phytoplankton. Cladoceran microfossils were enumerated as a means of corroborating extant eutrophication data from the sediment record. Inferences about the timing and trajectory of eutrophication were made using the cladoceran-based paleo-reconstruction. The cladoceran community of Lake Apopka began to change abruptly in both total abundance and relative percent abundance just before the lake shifted from macrophyte to algal dominance. Alona affinis, a mud-vegetation associated cladoceran, disappeared before the SDL was formed. Planktonic and benthic species also began to increase below the SDL, indicating an increase in production of both planktonic and benthic species. Chydorus cf. sphaericus, an indicator of nutrient loading, increased relative to all other cladocerans beginning in the layer below the SDL and continuing upcore. Changes in the transitional sediment layer formed before the lake switched to phytoplankton dominance, including an increase in total phosphorus concentration, suggest a more gradual eutrophication process than previously reported. Data from this study supported conclusions from other paleolimnological studies that suggested anthropogenic phosphorus loading was the key factor in the hypereutrophication of Lake Apopka. 相似文献
969.
970.