首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   47292篇
  免费   1264篇
  国内免费   1049篇
测绘学   1158篇
大气科学   3303篇
地球物理   9515篇
地质学   17911篇
海洋学   4541篇
天文学   9783篇
综合类   303篇
自然地理   3091篇
  2022年   1033篇
  2021年   1465篇
  2020年   1395篇
  2019年   1461篇
  2018年   1363篇
  2017年   1161篇
  2016年   1169篇
  2015年   832篇
  2014年   1105篇
  2013年   2080篇
  2012年   1458篇
  2011年   2054篇
  2010年   1811篇
  2009年   2335篇
  2008年   1872篇
  2007年   1925篇
  2006年   1886篇
  2005年   1368篇
  2004年   1412篇
  2003年   1390篇
  2002年   1362篇
  2001年   1117篇
  2000年   1011篇
  1999年   850篇
  1998年   926篇
  1997年   914篇
  1996年   625篇
  1995年   615篇
  1994年   569篇
  1993年   472篇
  1992年   492篇
  1991年   469篇
  1990年   516篇
  1989年   438篇
  1988年   413篇
  1987年   484篇
  1986年   464篇
  1985年   498篇
  1984年   600篇
  1983年   537篇
  1982年   495篇
  1981年   455篇
  1980年   453篇
  1979年   382篇
  1978年   381篇
  1977年   373篇
  1976年   329篇
  1975年   331篇
  1974年   386篇
  1973年   399篇
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
101.
Sukhanova  I. N.  Flint  M. V.  Sakharova  E. G.  Fedorov  A. V.  Makkaveev  P. N.  Nedospasov  A. A. 《Oceanology》2018,58(6):802-816
Oceanology - Based on the material obtained in the spring–summer season of 2016, the composition and quantitative distribution of phytoplankton in the Ob estuary and over the Kara Sea shelf...  相似文献   
102.
Solar System Research - Abstract—Results of the implementation of a single concept for the development of the double launch system (SDZ) are presented, in particular the SDZ-La5 for the...  相似文献   
103.
Bobylev  V. V.  Krisanova  O. I.  Bajkova  A. T. 《Astronomy Letters》2020,46(7):439-448
Astronomy Letters - Based on published data, we have produced a sample of 256 radio sources whose trigonometric parallaxes and proper motions were measured by VLBI. This sample contains Galactic...  相似文献   
104.
Doklady Earth Sciences - Studies of the opto-acoustic effects accompanying laser breakdown in water generated by focused laser and ultrasonic radiation are performed. For the first time, the...  相似文献   
105.
Ovsyuchenko  A. N.  Larkov  A. S.  Sysolin  A. I.  Rogozhin  E. A.  Sobisevich  A. L.  Chen  J.  Liu  J.  Qin  J. 《Doklady Earth Sciences》2020,492(1):361-365
Doklady Earth Sciences - In October 2019, extremely impressive, fresh ruptures of the surface on the mud volcano of Mount Karabetova were discovered. The ruptures are represented by all the main...  相似文献   
106.
Doklady Earth Sciences - More than 250 magnetic spherules 40–200 µm in size were mechanically separated from a ferromanganese crust sample of Fedorov Guyot in the Magellan Seamounts. The...  相似文献   
107.
Morozov  E. G.  Frey  D. I.  Polukhin  A. A.  Krechik  V. A.  Artemiev  V. A.  Gavrikov  A. V.  Kasian  V. V.  Sapozhnikov  F. V.  Gordeeva  N. V.  Kobylyansky  S. G. 《Oceanology》2020,60(5):573-588
Oceanology - Results of a mesoscale survey in the Powell Basin in the northern part of the Weddell Sea are analyzed. The survey included 24 CTD casts (6 × 4) with an interval of six nautical...  相似文献   
108.
109.
Subsurface tile drainage speeds water removal from agricultural fields that are historically prone to flooding. While managed drainage systems improve crop yields, they can also contribute tothe eutrophication of downstream ecosystems, as tile-drained systems are conduits for nutrients to adjacent waterways. The changing climate of the Midwestern US has already altered precipitation regimes which will likely continue into the future, with unknown effects on tile drain water and nutrient loss to waterways. Adding vegetative cover (i.e., as winter cover crops) is one approach that can retain water and nutrients on fields to minimize export via tile drains. In the current study, we evaluate the effect of cover crops on tile drain discharge and soluble reactive phosphorus (SRP) loads using bi-monthly measurements from 43 unique tile outlets draining fields with or without cover crops in two watersheds in northern Indiana. Using four water years of data (n = 844 measurements), we examined the role of short-term antecedent precipitation conditions and variation in soil biogeochemistry in mediating the effect of cover crops on tile drain flow and SRP loads. We observed significant effects of cover crops on both tile drain discharge and SRP loads, but these results were season and watershed specific. Cover crop effects were identified only in spring, where their presence reduced tile drain discharge in both watersheds and SRP loads in one watershed. Varying effects on SRP loads between watersheds were attributed to different soil biogeochemical characteristics, where soils with lower bioavailable P and higher P sorption capacity were less likely to have a cover crop effect. Antecedent precipitation was important in spring, and cover crop differences were still evident during periods of wet and dry antecedent precipitation conditions. Overall, we show that cover crops have the potential to significantly decrease spring tile drain P export, and these effects are resilient to a wide range of precipitation conditions.  相似文献   
110.
Sustainable fuels legislation and volatility in energy prices have put additional pressures on the forestry sector to intensify the harvesting of biomass for “advanced biofuel” production. To better understand how residual biomass removal after harvest affects forest hydrology in relatively low slope terrain, a Before-After-Control-Impact (BACI) study was conducted in the USDA Forest Service's Marcell Experimental Forest, Minnesota, USA. Hydrological measurements were made from 2010–2013 on a forested hillslope that was divided into three treatment blocks, where one block was harvested and residual biomass removed (Biomass Removed), the second was harvested and residual biomass left (Biomass Left), and the last block was left as an Unharvested Control. The pre-harvest period (2 years) was 2010–11 and post-harvest (2 years) was 2012–13. Water table elevation at the upslope and downslope position, subsurface runoff, and soil moisture were measured between May–November. Mixed effect statistical models were used to compare both the before-after and “control” treatment ratios (ratios between harvested hillslopes and the Unharvested Control hillslope). Subsurface runoff significantly increased (p < .05) at both harvested hillslopes but to a greater degree on the Biomass Left hillslope. Greater subsurface runoff volumes at both harvested hillslopes were driven by substantial increases during fall, with additional significant increases during summer on the Biomass Left hillslope. The hydrological connectivity, inferred from event runoff ratios, increased due to harvesting at both hillslopes but only significantly on the Biomass Left hillslope. The winter harvest minimized soil disturbance, resulting in no change to the effective hydraulic conductivity distribution with depth. Thus, the observed hydrological changes were driven by increased effective precipitation and decreased evapotranspiration, increasing the duration that both harvested hillslopes were hydrologically active. The harvesting of residual biomass appears to lessen hydrological connectivity relative to leaving residual biomass on the hillslope, potentially decreasing downstream hydrological impacts of similar forestry operations.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号