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   253853篇
  免费   4292篇
  国内免费   3339篇
测绘学   6758篇
大气科学   18766篇
地球物理   53147篇
地质学   87445篇
海洋学   20986篇
天文学   55653篇
综合类   983篇
自然地理   17746篇
  2021年   2218篇
  2020年   2566篇
  2019年   2827篇
  2018年   2983篇
  2017年   2674篇
  2016年   5402篇
  2015年   4159篇
  2014年   6868篇
  2013年   14158篇
  2012年   6245篇
  2011年   7407篇
  2010年   6453篇
  2009年   9109篇
  2008年   7982篇
  2007年   7374篇
  2006年   9591篇
  2005年   7615篇
  2004年   7510篇
  2003年   7001篇
  2002年   6640篇
  2001年   5940篇
  2000年   5904篇
  1999年   5189篇
  1998年   5221篇
  1997年   5011篇
  1996年   4662篇
  1995年   4411篇
  1994年   4087篇
  1993年   3832篇
  1992年   3616篇
  1991年   3589篇
  1990年   3755篇
  1989年   3504篇
  1988年   3294篇
  1987年   3843篇
  1986年   3401篇
  1985年   4214篇
  1984年   4727篇
  1983年   4401篇
  1982年   4307篇
  1981年   3911篇
  1980年   3634篇
  1979年   3506篇
  1978年   3479篇
  1977年   3276篇
  1976年   3044篇
  1975年   2959篇
  1974年   2917篇
  1973年   3076篇
  1972年   2026篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
Results of analyses of stomach and intestinal contents from hooded ( Cystophora cristata ) and harp ( Phoca groenlandica ) seals captured in the pack ice belt of the Greenland Sea in summer (July-August) in 2000 and winter (February-March) in 2001 revealed that the diet of both species were comprised of relatively few prey taxa. Pelagic amphipods of the genus Parathemisto , the squid Gonatus fabricii , polar cod ( Boreogadus saida ) and capelin ( Mallotus villosus ) constituted 63-99% of the observed diet biomass in both seal species, irrespective of sampling period, but their relative contribution to the diet varied both with species and sampling period/area. For hooded seals, G. fabricii and capelin were the dominant food items in winter 2001, but the summer 2000 diet comprised a mixture of this squid and polar cod. Parathemisto was most important for the harp seals during summer 2000; in winter 2001 the contribution from krill and capelin were comparable to that of Parathemisto . Multivariate analyses revealed differences in the intestinal contents of hooded and harp seals in areas where the two species' occurrence spatially overlapped. Different foraging depths of the two species may have contributed to the observed differences in diets.  相似文献   
994.
995.
996.
997.
As we enter the twenty-first century, effective coastal management will become more of a central issue for coastal users and governors alike. The new century sees the review of many of the UK's existing management plans, and the subsequent publication of revised versions. Some of these new editions will have to show changes from their predecessors because of natural processes of erosion and accretion, yet other changes will be because of human interference. In this, we mean the continued ignoring of previous management plan recommendations regarding new development and coastal usage. Coastal management needs to adapt and develop, yet along with this, increased education and awareness is also necessary to allow users to understand and appreciate what is being recommended in management plans.  相似文献   
998.
Rainfall thresholds for landsliding in the Himalayas of Nepal   总被引:5,自引:0,他引:5  
Landsliding of the hillslope regolith is an important source of sediment to the fluvial network in the unglaciated portions of the Himalayas of Nepal. These landslides can produce abrupt increases of up to three orders of magnitude in the fluvial sediment load in less than a day. An analysis of 3 years of daily sediment load and daily rainfall data defines a relationship between monsoonal rainfall and the triggering of landslides in the Annapurna region of Nepal. Two distinct rainfall thresholds, a seasonal accumulation and a daily total, must be overcome before landslides are initiated. To explore the geomorphological controls on these thresholds, we develop a slope stability model, driven by daily rainfall data, which accounts for changes in regolith moisture. The pattern of rainfall thresholds predicted by the model is similar to the field data, including the decrease in the daily rainfall threshold as the seasonal rainfall accumulation increases. Results from the model suggest that, for a given hillslope, regolith thickness determines the seasonal rainfall necessary for failure, whereas slope angle controls the daily rainfall required for failure.  相似文献   
999.
We present an evaluation of the procedure by which model prediction bias is examined in palaeolimnological transfer function inference models. We argue that most of the prediction biases commonly reported in the literature are, in fact, fallacious, and are the artificial consequence of the inappropriate manner in which residuals are traditionally examined. We show that the extent of the specious model bias is entirely predictable from first principles and is essentially determined by the strength of the predictive model. We suggest that the analysis of residuals should always be examined as a function of the model's predictions and we discuss the implications of the old and new approaches.  相似文献   
1000.
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