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   261569篇
  免费   4290篇
  国内免费   3300篇
测绘学   6824篇
大气科学   19170篇
地球物理   54485篇
地质学   91148篇
海洋学   21720篇
天文学   56969篇
综合类   994篇
自然地理   17849篇
  2021年   2223篇
  2020年   2576篇
  2019年   2830篇
  2018年   3925篇
  2017年   3745篇
  2016年   5917篇
  2015年   4178篇
  2014年   6892篇
  2013年   14184篇
  2012年   6679篇
  2011年   8222篇
  2010年   7481篇
  2009年   9973篇
  2008年   8651篇
  2007年   8257篇
  2006年   9571篇
  2005年   7812篇
  2004年   7616篇
  2003年   7125篇
  2002年   6640篇
  2001年   5920篇
  2000年   5888篇
  1999年   5187篇
  1998年   5215篇
  1997年   5017篇
  1996年   4657篇
  1995年   4401篇
  1994年   4086篇
  1993年   3831篇
  1992年   3614篇
  1991年   3584篇
  1990年   3748篇
  1989年   3506篇
  1988年   3292篇
  1987年   3837篇
  1986年   3397篇
  1985年   4216篇
  1984年   4724篇
  1983年   4397篇
  1982年   4305篇
  1981年   3921篇
  1980年   3637篇
  1979年   3504篇
  1978年   3481篇
  1977年   3274篇
  1976年   3040篇
  1975年   2956篇
  1974年   2913篇
  1973年   3071篇
  1972年   2023篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
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   
992.
993.
994.
995.
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   
996.
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   
997.
998.
Geography Education: The Glass Is Half Full and It's Getting Fuller   总被引:2,自引:0,他引:2  
This article examines the recent progress of geography education as an academic subfield of geography and as an area of activity in K–12 education. Challenges to both endeavors are described. A restructuring of geography education's research program that focuses on spatial thinking and reasoning and merges applied and theoretical research is proposed.  相似文献   
999.
ABSTRACT. Investigations of dooryard gardens, kitchen gardens, home gardens, and houselot gardens fall unequally into one of three groupings. The first are those that treat the plants in the gardens as biological entities and define a space considered a culturally controlled biological community or habitat. The second are those that consider plants cultural traits and the space defined by their positions a setting for household activities. The third conceives of plants as design elements within a garden or a landscape that frames a house or provides a setting for formal human performances. Recent decades have witnessed a broadening focus in the study of gardens, from spatial characteristics and biological content to social and cultural concerns such as reciprocity networks, contested spaces, and the concept of “dwelling.”  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