全文获取类型
收费全文 | 6273篇 |
免费 | 219篇 |
出版年
2020年 | 98篇 |
2019年 | 83篇 |
2018年 | 123篇 |
2017年 | 147篇 |
2016年 | 143篇 |
2015年 | 133篇 |
2014年 | 83篇 |
2013年 | 1486篇 |
2012年 | 180篇 |
2011年 | 220篇 |
2010年 | 238篇 |
2009年 | 250篇 |
2008年 | 222篇 |
2007年 | 196篇 |
2006年 | 188篇 |
2005年 | 161篇 |
2004年 | 80篇 |
2003年 | 115篇 |
2002年 | 102篇 |
2001年 | 90篇 |
2000年 | 103篇 |
1999年 | 98篇 |
1998年 | 73篇 |
1997年 | 56篇 |
1996年 | 68篇 |
1995年 | 59篇 |
1994年 | 55篇 |
1993年 | 66篇 |
1992年 | 59篇 |
1991年 | 38篇 |
1990年 | 36篇 |
1989年 | 58篇 |
1988年 | 46篇 |
1987年 | 56篇 |
1986年 | 52篇 |
1985年 | 75篇 |
1984年 | 96篇 |
1983年 | 83篇 |
1982年 | 65篇 |
1981年 | 72篇 |
1980年 | 77篇 |
1979年 | 70篇 |
1978年 | 71篇 |
1977年 | 56篇 |
1976年 | 75篇 |
1975年 | 77篇 |
1974年 | 49篇 |
1973年 | 39篇 |
1972年 | 36篇 |
1970年 | 31篇 |
排序方式: 共有6492条查询结果,搜索用时 62 毫秒
1.
2.
Evidence from several disciplines supports the proposition that following the Wisconsinan and an initial but brief moist phase of the Holocene, there was a gradual drying trend in the mid-continent Prairie Peninsula that peaked about 7000 years ago, a pattern that was not reversed until about 4000 years ago. Except for minor perturbations, conditions during the past four millennia have essentially been similar to those of the present. Studies in palynology and geomorphology have shown that the climatic regimes of the early/mid Post-glacial had a marked influence on both the biota and landscape erosional patterns, especially along the margins of the Prairie Peninsula. Measurable changes in the landscape and attendant biota show a time-transgressive pattern of drought related phenomena recorded palaeo-ecologically from west to east.Few studies of archaeological sites in the Prairie Peninsula have examined the evidence in and around the sites themselves for data reflecting local environmental dynamics. In fact, earlier models proposed for explaining the evolution of food-procuring systems in eastern North America virtually ignored potential changes in the natural environment. This paper examines data from two archaeological sites along the prairie-forest border in Missouri that contain long cultural sequences. Evidence is presented that pronounced changes in the natural environment occurred and that these biophysical variables are indeed critical for understanding the evolution of food-procuring societies in the American Midwest. 相似文献
3.
4.
5.
6.
7.
图形创意引发了设计基础教育的变革,作为现代教育训练的课题,可培养学生通过联想、想象和一些表现手法来观察和发现生活中的创意元素。 相似文献
8.
Trigger levels for fine suspended sediment (FSS) load in streams are required to identify rainfall-runoff events that have significantly elevated FSS loads, compared with expected background loads, as a result of catchment disturbance. Stream FSS load data collected in the mine-impacted Magela Creek catchment in the wet-dry tropics were used to derive water quality management trigger levels for two approaches – a Before-After-Control-Impact Paired difference design (BACIP) and a regression relationship between observed FSS load and corresponding event discharge characteristics. The results indicate that both the BACIP and regression relationship approaches behave similarly, with similar FSS load events elevated above trigger levels. Notwithstanding this, it is recommended that in order to reliably assess the location and magnitude of a catchment disturbance on FSS load, a combination of BACIP and regression relationship approaches must be adopted. In this study, an event with a FSS load above trigger levels associated with both BACIP and the regression relationship fitted for the downstream site is considered to be significantly elevated as a result of a disturbance within the mine-impacted region. While this technique cannot conclusively determine whether the cause of the disturbance is mine-related or natural (such as fire or bank erosion), it is an efficient statistical method of identifying events that warrant further investigation and management action, if required. 相似文献
9.
旅游能否成为一门独立学科,关键取决于能否形成一套完善的学科基础理论和方法论。本文分析了中外旅游研究方法的进展情况,对理论实证方法的优越性进行了阐释,将其在国内外旅游研究中不同应用程度作以比较,进而总结出国内旅游研究方法的方向。 相似文献
10.