全文获取类型
收费全文 | 84323篇 |
免费 | 1379篇 |
国内免费 | 665篇 |
专业分类
测绘学 | 2019篇 |
大气科学 | 6570篇 |
地球物理 | 17438篇 |
地质学 | 28039篇 |
海洋学 | 7175篇 |
天文学 | 19076篇 |
综合类 | 175篇 |
自然地理 | 5875篇 |
出版年
2020年 | 636篇 |
2019年 | 658篇 |
2018年 | 1313篇 |
2017年 | 1284篇 |
2016年 | 1766篇 |
2015年 | 1303篇 |
2014年 | 1821篇 |
2013年 | 4103篇 |
2012年 | 2003篇 |
2011年 | 2980篇 |
2010年 | 2558篇 |
2009年 | 3737篇 |
2008年 | 3388篇 |
2007年 | 3102篇 |
2006年 | 3103篇 |
2005年 | 2670篇 |
2004年 | 2778篇 |
2003年 | 2576篇 |
2002年 | 2433篇 |
2001年 | 2221篇 |
2000年 | 2148篇 |
1999年 | 1876篇 |
1998年 | 1879篇 |
1997年 | 1832篇 |
1996年 | 1564篇 |
1995年 | 1476篇 |
1994年 | 1334篇 |
1993年 | 1217篇 |
1992年 | 1165篇 |
1991年 | 1010篇 |
1990年 | 1207篇 |
1989年 | 1050篇 |
1988年 | 913篇 |
1987年 | 1143篇 |
1986年 | 1007篇 |
1985年 | 1267篇 |
1984年 | 1468篇 |
1983年 | 1396篇 |
1982年 | 1249篇 |
1981年 | 1207篇 |
1980年 | 1057篇 |
1979年 | 1032篇 |
1978年 | 1096篇 |
1977年 | 989篇 |
1976年 | 959篇 |
1975年 | 906篇 |
1974年 | 880篇 |
1973年 | 893篇 |
1972年 | 565篇 |
1971年 | 500篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
This paper outlines the application of a new data‐based mechanistic (DBM) modelling methodology to the characterization of the sediment transmission dynamics in a small upland reservoir, Wyresdale Park, Lancashire. The DBM modelling strategy exploits advanced statistical procedures to infer the dynamic model structure and its associated parameters directly from the instrumented data, producing a parametrically efficient, continuous time, transfer function model which relates suspended sediment load at the reservoir inflow to the outflow at the event scale. The associated differential equation model parameters have physical attributes which can be interpreted in terms of sediment transmission processes and associated reservoir trap efficiency. Sedigraph analysis suggests that wind‐induced resuspension episodically supplies an additional load to the reservoir outlet. The stochastic nature of the DBM model makes it ideal for evaluating the effects of uncertainty through Monte Carlo simulations (MCS) for discharge and sediment transmission. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
992.
993.
P.E. Villagra G.E. Defossé H.F. del Valle S. Tabeni M. Rostagno E. Cesca E. Abraham 《Journal of Arid Environments》2009,73(2):202-211
The complex interactions between human activity and natural processes determine non-linear dynamics in ecosystems that can difficult their management. Human settlements in arid lands contribute to the modification of disturbance regimes, including the introduction of new disturbances and the elimination of others. In consequence, they can alter the functional mechanisms that allow systems to overcome limiting factors, leading to desertification. In this revision, we evaluated the effects of the changes on disturbance regimes produced by the different forms of land transformation on the structure and function of ecosystems of the Monte Biogeographical Province, in Argentinean arid west. Two approaches were used: the analysis of land use history and the analysis of the effects of the main disturbances on the dynamics of different communities. We concluded that throughout the history of the Monte Desert, the joint action of natural and anthropic agents has resulted in complex dynamics that lead most area of the Monte to a moderate to severe status of desertification. The modification of the disturbance regime had strong consequences for several aspects of the dynamics of communities, such as species composition and diversity, water dynamics, soil conditions, trophic structure and productivity of Monte Desert ecosystems. However, disturbance regimes could be managed to promote favorable transitions in ecosystems and, therefore, could be a tool for optimizing productivity of agro-ecosystems, and recovering and conserving natural ecosystems. 相似文献
994.
The Geology and Petroleum Resources of Kansas: A Review From Alpha to Omega or From the Pleistocene to the Precambrian 总被引:1,自引:0,他引:1
Daniel F. Merriam 《Natural Resources Research》2010,19(4):293-316
The geological story of Kansas is told through the rocks that are present. It is a simple story in generalities but complex
in detail. Knowing the story, gives insight into understanding the occurrence and location of possible economic valuable minerals,
such as petroleum. This is a brief review of Kansas geology with respect to the known occurrence of oil and gas. Kansas is
part of the Midcontinent oil province with oil having been discovered 150 years ago and commercial production commencing in
1873. Although many prospects remain in Kansas, the state has gone from the number 1 producer in the U.S. in 1916 to 8th today.
Exploration for new oil and gas production therefore is going to have to be more imaginative and utilize new approaches and
techniques to find the elusive petroleum. There are possibilities however for the prospector who can search diligently. Although
the big fields probably have been discovered, the prospects today are deeper, in more undetectable traps, and in essentially
untested places. 相似文献
995.
996.
Ohne Zusammenfassung 相似文献
997.
After describing briefly some of the outstanding features of the uncommonly cold and wet year 1695, one of the coldest years
of the Little Ice Age, an annotated translation is presented of a contemporary review of the unusual weather events of the
year in Europe. The original was published in 1702 in Frankfurt am Main, Germany, in vol. XIV of the seriesTheatrum Europaeum. The annotation relates to the historical events of the year that were substantially affected by the weather, events to which
rather abstruse references are made in the aforementioned contemporary account. In addition to the fact that the contemporary
review describes the uncommon weather conditions of 1695, it appears to be the first extended weather review in history. 相似文献
998.
999.
Farmland on the Downs between Lewes and Brighton suffered severe erosion during the autumn of 1982. The erosion was widespread and affected a variety of topographic situations, but it was confined to areas of arable land and recently-sown grass leys. Erosion on the scale recorded during the autumn of 1982 has never previously been recorded from this area. Three major sites of erosion are described and explanations for the erosion are sought through an analysis of rainfall conditions experienced during autumn 1982 and in recent changes in agricultural land use on the Downs. It is concluded that, whilst total rainfall and the intensity of rainstorms were both unusually high, similar events are likely to recur several times a century. Evidence is presented which suggests that the ploughing up of permanent pasture, the removal of field boundaries and the increased adoption of autumn-sown cereals have all contributed to the onset of severe erosion. It appears that a major re-activation of erosion on the Downs may be commencing which threatens the long-term viability of farming in the area. 相似文献
1000.