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   202篇
  免费   11篇
  国内免费   1篇
测绘学   6篇
大气科学   7篇
地球物理   51篇
地质学   77篇
海洋学   15篇
天文学   7篇
综合类   1篇
自然地理   50篇
  2022年   1篇
  2021年   6篇
  2020年   9篇
  2019年   8篇
  2018年   5篇
  2017年   8篇
  2016年   6篇
  2015年   5篇
  2014年   10篇
  2013年   15篇
  2012年   4篇
  2011年   9篇
  2010年   9篇
  2009年   12篇
  2008年   9篇
  2007年   9篇
  2006年   8篇
  2005年   10篇
  2004年   9篇
  2003年   5篇
  2002年   5篇
  2001年   5篇
  2000年   3篇
  1999年   5篇
  1998年   2篇
  1997年   7篇
  1995年   2篇
  1994年   4篇
  1993年   2篇
  1992年   2篇
  1991年   1篇
  1989年   1篇
  1988年   2篇
  1986年   2篇
  1985年   1篇
  1984年   1篇
  1983年   3篇
  1981年   1篇
  1978年   1篇
  1976年   2篇
  1975年   1篇
  1974年   1篇
  1973年   1篇
  1966年   1篇
  1955年   1篇
排序方式: 共有214条查询结果,搜索用时 15 毫秒
201.
In 2002, the state of Victoria, Australia increased its “no-take” marine protected areas (MPAs) 100 fold to cover over 5% of its coastal waters in a comprehensive, adequate and representative system of marine national parks and sanctuaries. Given the ambitious targets set for MPA establishment globally in 2003 at the World Summit for Sustainable Development this apparently remarkable achievement could be an example to other nations and states attempting to establish substantial MPA systems.This paper describes and discusses the factors that contributed to the establishment of the Victorian system and the relevance of these factors to other jurisdictions.  相似文献   
202.
An Nd:YAG-based airborne lidar system has been used to measure the optical properties of littoral waters off the northwest Scottish coast. The small-scale structure of subsurface scattering layers was also investigated. Methods of solving the Lidar Sensing Equation in the single scattering approximation are described and the values of the derived extinction indices presented. The extinction index averaged over a series of five flights to the northeast of the Gulf Stream was ε= 0.224 m -1 with a standard deviation of 0.212 m -1 . Further, it was demonstrated that, in coastal waters, optical inhomogeneities with dimensions between 50 m and 200 km obey the power law Sp ε ~ k -P with the parameter P close to two. In turbid or transparent areas, the water extinction index can change by several tenths of a percent with respect to the surrounding water mass. This suggests that the observed nonmonotonic behavior of the power spectra of the water extinction index fluctuations is caused by the outer scale of turbulence, in particular, by the bottom depth at the measurement site.  相似文献   
203.
This paper covers some of the pressing problems of the Arctic Ocean, the existing structures and how cooperation in the region may develop. The region is surrounded by the lands of Canada, Denmark, Finland, Iceland, Norway, the Russian Federation and the United States of America, who together formed the Arctic Council in recognition of the fact that the issues facing the north exceed the capabilities and capacities of any one country and that successful solutions must be built on common actions. The expense and difficulty of operating in extreme conditions of the Arctic forced much of the significant arctic research into cooperative bilateral or multilateral projects, usually with a discrete research objective. Global organizations including the Intergovernmental Oceanographic Commission (IOC) of the United Nations Educational, Scientific and Cultural Organization (UNESCO), the World Meteorological Organization (WMO), the International Council for Science (ICSU), the Food and Agriculture Organization (FAO), and the International Maritime Organization (IMO) sponsor specific climate related programs in the Arctic. The warming trends in the Arctic provide the motivation for renewed attention on the Arctic, which is changing rapidly both environmentally and socially. The climate and pollution issues, both anthropogenic in origin, have created some priorities in governmental thinking and led to limited additional funding in recent years. Industry needs to be responsive and innovative in incorporating stringent control and monitoring into development projects and in contributing to research and environmental management. Collective efforts by all interested governments will be needed to address the scarcity of research and observations, extending from the oceans to the coastal areas and drainage basins of the hinterlands. Arctic countries must also pursue global arrangements to protect the Arctic from the impacts of pollution from outside areas. The work started by the Arctic Council needs to be bolstered by the establishment of a regional convention for the Arctic, which will confer more responsibility upon governments to continue with the programs.  相似文献   
204.
205.
The differences of coalbed methane(CBM) desorption-diffusion from coal drilling-core under various drilling fluid medium are not considered in the present calculating methods of lost CBM quantity,which leads possibly to the inaccuracy of CBM quantity in coal seam.Here we took the desorption of CBM from coal core under drilling fluid medium as a pressure-swing process,and based on the Langmuir equation and Fick-first law,established the desorption-diffusion model and numerical modeling method of lost gas(inc...  相似文献   
206.
The water chemistry of lake systems on the edge of the Antarctic continent responds quickly to changes in the moisture balance. This is expressed as increasing salinity and decreasing lake water level during dry periods, and the opposite during wet periods. The diatom composition of the lakes also changes with these fluctuations in salinity and lake water depth. This is important, as their siliceous remains become incorporated into lake sediments and can provide long-term records of past salinity using transfer functions. In order to develop transfer functions, diatoms and water chemistry data were inter-calibrated from five different East Antarctic oases, namely the Larsemann Hills, the Bølingen Islands, the Vestfold Hills, the Rauer Islands and the Windmill Islands. Results indicate that salinity is the most important environmental variable explaining the variance in the diatom flora in East Antarctic lakes. In oligo- saline lakes the variance is mainly explained by lake water depth. This dataset was used to construct a weighted averaging transfer function for salinity in order to infer historical changes in the moisture balance. This model has a jack-knifed r2 of 0.83 and a RMSEP of 0.31. The disadvantage of this transfer function is that salinity changes in oligo-saline lakes are reconstructed inaccurately due to the edge effect and due to the low species turnover along the salinity gradient at its lower end. In order to infer changes in the moisture balance in these lakes, a second transfer function using weighted averaging partial least squares (with two components) for depth was constructed. This model has a jack-knifed r2 of 0.76 and a RMSEP of 0.22. Both transfer functions can be used to infer climate driven changes in the moisture balance in lake sediment cores from oligo-, hypo-, meso- and hyper-saline lakes in East Antarctic oases between 102–75°E. The transfer function for lake water depth is promising to track trends in the moisture balance of small freshwater lakes, where changes in shallow and deep-water sediments are readily reflected in changing diatom composition.  相似文献   
207.
208.
A study of certain small-scale fluted moraines, all less than 4m high but up to 400m long, in the Torridon area has shown that they are subglacial features composed of a clast-rich till, and formed in the Loch Lomond Stadial beneath ice with a maximum depth of 100-200m. The evidence from lithological analysis of till samples from the fluted moraines suggests that they were formed by subglacial deformation of a pre-existing till with little in the way of net down-glacier movement of the material.  相似文献   
209.
Recent rapid salinity rise in three East Antarctic lakes   总被引:1,自引:0,他引:1  
Research in East Antarctica has shown several recent environmental changes that may be linked to human impacts on climate. In order to detect the influence and context of these changes on coastal aquatic ecosystems we examined lake sediment cores from three lakes in the Windmill Islands, East Antarctica; Beall Lake, Holl Lake and ȁ8Lake Mȁ9. Cores were sectioned at␣2.5 mm intervals. Their diatom species composition was examined to detect changes in lake salinity using a diatom-salinity transfer function, and their algal pigment content was examined to detect photoautotrophic community responses to environmental change. Results showed that Holl Lake originated in a depression exposed by Holocene recession of the continental ice sheet and that Beall Lake and Lake M originated as isolated marine basins formed by changes in relative sea level. A general late Holocene trend of declining lake salinity was evident in all three lakes, interrupted by one short-term high salinity event in Beall Lake. This is consistent with a long-term positive moisture balance. This general decline in salinity has been followed by a remarkable recent rapid increase in salinity in all three lakes in the last few decades. We speculate that this rapid increase in salinity might be linked to changes taking place in the region including feedbacks resulting from decreasing sea ice extent as recorded in the nearby Law Dome ice core, and positive feedbacks in the catchments whereby reduced snow cover has led to decreased albedo, which in turn has caused increased evaporation and sublimation. Collectively these changes have shifted the lakes across a threshold from positive to negative moisture balance. A minor, but not rapid shift in the abundance of diatom pigments relative to pigments from green algae and cyanobacteria was also detected suggesting that some changes in photoautotrophic community composition have occurred. Measurements of modern nutrient levels are also higher than would be expected in Beall Lake and Holl Lake, given the extremely low sediment accumulation rates. This may be associated with a c. 300% increase in the population of Adélie penguins in the Windmill Islands recorded since the 1950s, or may a first signs of a rapid increase in catchment development and associated lake productivity as experienced in Antarctic and Arctic lakes subject to recent rapid regional warming. The most marked feature of the records is the rapid increase in salinity in all three lakes in␣the last few decades, which has occurred in lakes both with and without resident penguin populations.Dominic A. Hodgson and Donna Roberts contributed equally to this work  相似文献   
210.
The equilibrium form of the fluvial long profile has been used to elucidate a wide range of aspects of landscape history including tectonic activity in tectonic collision zones, and in continental margin and other intraplate settings, as well as other base‐level changes such as due to sealevel fluctuations. The Hack SL form of the long profile, which describes a straight line on a log–normal plot of elevation (normal) versus distance (logarithmic), is the equilibrium long profile form that has been most widely used in such studies; slope–area analysis has also been used in recent years. We show that the SL form is a special case of a more general form of the equilibrium long profile (here called the DS form) that can be derived from the power relationship between stream discharge and downstream distance, and the dependence of stream incision on stream power. The DS form provides a better fit than the SL form to river long profiles in an intraplate setting in southeastern Australia experiencing low rates of denudation and mild surface uplift. We conclude that, if an a priori form of the long profile is to be used for investigations of regional landscape history, the DS form is preferable. In particular, the DS form in principle enables equilibrium steepening due to an increase in channel substrate lithological resistance (parallel shift in the DS plot) to be distinguished from disequilibrium steepening due to long profile rejuvenation (disordered outliers on the DS plot). Slope–area analysis and the slope–distance (DS) approach outlined here are complementary approaches, reflecting the close relationship between downstream distance and downstream catchment area. Copyright © 2006 John Wiley & Sons, Ltd.  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