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   29901篇
  免费   666篇
  国内免费   3195篇
安全科学   1109篇
废物处理   1344篇
环保管理   3190篇
综合类   10333篇
基础理论   6628篇
环境理论   4篇
污染及防治   7302篇
评价与监测   1633篇
社会与环境   1819篇
灾害及防治   400篇
  2023年   233篇
  2022年   588篇
  2021年   537篇
  2020年   472篇
  2019年   426篇
  2018年   1101篇
  2017年   1148篇
  2016年   1168篇
  2015年   794篇
  2014年   957篇
  2013年   1975篇
  2012年   1288篇
  2011年   1899篇
  2010年   1401篇
  2009年   1478篇
  2008年   1658篇
  2007年   1894篇
  2006年   906篇
  2005年   812篇
  2004年   733篇
  2003年   778篇
  2002年   749篇
  2001年   789篇
  2000年   597篇
  1999年   482篇
  1998年   393篇
  1997年   356篇
  1996年   338篇
  1995年   328篇
  1994年   347篇
  1993年   279篇
  1992年   272篇
  1991年   268篇
  1990年   275篇
  1989年   245篇
  1988年   191篇
  1987年   187篇
  1986年   187篇
  1985年   183篇
  1984年   216篇
  1983年   196篇
  1982年   202篇
  1981年   214篇
  1980年   169篇
  1979年   180篇
  1968年   156篇
  1967年   184篇
  1966年   154篇
  1965年   148篇
  1964年   151篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
Classification and distribution of mangrove vegetation are vital information for the proper development of a mangrove management plan. In this study, classification for the mangroves of the district of Kemaman were done using both 1 : 5000 aerial photographs and Landsat TM imageries. The coverage by aerial photographs is limited to the coastal and estuarine areas only. Thus, for areas further upstream of the aerial photo coverage, Landsat TM imageries were used. Analysis of aerial photographs and remote sensing images revealed that the mangroves of Kemaman could be classified into 14 different classes of vegetation. All the 14 classes were identified from areas covered by the aerial photographs. For areas covered by the Landsat images only 7 classes of vegetation were identified. The accuracy for aerial photograph and Landsat images are 91.2% and 87.8%, respectively. It can be concluded that although both techniques are useful in determining the mangrove vegetation classes, the large 1 : 5000 aerial photographs are more accurate and provided more detailed information comparatively.  相似文献   
42.
可持续旅游是 80年代中期以后提出的关于旅游业发展的新思想。经过十多年的不断拓展深化 ,已经得到世界各国的广泛认同 ,并作为跨世纪旅游业发展的战略指导思想。本文从可持续旅游的由来和对可持续旅游内涵实质的界定角度出发 ,结合可持续旅游实现途径的探讨 ,以井冈山风景名胜区为例 ,对风景区旅游业可持续发展进行了深入的探讨 ,为景区的发展规划提出了战略性构想 ,以期能对井冈山风景名胜区旅游资源的可持续利用和旅游业的可持续发展起到抛砖引玉的作用  相似文献   
43.
由于我国对企业特别是海外来大陆投资的企业,缺乏严格的安全管理手段,致使不少企业屡次出现安全事故。为了减少企业安全事故的发生,提高企业安全度,要求企业在贯彻“安全第一,预防为主”的基本方针的基础上。建立“以人为本。以制度和伦理道德为约束的人性化的安全管理系统”。本文给出了该系统的模型,并对此模型进行了分析和解释,探讨了相关的几个理论基础,最后给出了人性化安全管理的实质。通过该系统的管理执行,使企业在尊重人权、尊重生命的前提下发展壮大,最终实现安全的全球化。  相似文献   
44.
Assessment of environmental changes in the Orinoco River delta   总被引:1,自引:0,他引:1  
Major anthropogenic driven changes in the hydrologic and sedimentation patterns of the Orinoco River have had an impact on environmental conditions in the delta. The abrupt water flow reduction from 3,600 to 200 m3 s–1 in one of its major distributaries resulting from dam construction forced its transformation from a fresh-water body into a tidal channel with an increase in salinity level (as far as 100 km upstream) and with well-mixed water at the mouth and estuarine connection to the Paria Gulf. Three different sectors along this distributary can be identified (indicated by the Na/Cl ratio in the water). As a result, noticeable changes have occurred in the mangrove community which moved about 60 km further upstream. The changes have also promoted the formation of new islands of sediment progradation at the mouth of this distributary, where successional colonization and species replacement by different species of grasses and mangroves take place. Electronic Publication  相似文献   
45.
The article puts forward the process and means of regional water and land balance research, and then from two scenarios which are the balances under natural regulation and human intervention, calculated and analysed the balance between water and land on Ningxia Plain. For the balance under natural regulation named farmland water balance, using farmland water resource balance equation, the research estimated the monthly farmland" water balance of 8 major crops for all of the 12 counties on Ningxia Plain in the period of 1960-2001; for the balance under human intervention, the research estimated land-use water balance equation of the counties in 2000, and calculated the balance between land use and water resources including irrigating water of all the 12 counties on Ningxia Plain. Results showed that ①precipitation can not meet the water demand of the crops for growth and development on Ningxia Plain, and water shortage is the primary character of farmland water balance under natural regulation. ②the diversity of water and land balance of different counties is distinctly influenced by the crop structure, water quantity for irrigation and irrigation level. ③Irrigation water could meet the crop water demand on Ningxia Plain in 2000, but there was not much space to expand irrigating cultivated land.  相似文献   
46.
47.
New experimental data on biological productivity of plant communities in oligotrophic and mesotrophic bogs of the middle taiga subzone over the past five years are presented. The relationship between net primary production and the stock of live phytomass is estimated. The stock of necromass in oligotrophic bog ecosystems increases from west to east, while the stock of live phytomass and net primary production decrease.  相似文献   
48.
Although stewardship has been widely defined and used in environmental management and planning, there is a dearth of studies that describe how the lay public perceives this concept. A national sample of residents in 14 states who live near DOE nuclear facilities were interviewed to delineate public understanding and awareness of the stewardship program of the U.S. Department of Energy (DOE). This study discusses the findings of the survey and discusses how institutional trust influences public participation and resident’s choices of potential stewards. Almost 40% of the respondents could not define stewardship; those who did, believed that ‘responsibility,’ ‘management,’ and ‘accountability’ are key elements of stewardship. In addition, about a third of the respondents identified Federal groups and the DOE as potential stewards. Readers should send their comments on this paper to: BhaskarNath@aol.com within 3 months of publication of this issue  相似文献   
49.
Studies on paleosols under an archaeological landmark of a rare type (a complex of kurgans with “whiskers”) dating from the Early Iron Age (the fourth century AD) have been performed in the steppe zone of the Transural Plateau. The size and shape of third-order soil polygons under stony ridges (“whiskers”) between the kurgans have been described in detail. The results have shown that the paleosol under the kurgans erected at the turn of the Late Sarmatian and Hun times (1600 years ago) is characterized by a higher humus content and deeper location of the carbonate horizon, compared to the recent soil. This indicates that an increase in atmospheric humidity took place in the fourth century AD.  相似文献   
50.
Results of groundwater and seawater analyses for radioactive (3H, 222Rn, 223Ra, 224Ra, 226Ra, and 228Ra) and stable (D and 18O) isotopes are presented together with in situ spatial mapping and time series 222Rn measurements in seawater, direct seepage measurements using manual and automated seepage meters, pore water investigations using different tracers and piezometric techniques, and geoelectric surveys probing the coast. This study represents first time that such a new complex arsenal of radioactive and non-radioactive tracer techniques and geophysical methods have been used for simultaneous submarine groundwater discharge (SGD) investigations. Large fluctuations of SGD fluxes were observed at sites situated only a few meters apart (from 0 cm d−1 to 360 cm d−1; the unit represents cm3/cm2/day), as well as during a few hours (from 0 cm d−1 to 110 cm d−1), strongly depending on the tidal fluctuations. The average SGD flux estimated from continuous 222Rn measurements is 17 ± 10 cm d−1. Integrated coastal SGD flux estimated for the Ubatuba coast using radium isotopes is about 7 × 103 m3 d−1 per km of the coast. The isotopic composition (δD and δ18O) of submarine waters was characterised by significant variability and heavy isotope enrichment, indicating that the contribution of groundwater in submarine waters varied from a small percentage to 20%. However, this contribution with increasing offshore distance became negligible. Automated seepage meters and time series measurements of 222Rn activity concentration showed a negative correlation between the SGD rates and tidal stage. This is likely caused by sea level changes as tidal effects induce variations of hydraulic gradients. The geoelectric probing and piezometric measurements contributed to better understanding of the spatial distribution of different water masses present along the coast. The radium isotope data showed scattered distributions with offshore distance, which imply that seawater in a complex coast with many small bays and islands was influenced by local currents and groundwater/seawater mixing. This has also been confirmed by a relatively short residence time of 1–2 weeks for water within 25 km offshore, as obtained by short-lived radium isotopes. The irregular distribution of SGD seen at Ubatuba is a characteristic of fractured rock aquifers, fed by coastal groundwater and recirculated seawater with small admixtures of groundwater, which is of potential environmental concern and has implications on the management of freshwater resources in the region.  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