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   283篇
  免费   6篇
  国内免费   5篇
安全科学   17篇
废物处理   11篇
环保管理   87篇
综合类   22篇
基础理论   50篇
环境理论   1篇
污染及防治   60篇
评价与监测   32篇
社会与环境   12篇
灾害及防治   2篇
  2023年   2篇
  2022年   3篇
  2019年   5篇
  2018年   6篇
  2017年   4篇
  2016年   11篇
  2015年   9篇
  2014年   6篇
  2013年   45篇
  2012年   6篇
  2011年   11篇
  2010年   11篇
  2009年   9篇
  2008年   10篇
  2007年   8篇
  2006年   16篇
  2005年   11篇
  2004年   2篇
  2003年   6篇
  2002年   3篇
  2001年   9篇
  2000年   3篇
  1999年   6篇
  1998年   5篇
  1997年   3篇
  1996年   5篇
  1995年   4篇
  1994年   2篇
  1993年   3篇
  1992年   3篇
  1991年   5篇
  1990年   2篇
  1989年   4篇
  1988年   5篇
  1987年   6篇
  1986年   4篇
  1985年   5篇
  1984年   3篇
  1983年   3篇
  1982年   2篇
  1981年   5篇
  1980年   4篇
  1979年   2篇
  1977年   4篇
  1975年   3篇
  1974年   2篇
  1973年   2篇
  1972年   2篇
  1970年   1篇
  1968年   1篇
排序方式: 共有294条查询结果,搜索用时 15 毫秒
241.
The response of an ecosystem to perturbations is mediated by both antagonistic and facilitative interactions between species. It is thought that a community's resilience depends crucially on the food web--the network of trophic interactions--and on the food web's degree of compartmentalization. Despite its ecological importance, compartmentalization and the mechanisms that give rise to it remain poorly understood. Here we investigate several definitions of compartments, propose ways to understand the ecological meaning of these definitions, and quantify the degree of compartmentalization of empirical food webs. We find that the compartmentalization observed in empirical food webs can be accounted for solely by the niche organization of species and their diets. By uncovering connections between compartmentalization and species' diet contiguity, our findings help us understand which perturbations can result in fragmentation of the food web and which can lead to catastrophic effects. Additionally, we show that the composition of compartments can be used to address the long-standing question of what determines the ecological niche of a species.  相似文献   
242.
We propose a new method of cost-benefit analysis to investigate whether the new air pollution control equipment-electrocyclone vis-a-vis cyclone has the potential for practical use. When the flow rate of waste gas is 1000 m3/min, the cost of cyclone is then compared with that of electrocyclone provided that the benefit side is being fixed. The results show that the capital cost of electrocyclone is higher than that of cyclone, but the operating cost of electrocyclone is much lower than that of cyclone. Straight-line depreciation method is used to calculate the depreciation of capital cost per year. The total cost of electrocyclone is NT$ 160 290 per year which is cheaper than that of cyclone NT$ 225 356.  相似文献   
243.
244.
The need to understand the processes contributing to marsh sedimentation has become more urgent with the recent recognition of the role of tidal marshes as sea defences, as well as the many restoration efforts currently under way. This study was designed to build upon previous sedimentation work at Scolt Head Island by Combining techniques for measuring short-term sedimentation, with detailed assessment of hydroperiod, previously used only in comparison with longer-term accretion measurements or in micro-tidal systems. Measurements of water level, sediment deposition (at various distances from the creek margin) and suspended sediment concentration (SSC) (creek margin and an interior site) were made at Hut Marsh over three sequential over-marsh tides during May 1994. Sediment trap data show a significant trend of declining sediment deposition away from the creek when data from all three tides are combined. All tides show higher SSC on the flood tide than on the ebb tide at the creek margin location. There is little difference in flood and ebb SSCs at the interior site. An order of magnitude decrease in sediment deposition within 20 m on the creek shows the rapidity with which sediment is deposited on these marshes. Higher tides influence both the magnitude and pattern of marsh surface sediment deposition. Increased creek velocities on higher tides provide more potential for resuspension within the creek and increase the supply of sediment to the marsh surface. This study suggests that the design of tidal creeks may be essential for the development of sustainable coastal marshes in restoration projects.  相似文献   
245.
Cosolvent flushing is a technique that has been proposed for the removal of hydrophobic organic contaminants in the subsurface. Cosolvents have been shown to dramatically increase the solubility of such compounds compared to the aqueous solubility; however, limited data are available on the effectiveness of cosolvents for field-contaminated media. In this work, we examine cosolvent flushing for the removal of polycyclic aromatic hydrocarbons (PAHs) in soil from a former manufactured gas plant (FMGP). Batch studies confirmed that the relationship between the soil-cosolvent partitioning coefficient (K(i)) and the volume fraction of cosolvent (f(c)) followed a standard log-linear equation. Using methanol at an fc of 0.95, column studies were conducted at varying length scales, ranging from 11.9 to 110 cm. Removal of PAH compounds was determined as a function of pore volumes (PVs) of cosolvent flushed. Despite using a high f(c), rate and chromatographic effects were observed in all the columns. PAH effluent concentrations were modeled using a common two-site sorption model. Model fits were improved by using MeOH breakthrough curves to determine fitted dispersion coefficients. Fitted mass-transfer rates were two to three orders of magnitude lower than predicted values based on published data using artificially contaminated sands.  相似文献   
246.
We analyzed Hg species distribution patterns among ecosystem compartments in the Everglades at the landscape level in order to explore the implications of Hg distribution for Hg bioaccumulation and to investigate major biogeochemical processes that are pertinent to the observed Hg distribution patterns. At an Everglade-wide scale, THg concentrations were significantly increased in the following order: periphyton相似文献   
247.
Abstract: Stream and riparian managers must effectively allocate limited financial and personnel resources to monitor and manage riparian ecosystems. They need to use management strategies and monitoring methods that are compatible with their objectives and the response potential of each stream reach. Our objective is to help others set realistic management objectives by comparing results from different methods used to document riparian recovery across a diversity of stream types. The Bureau of Land Management Elko Field Office, Nevada, used stream survey, riparian proper functioning condition (PFC) assessment, repeat photographic analysis, and stream and ecological classification to study 10 streams within the Marys River watershed of northeast Nevada during all or parts of 20 years. Most riparian areas improved significantly from 1979 to 1992‐1993 and then additionally by 1997‐2000. Improvements were observed in riparian and habitat condition indices, bank cover, and stability, pool quality, bank angle, and depth of undercut bank. Interpretation of repeat photography generally confirmed results from stream survey and should be part of long‐term riparian monitoring. More attributes of Rosgen stream types C and E improved than of types B and F. A and Gc streams did not show significant improvement. Alluvial draws and alluvial valleys improved in more ways than V‐erosional canyons and especially V‐depositional canyons. Stream survey data could not be substituted for riparian PFC assessment. Riparian PFC assessments help interpret other data.  相似文献   
248.
249.
The concentrations of six heavy metals (HMs) and 16 US EPA priority polycyclic aromatic hydrocarbons (PAHs) in sediment samples of the confluence of rivers Niger and Benue were investigated. The ecological risk assessment of the contaminants was carried out. The results showed that the sediment samples were heavily polluted with iron and moderately polluted with Cd while other metals posed no pollution problem when compared with USEPA sediment quality guidelines. Only six out of the 16 priority PAHs were detected in the samples, and source apportionment of the PAHs indicated that they are of pyrogenic origin. The ∑PAHs in the samples were lower than many of similar studies and were of no pollution risk. The ecological risk assessment result of the heavy metals showed that the sediments were of considerable risk due majorly to Cd levels. The HM concentration results statistically showed significant difference between seasons at probability value (P < .05). Data analysis by PCA classified the metals into three different components according to sources. The levels of HMS and PAHs detected in the sediments were correlated for source identification, and the correlation showed that the majority of the pollutants were mainly from anthropogenic sources. There is increasing level of anthropogenic activities at the vicinity of the confluence due to urbanization which may call for periodic monitoring of the sediment quality.  相似文献   
250.
This paper reports the results of research conducted for the U.S. Environmental Protection Agency in connection with the Ohio River Basin Energy Study (U.S.-EPA Grant No. 805585, Subcontract No. R805588). The purpose of the research is to estimate monetary losses to agricultural producers in the study region from airborne residuals. Estimates of physical crop losses were provided under the direction of Dr. Orie Loucks at The Institute of Ecology and were used for estimating the related monetary losses. Economic losses are defined as producer surplus losses to the agricultural sector due to reductions in productivity from airborne residuals. Research results support the following general conclusions; monetary losses to agricultural producers in the region are on the order of 12% of the present discounted value of clean air production through the year 2000. Losses from utilities alone are on the order of 4.8%. Losses are highly concentrated in the ORBES portions of Illinois, Indiana, and Ohio and primarily related to soybean and corn production. The overwhelming monetary losses are attributable to O3 concentrations in the region. Compliance and noncompliance with state implementation plans (SIP) does not significantly change the degree of monetary losses. Only high growth in electric demand produces annual losses which rise to the year 2000 and produces significantly greater losses among alternative scenarios.  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