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   17449篇
  免费   283篇
  国内免费   322篇
安全科学   613篇
废物处理   737篇
环保管理   2593篇
综合类   2936篇
基础理论   4484篇
环境理论   7篇
污染及防治   4548篇
评价与监测   994篇
社会与环境   1017篇
灾害及防治   125篇
  2022年   160篇
  2021年   162篇
  2020年   181篇
  2019年   141篇
  2018年   282篇
  2017年   256篇
  2016年   397篇
  2015年   318篇
  2014年   447篇
  2013年   1404篇
  2012年   604篇
  2011年   882篇
  2010年   664篇
  2009年   760篇
  2008年   824篇
  2007年   868篇
  2006年   719篇
  2005年   605篇
  2004年   621篇
  2003年   563篇
  2002年   537篇
  2001年   642篇
  2000年   518篇
  1999年   316篇
  1998年   209篇
  1997年   228篇
  1996年   222篇
  1995年   266篇
  1994年   209篇
  1993年   208篇
  1992年   174篇
  1991年   182篇
  1990年   178篇
  1989年   180篇
  1988年   153篇
  1987年   133篇
  1986年   156篇
  1985年   143篇
  1984年   190篇
  1983年   147篇
  1982年   171篇
  1981年   158篇
  1980年   131篇
  1979年   151篇
  1978年   94篇
  1977年   98篇
  1975年   88篇
  1974年   93篇
  1973年   86篇
  1972年   96篇
排序方式: 共有10000条查询结果,搜索用时 406 毫秒
861.
Why social values cannot be changed for the sake of conservation   总被引:1,自引:0,他引:1       下载免费PDF全文
The hope for creating widespread change in social values has endured among conservation professionals since early calls by Aldo Leopold for a “land ethic.” However, there has been little serious attention in conservation to the fields of investigation that address values, how they are formed, and how they change. We introduce a social–ecological systems conceptual approach in which values are seen not only as motivational goals people hold but also as ideas that are deeply embedded in society's material culture, collective behaviors, traditions, and institutions. Values define and bind groups, organizations, and societies; serve an adaptive role; and are typically stable across generations. When abrupt value changes occur, they are in response to substantial alterations in the social–ecological context. Such changes build on prior value structures and do not result in complete replacement. Given this understanding of values, we conclude that deliberate efforts to orchestrate value shifts for conservation are unlikely to be effective. Instead, there is an urgent need for research on values with a multilevel and dynamic view that can inform innovative conservation strategies for working within existing value structures. New directions facilitated by a systems approach will enhance understanding of the role values play in shaping conservation challenges and improve management of the human component of conservation.  相似文献   
862.
Studies in three typical forest biotopes of the Valdai Upland were performed to evaluate the stocks and surface area of coarse woody debris from spruce and birch (in linear transects), its colonization by xylotrophic fungi (during reconnaissance surveys), and CO2 emission (by a chamber method). The stock and surface area were minimum in a paludal birch forest (46.4 m3/ha and 960 m2/ha) and maximum in a decay area of spruce forest (256.1 m3/ha and 3761 m2/ha, respectively). The assemblages of wood-decay fungi had a composition typically found in southern taiga forests. The total CO2 flux varied from 145 kg C-CO2/ha per year in the paludal birch forest to 462 kg C-CO2/ha per year in small herb–green moss spruce forest. It is concluded that air temperature is an informative predictor of seasonal C-CO2 flux rate from coarse woody debris.  相似文献   
863.
We evaluated the effect of ultrasound radiation on lipase activity. The experiments were conducted using a low‐cost crude enzyme extract. We then used this lipase to treat dairy effluent. The results of our study presented an increase of approximately 430% in the concentration of free fatty acids.  相似文献   
864.
Best management practices (BMPs) are widely used to mitigate impacts of increased impervious surfaces on stormwater runoff. However, there is limited detailed and up‐to‐date information available on the cost of designing, constructing, and maintaining BMPs over their lifetime. The objective of this study is to analyze BMPs recently constructed by the Virginia Department of Transportation (VDOT) to quantify their total cost per pound of phosphorus removed annually. A motivating factor for the study is recent changes to regulatory guidelines in Virginia which allow for full or partial substitution of purchased nutrient credits in lieu of constructing onsite BMPs to achieve compliance with stormwater quality regulations. Results of the analysis of nine BMPs found their cost ranged from $20,100 to $74,900, in 2014 dollars, per pound ($44,313‐$165,126 per kg) of phosphorus removed. Based on these results and assuming current credit prices procured by VDOT, purchasing nutrient credits is a cost‐effective option for the agency, especially when factoring in the cost of additional right of way for the BMP. Based on this finding, we expect compliance with stormwater quality regulations through credit purchases to become more widely used in Virginia. Moving forward, we suggest more direct tracking of BMP costs to support comparisons between BMP costs across a range of types and conditions to credit purchases for meeting stormwater regulations.  相似文献   
865.
The National Flood Interoperability Experiment (NFIE) was an undertaking that initiated a transformation in national hydrologic forecasting by providing streamflow forecasts at high spatial resolution over the whole country. This type of large‐scale, high‐resolution hydrologic modeling requires flexible and scalable tools to handle the resulting computational loads. While high‐throughput computing (HTC) and cloud computing provide an ideal resource for large‐scale modeling because they are cost‐effective and highly scalable, nevertheless, using these tools requires specialized training that is not always common for hydrologists and engineers. In an effort to facilitate the use of HTC resources the National Science Foundation (NSF) funded project, CI‐WATER, has developed a set of Python tools that can automate the tasks of provisioning and configuring an HTC environment in the cloud, and creating and submitting jobs to that environment. These tools are packaged into two Python libraries: CondorPy and TethysCluster. Together these libraries provide a comprehensive toolkit for accessing HTC to support hydrologic modeling. Two use cases are described to demonstrate the use of the toolkit, including a web app that was used to support the NFIE national‐scale modeling.  相似文献   
866.
Hydraulic events are a leading cause of bridge failures. While these hydraulic events are accounted for in bridge design, changing environmental and land use conditions require continual updating of this risk. For example, after a bridge has been constructed, streamflow can change in unanticipated ways as a result of land use changes, geomorphic changes, and climate change. The objective of this research was to create a screening method able to quickly and inexpensively estimate overtopping risk across a collection of bridges based on the current streamflow conditions. The method uses a geographic information system, nationally available and standardized datasets, and recent regression equations to quantify bridge vulnerability to overtopping for flooding with varying return periods. This screening method could also be used to assist decision makers in updating the Waterway Adequacy field in the National Bridge Inventory, which indicates the overtopping risk of bridges. The method was applied to a portion of the Hampton Roads region of Virginia, United States that includes 475 bridges. The results of the analysis, when combined with transportation data for bridges, aid decision makers to assign further resources to complete more detailed analyses of bridges identified as being at risk for overtopping.  相似文献   
867.
868.
One uncertainty associated with large dam removal is the level of downstream sediment deposition and associated short‐term biological effects, particularly on salmonid spawning habitat. Recent studies report downstream sediment deposition following dam removal is influenced by proximity to the source and river transport capacity. The impacts of dam removal sediment releases are difficult to generalize due to the relatively small number of dam removals completed, the variation in release strategies, and the physical nature of systems. Changes to sediment deposition and associated streambed composition in the Elwha River, Washington State, were monitored prior to (2010‐2011) and during (2012‐2014) the simultaneous removal of two large dams (32 and 64 m). Changes in the surface layer substrate composition during dam removal varied by year and channel type. Riffles in floodplain channels downstream of the dams fined and remained sand dominated throughout the study period, and exceeded levels known to be detrimental to incubating salmonids. Mainstem riffles tended to fine to gravel, but appear to be trending toward cobble after the majority of the sediment was released and transported through system. Thus, salmonid spawning habitats in the mainstem appear to have been minimally impacted while those in floodplain channels appear to have been severely impacted during dam removal.  相似文献   
869.
By discharging excess stormwater at rates that more frequently exceed the critical flow for stream erosion, conventional detention basins often contribute to increased channel instability in urban and suburban systems that can be detrimental to aquatic habitat and water quality, as well as adjacent property and infrastructure. However, these ubiquitous assets, valued at approximately $600,000 per km2 in a representative suburban watershed, are ideal candidates to aid in reversing such cycles of channel degradation because improving their functionality would not necessarily require property acquisition or heavy construction. The objective of this research was to develop a simple, cost‐effective device that could be installed in detention basin outlets to reduce the erosive power of the relatively frequent storm events (~ < two‐year recurrence) and provide a passive bypass to maintain flood control performance during infrequent storms (such as the 100‐year recurrence). Results from a pilot installation show that the Detain H2O device reduced the cumulative sediment transport capacity of the preretrofit condition by greater than 40%, and contributed to reduced flashiness and prolonged baseflows in receiving streams. When scaling the strategy across a watershed, these results suggest that potential gains in water quality and stream channel stability could be achieved at costs that are orders of magnitude less than comparable benefits from newly constructed stormwater control measures.  相似文献   
870.
The ability to accurately simulate flow and nutrient removal in treatment wetlands within an agricultural, watershed‐scale model is needed to develop effective plans for meeting nutrient reduction goals associated with protection of drinking water supplies and reduction of the Gulf of Mexico hypoxic zone. The objectives of this study were to incorporate new equations for wetland hydrology and nutrient removal in Soil and Water Assessment Tool (SWAT), compare model performance using original and improved equations, and evaluate the ramifications of errors in watershed and tile drain simulation on prediction of NO3‐N dynamics in wetlands. The modified equations produced Nash‐Sutcliffe Efficiency values of 0.88 to 0.99 for daily NO3‐N load predictions, and percent bias values generally less than 6%. However, statistical improvement over the original equations was marginal and both old and new equations provided accurate simulations. The new equations reduce the model's dependence on detailed monitoring data and hydrologic calibration. Additionally, the modified equations increase SWAT's versatility by incorporating a weir equation and an irreducible nutrient concentration and temperature coefficient. Model improvements enhance the utility of SWAT for simulating flow and nutrients in wetlands and other impoundments, although performance is limited by the accuracy of inflow and NO3‐N predictions from the contributing watershed. Editor's note : This paper is part of the featured series on SWAT Applications for Emerging Hydrologic and Water Quality Challenges. See the February 2017 issue for the introduction and background to the series.  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