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   1519篇
  免费   57篇
  国内免费   159篇
安全科学   116篇
废物处理   84篇
环保管理   884篇
综合类   369篇
基础理论   90篇
污染及防治   58篇
评价与监测   65篇
社会与环境   15篇
灾害及防治   54篇
  2024年   1篇
  2023年   9篇
  2022年   14篇
  2021年   16篇
  2020年   23篇
  2019年   5篇
  2018年   10篇
  2017年   12篇
  2016年   18篇
  2015年   14篇
  2014年   19篇
  2013年   25篇
  2012年   16篇
  2011年   30篇
  2010年   29篇
  2009年   24篇
  2008年   57篇
  2007年   13篇
  2006年   21篇
  2005年   17篇
  2004年   38篇
  2003年   70篇
  2002年   159篇
  2001年   142篇
  2000年   174篇
  1999年   134篇
  1998年   103篇
  1997年   80篇
  1996年   214篇
  1995年   60篇
  1994年   17篇
  1993年   12篇
  1992年   5篇
  1991年   5篇
  1990年   4篇
  1989年   9篇
  1988年   3篇
  1987年   2篇
  1986年   1篇
  1985年   22篇
  1984年   38篇
  1983年   35篇
  1982年   4篇
  1981年   2篇
  1980年   7篇
  1978年   5篇
  1977年   1篇
  1975年   1篇
  1971年   14篇
  1967年   1篇
排序方式: 共有1735条查询结果,搜索用时 906 毫秒
491.
The US Army Engineering Research Development Center (ERDC) uses a modified form of the Revised Universal Soil Loss Equation (RUSLE) to estimate spatially explicit rates of soil erosion by water across military training facilities. One modification involves the RUSLE support practice factor (P factor), which is used to account for the effect of disturbance by human activities on erosion rates. Since disturbance from off-road military vehicular traffic moving through complex landscapes varies spatially, a spatially explicit nonlinear regression model (disturbance model) is used to predict the distribution of P factor values across a training facility. This research analyzes the uncertainty in this model's disturbance predictions for the Fort Hood training facility in order to determine both the spatial distribution of prediction uncertainty and the contribution of different error sources to that uncertainty. This analysis shows that a three-category vegetation map used by the disturbance model was the greatest source of prediction uncertainty, especially for the map categories shrub and tree. In areas mapped as grass, modeling error (uncertainty associated with the model parameter estimates) was the largest uncertainty source. These results indicate that the use of a high-quality vegetation map that is periodically updated to reflect current vegetation distributions, would produce the greatest reductions in disturbance prediction uncertainty.  相似文献   
492.
The National Vegetation Classification Standard (NVCS) was implemented at two US National Park Service (NPS) sites in Texas, the Padre Island National Seashore (PINS) and the Lake Meredith National Recreation Area (LMNRA), to provide information for NPS oil and gas management plans. Because NVCS landcover classifications did not exist for these two areas prior to this study, we created landcover classes, through intensive ground and aerial reconnaissance, that characterized the general landscape features and at the same time complied with NVCS guidelines. The created landcover classes were useful for the resource management and were conducive to classification with optical remote sensing systems, such as the Landsat Thematic Mapper (TM). In the LMNRA, topographic elevation data were added to the TM data to reduce confusion between cliff, high plains, and forest classes. Classification accuracies (kappa statistics) of 89.9% (0.89) and 88.2% (0.87) in PINS and LMNRA, respectively, verified that the two NPS landholdings were adequately mapped with TM data. Improved sensor systems with higher spectral and spatial resolutions will ultimately refine the broad classes defined in this classification; however, the landcover classifications created in this study have already provided valuable information for the management of both NPS lands. Habitat information provided by the classifications has aided in the placement of inventory and monitoring plots, has assisted oil and gas operators by providing information on sensitive habitats, and has allowed park managers to better use resources when fighting wildland fires and in protecting visitors and the infrastructure of NPS lands.  相似文献   
493.
The amount of ecological restoration required to mitigate or compensate for environmental injury or habitat loss is often based on the goal of achieving ecological equivalence. However, few tools are available for estimating the extent of restoration required to achieve habitat services equivalent to those that were lost. This paper describes habitat equivalency analysis (HEA), a habitat-based “service-to-service” approach for determining the amount of restoration needed to compensate for natural resource losses, and examines issues in its application in the case of salt marsh restoration. The scientific literature indicates that although structural attributes such as vegetation may recover within a few years, there is often a significant lag in the development of ecological processes such as nutrient cycling that are necessary for a fully functioning salt marsh. Moreover, natural variation can make recovery trajectories difficult to define and predict for many habitat services. HEA is an excellent tool for scaling restoration actions because it reflects this ecological variability and complexity. At the same time, practitioners must recognize that conclusions about the amount of restoration needed to provide ecological services equivalent to those that are lost will depend critically on the ecological data and assumptions that are used in the HEA calculation.  相似文献   
494.
A fuzzy decision analysis method for integrating ecological indicators was developed. This was a combination of a fuzzy ranking method and the analytic hierarchy process (AHP). The method was capable of ranking ecosystems in terms of environmental conditions and suggesting cumulative impacts across a large region. Using data on land cover, population, roads, streams, air pollution, and topography of the Mid-Atlantic region, we were able to point out areas that were in relatively poor condition and/or vulnerable to future deterioration. The method offered an easy and comprehensive way to combine the strengths of fuzzy set theory and the AHP for ecological assessment. Furthermore, the suggested method can serve as a building block for the evaluation of environmental policies.  相似文献   
495.
Applying Ecological Risk Principles to Watershed Assessment and Management   总被引:6,自引:0,他引:6  
Considerable progress in addressing point source (end of pipe) pollution problems has been made, but it is now recognized that further substantial environmental improvements depend on controlling nonpoint source pollution. A watershed approach is being used more frequently to address these problems because traditional regulatory approaches do not focus on nonpoint sources. The watershed approach is organized around the guiding principles of partnerships, geographic focus, and management based on sound science and data. This helps to focus efforts on the highest priority problems within hydrologically-defined geographic areas. Ecological risk assessment is a process to collect, organize, analyze, and present scientific information to improve decision making. The U.S. Environmental Protection Agency (EPA) sponsored three watershed assessments and found that integrating the watershed approach with ecological risk assessment increases the use of environmental monitoring and assessment data in decision making. This paper describes the basics of the watershed approach, the ecological risk assessment process, and how these two frameworks can be integrated. The three major principles of watershed ecological risk assessment found to be most useful for increasing the use of science in decision making are (1) using assessment endpoints and conceptual models, (2) holding regular interactions between scientists and managers, and (3) developing a focus for multiple stressor analysis. Examples are provided illustrating how these principles were implemented in these assessments.  相似文献   
496.
California and other regions in the United States are becoming more populated and ethnically diverse, and thus, ecological impacts on the wildland–urban interface are a significant policy concern. In a socioeconomic assessment focused on the geographic regions surrounding four national forests in southern California, population projections are being formulated to assist in the update of forest plans. In southern California, the projected trend of explosive population growth combined with increased ethnic and racial diversity indicates four challenges for environmental management. First, patterns of recreation use on wildlands are likely to change, and management of these areas will have to address new needs. Second, as land-management agencies face changing constituencies, new methods of soliciting public involvement from ethnic and racial groups will be necessary. Third, growth in the region is likely to encroach upon wildland areas, affecting water, air, open space, and endangered species. Fourth, in order to address all these concerns in a climate of declining budgets, resource management agencies need to strengthen collaborative relationships with other agencies in the region. How environmental managers approach these changes has widespread implications for the ecological sustainability of forests in southern California.  相似文献   
497.
Community involvement is fundamental to the management of multijurisdictional river basins but, in practice, is very difficult to achieve. The Murray-Darling basin, in Australia, and the Mekong River basin in Southeast Asia are both cooperatively managed multijurisdictional river basins where the management authorities have expressed an aim of community involvement. In the Murray-Darling basin vigorous efforts have promoted a culture of community consultation throughout each of the state jurisdictions involved, although true participation has not necessarily been achieved. In the Mekong basin the community is much more diverse and the successes so far have been largely at the local level, involving action in subsections of the basin. These case studies suggest that community involvement in the form of community consultation across large multijurisdictional river basins is achievable, but more comprehensive participation is not necessarily possible.  相似文献   
498.
分析探讨了浙江鄞县42级地震及历史上浙北中强震前的地震空区现象,认为孕震空区可能是浙北中等及中强震的中期前兆,并提出浙北孕震空区的特征及判别方法。  相似文献   
499.
2 (1 and 24 h), SO2 (1 and 24 h), O3 (1 h) and PM10 (24 h). The AQI is linked to the new air quality guidelines in Finland. The AQI is based on acute health effects, but long-term effects on nature and materials are also taken into consideration. Subindices are calculated hourly for all pollutants and for a given hour the highest subindex becomes the AQI. AQI values are currently calculated for the center of Helsinki and for typical suburban areas.  相似文献   
500.
/ Recreation satisfaction is a complex psychological construct that is difficult to define and measure. Recent approaches suggest that overall satisfaction may be a function of multiple satisfactions derived from specific elements of a recreation experience such as the situational characteristics of a recreation setting or activity and the recreationist's subjective evaluations of the experience. In this paper, a path model of whitewater boating satisfaction was tested using data from a survey of 1210 commercial and 111 private boaters on the Cheat River of West Virginia. The pathmodel included the direct and mediating effects of situational variables and the subjective evaluations of boaters and explained 52% and 54% of the variation in satisfaction of commercial and private boaters, respectively. Factors related to the satisfaction of both groups included a composite variable representing opportunities for challenge, excitement, and skill testing on the river trip; water flow levels; and crowding perceptions. In combination, water flow level and boater's perceptions of opportunities to experience challenge, excitement, and test boating skills were the most important variables for explaining satisfaction of both groups. Additional factors affecting commercial, but not private, boater satisfaction included the motive of escaping the usual demands of life and a social interaction variable. Among private boaters, perceptions of the environmental conditions also contributed to overall satisfaction. The results support the multiple satisfaction approach of previous research. River management implications are discussed.KEY WORDS: Whitewater; River recreation; Satisfaction  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