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   133篇
  免费   0篇
安全科学   5篇
环保管理   24篇
基础理论   98篇
污染及防治   2篇
评价与监测   2篇
灾害及防治   2篇
  2014年   12篇
  2013年   7篇
  2012年   6篇
  2011年   8篇
  2010年   10篇
  2009年   5篇
  2008年   3篇
  2007年   13篇
  2006年   8篇
  2005年   8篇
  2004年   5篇
  2003年   2篇
  2002年   4篇
  1999年   5篇
  1997年   4篇
  1996年   2篇
  1995年   2篇
  1993年   1篇
  1992年   4篇
  1991年   2篇
  1990年   4篇
  1989年   1篇
  1988年   4篇
  1987年   2篇
  1986年   2篇
  1984年   4篇
  1983年   1篇
  1982年   2篇
  1981年   1篇
  1978年   1篇
排序方式: 共有133条查询结果,搜索用时 46 毫秒
111.
Developing countries have increased their activity in smelting and refining of copper in order to retain a greater proportion of the value added and to achieve more control over production and marketing. This article examines the technology and economics of copper smelting and refining and considers the advantages and disadvantages to developing countries of increased investment in copper ore processing.
Les pays en développement ont accru leur activité dans le domaine de la fonte et du raffinage du cuivre dans le but de conserver une part plus grande de la valeur ajoutée et d'établir un contrôle plus efficace sur la production et la commercialisation de cette ressource. Cet article étudie les aspects techniques et économiques de la fonte et du raffinage du cuivre. Il compare les avantages et désavantages, pour les pays en développement, de la croissance des investissements dans le traitement du minerai de cuivre.
Los países en desarrollo han incrementado las actividades de fundición y refinado de cobre con el objeto de retener una porción mayor del valor agregado y de ejercer más control sobre la producción y el mercado. Este artículo examina la tecnología y economía de la fundición y refinado del cobre y considera las ventajas y desventajas de mayores inversiones en el procesado del mineral de cobre por los países en desarrollo.  相似文献   
112.
Abstract: Different deforestation agents, such as small farmers and large agricultural businesses, create different spatial patterns of deforestation. We analyzed the proportion of deforestation associated with different‐sized clearings in the Brazilian Amazon from 2002 through 2009. We used annual deforestation maps to determine total area deforested and the size distribution of deforested patches per year. The size distribution of deforested areas changed over time in a consistent, directional manner. Large clearings (>1000 ha) comprised progressively smaller amounts of total annual deforestation. The number of smaller clearings (6.25–50.00 ha) remained unchanged over time. Small clearings accounted for 73% of all deforestation in 2009, up from 30% in 2002, whereas the proportion of deforestation attributable to large clearings decreased from 13% to 3% between 2002 and 2009. Large clearings were concentrated in Mato Grosso, but also occurred in eastern Pará and in Rondônia. In 2002 large clearings accounted for 17%, 15%, and 10% of all deforestation in Mato Grosso, Pará, and Rondônia, respectively. Even in these states, where there is a highly developed agricultural business dominated by soybean production and cattle ranching, the proportional contribution of large clearings to total deforestation declined. By 2009 large clearings accounted for 2.5%, 3.5%, and 1% of all deforestation in Mato Grosso, Pará, and Rondônia, respectively. These changes in deforestation patch size are coincident with the implementation of new conservation policies by the Brazilian government, which suggests that these policies are not effectively reducing the number of small clearings in primary forest, whether these are caused by large landholders or smallholders, but have been more effective at reducing the frequency of larger clearings.  相似文献   
113.
Abstract: The new approaches advocated by the conservation community to integrate conservation and livelihood development now explicitly address landscape mosaics composed of agricultural and forested land rather than only protected areas and largely intact forests. We refer specifically to a call by Harvey et al. (2008) to develop a new approach based on six strategies to integrate biodiversity conservation with sustainable livelihoods in Mesoamerican landscape mosaics. We examined the applicability of this proposal to the coffee agroforests of the Western Ghats, India. Of the six strategies, only one directly addresses livelihood conditions. Their approach has a clear emphasis on conservation and, as currently formulated risks repeating the failures of past integrated conservation and development projects. It fails to place the aspirations of farmers at the core of the agenda. Thus, although we acknowledge and share the broad vision and many of the ideas proposed by this approach, we urge more balanced priority setting by emphasizing people as much as biodiversity through a careful consideration of local livelihood needs and aspirations.  相似文献   
114.
A survey of one of the camps still holding refugees from Iraq who crossed into Turkey in the spring of 1991 showed that the majority of the population was under 15 years of age and that increased mortality occurred during the first 30 days after the refugees left their homes in Iraq. Infants, young children, and the elderly suffered the highest mortality, with infant mortality rates (IMRs) over the first month of the crisis approximately 18–29 times the MR in Iraq in the late 1980s. Still unexplained is a greater than two-fold excess mortality among males compared with females. Other demographic and health findings are also reported.  相似文献   
115.
116.
A key ingredient in the expansion of agricultural output in many developing countries is the availability of reliable and cost-effective energy supplies for irrigation. Such supplies are also fundamental to any strategy to stimulate rural economic development. This paper compares the costs of water pumping through the use of commonly-used, small diesel-driven pumpsets versus a decentralized generator connected to a small electrified grid. An example representative of irrigated cropping systems found in the Senegal River Basin is used to compare costs. The results show that pumping costs from a decentralized power supply and minigrid are roughly comparable with those of direct-drive diesel pumpsets. Further, excess capacity would be available to provide power for small industries and other rural needs. In many sparsely populated rural locations where low capacity factors make central grid extension uneconomic, a minigrid with a guaranteed load of irrigation pumping can be an attractive alternative for providing power for rural development.  相似文献   
117.
Abstract:  Urbanization negatively affects natural ecosystems in many ways, and aquatic systems in particular. Urbanization is also cited as one of the potential contributors to recent dramatic declines in amphibian populations. From 2000 to 2002 we determined the distribution and abundance of native amphibians and exotic predators and characterized stream habitat and invertebrate communities in 35 streams in an urbanized landscape north of Los Angeles (U.S.A.). We measured watershed development as the percentage of area within each watershed occupied by urban land uses. Streams in more developed watersheds often had exotic crayfish ( Procambarus clarkii ) and fish, and had fewer native species such as California newts ( Taricha torosa ) and California treefrogs ( Hyla cadaverina ). These effects seemed particularly evident above 8% development, a result coincident with other urban stream studies that show negative impacts beginning at 10–15% urbanization. For Pacific treefrogs ( H. regilla ), the most widespread native amphibian, abundance was lower in the presence of exotic crayfish, although direct urbanization effects were not found. Benthic macroinvertebrate communities were also less diverse in urban streams, especially for sensitive species. Faunal community changes in urban streams may be related to changes in physical stream habitat, such as fewer pool and more run habitats and increased water depth and flow, leading to more permanent streams. Variation in stream permanence was particularly evident in 2002, a dry year when many natural streams were dry but urban streams were relatively unchanged. Urbanization has significantly altered stream habitat in this region and may enhance invasion by exotic species and negatively affect diversity and abundance of native amphibians.  相似文献   
118.
Abstract:  A World Conservation Union (IUCN) regional red list is an objective assessment of regional extinction risk and is not the same as a list of conservation priority species. Recent research reveals the widespread, but incorrect, assumption that IUCN Red List categories represent a hierarchical list of priorities for conservation action. We developed a simple eight-step priority-setting process and applied it to the conservation of bees in Ireland. Our model is based on the national red list but also considers the global significance of the national population; the conservation status at global, continental, and regional levels; key biological, economic, and societal factors; and is compatible with existing conservation agreements and legislation. Throughout Ireland, almost one-third of the bee fauna is threatened (30 of 100 species), but our methodology resulted in a reduced list of only 17 priority species. We did not use the priority species list to broadly categorize species to the conservation action required; instead, we indicated the individual action required for all threatened, near-threatened, and data-deficient species on the national red list based on the IUCN's conservation-actions template file. Priority species lists will strongly influence prioritization of conservation actions at national levels, but action should not be exclusive to listed species. In addition, all species on this list will not necessarily require immediate action. Our method is transparent, reproducible, and readily applicable to other taxa and regions.  相似文献   
119.
Science, Scientists, and Policy Advocacy   总被引:1,自引:1,他引:1  
  相似文献   
120.
Abstract:  Many carnivore populations escaped extinction during the twentieth century as a result of legal protections, habitat restoration, and changes in public attitudes. However, encounters between carnivores, livestock, and humans are increasing in some areas, raising concerns about the costs of carnivore conservation. We present a method to predict sites of human-carnivore conflicts regionally, using as an example the mixed forest-agriculture landscapes of Wisconsin and Minnesota (U.S.A.). We used a matched-pair analysis of 17 landscape variables in a geographic information system to discriminate affected areas from unaffected areas at two spatial scales (townships and farms). Wolves ( Canis lupus ) selectively preyed on livestock in townships with high proportions of pasture and high densities of deer ( Odocoileus virginianus ) combined with low proportions of crop lands, coniferous forest, herbaceous wetlands, and open water. These variables plus road density and farm size also appeared to predict risk for individual farms when we considered Minnesota alone. In Wisconsin only, farm size, crop lands, and road density were associated with the risk of wolf attack on livestock. At the level of townships, we generated two state-wide maps to predict the extent and location of future predation on livestock. Our approach can be applied wherever spatial data are available on sites of conflict between wildlife and humans.  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