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   28872篇
  免费   192篇
  国内免费   165篇
安全科学   520篇
废物处理   1506篇
环保管理   3809篇
综合类   3981篇
基础理论   8207篇
环境理论   6篇
污染及防治   6602篇
评价与监测   2152篇
社会与环境   2327篇
灾害及防治   119篇
  2022年   131篇
  2021年   162篇
  2020年   128篇
  2019年   178篇
  2018年   1777篇
  2017年   1691篇
  2016年   1605篇
  2015年   431篇
  2014年   456篇
  2013年   1380篇
  2012年   1011篇
  2011年   2121篇
  2010年   1452篇
  2009年   1306篇
  2008年   1756篇
  2007年   2156篇
  2006年   758篇
  2005年   648篇
  2004年   722篇
  2003年   721篇
  2002年   729篇
  2001年   780篇
  2000年   517篇
  1999年   309篇
  1998年   285篇
  1997年   241篇
  1996年   242篇
  1995年   276篇
  1994年   299篇
  1993年   233篇
  1992年   268篇
  1991年   245篇
  1990年   267篇
  1989年   258篇
  1988年   203篇
  1987年   180篇
  1986年   161篇
  1985年   173篇
  1984年   215篇
  1983年   194篇
  1982年   201篇
  1981年   191篇
  1980年   145篇
  1979年   165篇
  1978年   135篇
  1977年   119篇
  1975年   123篇
  1974年   117篇
  1973年   114篇
  1972年   137篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
962.
A resource survey and planning method for parks, reserves, and other environmentally significant areas (ESAs) is presented in the context of a holistic balanced approach to land use and environmental management. This method provides a framework for the acquisition, analysis, presentation, and application of diverse ecological data pertinent to land use planning and resource management within ESAs. Through the independent analysis and subsequent integration of abiotic, biotic, and cultural or ABC information, land areas within an ESA are identified in terms of their relative environmental significance and environmental constraints. The former term encompasses wildlife, historic, and other resource values, while the latter term reflects biophysical hazards and sensitivities, and land use conflicts. The method thus calls for a matching of an ESA's distinctive attributes with appropriate land use and institutional arrancements through an analysis of available acts, regulations, agencies, and other conservation and land use management mechanisms. The method culminates with a management proposal showing proposed park or reserve allocations, buffer areas, or other land use controls aimed at preserving an ESA's special ecological qualities, while providing for resource development. The authors suggest that all resource management decisions affecting ESA's should be governed by a philosophical stance that recognizes a spectrum of broad land use types, ranging from preservation to extractive use and rehabilitation.  相似文献   
963.
964.
The breakpoint rainfall hydrology and pesticide options of the field scale model CREAMS (Chemicals, Runoff, and Erosion from Agricultural Management Systems) were used to predict average concentrations of hexazinone [3 cyclohexyl-6-(dimethyl-amino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione] in stormflow from four forested watersheds in the upper Piedmont region of Georgia. Predicted concentrations were compared with measured concentrations recorded over a 13-month period. CREAMS accurately predicted hexazinone concetrations in the initial stormflow events which also contained the highest concentrations. The model underestimated the hexazinone concentrations in stormflow two months and greater following pesticide application. In a companion study, the daily rainfall option of the CREAMS model was used to evaluate the reltive risk associated with the maximum expected concentration of hexazinone, bromacil (5-bromo-3 sec-butyl-6 methyuracil), picloram (4-amino-3,5,6 trichloropicolinic acid), dicamba (3,6-dichloro-0-anisic acid), and triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid} in stormflow from small forested watersheds. The model predicted the following order of potential residue appearance in stormflow: bromacil>triclopyr>hexazinone>picloram>dicamba. Subsurface movement of residues via interflow and deep leaching losses are not simulated by the version of CREAMS used in these studies.  相似文献   
965.
In July 1982 the US Government announced that it would not sign the new Convention on the Law of the Sea adopted at the eleventh session of the Third United Nations Conference on the Law of the Sea in April 1982. This article investigates the possible consequences of the failure to achieve a consensus between the Group of 77 and the western industrialised States on a regime to govern the deep-sea mining of manganese nodules. After considering the range of possible outcomes, the author identifies the benefits and costs involved in operating under the two alternative regimes, that provided by the new Convention and related Conference resolutions and the alternative Reciprocating States Regime based on unilateral legislation co-ordinated by a Mini-Treaty. En juillet 1982, le gouvernement des Etats-Unis a annoncé qu'il ne signerait pas la nouvelle Convention sur le droit de la mer qui a été adoptée à la llème session de la troisième Conférence des Nations Unies sur le droit de la mer en avril 1982. Cet article étudie les conséquences possibles de l'absence d'un consensus entre le Groupe des 77 et les nations industrielles occidentales sur un régime devant gouverner l'exploitation des nodules de manganèse que l'on trouve sur les fonds marins profonds. Après avoir consideéré tout un éventail de résultantes éventuelles, l'auteur dresse un état des avantages et désavantages que présentent les deux régimes possibles, c'est-à-dire le régime prévu par la nouvelle Convention et les résolutions concomitantes adoptées par la Conférence et le régime de réciprocité entre états basé sur une législation unilatérale et sanctionné par un mini-traité. En julio de 1982 el gobierno de los Estados Unidos anunció que no firmaría la nueva Convención sobre el Derecho del Maradoptada en la undécima sesión de la Tercera Conferencia de las Naciones Unidas sobre el Derecho del Mar en abril de 1982. Este artículo investiga las posibles consecuencias de la falta de consenso entre el Grupo de los 77 y los estados industriales occidentales sobre el régimen que gobierna la extracción de nódulos de manganeso en mares profundos. Después de considerar el posible rango de consecuencias el autor evalúa los costos y beneficios resultantes de operar bajo dos regímenes alternativos, aquel definido por la nueva convención y las resoluciones relacionadas y el Régimen de Reciprocidad entre Estados basado en legislación unilateral coordinados por el Mini-Tratado.  相似文献   
966.
967.
968.
Bioproduction strategies for rare hexose sugars   总被引:1,自引:0,他引:1  
A new strategy for the bioproduction of all ketohexoses was developed using hexitols as intermediates. Biocatalysts used to employ the strategy were D-tagatose 3-epimerase, which epimerizes ketohexoses at the C-3 position, and oxidoreductases, which catalyze oxidation-reduction reactions between ketohexoses and the corresponding hexitols. Arranging all the ketohexoses and hexitols in a symmetric ring and connecting them with 20 biochemical reactions, I was able to construct a design for the bioproduction of all the rare ketohexoses. Various aldose isomerases transform ketohexoses into the corresponding aldohexoses, so the strategy is useful for the bioproduction of all the rare hexose sugars. Furthermore, the design revealed that there are four routes to the L-hexose world from the D-hexose one.  相似文献   
969.
The stability of cohesive sediment deposits during a rare storm is a critical component in the evaluation of remedial options at a contaminated sediment site. Estimating scour depths during a rare storm, and the resulting contaminant concentrations in the surficial layer of the bed, is necessary for comparing the efficacy of various remedial alternatives. Evaluation of sediment stability is accomplished using sediment transport analyses that employ quantitative procedures. Qualitative analyses or conceptual models can be useful for developing and validating quantitative analysis tools; however, qualitative techniques alone generally are insufficient for conducting defensible remedial alternative evaluations. The level of analysis used for a specific site depends on data availability, required level of accuracy, and time and budget constraints. A tier 1 analysis involves the use of approximate equations to produce order-of-magnitude estimates of scour depths during a rare storm. The second tier of this analysis scheme employs the development and application of a sediment transport model to evaluate bed stability. State-of-the-science sediment transport models have been effectively used as management tools for evaluating remedial options at several contaminated sediment sites. It should not be presumed that rare storm events cause catastrophic impacts at the site under review. Two case studies demonstrate that a rare storm is not necessarily catastrophic; significant increases in surficial bed concentrations caused by reexposure of elevated concentrations buried at depth in the bed will not necessarily occur during a rare storm. However, it is important to note that sediment stability is site-specific.  相似文献   
970.
Under the United States Clean Air Act Amendments of 1977, a class I designation safeguards wilderness areas from the negative effects of new sources of air pollution. We monitored streamwater chemistry in the class I Lye Brook Wilderness in southwestern Vermont from May 1994 through August 1995. Stream samples were collected biweekly at nine sampling locations throughout the wilderness and were analyzed for major cations and anions, dissolved organic carbon, pH, and acid-neutralizing capacity. Eight of nine sites sampled had mean annual acid neutralizing capacity values below zero. During the study period, decreases in streamwater acid neutralizing capacity values were caused primarily by SO4 2−. At some sites, however, NO3 and naturally occurring, weak organic acids were seasonally important. During high discharge, the low pH and high concentrations of inorganic monomeric Al were at levels that are toxic to acid-sensitive aquatic species. Watershed mass balances were calculated to determine annual gains or losses for measured ions. These budgets indicate that S inputs and outputs were nearly equal, there was a net loss of base cations, and a net gain in N. How long these watersheds can continue to assimilate additional N inputs is unknown.  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