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   12946篇
  免费   1284篇
  国内免费   2244篇
安全科学   2723篇
废物处理   304篇
环保管理   2325篇
综合类   6316篇
基础理论   1507篇
环境理论   16篇
污染及防治   874篇
评价与监测   842篇
社会与环境   759篇
灾害及防治   808篇
  2024年   56篇
  2023年   258篇
  2022年   459篇
  2021年   550篇
  2020年   518篇
  2019年   491篇
  2018年   424篇
  2017年   530篇
  2016年   642篇
  2015年   653篇
  2014年   691篇
  2013年   947篇
  2012年   1034篇
  2011年   1125篇
  2010年   748篇
  2009年   787篇
  2008年   663篇
  2007年   839篇
  2006年   742篇
  2005年   628篇
  2004年   477篇
  2003年   462篇
  2002年   394篇
  2001年   331篇
  2000年   276篇
  1999年   238篇
  1998年   165篇
  1997年   188篇
  1996年   150篇
  1995年   132篇
  1994年   124篇
  1993年   103篇
  1992年   81篇
  1991年   64篇
  1990年   52篇
  1989年   46篇
  1988年   47篇
  1987年   35篇
  1986年   25篇
  1985年   26篇
  1984年   13篇
  1983年   16篇
  1982年   31篇
  1981年   31篇
  1980年   29篇
  1979年   26篇
  1978年   21篇
  1977年   12篇
  1972年   24篇
  1971年   27篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
961.
: In general, the choice among reservoirs for water supply or flow augmentation is a multiobjective problem. Choices are based in part on the yield available from water supply reservoirs or, in the case of flow augmentation reservoirs, on the increase in low flows at downstream locations. Detailed estimates of these effects may be too costly for basin planning purposes. Thus this paper presents methods for rapid estimation of those quantities for New Hampshire. For water supply reservoirs, a composite empirical relation between Y95 (the draft available 95 percent of the time) and storage ratio, S*, is developed from previous studies in the region. For flow augmentation reservoirs, empirical relations between S* and degree of regulation, R*, are applied to each upstream regulating reservoir. Values of regulation arc then summed and divided by the mean flow at the downstream reach of interest. This parameter, (ΓR)*, is then related to increase in flow available 95 percent of the time by an empirical relation.  相似文献   
962.
Velocity and depth preference curves for juvenile and adult red shiners (Notropis lutrensis) were developed for each season of the year. The incremental methodology, developed by the U.S. Fish and Wildlife Service for quantifying the amount of physical habitat available for fish, was applied as a basis for recommending minimum stream flows for life stages of red shiners and channel catfish (Ictalurus punctatus) in the Washita River below Foss Reservoir. The minimum stream flow required to support populations of red shiners and channel catfish was estimated to be 0.60 m3/s during all months except April and May, during which flushing flows of 3.0 m3/s were recommended. Under drought conditions a contingency flow of 0.18 m3/s was recommended for August through March, a flushing flow of 2.4 m3/s for April and May, and a flow of 0.30 m3/s for channel catfish spawning in June and July. Standing crop of adult red shiners per weighted usable area averaged 2.73 g/m2 (27 kg/ha) for two sites on the Washita River. A positive relation between standing crop and weighted usable area was suggested. Use of the incremental methodology on the study reach below Foss Reservoir required 96 person-hours of labor, excluding training, travel, and developing preference curves, and a total cost of $1,762, of which $1,200 was for the purchase of equipment.  相似文献   
963.
ABSTRACT: A method of project planning for systematically assessing and recording impacts of alternative timber harvest startegies for subareas of a sale planning area is presented. This method uses an interdisciplinary team composed of specialists and decisionmaker to develop and analyze treatment alternatives and uses impact prediction matrices with explicit evaluation criteria to illustrate and record ratings of impacts on resource factors. The method is a system for organizing, for discovering appropriate alternatives, for facilitating interaction, and for documenting environmental analysis. It provides for decisionmaker control over the project, for exchange of value perspectives, and for reduction in conflict between specialists. The quality of decisions which result from the method presented depends upon the information available and the effectiveness of interaction between all members of the interdisciplinary team.  相似文献   
964.
ABSTRACT: For many years, Federal water resources projects have been subjected to benefit-cost analysis to establish their economic feasibility. Several years ago social well-being was added as a consideration. This paper discusses the state-of-the-art in analyzing and evaluating aspects of social well-being. It stresses current short-falls and advocates a direction for further efforts.  相似文献   
965.
ABSTRACT. Transient, two-dimensional solutions are developed which describe the movement and distribution of a conservative substance in a stream-aquifer system. The solutions are obtained by solving sequentially the groundwater flow and mass transport equations. A variational approach in conjunction with the finite element method is used to solve the groundwater flow equation. Galerkin's approach coupled with the finite element method is used to solve the mass transport equation. Linear approximated triangular elements and a centered scheme of numerical integration are employed to calculate the hydraulic head distribution and the concentration of solute in the flow region. The linear approximation used to define the concentration function within each element is not appropriate for cases involving steep concentration gradients. For such cases, higher order approximations are necessary to assure the continuity of gradients across interelemental boundaries. Numerical examples that illustrate the applicability of the model are presented.  相似文献   
966.
To document environmental impact predictions for land development, as required by United States government regulatory agencies, vegetation studies are conducted using a variety of methods. Density measurement (stem counts) is one method that is frequently used. However, density measurement of shrub and herbaceous vegetation is time-consuming and costly. As an alternative, the Braun-Blanquet cover-abundance scale was used to analyze vegetation in several ecological studies. Results from one of these studies show that the Braun-Blanquet method requires only one third to one fifth the field time required for the density method. Furthermore, cover-abundance ratings are better suited than density values to elucidate graphically species-environment relationships. For extensive surveys this method provides sufficiently accurate baseline data to allow environmental impact assessment as required by regulatory agencies.  相似文献   
967.
ABSTRACT: A simple simulator was constructed, calibrated, programmed and used in management studies of a hydraulic system containing two pumping stations in series, connected by canals and reservoirs. The features of the model, its calibration and use are described, and comparisons between measured and computed hydraulic behavior are presented.  相似文献   
968.
ABSTRACT: The density and distribution of land uses has important consequences for the planning of sewerage systems and for the costs of these systems. This paper examines these consequences using a simplified service area model. The model determines the area to be served by a central waste treatment plant, where alternative on-lot disposal systems are also available. The model is applied to various urban area configurations, which are summarized by their total populations and by their population density distributions. Both minimum regional cost and minimum local cost service area configurations are determined. In addition, the sensitivity of the model to the parameters of the cost and population density functions is assessed. It is found that the model is most sensitive to the parameters of the collection cost function.  相似文献   
969.
ABSTRACT: Current policies for correcting the problem of irrigation return flow pollution tend to attack the symptoms of the problem, rather than its cause. The present institutional arrangement for allocating irrigation water is seen as the source of the problem. This paper examines the water quality benefits of altering the institutional arrangement to allow for irrigation water transfers through a rental market. It is conceptualized that by creating a water rental market an opportunity cost would be associated with the use of irrigation water such that profit maximizing farmers would be induced to use his water supply more efficiently and rent the surplus to other irrigators, thus reducing return flow pollution. It is shown that a water rental market could increase water quality in the Yakima River in southcentral Washington by 31 percent as well as increase farm incomes and crop production.  相似文献   
970.
New concepts and insights concerning human and natural systems in the coastal zone are emerging from recent energy analysis and synthesis studies. By using new concepts for measuring the quality of energy, one can express the work of ecosystems and human economies in equivalent terms. From energy models and new energy quality evaluations, one can learn what kinds of coastal systems maximize power, are competitive, economically vital, and likely to have a survival advantage.Energy analysis and synthesis is aided by energy circuit diagrams. Models of the coastal zone that emphasize the change in external driving functions related to world energy sources provide insights and some predictive abilities that are not found in economic studies, since money flows alone do not evaluate external driving energies.This paper suggests four procedures for coastal planning: 1) calculation of investment ratio in units of equal quality to determine which projects are economic in a broad sense, 2) development of energy signatures for coastal ecosystems, 3) determination of which interface ecosystems develop the best energy flows, and 4) development of regional models that include the main features of human and natural ecosystems.  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