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   5000篇
  免费   147篇
  国内免费   61篇
安全科学   273篇
废物处理   168篇
环保管理   1268篇
综合类   540篇
基础理论   1387篇
环境理论   8篇
污染及防治   1013篇
评价与监测   323篇
社会与环境   173篇
灾害及防治   55篇
  2024年   14篇
  2023年   69篇
  2022年   59篇
  2021年   65篇
  2020年   66篇
  2019年   69篇
  2018年   122篇
  2017年   134篇
  2016年   181篇
  2015年   121篇
  2014年   162篇
  2013年   411篇
  2012年   237篇
  2011年   307篇
  2010年   203篇
  2009年   236篇
  2008年   263篇
  2007年   257篇
  2006年   229篇
  2005年   198篇
  2004年   184篇
  2003年   149篇
  2002年   142篇
  2001年   88篇
  2000年   96篇
  1999年   68篇
  1998年   76篇
  1997年   61篇
  1996年   65篇
  1995年   80篇
  1994年   74篇
  1993年   68篇
  1992年   58篇
  1991年   40篇
  1990年   36篇
  1989年   37篇
  1988年   30篇
  1987年   39篇
  1986年   41篇
  1985年   39篇
  1984年   44篇
  1983年   45篇
  1982年   52篇
  1981年   43篇
  1980年   32篇
  1979年   15篇
  1978年   29篇
  1977年   16篇
  1976年   13篇
  1971年   9篇
排序方式: 共有5208条查询结果,搜索用时 15 毫秒
81.
ABSTRACT: Most water-resouree investigations in semiarid basins of the Great Basin in western North America conclude that ground-water recharge from direct precipitation on the valley floor is negligible. However, many of these basins contain large areas covered by unvegetated, active sand dunes that may act as conduits for ground-water recharge. The potential for this previously undocumented recharge was investigated in an area covered by sand dunes in Desert Valley, northwestern Nevada, using a deep percolation model. The model uses daily measurements of precipitation and temperature th determine energy and moisture balances, from which estimates of long-term mean annual recharge are made. For the study area, the model calculated a mean annual recharge rate of as much as 1.3 inches per year, or 17 percent of the long-term mean precipitation. Model simulations also indicate that recharge would be virtually zero if the study area were covered by vegetation rather than dunes.  相似文献   
82.
In this article the authors assert that successful long-term pollution prevention will require changes in the culture and business practices of an organization such that the term “pollution prevention” eventually becomes obsolete. They envision firms evolving to a point where actions that today are referred to as pollution prevention will simply be part of standard business practices and thoroughly embedded in the firms' culture. Although pollution prevention projects and programs need to be highly visible at the outset to communicate throughout the organization and to demonstrate the benefits that can be gained, companies should adopt a long-term strategy aimed at making fundamental changes in the way they do business, rather than relying solely on piecemeal projects and special plans or programs. Such evolution of the firm will maximize the chances of meeting the dual challenges of business competitiveness and environmental stewardship. This article presents a vision for integrating environmental performance with business objectives and suggests practical steps to begin moving toward that vision.  相似文献   
83.
ABSTRACT: A procedure using detrended kriging has been developed to calculate daily values of mean areal precipitation (MAP) for input to hydrologic models. The important features of this procedure that overcome weaknesses in existing MAP procedures are: (1) specific precipitation-elevation relationships are determined for each time period as opposed to using relationships based on climatological averages, (2) spatial variability is incorporated by estimating precipitation for each grid cell over a watershed, (3) the spatial correlation structure of precipitation is explicitly modeled, and (4) station weights for precipitation estimates are determined objectively and optimally. Detailed cross-validation testing of the procedure was done for the Reynolds Creek research watershed in southwestern Idaho. The procedure is suitable for use in operational streamflow forecasting.  相似文献   
84.
Summary In order to consider appropriate sustainable development, it is first necessary to understand the problems of developments that are inappropriate for specific population groups. Variables, such as, the culture of the group, its historical experiences, demographic and economic resources, population density, and remoteness or urban proximity should be scrutinized. This is a case study of one population group with a history of resource dependency.Inchelium is a small Native American community on the Colville Indian Reservation in Washington, USA. The construction of a large dam ended fishing in Inchelium. Changes due to the dam have led to exploitation and degradation of the community's lifestyle. Several attempts at viable alternative economic development have led to disagreements about cultural and environmental issues. Lack of sustainable development has fostered a non-viable economy. In this economy half of the employable persons are unemployed, and 80 percent are dependent upon some form of government financial and service aid.These problems are related to the effects of colonization found on all North American Indian reservations. A dependency relationship with the federal government has resulted in qualitative changes in the use of natural resources, from subsistence to extraction.Dr Pamela Elkind is Department Chairperson and Professor of Environmental Sociology at Eastern Washington University. David Lundgren is based in the Department of Sociology at Stevenson College. This paper was first delivered at the Global Forum '94 Conference in Manchester, UK.  相似文献   
85.
ABSTRACT: Following the Midwest flood of 1993, a study was initiated along a 39-mile segment of the Missouri River to determine if there was an association between woody corridors and levee stability. A systematic sample of levee failures revealed that primary levees which did not fail had a significantly wider woody corridor than failed levees. Analysis of the total inventory of failed levees revealed that as the width of the woody corridor decreased, the length of the levee failure increased. Number of levee failures and their severity of damage could be reduced if woody corridors were at least 300 feet wide.  相似文献   
86.
ABSTRACT: Accurate data about nutrient concentrations in wastewater treatment plant effluents are needed for river basin water-quality studies. As part of the U.S. Geological Survey's National Water-Quality Assessment Program in the South Platte River Basin, nutrient data were requested from 31 wastewater-treatment plants located in the basin. This article describes the types of nutrient data available from the plants, examines the variability of effluent nutrient concentrations, and discusses methods for estimation of nutrient concentrations where data are lacking. Ammonia was monitored at 88 percent of the plants, nitrite plus nitrate was monitored at 40 percent of the plants, and organic nitrogen and phosphorus were monitored at less than 25 percent of the plants. Median total nitrogen concentrations and median total phosphorus concentrations were small compared to typical literature estimates for wastewater-treatment plants with secondary treatment. Nutrient concentrations in effluent from wastewater-treatment plants varied widely between and within plants. For example, ammonia concentrations varied as much as 5 mg/L during a day, as much as 10 mg/L from day to day, and as much as 30 mg/L from summer to winter within a plant. In the South Platte River Basin, estimates of median annual ammonia and nitrite plus nitrate concentrations can be improved based on plant processes; and nitrite plus nitrate and organic nitrogen concentrations can be estimated based on ammonia concentrations. However, to avoid large estimation errors, more complete nutrient data from wastewater-treatment plants are needed for integration into river basin water quality studies. The paucity of data hinders attempts to evaluate the relative importance of point source and nonpoint source nutrient loadings to rivers.  相似文献   
87.
88.
ABSTRACT: As part of the U.S. Environmental Protection Agency's effort to determine the long-term effects of acidic deposition on surface water chemistry, annual runoff was estimated for about 1000 ungaged sites in the eastern U.S. using runoff contour maps. One concern in using contour maps was that a bias may be introduced in the runoff estimates due to the size of the 1000 ungaged sites relative to the size of the watersheds used in developing the maps. To determine if a bias was present the relationship between the annual runoff (expressed as depth) and the watershed area for the Northeast (NE) and Southern Blue Ridge Province (SBRP) was tested using five regional data bases. One short-term data base (1984 Water Year, n = 531) and two long-term data bases (1940–57, n = 134 and 1951–80, n = 342) were used in the NE. In the SBRP one short-term database (1984 Water Year, n = 531) and one long-term data base (1951–80, n = 60) were used. For the NE and the SBRP, runoff was not directly correlated with watershed area using the five regional databases. Also, runoff normalized by precipitation was not related to watershed area.  相似文献   
89.
ABSTRACT: Water level fluctuations of the Great Lakes often have created regional controversies among the states and Canadian provinces that share this vast resource. Even though the 100-year range of their water levels is only four to five feet, episodes of high and low Great Lakes water levels have been a recurring problem throughout the twentieth century. The possibility of increased diversion and consumptive use has exacerbated the existing conflicts over how to manage this water resource. A research project evaluated the effects of interbasin diversion on the Great Lakes system and on the industries that depend on the maintenance of historical water levels, namely hydropower and commercial navigation. The simulation approach employed in this research and some of the important findings are presented. The approach is similar to that used in recent government studies of Great Lakes water level regulation. Several significant modifications were made specifically addressing the diversion issue. Aggregate annual impacts to hydropower and shipping resulting from a diversion of 10,000 cubic feet per second were found to vary from 60 to 100 million dollars. Increases in impacts as a function of diversion rate are nonlinear for the navigation industry.  相似文献   
90.
ABSTRACT: The spatial and temporal variability of hydroclimatic elements were investigated in the central and northern Rocky Mountains (Colorado, Idaho, Montana, Utah, and Wyoming) during the 1951–1985 period. The three hydroclimatic elements studied were total water-year (October 1-September 30) streamflow (ST), winter (October 1-March 31) accumulated precipitation (PR), and April 1 snowpack (SN). An analysis of 14 virgin watersheds showed wide spatial djfferences in the temporal variability of SN, PR, and ST, and these were found to be caused largely by basin exposure to moist air flows. The more stable (low variability) basins were those exposed to prevailing northerly to westerly flow, while unstable (high variability) basins were exposed to occasional southwesterly to southeasterly moist flow. Snowpack was the better indicator of ST in 11 of the 14 watersheds, explaining 37 to 87 percent of the ST variance. Analysis of the spatial variability, based on all SN and PR data from across the study area, revealed 11 discrete climatic regions. Both SN and PR exhibited coherent regions of stable and unstable temporal variability. The average variability between stable and unstable regions differed by a factor of two, and the differences were best explained by the exposure of the mountain barrier to moist air flows.  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