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   2693篇
  免费   76篇
  国内免费   23篇
安全科学   123篇
废物处理   109篇
环保管理   821篇
综合类   259篇
基础理论   640篇
环境理论   1篇
污染及防治   568篇
评价与监测   157篇
社会与环境   79篇
灾害及防治   35篇
  2023年   17篇
  2022年   22篇
  2021年   35篇
  2020年   24篇
  2019年   32篇
  2018年   50篇
  2017年   46篇
  2016年   61篇
  2015年   53篇
  2014年   53篇
  2013年   287篇
  2012年   107篇
  2011年   137篇
  2010年   110篇
  2009年   119篇
  2008年   155篇
  2007年   144篇
  2006年   126篇
  2005年   95篇
  2004年   77篇
  2003年   97篇
  2002年   86篇
  2001年   56篇
  2000年   45篇
  1999年   24篇
  1998年   43篇
  1997年   28篇
  1996年   38篇
  1995年   46篇
  1994年   31篇
  1993年   44篇
  1992年   36篇
  1991年   15篇
  1990年   23篇
  1989年   17篇
  1988年   24篇
  1987年   15篇
  1986年   19篇
  1985年   29篇
  1984年   23篇
  1983年   24篇
  1982年   34篇
  1981年   34篇
  1980年   32篇
  1979年   21篇
  1978年   23篇
  1977年   12篇
  1976年   8篇
  1974年   8篇
  1972年   10篇
排序方式: 共有2792条查询结果,搜索用时 15 毫秒
51.
ABSTRACT: A study of the length of the growing and dormant season in Pennsylvania by isoline interpolation from climatological data. Maps of the beginning and ending of the growing and dormant seasons, length of growing season and ratio of growing to dormant season are included.  相似文献   
52.
An important goal of the Reagan administration has been to shift responsibility for many public programs from the US federal government to the states. This New Federalism seeks to restore a proper balance to the federal system and to ensure an effective working partnership between the states and the federal government. Such a partnership is especially important for many environmental laws because these laws often give states primary responsibility for the control and abatement of pollution.This research examines the extent to which the Reagan administration has succeeded in improving intergovernmental environmental relations in terms of state implementation of the Clean Water Act. Data from a 1985 survey of directors of state water quality control programs are compared with responses to a similar survey that the US General Accounting Office conducted in 1979. The latter survey found considerable dissatisfaction on the part of state directors with the quality of their relations with the US Environmental Protection Agency. Although some improvement can be noted between 1979 and 1985, the Reagan administration's efforts to improve intergovernmental relations appear to have been of limited consequence, to the possible detriment of effective implementation of the Clean Water Act.  相似文献   
53.
ABSTRACT: Export coefficients (kg/km2/yr) for dissolved ortho-phosphate (OP), total phosphorus (TP), total inorganic nitrogen (TIN), and total nitrogen (TN) were derived for watersheds in Wisconsin using data bases available for 17 basins from the U.S. Environmental Protection Agency — National Eutrophication Survey, U.S. Geological Survey, and the Wisconsin Department of Natural Resources. Three general land use categories, representative of most regions in Wisconsin, were established: forest, mixed, and agricultural. Data for the 17 basins indicated greater exports of OP. TP, TIN, and TN as the percentage of forest decreased and agriculture increased. These region-specific coefficients are compared to the values reported in the literature representing much broader areas of the U.S.  相似文献   
54.
Reduction of sulphur deposition causesrecovery of acidified surface waters. Processes in thecatchment delay recovery. The acidification model MAGICwas applied to the Vikedal and Tovdal rivers in southernNorway. Response in water chemsitry is delayed by 10–20 yr. The delay is due to release of old sulphate atVikedal and cation exchange at Tovdal. Assuming that theGothenburg protocol is fully implemented by the year2010, much of the predicted increase of ANC will occur inthe next 10 yr with a levelling off by about 2040. Ifnitrogen leaching increases in the future, however,recovery of ANC will not be as rapid, nor as complete.Critical load for acidity calculated by steady-statemodels is confirmed by the MAGIC predictions. Futurerequirement for mitigation measures such as liming willdecrease in the future as acid deposition decreases. Bythe year 2046 the liming requirement will be reduced byabout 45% at Vikedal and 65% at Tovdal. One of the mainpurposes of the Norwegian national monitoring programmeis to provide documentation of changes in environmentalquality due to long-range transported air pollutants.Modelling applications such as this clearly show that thedata fill this purpose.  相似文献   
55.
In Australia, problems of dryland and stream salinity have recently become the focus of a National Action Plan. In many river catchments, preliminary stream salt load and salinity targets have been set to define maximum permissible export levels in 2015. Afforestation has been proposed as a strategy for meeting these targets, although several studies suggest that widespread commercial tree plantations are likely to deliver net dis-benefits. However, the impacts on stream salt loads of more localised tree plantings in high salt yielding areas have not been quantified. In this paper we use a simple empirical model to predict the effects of various strategic and non-strategic tree planting scenarios on flows and salt loads in the mid-Macquarie catchment, New South Wales. A simple salt routing model is then used to estimate the effect of these changes on salt loads at the end-of-valley monitoring site for the Macquarie catchment. Results suggest that widespread land management interventions will be required to meet the preliminary salt load targets for this catchment. On their own, small-scale, strategic tree planting in high salt export areas of the mid-Macquarie area will not have a significant impact on salt loads at the end-of-valley monitoring site. While widespread tree plantings may reduce salt loads in the longer term, they are likely to cause streamflow losses in the shorter term. Thus, stream salinities are expected to rise initially, due to the different response times of groundwater and surface water systems to land use change.  相似文献   
56.
A variety of novel polymeric materials ranging from elastomers to tough, rigid plastics have been prepared by the cationic copolymerization of regular soybean oil, low-saturation soybean oil, or conjugated low-saturation soybean oil with various alkene commonomers. Using appropriate compositions and reaction conditions, 70–100% of the soybean oil is covalently incorporated into the cross-linked polymer networks, contributing significantly to cross-linking during copolymerization. The resulting thermosets exhibit thermophysical and mechanical properties that are competitive with those of their petroleum-based counterparts. In addition, good damping and shape memory properties have been obtained by controlling the degree of cross-linking and the rigidity of the polymer backbone. New materials with similar characteristics have also been produced from other biological oils, including tung, and fish oils using the same technique. The new, more valuable properties of these bioplastics suggest numerous promising applications of these novel polymeric materials.  相似文献   
57.
Starch-PVOH cast films were prepared with and without crosslinking agent (hexamethoxymethylmelamine) in the absence of plasticizer. Moisture absorption in films without crosslinking agent at a low relative humidity was similar to that of PVOH and increased as the relative humidity increased. Films with crosslinking agent showed moisture absorption linearly proportional to the relative humidity. Significant improvement in resistance to water disintegration for crosslinked starch-PVOH films was observed. While the tensile strength decreased with increased relative humidity, crosslinking significantly improved the tensile strength. Increased PVOH content improved elongation of films even when the relative humidity was 80% or higher. Biodegradation studies revealed that the degradation rate was negatively correlated with the PVOH content in films and crosslinking generated more converged degradation curves. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable.  相似文献   
58.
59.
60.
Food habits of white-tailed deer (Odocoileus virginianus) were examined from January to November 1984 via fecal-pellet analysis at Valley Forge National Historical Park (VFNHP), which represents an island habitat for deer surrounded by extensive urbanization, in southeastern Pennsylvania. In addition, use of fields by deer was compared to food habits. Herbaceous vegetation (forbs, leaves of woody plants, and conifer needles) was the predominant food type in all seasons except fall. Acorns and graminoids (grasses and sedges) were important food resources in fall and spring, respectively. Use of woody browse (twigs) was similar among seasons. Field use was relatively high during fall, winter without snow cover (<20 cm), and spring when food resources in fields were readily available. In contrast, use of fields was lowest in summer when preferred woodland foods were available and in winter with snow cover when food in fields was not readily accessible. Patterns of food-type use by deer at VFNHP indicate the year-round importance of nonwoody foods and field habitats to deer populations on public lands such as national parks in the northeastern United States.  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