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   4675篇
  免费   96篇
  国内免费   73篇
安全科学   213篇
废物处理   233篇
环保管理   1258篇
综合类   395篇
基础理论   1066篇
环境理论   5篇
污染及防治   1127篇
评价与监测   329篇
社会与环境   170篇
灾害及防治   48篇
  2023年   21篇
  2022年   19篇
  2021年   47篇
  2020年   29篇
  2019年   59篇
  2018年   69篇
  2017年   88篇
  2016年   109篇
  2015年   109篇
  2014年   104篇
  2013年   537篇
  2012年   174篇
  2011年   246篇
  2010年   188篇
  2009年   186篇
  2008年   254篇
  2007年   254篇
  2006年   230篇
  2005年   161篇
  2004年   181篇
  2003年   186篇
  2002年   166篇
  2001年   78篇
  2000年   104篇
  1999年   96篇
  1998年   73篇
  1997年   64篇
  1996年   72篇
  1995年   71篇
  1994年   61篇
  1993年   65篇
  1992年   72篇
  1991年   43篇
  1990年   51篇
  1989年   42篇
  1988年   32篇
  1987年   38篇
  1986年   44篇
  1985年   40篇
  1984年   44篇
  1983年   49篇
  1982年   38篇
  1981年   54篇
  1980年   44篇
  1979年   22篇
  1978年   24篇
  1977年   17篇
  1976年   12篇
  1975年   14篇
  1971年   13篇
排序方式: 共有4844条查询结果,搜索用时 31 毫秒
931.
932.
933.
Lets' save Mono Lake   总被引:1,自引:1,他引:0  
  相似文献   
934.
935.
The present study attempted to assess the extent to which job incumbents engage in performance relevant self-regulatory activities. Specifically, the degrees to which 62 insurance salespersons; (1) set performance goals, (2) monitored performance levels, and (3) exercised contingent self-reward or punishment with respect to goal achievement or goal failure were measured; and, the consequences that such activities had on performance were examined. The results of this study indicate that, in general, subjects set goals, but typically of a distal/result-orientated, as opposed to a proximal/behaviourally orientated-nature. Goal setting per se had no effects on sales performance, although an objective measure of goal difficulty was related to this criterion. With respect to self-monitoring, it was discovered that the greatest amount of performance feedback came from oneself and one's supervisor, as opposed to one's co-workers. In terms of performance, there was an interaction of feedback source and feedback preference such that the ability to generate internal feedback was related to performance only for individuals who had a preference for such internal feedback. There appeared to be no substantial amount of contingent self-reward occurring within this organization, although contingent self-punishment (in the form of self-criticism) was in evidence; and, such self-admonishment seemed to have detrimental effects on performance. While the three classes of variables independently explained 26 per cent of the variance in performance, the interactive effects suggested by theories of self-regulation (Bandura, 1977; Kanfer, 1971) were not in evidence.  相似文献   
936.
We investigated experimental warming and simulated grazing (clipping) effects on rangeland quality, as indicated by vegetation production and nutritive quality, in winter-grazed meadows and summer-grazed shrublands on the Tibetan Plateau, a rangeland system experiencing climatic and pastoral land use changes. Warming decreased total aboveground net primary productivity (ANPP) by 40 g x m(-2) x yr(-1) at the meadow habitats and decreased palatable ANPP (total ANPP minus non-palatable forb ANPP) by 10 g x m(-2) x yr(-1) at both habitats. The decreased production of the medicinal forb Gentiana straminea and the increased production of the non-palatable forb Stellera chamaejasme with warming also reduced rangeland quality. At the shrubland habitats, warming resulted in less digestible shrubs, whose foliage contains 25% digestible dry matter (DDM), replacing more digestible graminoids, whose foliage contains 60% DDM. This shift from graminoids to shrubs not only results in lower-quality forage, but could also have important consequences for future domestic herd composition. Although warming extended the growing season in non-clipped plots, the reduced rangeland quality due to decreased vegetative production and nutritive quality will likely overwhelm the improved rangeland quality associated with an extended growing season. Grazing maintained or improved rangeland quality by increasing total ANPP by 20-40 g x m(-2) x yr(-1) with no effect on palatable ANPP. Grazing effects on forage nutritive quality, as measured by foliar nitrogen and carbon content and by shifts in plant group ANPP, resulted in improved forage quality. Grazing extended the growing season at both habitats, and it advanced the growing season at the meadows. Synergistic interactions between warming and grazing were present, such that grazing mediated the warming-induced declines in vegetation production and nutritive quality. Moreover, combined treatment effects were nonadditive, suggesting that we cannot predict the combined effect of global changes and human activities from single-factor studies. Our findings suggest that the rangelands on the Tibetan Plateau, and the pastoralists who depend on them, may be vulnerable to future climate changes. Grazing can mitigate the negative warming effects on rangeland quality. For example, grazing management may be an important tool to keep warming-induced shrub expansion in check. Moreover, flexible and opportunistic grazing management will be required in a warmer future.  相似文献   
937.
938.
939.
The spatial and temporal variability of transparency measured for 15 years at 7 stations near the San Diego sewage outfall has been investigated and compared to the temporal variability of sewage suspended solid discharge and flow rate. the purpose of the time-series analyses was to distinguish natural from human (sewage discharge) causes of temporal changes in transparency. the results show that: 1) variations in transparency are highly correlated over the entire area, but there is a gradient in means and variability in the direction perpendicular to the coast; 2) there are no long term trends for increase or decrease in the water clarity at any of the stations; 3) most of the variance of transparency is contained in the seasonal frequency band; 4) over the same time period sewage discharge has significantly increased and suspended solids decreased; 5) most of the variance of these human-caused properties is in the interannual frequency band; (6) there is no correlation at any time-lag between water clarity and suspended solid discharge or flow. These results lead to the conclusion that these anthropogenic properties are not affecting transparency, while natural factors such as seasonality and distance from coast do.  相似文献   
940.
Environmental fungi become a potential Indoor Air Quality (IAQ) problem when adequate moisture and nutrients are present in building materials. Because of their potential to rapidly spread contamination throughout a building, ventilation system materials are of particular significance as potential microbial contamination sources. Current recommendations are to discard fibrous glass insulation that appears to be wet or moldy. Unfortunately, this advice is not always followed. Instead, cleaning is sometimes being used in buildings to remediate fibrous glass duct liner that is already contaminated with microbial growth. The objectives of this research program were to: 1) determine, under dynamic test conditions, whether fungal spore levels on heating, ventilating, and air-conditioning (HVAC) duct material surfaces could be substantially reduced by thorough vacuum cleaning, 2) evaluate whether subsequent fungal growth could be limited or contained by mechanical cleaning, and 3) provide data concerning the advisability of cleaning duct materials. The constant high relative humidity (RH) environment to which the test materials were exposed during this study was selected as a favorable growth environment that is frequently found in Southeastern United States HVAC systems. The results showed that, following cleaning, the levels of the two test fungi, A. versicolor and P. chrysogenum, recovered to preclean levels within 6 weeks. Therefore, mechanical cleaning by contact vacuuming alone was able to only temporarily reduce the surface fungal load. The current guidelines to discard contaminated materials should be followed.  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