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   18784篇
  免费   1599篇
  国内免费   1619篇
安全科学   4101篇
废物处理   365篇
环保管理   4807篇
综合类   7453篇
基础理论   1688篇
环境理论   10篇
污染及防治   797篇
评价与监测   916篇
社会与环境   993篇
灾害及防治   872篇
  2024年   25篇
  2023年   206篇
  2022年   327篇
  2021年   515篇
  2020年   629篇
  2019年   481篇
  2018年   366篇
  2017年   603篇
  2016年   698篇
  2015年   690篇
  2014年   800篇
  2013年   1050篇
  2012年   1225篇
  2011年   1437篇
  2010年   1018篇
  2009年   1153篇
  2008年   821篇
  2007年   1203篇
  2006年   1222篇
  2005年   954篇
  2004年   923篇
  2003年   801篇
  2002年   728篇
  2001年   586篇
  2000年   588篇
  1999年   509篇
  1998年   350篇
  1997年   325篇
  1996年   263篇
  1995年   277篇
  1994年   183篇
  1993年   166篇
  1992年   146篇
  1991年   93篇
  1990年   62篇
  1989年   47篇
  1988年   47篇
  1987年   47篇
  1986年   40篇
  1985年   33篇
  1984年   34篇
  1983年   24篇
  1982年   35篇
  1981年   23篇
  1980年   37篇
  1979年   44篇
  1978年   32篇
  1977年   20篇
  1972年   24篇
  1971年   24篇
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
31.
朱浩  彭雨  鄂加强  彭亮 《环境工程学报》2008,2(8):1083-1086
针对锌精馏铅塔动态过程中的非线性、非最小相位特征、不稳定性、时滞和负荷干扰,基于模糊控制策略下给出了锌精馏铅塔燃烧系统新的控制方法.运用模糊控制系统对锌精馏铅塔的烟气温度进行仿真研究和实时控制结果表明,该研究所设计的模糊控制器能够克服许多干扰因素,产生了良好的控制效果.  相似文献   
32.
灰色协调度模型在产业用水系统分析中的应用   总被引:1,自引:0,他引:1  
揭示产业用水系统的时空动态发展规律,探讨其内部的协调关系,是开展节水型社会建设规划,实现水资源可持续利用的关键。然而,目前对产业用水系统的协调性分析还缺乏较为有效的理论和方法。以往在利用协调度模型进行用水系统有序度研究时,通常采用两段叙述的线性分段函数表示,较不适用于描述产业用水系统内部要素的非线性结构。因此,针对产业用水系统的特性,利用灰关联原理建立有序度函数,以建立更适于用水系统分析的灰色协调度模型。利用该模型对上海市1997~2005年产业用水系统进行实证研究,结果显示:上海市产业用水系统基本有序,发展比较协调,其用水综合效益较大,但距离最优状态尚有潜力可挖;其中,农业用水子系统的发展较为欠缺,在未来用水规划中应予以重视。  相似文献   
33.
对武钢第一炼钢厂100t转炉OG系统设备存在的问题进行了分析、研究,结合生产实际进行了一系列的技术改进和工艺参数调整,提高了OG系统效率。  相似文献   
34.
The authors present the beginnings of a planning support system (PSS) for agri-environmental measures exemplified by a virtual implementation of Colorfields and blooming strips on model farms, based on real-world data. This paper starts with an introduction to the Colorfields, a concept for transdisciplinary and sustainable landscape design of set-aside land. Colorfields comprise of blooming strips of flowering annual or biennial plants, which are designed and drilled in pattern on fallow land creating Land Art. The temporary scenic arrangements of the Colorfields combine the advantages of ecological strips, e.g. providing habitats for insects (especially bees), improving soil fertility through the cultivation of intercrops, with improvements of the social recognition of farmers as producers of pleasant landscapes instead of monoculture fields.The prototype of the PSS uses two software tools of different scientific origin, the bio-economic modeling system MODAM and the landscape visualization system Lenné3D, which are linked based on geo-data. The resulting system helps to assess the economic effects and visualizes the effects of the specific landuse patterns under different scenarios.The economic assessment of blooming strips on arable land and of one Colorfield on fallow land shows that these measures prove to be profitable from an economic viewpoint assuming the current area payments for the obligatory European Union set-aside program. Furthermore, the visualizations enable the design to be tested virtually by exploring the resultant scenery. They provide artists, planners and stakeholders including farmers with a tool to virtually wander through landscape scenarios supporting a collaborative design and a shared vision for the community.The results of the two model farms and previous case studies for Colorfields demonstrate how current policy conditions could be used for the improvement of environmental and scenic qualities. Furthermore, the ability of the tools, MODAM and Lenné3D, suggests to support and promote these activities.  相似文献   
35.
Abstract:  We examined factors that may independently or synergistically contribute to amphibian population declines. We used epidemiologic case–control methodology to sample and analyze a large database developed and maintained by the Arizona Game and Fish Department that describes historical and currently known ranid frog localities in Arizona, U.S.A. Sites with historical documentation of target ranid species ( n = 324) were evaluated to identify locations where frogs had disappeared during the study period (case sites) and locations where frog populations persisted (control sites). Between 1986 and 2003, 117 (36%) of the 324 sites became case sites, of which 105 were used in the analyses. An equal number of control sites were sampled to control for the effects of time. Risk factors, or predictor variables, were defined from environmental data summarized during site surveys and geographic information system data layers. We evaluated risk factors with univariate and multifactorial logistic-regression analyses to derive odds ratios (OR). Odds for local population disappearance were significantly related to 4 factors in the multifactorial model. Disappearance of frog populations increased with increasing elevation (OR = 2.7 for every 500 m, p < 0.01). Sites where disappearances occurred were 4.3 times more likely to have other nearby sites that also experienced disappearances (OR = 4.3, p < 0.01), whereas the odds of disappearance were 6.7 times less (OR = 0.15, p < 0.01) when there was a source population nearby. Sites with disappearances were 2.6 times more likely to have introduced crayfish than were control sites (OR = 2.6, p = 0.04). The identification of factors associated with frog disappearances increases understanding of declines occurring in natural populations and aids in conservation efforts to reestablish and protect native ranids by identifying and prioritizing implicated threats.  相似文献   
36.
中国可持续发展综合评价研究   总被引:19,自引:3,他引:16  
可持续发展作为一个动态、开放的复杂巨系统。人口、资源、经济、环境和科技构成了其关键要素。本文基于PREEST系统模型。首先提出了一套中国可持续发展综合评价指标体系:同时。借助于主成分分析法和隶属度分析法。就中国1987~2001年度的综合发展指数与协调发展指数进行了实证分析;最后。基于研究成果。对中国未来的可持续发展提出了若干政策建议。  相似文献   
37.
According to the United Nations population projections, the population of the elderly is expected to roughly triple in China from 2000 to 2050, particularly when the generations who were born between the 1950s and 1970s move through the age structure, and also because people are living longer and fertility rates have fallen, population aging is expected to put pressure on government's fiscal balance through higher old-age security benefits and health-care expenditures. This work draws together the broad range of elements involved within a consistent framework, based on a computable dynamic general equilibrium model with an overlapping generation structure. Further analysis using model simulation illustrates that the alternative schemes for the benefit rate, retirement age and technological progress are likely to be beneficial, and that an obvious slow-down in the growth of living standards is likely to be avoided.  相似文献   
38.
介绍了冲击水浴喷雾式除尘器在柳钢焦化厂的应用试验情况,制作安装,调试运行后检测鉴定表明,冲击水浴喷雾式除尘器应用在冶金焦末粉尘上,不但操作简单,维护方便,除尘效率高,效果好,而且成本低廉,只有电除尘器或布袋除尘器的20%左右。  相似文献   
39.
长江流域的昆虫病毒资源及其开发利用   总被引:1,自引:0,他引:1  
在长江流域已发现约130种昆虫病毒,其中多数属杆状病毒科。昆虫病毒作为杀虫剂在本地区广泛应用,防治棉花、森林、茶树、蔬菜和牧草上的害虫,克服了化学农药的一些缺点,取得了良好的生态效益和经济效益。近年来还进行了杆状病毒表达载体的研究,该系统的研制已用于高水平表达具有生物活性的外源基因产物,获得医药产品,或者组建更有效的基因工程病毒杀虫剂。  相似文献   
40.
ABSTRACT: Two dynamic programming models — one deterministic and one stochastic — that may be used to generate reservoir operating rules are compared. The deterministic model (DPR) consists of an algorithm that cycles through three components: a dynamic program, a regression analysis, and a simulation. In this model, the correlation between the general operating rules, defined by the regression analysis and evaluated in the simulation, and the optimal deterministic operation defined by the dynamic program is increased through an iterative process. The stochastic dynamic program (SDP) describes streamflows with a discrete lag-one Markov process. To test the usefulness of both models in generating reservoir operating rules, real-time reservoir operation simulation models are constructed for three hydrologically different sites. The rules generated by DPR and SDP are then applied in the operation simulation model and their performance is evaluated. For the test cases, the DPR generated rules are more effective in the operation of medium to very large reservoirs and the SDP generated rules are more effective for the operation of small reservoirs.  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