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   10743篇
  免费   1014篇
  国内免费   1884篇
安全科学   2263篇
废物处理   231篇
环保管理   1860篇
综合类   5265篇
基础理论   1289篇
环境理论   16篇
污染及防治   713篇
评价与监测   752篇
社会与环境   659篇
灾害及防治   593篇
  2024年   69篇
  2023年   279篇
  2022年   434篇
  2021年   479篇
  2020年   453篇
  2019年   438篇
  2018年   371篇
  2017年   469篇
  2016年   553篇
  2015年   543篇
  2014年   587篇
  2013年   767篇
  2012年   836篇
  2011年   948篇
  2010年   612篇
  2009年   638篇
  2008年   546篇
  2007年   681篇
  2006年   598篇
  2005年   484篇
  2004年   369篇
  2003年   361篇
  2002年   308篇
  2001年   246篇
  2000年   197篇
  1999年   202篇
  1998年   133篇
  1997年   145篇
  1996年   127篇
  1995年   93篇
  1994年   109篇
  1993年   79篇
  1992年   62篇
  1991年   53篇
  1990年   40篇
  1989年   36篇
  1988年   40篇
  1987年   22篇
  1986年   14篇
  1985年   20篇
  1984年   13篇
  1983年   12篇
  1982年   25篇
  1981年   20篇
  1980年   19篇
  1979年   20篇
  1978年   14篇
  1977年   10篇
  1972年   18篇
  1971年   20篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
691.
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   
692.
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   
693.
ABSTRACT: A nonlinear multilevel transportation model is developed to study large-scale allocations in a water resources system. The model uses a modified transportation matrix formulated with nonlinear cost functions as the basic subregional model and the goal coordination method for multilevel decomposition and optimization of the overall regional system. The model is applied to projected water requirements for Salt Lake County in 1985. Sources of water supply - surface water, ground water, import water, and reuse of reclaimed wastewater on a restricted basis - are available to satisfy water requirements for municipal, industrial, and agricultural sectors in four subregions. The conjugate gradient projection method is used to optimize the first level subregional models having cost functions of the form of C = aXb, and the second level problem is solved using the conjugate gradient method.  相似文献   
694.
Potential increase in fire hazard as a result of timber harvesting is a concern of forest managers throughout the United States. Treating fuels can help reduce unacceptable fire hazards. To evaluate alternative fuel treatments, managers need to know their effects on fire hazard. A decision analysis approach to estimating fire hazard in terms of expected burned area was applied to a watershed in the Siskiyou National Forest (Oregon). Three treatment alternatives (do nothing and two levels of yarding unmerchantable material) were evaluated, and the effects of the treatments were projected over a 90-yr period. Initially, the effects of applying a treatment are small. After 50 years of treatment, the most intense alternative can be expected to show almost a 50% reduction in burned area compared to no treatment. The procedure also estimates burned are by fire size and fire intensity classes. Managers may find this useful for estimating expected fire effects associated with a particular fuel treatment regime.  相似文献   
695.
In the Southwestern United States, increasing demand for firewood has quickly promoted pinyon-juniper woodlands to commercial status. Slow recovery rates for pinyon and juniper and inadequate mensuration data present significant obstacles to predictive management efforts. Many National Forest districts have witnessed continued fuel use for at least the past 100 years. To explore the need for long-term analysis at the district level, we have developed FORMAN I, a computer simulation written in FORTRAN IV that models prolonged fuel harvesting and its impact on pinyon-juniper woodlands. The technique is well-suited for historical analyses and we comply with an initial application that involves the suggested impact of prehistoric peoples on a marginal woodland in Chaco Canyon, New Mexico. To accentuate the significance of the simulation, we have deliberately overestimated woodland parameters while maintaining conservative annual rates for firewood procurement. A low-density woodland (less than 14.8 cords/ha) is completely depleted within 200 years when subjected to tenth-through-twelfth century estimates of human demography for the canyon. Interesting conclusions are drawn from the lack of pinyon-juniper recovery at Chaco over the past millennium. Traditional assumptions, such as the pristine state of Southwestern vegetation prior to historic settlement and subsequent invasion of marginal grasslands by pinyon and juniper, are questioned.  相似文献   
696.
ABSTRACT: Gaged watersheds can provide information as to geomorphic, and geologic influence on the spatial variability of rainfall-runoff relationships. However, correlations between raingages distributed throughout the basin, and stream discharge are influenced by both storm patterns and drainage basin characteristics. Factor analysis has been applied to rainfall-runoff relationship to isolate the storm pattern from a basin response factor. Comparing two periods of time separated by eight years reveals relative stability in the rainfall attenuation (basin response) factor, while storm patterns for the two periods of record are quite disparate.  相似文献   
697.
ABSTRACT: Data splitting is used to compare methods of determining “homogeneous” hydrologic regions. The methods compared use cluster analysis based on similarity of hydrologic characteristics or similarity of characteristics of a stream's drainage basin. Data for 221 stations in Arizona are used to show that the methods, which are a modification of DeCoursey's scheme for defining regions, improve the fit of estimation data to the model, but that is is necessary to have an independent measure of predictive accuracy, such as that provided by data splitting, to demonstrate improved predictive accuracy. The methods used the complete linkage algorithm for cluster analysis and computed weighted average estimates of hydrologic characteristics at ungaged sites.  相似文献   
698.
The metabolic fate of 14C-phenyl-labeled herbicide clodinafop-propargyl (CfP) was studied for 28 days in lab assays using a soil from Germany (Ap horizon, silt loam, and cambisol). Mineralization amounted to 12.40% of applied 14C after 28 days showing a distinct lag phase until day 7 of incubation. Portions of radioactivity extractable by means of 0.01 M CaCl2 solution (bioavailable fraction) decreased rapidly and were 4.41% after 28 days. Even immediately after application, only 57.31% were extracted with the aqueous solvent. Subsequent extraction using accelerated solvent extraction (ASE; acetonitrile/water 4:1, v/v) released 39.91% of applied 14C with day 0 and 26.16% with day 28 of incubation from the samples. Non-extractable portions of radioactivity thus, increased with time amounting to 11.99% (day 0) and 65.00% (day 28). A remarkable increase was observed between 14 and 28 days correlating with the distinct increase of mineralization. No correlation was found throughout incubation with general microbial activity as determined by DMSO reduction. Analysis of the CaCl2 and ASE extracts by radio-TLC, radio-HPLC and GC/MS revealed that CfP was rapidly cleaved to free acid clodinafop (Cf), which was further (bio-) transformed; DT50 values (based on radio-TLC detection of the parent compound) were far below 1 day (CfP) and about 7 days (Cf). TLC analysis pointed to 2-(4-hydroxyphenoxy)-propionic acid as further metabolite. Due to fractionation of non-extractable residues, most of the 14C was associated with fulvic and humic acids, portions in humin fractions and non-humics were moderate and low, respectively. Using a special strategy, which included pre-incubation of the soil with CfP and then mineralization of 14C-CfP as criterion, a microorganism was isolated from the soil examined. The microorganism grew using CfP as sole carbon source with concomitant evolution of 14CO2. The bacterium was characterized by growth on commonly used carbon sources and by 16S rDNA sequence analysis. The sequence exhibited high similarity with that of Rhodococcus wratislaviensis (99.56%; DSM 44107, NCIMB 13082).  相似文献   
699.
There is an international divide between net emissions importers and net emissions exporters, with industrialised nations mainly falling into the former and emerging economies the latter. Integrating emissions transfers into climate policy, so as not to disadvantage export-intensive countries, has been suggested to increase participation in international emissions reduction commitments. Consumption-based scenarios are presented for the UK identifying the geographic and sectorial source of emissions to meet future consumer demands given the current international climate policy landscape. The analysis is applied to the UK yet the discussion is applicable to international climate policy; assigning national responsibility for global emissions reductions; and extending the mitigation potential for net importing countries. Two trajectories for UK consumption emissions are calculated in which (1) international reduction targets are consistent with those pledged today equating to four degrees of temperature rise and (2) international reduction targets achieve a two degree future. By 2050 it is estimated that UK consumption emissions are 40–260% greater than UK territorial emissions depending on the strength of global reduction measures, and assuming the UK meets its 80% reduction in 1990 emissions by 2050 target. Cumulative emissions are presented alongside emissions trajectories, recognising that temperature rise is directly related to every tonne of carbon emitted. Whilst this paper argues that the current UK emissions targets underestimate the UK's contribution to global mitigation for two degrees, it shows how expanding the focus of policy towards consumption introduces new opportunities for reduction strategies at scale. The paper advocates the implementation of consumption-based emissions accounting which reveals underexploited policy interventions and increases the potential to break down barriers that exist between industrialised and emerging economies in international climate policy.  相似文献   
700.
Rail transit is generally acknowledged as an alternative transport mode in contributing towards sustainable mobility. In addition to minimising negative externalities, rail transit has sustainable land-use opportunities to integrate transport- and spatial planning. The objective of this paper is to determine the impact of integrative light rail scenarios and their ability to curtail private vehicle driven urban sprawl in the Flemish rhombus.The paper proposes three light rail scenarios: an infrastructural scenario; tramification scenario; and spatial rail scenario, each covering a specific landscape structure to reorganise the dispersed spatial environment in Flanders in the long-term. We used the participatory multi-actor multi-criteria analysis (MAMCA) which incorporates the objectives of all involved stakeholders to assess the impact of the scenarios.The infrastructural alternative scenario gained most support among the involved stakeholders, on the grounds of improved multimodality, enhanced user amenities, reduced implementation costs, moderated greenhouse gas emissions and mitigated infrastructural barrier effects.Despite the merits of the infrastructural scenario in terms of stakeholder objectives, few possibilities are included to elaborate upon sustainable land-use development. In response to the low performance of this assessment criterion, catalyst measures are discussed to support the implementation.  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