全文获取类型
收费全文 | 23196篇 |
免费 | 587篇 |
国内免费 | 2156篇 |
专业分类
安全科学 | 641篇 |
废物处理 | 1410篇 |
环保管理 | 2548篇 |
综合类 | 5933篇 |
基础理论 | 5885篇 |
环境理论 | 1篇 |
污染及防治 | 5121篇 |
评价与监测 | 1935篇 |
社会与环境 | 2222篇 |
灾害及防治 | 243篇 |
出版年
2023年 | 191篇 |
2022年 | 499篇 |
2021年 | 457篇 |
2020年 | 421篇 |
2019年 | 383篇 |
2018年 | 1816篇 |
2017年 | 1794篇 |
2016年 | 1680篇 |
2015年 | 649篇 |
2014年 | 725篇 |
2013年 | 1241篇 |
2012年 | 1210篇 |
2011年 | 2146篇 |
2010年 | 1362篇 |
2009年 | 1296篇 |
2008年 | 1658篇 |
2007年 | 1872篇 |
2006年 | 626篇 |
2005年 | 515篇 |
2004年 | 496篇 |
2003年 | 534篇 |
2002年 | 546篇 |
2001年 | 500篇 |
2000年 | 391篇 |
1999年 | 221篇 |
1998年 | 201篇 |
1997年 | 164篇 |
1996年 | 178篇 |
1995年 | 173篇 |
1994年 | 146篇 |
1993年 | 130篇 |
1992年 | 126篇 |
1991年 | 112篇 |
1990年 | 96篇 |
1989年 | 83篇 |
1988年 | 81篇 |
1987年 | 75篇 |
1986年 | 60篇 |
1985年 | 64篇 |
1984年 | 78篇 |
1983年 | 69篇 |
1982年 | 66篇 |
1981年 | 66篇 |
1980年 | 47篇 |
1979年 | 47篇 |
1978年 | 40篇 |
1977年 | 39篇 |
1976年 | 36篇 |
1973年 | 44篇 |
1972年 | 54篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Winer N 《Disasters》1989,13(1):1-8
Food security in Ethiopia is discussed in the context of the repeated famines and the international responses both to them and to the socialist agricultural policies being pursued by Ethiopia. Increasing concern has been expressed by the international donor community regarding the ability of Ethiopia to absorb development funds without a major shift in emphasis in agricultural policy-making. The background to Ethiopia's present vulnerability is shown both in terms of the size of the vulnerable population and in terms of the poor performance of the agricultural sector in the last decade. The author looks at the present agricultural and marketing policy reforms and questions whether they are sufficient to generate the sort of international response needed to create the level of food security that would be required to avert future famines. 相似文献
62.
Relationships between porewater nutrients and seagrasses in a subtropical carbonate environment 总被引:9,自引:0,他引:9
The feeding, diet and egg production of the copepod Acartia tonsa were dermined during ten experiments in Los Angeles Harbor, California, between November 1986 and October 1987. Copepods were incubated in situ, in quasi-natural food environments. Water temperatures ranged from 14.6 to 21.5°C. Particulate organic carbon and nitrogen (POC and PON) were high (534 to 3710 g Cl-1, 51 to 459 Nl-1) but dominated by small (<8 m diam) particles. Plankton (phytoplankton and microzooplankton) C-biomass composed about 10% of the total POC and was usually dominated by particles >8 m. Plankton biomass was always low. Daily ingestion rates ranged from 3 to 96% of body C; egg production ranged from 4 to 35% of body carbon. Mean ingestion and egg production rates during spring-summer were 1.9 and 1.5 times higher than average for the entire study, respectively. The average gross efficiency of egg production for the study was 80%; the spring-summer mean was 41%. Bivariate and multiple-regression analyses revealed that the ingestion rate was dependent upon both temperature and food availability, but that, below 21°C, egg production depended more upon temperature than upon food concentration. To detect dietary preferences, the composition of diet was compared with that of the food supply. Selective feeding was infrequent, but the diet was often dominated by dinoflagellates and ciliates. It would appear that within metabolic limits governed by temperature, the feeding response of A. tonsa is dependent upon food concentration, while egg production depends more on qualitative attributes of the food supply. 相似文献
63.
Previous epidemiological studies have shown that dental fluorosis is endemic in the lowland, dry zone of Sri Lanka, which is considered to be an area in which excessive quantities of fluorides are present in the drinking water supplies. It has been found that kaolinitic clay forms a suitable raw material in the defluoridation of water.It is shown that there is a noticeable effect of selenium and media pH on the reactions involved in the interaction of fluoride with clay. In this study, 1 mM fluoride solutions containing SeO
3
2–
(selenite) concentrations of 0 mM, 0.1 mM, 0.5 mM and 1 mM were used in the reactions with kaolinitic clay. The effect of pH was monitored in the range 4 to 8. It was observed that fluoride adsorption was maximum at a pH of 5.6 without either SeO
3
2–
or SeO
4
2–
, the adsorption capacity being 15.2 mol F– g–1 clay. However, when the SeO
3
2–
concentration was increased up to 0.5 mM at this optimum pH, the adsorption capacity reduced to 12.8 mol F– g–1 clay. Monitoring of the effect of SeO
4
2–
and media pH on fluoride adsorption showed that when the SeO
4
2–
concentration increases from zero to 0.1 mM, there is a reduction of fluoride adsorption capacity. However, when the SeO
4
2–
concentration is further increased from 0.1 mM to 1.0 mM, there was an increase in the fluoride adsorption capacity, indicating a more consistent effect of SeO
3
2–
on fluoride-kaolinitic clay interaction than SeO
4
2–
.Fluoride concentrations in drinking water supplies have a marked effect on dental health and the geochemistry of selenium appears to play an important role in the geochemical mobility of fluoride ions. 相似文献
64.
Cape JN Sheppard LJ Fowler D Harrison AF Parkinson JA Dao P Paterson IS 《Environmental pollution (Barking, Essex : 1987)》1992,75(2):229-236
Radioactive sulphate (35SO4) was applied to the soil below a Scots pine forest on 23 June 1989, and its movement into the canopy and into throughfall and stemflow was measured over 4 months. The specific activity, Bq (mg S)(-1), of the canopy increased monotonically; uptake by current-year (1989) expanding needles was initially twice as fast as by older needles or live twigs. By 10 October the canopy average specific activity was 62 Bq (mg S)(-1). The specific activity of net throughfall (throughfall + stemflow - rain), deduced from measurements from six throughfall collectors, six stemflow collectors and two rain collectors, fell rapidly from 12.6 Bq (mg S)(-1) in late July to <1 Bq (mg S)(-1) in mid-August. The results suggest (assuming rapid equilibration of 35S with sulphate in soil) that root-derived sulphate contributed c. 3% of sulphate in net throughfall and that dry deposition of SO2 and sulphate particles contributed c. 97% of the 0.56 g S m(-2) measured in net throughfall over the period. Simultaneous measurements of SO2 at canopy height and of NH3 above and within the canopy gave mean concentrations of 5.9 and 0.86 microg m(-3), respectively, sufficient to account for the sulphate measured in net throughfall only if codeposition of NH3 and SO2 occurred to canopy surfaces. The large values of specific activity observed in July, however, indicate that throughfall composition may be closely related to recent soil input of sulphate, and that equilibrium cannot be safely assumed. The possibility of a significant contribution of soil-derived sulphate to sulphate deposition in net throughfall cannot be ruled out on the basis of this experiment. 相似文献
65.
Eshleman KN Wigington PJ Davies TD Tranter M 《Environmental pollution (Barking, Essex : 1987)》1992,77(2-3):287-295
Field studies of chemical changes in surface waters associated with rainfall and snowmelt events have provided evidence of episodic acidification of lakes and streams in Europe and North America. Modelling these chemical changes is particularly challenging because of the variability associated with hydrological transport and chemical transformation processes in catchments. This paper provides a review of mathematical models that have been applied to the problem of episodic acidification. Several empirical approaches, including regression models, mixing models and time series models, support a strong hydrological interpretation of episodic acidification. Regional application of several models has suggested that acidic episodes (in which the acid neutralizing capacity becomes negative) are relatively common in surface waters in several regions of the US that receive acid deposition. Results from physically based models have suggested a lack of understanding of hydrological flowpaths, hydraulic residence times and biogeochemical reactions, particularly those involving aluminum. The ability to better predict episodic chemical responses of surface waters is thus dependent upon elucidation of these and other physical and chemical processes. 相似文献
66.
Wigington PJ Davies TD Tranter M Eshleman KN 《Environmental pollution (Barking, Essex : 1987)》1992,78(1-3):29-35
Episodic acidification is practically a ubiquitous process in streams and drainage lakes in Canada, Europe and the United States. Depressions of pH are often smaller in systems with low pre-episode pH levels. Studies on European surface waters have reported episodes most frequently with minimum pH levels below 4.5. In Canada and the United States, studies have also reported a number of systems that have had minimum pH levels below 4.5. In all areas, change in water flowpath during hydrological events is a major determinant of episode characteristics. Episodic acidification is also controlled by a combination of other natural and anthropogenic factors. Base cation decreases are an important contributor to episodes in circumneutral streams and lakes. Sulphate pulses are generally important contributors to episodic acidification in Europe and Canada. Nitrate pulses are generally more important to episodic acidification in the Northeast United States. Increases in organic acids contribute to episodes in some streams in all areas. The sea-salt effect is important in near-coastal streams and lakes. In Canada, Europe and the United States, acidic deposition has increased the severity (minimum pH reached) of episodes in some streams and lakes. 相似文献
67.
Environment, Development and Sustainability - Ecological welfare performance is a measure of the relationship between input ecological resources and output welfare, and it reflects the sustainable... 相似文献
68.
Russian Journal of Ecology - The aim of this study is to explore the effects of the litter cover thickness and cleaning methods on seed germination and seedling growth of Pinus densata. The overall... 相似文献
69.
世界能源百年变局与国家能源安全 总被引:1,自引:0,他引:1
在百年未有之大变局的背景下,我国当前所面临的能源形势日趋复杂,能源安全愈加成为攸关国家经济发展的重大战略。新形势下,无论是学科层面上世界能源地理与国家能源安全的科学问题、研究对象、研究方法等,还是国家现实需求上全球能源治理体系与国际能源合作等问题,都亟待专家学者们深入交流与讨论。通过“一问一答”的访谈方式,展示主持人与九位学界或业界访谈嘉宾基于不同视角的问答内容。分别围绕能源安全的内涵与全球能源治理模式、我国能源安全面临的挑战与对策、全球能源地缘政治形势的新变化、世界能源生产空间和消费空间不平等地理格局下的能源安全、全球能源低碳转型趋势下的中国能源高质量发展、重大突发公共卫生事件影响下的全球能源供需格局变化、未来能源地理学的研究前景与重要研究方向等一系列内容发表观点。以期达成一定共识,从而更好地研讨世界能源地理学的关键问题,并积极应对世界能源的百年变局,提出保障国家能源安全的应对措施等。 相似文献
70.
黑河流域底栖动物群落结构及水质评价 总被引:1,自引:0,他引:1
为了解黑河流域上中游底栖动物群落结构特征及其与环境因子之间的关系,于2018年8月对研究区域17个采样点的底栖动物和水体理化指标进行调查研究.共鉴定出底栖动物43种,其中节肢动物34种(79.1%),软体动物7种(16.3%),环节动物2种(4.6%);就整个研究区域而言,优势种为大蚊(Tipulidae)、豆娘幼虫(Damselfly)、水蜘蛛(Argyroneta)、耳萝卜螺(Radix auricularia)、琥珀螺(Suecinea sp.)、白旋螺(Gyraulusalbus),干、支流优势种分布趋势为干流(6种)优于支流(5种);底栖动物平均密度和平均生物量分别为77ind/m2和3.7423g/m2现存量存在显著空间差异,整体上干流(1032ind/m2、60.0963g/m2)大于支流(276ind/m2、3.5233g/m2),物种数干流(36种)大于支流(15种);单因素方差分析显示,Shannon-Wiener多样性指数、Margalef丰富度指数及Pielou均匀度指数分布特征为干流大于支流.黑河流域上游支流和中游干流不同河段底栖动物物种组成呈空间异质性,而多样性指数对物种组成依赖性强,但因黑河流域底栖动物物种组成受自然因素和人类活动双重影响程度差异较大,使得Shannon-Wiener多样性指数、Margalef丰富度指数及Pielou均匀度指数不适合黑河水质评价.依据BI指数和综合污染指数评价表明,黑河上游支流水质优于中游干流.根据底栖动物与环境因子之间的相关矩阵分析并结合RDA分析表明:BOD5、水温(WT)、电导率(EC)、DO、溶解性总固体(TDS)、CODMn及盐度是影响底栖动物群落结构的主要环境因子. 相似文献