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   3186篇
  免费   271篇
  国内免费   729篇
安全科学   167篇
废物处理   95篇
环保管理   834篇
综合类   1978篇
基础理论   409篇
环境理论   1篇
污染及防治   150篇
评价与监测   118篇
社会与环境   381篇
灾害及防治   53篇
  2024年   17篇
  2023年   73篇
  2022年   143篇
  2021年   154篇
  2020年   150篇
  2019年   128篇
  2018年   100篇
  2017年   147篇
  2016年   127篇
  2015年   133篇
  2014年   166篇
  2013年   202篇
  2012年   213篇
  2011年   242篇
  2010年   180篇
  2009年   180篇
  2008年   141篇
  2007年   234篇
  2006年   178篇
  2005年   151篇
  2004年   136篇
  2003年   122篇
  2002年   101篇
  2001年   82篇
  2000年   101篇
  1999年   84篇
  1998年   50篇
  1997年   62篇
  1996年   60篇
  1995年   50篇
  1994年   50篇
  1993年   51篇
  1992年   28篇
  1991年   35篇
  1990年   22篇
  1989年   13篇
  1988年   6篇
  1987年   4篇
  1986年   5篇
  1985年   4篇
  1984年   3篇
  1980年   6篇
  1979年   12篇
  1978年   4篇
  1977年   4篇
  1974年   2篇
  1973年   7篇
  1972年   5篇
  1971年   9篇
  1970年   2篇
排序方式: 共有4186条查询结果,搜索用时 15 毫秒
991.
可再生能源资源评价体系与方法   总被引:8,自引:0,他引:8  
本文根据可再生能源资源的特性以及与技术、市场之间的内在联系,构建了可再生能源的资源评价体系,分析了资源量、生产率、生产经济性及不确定性分析等各环节中的具体评价方法和指标框架,并提出了最小和最大经济获利开发量以度量资源生产的经济性潜力  相似文献   
992.
ABSTRACT: Federal spending on water resource development projects exceeds $10 billion annually. This paper examines the economic theory and practice on which the Federal water resource development plans are based. Existing theoretical and applied problems result in overinvestment. These include 1) no standard of value for the tradeoff of environmental objectives with economic objectives; 2) benefits based on “willingness to pay,” but beneficiaries pay only a fraction of a project's costs; 3) beneficiaries “shop around” among program purposes in order to reduce their commitments; and 4) benefit/cost (B/C) analyses are based on a discount rate, which is consistently below the Federal borrowing rate. Furthermore, the Government Accounting Office (GAO) frequently finds that the agency regulations are inadequate and result in inconsistent and questionable benefit computations. The President has proposed a series of water policy reforms, to reduce some of the apparent overinvestment in water resource development, but fundmental corrective action rests with the members of Congress.  相似文献   
993.
ABSTRACT: At a time in history when water resource development in the United States is being condemned as both economically inefficient and an environmental disaster, perhaps economists need to look back at previous development to see what the fruits of water development (be they sweet or sour) have been. The Boise Project of south-western Idaho is 70 years old and to some people it represents a gross error in resource use, while to others it represents a means of livelihood and well being. A recent research project at the University of Idaho attempted to measure not only the direct economic income benefits of the project (from irrigation), but also the indirect or secondary income benefits (from the food processing industry). Periodic regional input-output tables were constructed to assess the income generated from irrigation and food processing over the period from 1946 to 1970. Input-output analysis allowed researchers to evaluate the direct and indirect impacts of both the irrigation sector and the food processing sector and to compare their growth over time.  相似文献   
994.
ABSTRACT The American Indian occupies a unique place in the federal system of government There are indications that this relationship will continue and that Indian reservations are at the threshold of economic development. As this occurs, the nature and extent of Indian water rights becomes more important to Indian and non-Indian alike. The determination of these rights is a matter of more than judicial decisions. To a large degree the determination of these rights will rest in the non-judicial arena and will be influenced by the perceptions of those rights held by Indians and water allocation officials, both state and federal. If the perceptions of these political actors are not congruent, then political conflict will occur as the rights become more important. To depend solely upon the judicial system to resolve these conflicts entails risks and costs to both Indians and to allocating officials. Indians are taking seriously the federal policy of Indian self-determination, and water allocation officials run decided risks in failure to realize this. An alternative suggested is to include Indians as consulting parties when decisions are being made that affect Indian interests.  相似文献   
995.
ABSTRACT. A hybrid computer program was developed to predict the water and salt outflow from a river basin in which irrigation is the major user of water. The model combines a chemical model which predicts the quality of water percolated through a soil profile with a general hydrologic model. The chemical model considers the reactions that occur in the soil, including the exchange of calcium, magnesium, and sodium cations on the soil complex, and the dissolution and precipitation of gypsum and lime. The chemical composition of the outflow is a function of these chemical processes within the soil, plus the blending of undiverted inflows, evaporation, transpiration, and the mixing of sub surface return flows with groundwater. The six common ions of western waters, namely calcium (Ca++), magnesium (Mg++), sodium (Na+), sulfate (SO4=), chloride (Cl?), and bicarbonate (HCO3?) were considered in the study. Total dissolved solids (TDS) outflow was obtained by adding the individual ions. The overall model operates on a monthly time unit. The model was tested on a portion of the Little Bear River basin in northern Utah. The model successfully simulated measured outflows of water and each of the six ions for a 24-month period. The usefulness of the model was demonstrated by a management study of the prototype system. For example, preliminary results indicated that the available water supply could be used to irrigate additional land without unduly increasing the salt outflow from the basin. With minor adjustments the model can be applied to other hydrologic areas.  相似文献   
996.
ABSTRACT Attitudes toward the development of the American West have undergone important changes over the past century just as the nature of water resources as factors in development have changed. Viewing these changes processually, stages for water resources definition and use can be identified in the total process of western cultural development. The first stage involves the value of water resource development as a stimulus to population and economic growth in the West. The second stage, still in process', adopts a dominant cultural norm which sees water resource development as inevitable if not necessary to keep up with growth. A third stage to this evolutionary process is incipient. Future cultural values and thinking with respect to water resource development will be to look at development as a means for controlling or managing both the location and quantity of population and economic growth. To this end planners will have to become concerned with the questions of human adaptation. Concern will have to be given to the problems of getting a living which enables individuals to meet the subsistence needs of self and family, to establishing community which provides for cooperation among individuals and the management of conflict, to establishing improved communication which promotes interpersonal interaction, and for fostering innovation which provides the new ideas necessary to adapt to new environmental situations.,  相似文献   
997.
ABSTRACT. Interest in the geochemistry of groundwater is increasing owing to the great number of current projects involving underground liquid waste storage, artificial recharge of potable water, accidental contamination of groundwater bodies, sanitary landfills, and pollution monitoring. Geochemical techniques used to facilitate the understanding of a groundwater system range from extremely simple to those requiring sophisticated theories, equipment, and procedures. An interpretation of the simple trilinear diagram for samples collected from the Yucatan Peninsula of Mexico provided evidence that the fresh-water body was only a few tens of meters thick and was underlain everywhere by an extensive body of salt water. A geochemical technique that has been used effectively to identify the source of salt water in coastal aquifers is measurement of the carbon-14 concentrations. Carbon-14 has been used in a regional carbonate aquifer to determine the velocity of groundwater movement, rates of chemical reactions, and distribution of hydraulic conductivity. The application of principles of irreversible thermodynamics to groundwater systems provides a basis for constructing models which permit prediction, over both time and space, of changes in head distribution and chemical character of the water resulting from imposed stresses on the system. In essence, proper application of irreversible thermodynamics combines the potential theory of Hubbert with principles of reversible chemical thermodynamics, such as solution of carbonate minerals, to describe and explain controlling chemical reactions and processes of groundwater systems.  相似文献   
998.
A procedure is outlined which allows consideration of both objective and subjective indicators to establish priorities in plan implementation of water resource development. The objective procedure utilizes stepwise multiple discriminant analysis to predict community performance regarding planned project implementation, based on previous project implementation in the Northeast. The subjective procedure incorporates prior probabilities developed by the planner, based on observation and experience gained through the planning process. The proposed analysis could eliminate waste through better allocation of planning funds to implementation studies exhibiting higher probability of early implementation.  相似文献   
999.
为量化2000-2016年长江经济带水资源消耗量对经济增长的约束程度,本文在科布道格拉斯生产函数中加入水资源要素,构建改进的Romer增长阻尼模型进行实证分析。结果显示:①研究期内长江经济带水资源增长阻尼系数总体稳定,水资源对经济增长的约束作用程度经历了明显的强弱变化;②研究期内水资源增长阻尼显著降低并趋向平稳,说明水资源对长江经济带经济增长的约束作用减弱并逐渐稳定;③浙江、上海、江西等7个省份存在明显的水资源增长阻尼,区域差异显著。由此,本文建议:长江经济带各省市应提升水资源利用效率,同时加大资本投入、优化劳动力结构,在区域协调发展方面可构建水权交易机制,以期使长江经济带在水资源"大保护"约束下实现经济的持续发展。  相似文献   
1000.
In northeastern America, thousands of kilometers of utility rights-of-way (ROWs) have to be managed to prevent the establishment of a tall vegetation cover that does not comply with safety and maintenance regulations. Recent decades have seen the emergence of ecologically based vegetation control strategies to reduce environmental impacts as well as maintenance costs. One such strategy is to take advantage of competitive herbaceous covers to limit tree invasion. This approach, however, as well as its fundamental underlying principles, has been little scrutinized. In this article, (1) we present the main ecological concepts supporting the use of a herbaceous cover to limit tree invasion, emphasizing naturally forested ecosystems of northeastern America. They include reported evidence of stable plant communities and an overview of potential underlying mechanisms of inhibition. (2) We then review field applications, specifically testing the ability of seeded herbaceous covers to control tree invasion in ROWs. (3) We discuss unresolved issues relevant to management and research. The available evidence suggests that seeding herbaceous covers in ROWs can help control tree invasion, but many issues still limit broad-scale applications. The various interactions that govern plant community dynamics are far from being fully understood, so selecting species still largely depends on an empirical approach. Patterns of resistance to tree invasion must be investigated over a wide range of spatial, historical, and environmental contexts to determine effective management and seeding practices that will lead to broad-scale applications. We suggest establishing communities rather than single dominant species and using as much as possible native species to limit risks of invasion.Published online  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