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   457篇
  免费   22篇
  国内免费   5篇
安全科学   22篇
废物处理   1篇
环保管理   222篇
综合类   81篇
基础理论   58篇
污染及防治   6篇
评价与监测   21篇
社会与环境   56篇
灾害及防治   17篇
  2022年   1篇
  2021年   3篇
  2020年   6篇
  2019年   13篇
  2018年   7篇
  2017年   11篇
  2016年   20篇
  2015年   13篇
  2014年   6篇
  2013年   20篇
  2012年   19篇
  2011年   15篇
  2010年   15篇
  2009年   19篇
  2008年   17篇
  2007年   28篇
  2006年   18篇
  2005年   20篇
  2004年   22篇
  2003年   22篇
  2002年   18篇
  2001年   9篇
  2000年   14篇
  1999年   17篇
  1998年   19篇
  1997年   10篇
  1996年   8篇
  1995年   4篇
  1994年   7篇
  1993年   10篇
  1992年   1篇
  1991年   6篇
  1990年   6篇
  1989年   2篇
  1988年   5篇
  1987年   6篇
  1986年   1篇
  1985年   2篇
  1984年   4篇
  1983年   4篇
  1982年   3篇
  1981年   4篇
  1980年   5篇
  1979年   4篇
  1978年   3篇
  1977年   3篇
  1976年   4篇
  1973年   2篇
  1972年   2篇
  1971年   5篇
排序方式: 共有484条查询结果,搜索用时 234 毫秒
331.
ABSTRACT: Appraisal-level, engineering economic analyses for two types of irrigation systems are compared: (1) a surface-water irrigation system, and (2) a ground-water recharge irrigation system. An artificial recharge irrigation system is an economically viable alternative to surface-distributed irrigation in a conjunctive irrigation plan if electric power rates remain sufficiently low. As electric rates increase, this viability generally decreases until it becomes completely unattractive at rates of about 0.012 dollars per kilowatt hour.  相似文献   
332.
ABSTRACT: The Hydrologic Engineering Center, Corps of Engineers, has been engaged in research, training, and project assistance in non-structural flood control planning for Corps offices across the United States since 1975. Lessons learned from this experience deal with the role of nonstructural measures in flood plain management, the role of creativity in analysis, the role of analysis, and tools for analysis. The role of nonstructural measures in flood control planning depends upon the scale of the problem, the nature of the measure, the degree of protection desired, and whether damage is to existing or future property. An earnest seeking for nonstructural opportunities, a field presence for their formulation, and compatbility with local infrastructure plans are prerequisite to creative use of nonstructural measures. Analysis is a necessary complement of creativity. Several tools for nonstructural analysis have been developed and applied to flood problems involving several hundred and several thousand structures.  相似文献   
333.
Forests and competing land uses in Kenya   总被引:1,自引:0,他引:1  
Indigenous forests in Kenya, as in other developing countries, are under heavy pressure from competing agricultural land uses and from unsustainable cutting. The problem in Kenya is compounded by high population growth rates and an agriculturally based economy, which, even with efforts to control birth rates and industrialize, will persist into the next century. Both ecological and economic consequences of these pressures need to be considered in land-use decision making for land and forest management to be effective. This paper presents one way to combine ecological and economic considerations. The status of principal forest areas in Kenya is summarized and competing land uses compared on the basis of ecological functions and economic analysis. Replacement uses do not match the ecological functions of forest, although established stands of tree crops (forest plantations, fuel wood, tea) can have roughly comparable effects on soil and water resources. Indigenous forests have high, although difficult to estimate, economic benefits from tourism and protection of downstream agricultural productivity. Economic returns from competing land uses range widely, with tea having the highest and fuel wood plantations having returns comparable to some annual crops and dairying. Consideration of ecological and economic factors together suggests some trade-offs for improving land allocation decisions and several management opportunities for increasing benefits or reducing costs from particular land uses. The evaluation also suggests a general strategy for forest land management in Kenya.The views and interpretations expressed in this article are those of the authors and should not be attributed to the World Bank, its affiliated organizations, or any individual acting on their behalf.  相似文献   
334.
ABSTRACT: During recent years many countries have moved to rationalize the management of state owned or state controlled resources. In Victoria, Australia, water, in particular, has received a great deal of attention. Subsidies to construct, operate, and rehabilitate irrigation facilities have been reduced, and there have been numerous consolidations of small water supply authorities to achieve economies of scale. In addition, Victoria has taken the unusual step of using auctions to allocate new irrigation water entitlements. The six water auctions that took place during 1988 and early 1989 were perceived as an efficient and equitable mechanism to allocate limited water supplies to their highest value use. While the agricultural areas where the different auctions occurred were similar, there was substantial variation in the prices paid, which reflects varying demands for additional irrigation water. This paper will discuss the following topics: the auction process used and the results of the auctions, the efficiency of the auction procedures, and the factors influencing the demand for additional irrigation water in the different auctions. In addition, the implications of using auctions to allocate water supplies for future water management in Victoria will be discussed.  相似文献   
335.
ABSTRACT: The deliberate underwatering of a larger land area, as practiced in Southern Asia, has provided impetus for a systematic investigation into the effects of designing projects for crop water deficits on Benefit-Cost performance. The study began with the derivation, from published experimental results, of functions relating ultimate crop yield to the magnitude and timing of water deficits, i.e., of the productivity of irrigation water. To obtain the net benefit of the project, the relation between the harvested area and output and the on-farm production costs was then suggested. The cost of supplying the irrigation water to the proposed area and of distributing and applying it to the field was determined, thus completing the Benefit-Cost equation. A computer simulation model was then established to search for the irrigation project design capacity and area to maximize the net present value in the Benefit-Cost analysis for the development proposed.  相似文献   
336.
ABSTRACT: The problem of allocating scarce water resources among competing uses and users over time in Hawaii is addressed within the context of analytical institutional economics. The nature of this problem has been, in recent years, highly complicated by important institutional changes that control operating decisions for water supply and water pollution. Whereas the imbalance in governmental initiatives regarding changes in the system of water rights (predominantly state) and environmental laws (predominantly federal) are based on U.S. constitutional provisions, the more fundamental roots of the crucial legal doctrines involved have been traced back to the common property concept. This suggests that the more promising opportunities for meeting the water policy challenges of the state are to be found in the historical common property system (ahupua'a) of Hawaii.  相似文献   
337.
ABSTRACT: Dams were built by the U. S. Army Corps of Engineers on the Kaskaskia River at Shelbyville and Carlyle in Illinois, in 1969 and 1967, respectively. The operation of the Shelbyville and Carlyle Lakes has changed over the years because of considerably lower bankfull channel capacities downstream of the dams than were adopted in the project designs. This study was conducted to review the present operation policy. Intent was to derive a policy for maximizing the overall benefits (or minimizing the overall damages) and to compare these benefits or damages with those with the present policy. The operating rules were optimized through a simulation model which was structured considering the physical nature of the system and the desirable operation in the best interest of various beneficial uses. The expected annual value of overall benefits from recreation and agriculture is shown to increase by $0.2 million with the optimal policy. With the optimal operation, the overall damages are reduced by 76 percent on the average over the 24 years of flow record at Shelbyville and Carlyle.  相似文献   
338.
ABSTRACT: There is a serious shortage of yield data for Jojoba (SIMMONDSIA CHINENSIS) from mature and cultivated plantations of ten years or older. Thus, the optimal quantities of water and fertilizers that are needed annually by a Jojoba plantation to achieve maximum profits were estimated from existing plantations. A computer model (IMSL subroutine ZXMWD) was used to determine a Mitscherlich yield function. An economic profit model that incorporates the yield function was then designed to maximize the profits. According to the net present value (NPV) criterion, the maximum yearly net profit is about $7800 per ha, which is achieved during the 11th growth year. The computed cumulative NPV is approximately $21,000 per ha for the entire 15-year period. Similar profits were calculated when the model was tested to examine the NPV using different assumptions with respect to varying costs.  相似文献   
339.
ABSTRACT: This paper presents a new methodology to calculate economic losses from hypothetical, extreme flood events, such as the Probable Maximum Flood. The methodology uses economic data compiled from already-available secondary sources, such as U.S. Census data on magnetic tapes, utilizing microcomputer and other electronic media. Estimates of land elevations are obtained from topographic maps, and flood elevations axe estimated using, for example, a dam breach and flood routing (DAMBRK) model (Fread, 1984). The calculations are performed at a disaggregate spatial scale, by various land use and industrial classification categories. The basic areal units are city blocks (for urbanized areas), enumeration districts, and Census tracts. Depth-damage functions, which provide an estimate of damages as a proportion of the existing value of the structure, are estimated statistically. Computer software (called DAMAGE) is used to combine the economic, flood elevation, and depth-damage information to compute economic losses for different possible flood stages and for different inflow events. Two case studies are presented as illustrations of the method.  相似文献   
340.
A model describing parents’ preferences to relieve their own and their children's acute illnesses is estimated using stated-preference data. Estimated marginal rates of substitution (MRS) between child and parent illness are about two, indicating that parents value children's illness attributes twice as highly as their own. The MRS is larger for younger children, falls toward unity as the child approaches adulthood, and appears to reflect parental altruism rather than parent–child differences in initial health or illness costs. Intra-family allocations may compensate for chronic health impairments. Parents’ willingness to pay to avoid own or child illness increases with income, declines with fertility, increases at a decreasing rate with duration and number of symptoms, and depends on perceived discomfort and activity restrictions. Current methods of assessing morbidity benefits of environmental regulations may understate substantially the value of children's health, particularly in African-American families.  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