全文获取类型
收费全文 | 857篇 |
免费 | 33篇 |
国内免费 | 9篇 |
专业分类
安全科学 | 30篇 |
废物处理 | 19篇 |
环保管理 | 312篇 |
综合类 | 93篇 |
基础理论 | 256篇 |
污染及防治 | 128篇 |
评价与监测 | 34篇 |
社会与环境 | 17篇 |
灾害及防治 | 10篇 |
出版年
2023年 | 5篇 |
2022年 | 17篇 |
2021年 | 14篇 |
2020年 | 24篇 |
2019年 | 18篇 |
2018年 | 21篇 |
2017年 | 40篇 |
2016年 | 30篇 |
2015年 | 23篇 |
2014年 | 33篇 |
2013年 | 53篇 |
2012年 | 44篇 |
2011年 | 50篇 |
2010年 | 40篇 |
2009年 | 33篇 |
2008年 | 41篇 |
2007年 | 39篇 |
2006年 | 37篇 |
2005年 | 23篇 |
2004年 | 36篇 |
2003年 | 34篇 |
2002年 | 29篇 |
2001年 | 19篇 |
2000年 | 13篇 |
1999年 | 15篇 |
1998年 | 13篇 |
1997年 | 16篇 |
1996年 | 12篇 |
1995年 | 11篇 |
1994年 | 11篇 |
1993年 | 13篇 |
1992年 | 10篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1989年 | 12篇 |
1988年 | 4篇 |
1987年 | 9篇 |
1986年 | 6篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1971年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有899条查询结果,搜索用时 515 毫秒
871.
People on the land: changes in global population and croplands during the 20th century 总被引:2,自引:0,他引:2
This study reviews the major changes in global distribution of croplands during the 20th century. During the 20th century, the cropland base diminished greatly (from approximately 0.75 ha person-1 in 1900 to approximately 0.35 ha person-1 in 1990). This loss of croplands was not globally uniform: more than half the world's population, living in developing nations, lost nearly two-thirds of their per capita cropland base. The distribution of croplands has become increasingly skewed--in 1990, 80% of the population lived off less than 0.35 ha person-1. While agricultural yields have generally increased, they have barely kept pace with population growth in developing nations. Overall, the global food production system is becoming increasingly vulnerable to regional disruptions because of our increasing reliance on expensive technological options to increase agricultural production, or on global food trade. 相似文献
872.
873.
874.
Evolutionary simulation-optimization (ESO) techniques can be adapted to model a wide variety of problem types in which system components are stochastic. Grey programming (GP) methods have been previously applied to numerous environmental planning problems containing uncertain information. In this paper, ESO is combined with GP for policy planning to create a hybrid solution approach named GESO. It can be shown that multiple policy alternatives meeting required system criteria, or modelling-to-generate-alternatives (MGA), can be quickly and efficiently created by applying GESO to this case data. The efficacy of GESO is illustrated using a municipal solid waste management case taken from the regional municipality of Hamilton-Wentworth in the Province of Ontario, Canada. The MGA capability of GESO is especially meaningful for large-scale real-world planning problems and the practicality of this procedure can easily be extended from MSW systems to many other planning applications containing significant sources of uncertainty. 相似文献
875.
876.
877.
878.
879.
880.
A model for predicting community mosaics and wildlife diversity resulting from fire disturbance to a forest ecosystem is presented. It applies an algorithm that delineates the size and shape of each patch from grid-based input data and calculates standard diversity measures for the entire mosaic of community patches and their included animal species. The user can print these diversity calculations, maps of the current community-type-age-class mosaic, and maps of habitat utilization by each animal species. Furthermore, the user can print estimates of changes in each resulting from natural disturbance. Although data and resolution level independent, the model is demonstrated and tested with data from the Lewis and Clark National Forest in Montana. 相似文献