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   31098篇
  免费   320篇
  国内免费   333篇
安全科学   952篇
废物处理   1439篇
环保管理   3841篇
综合类   5356篇
基础理论   8093篇
环境理论   18篇
污染及防治   8029篇
评价与监测   2070篇
社会与环境   1773篇
灾害及防治   180篇
  2023年   175篇
  2022年   326篇
  2021年   353篇
  2020年   287篇
  2019年   354篇
  2018年   478篇
  2017年   482篇
  2016年   759篇
  2015年   598篇
  2014年   898篇
  2013年   2430篇
  2012年   1075篇
  2011年   1463篇
  2010年   1175篇
  2009年   1208篇
  2008年   1447篇
  2007年   1494篇
  2006年   1279篇
  2005年   1109篇
  2004年   994篇
  2003年   1081篇
  2002年   958篇
  2001年   1249篇
  2000年   871篇
  1999年   532篇
  1998年   357篇
  1997年   369篇
  1996年   367篇
  1995年   430篇
  1994年   452篇
  1993年   359篇
  1992年   376篇
  1991年   358篇
  1990年   390篇
  1989年   341篇
  1988年   296篇
  1987年   280篇
  1986年   223篇
  1985年   248篇
  1984年   265篇
  1983年   256篇
  1982年   241篇
  1981年   222篇
  1980年   175篇
  1979年   194篇
  1978年   176篇
  1975年   140篇
  1974年   117篇
  1972年   130篇
  1971年   131篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
511.
Agricultural change has transformed large areas of traditional farming landscapes, leading to important changes in the species community assemblages in most European countries. We suspect that the drastic changes in land-use that have occurred in Andalusia (southern Spain) over recent decades, may have affected the distribution and abundance of game species in this region. This article compares the distribution of the main game species in Andalusia during the 1960s and 1990s, using data from maps available from the Mainland Spanish Fish, Game and National Parks Service and from recent datasets on hunting yield distributions, respectively. Big-game and small-game species were significantly segregated in southern Spain during the 1990s, as two clearly independent chorotypes (groups of species whose abundances are similarly distributed) were obtained from the classification analysis. In contrast, big-game and small-game species were not significantly segregated several decades ago, when there was only one chorotype consisting of small-game species and wild boar. The other three ungulates did not constitute a significant chorotype, as they showed positive correlations with some species in the group mentioned above. These changes seem to be a consequence of the transformations that have occurred in the Iberian Mediterranean landscape over the last few decades. The abandoning of traditional activities, and the consequent formation of dense scrubland and woodland, has led to an expansion of big-game species, and a decrease of small-game species in mountain areas. Moreover, agricultural intensification has apparently depleted small-game species populations in some agricultural areas. On the other hand, the increasingly intensive hunting management could be artificially boosting this segregation between small-game and big-game species. Our results suggest that the conservation and regeneration of traditional agricultural landscapes (like those predominating in the 1960s) should be a priority for the conservation of small-game species.  相似文献   
512.
Excess crude protein (CP) in dairy cow diets is excreted mostly as urea nitrogen (N), which increases ammonia (NH) emissions from dairy farms and heightens human health and environmental concerns. Feeding less CP and more tannin to dairy cows may enhance feed N use and milk production, abate NH emissions, and conserve the fertilizer N value of manure. Lab-scale ventilated chambers were used to evaluate the impacts of CP and tannin feeding on slurry chemistry, NH emissions, and soil inorganic N levels after slurry application to a sandy loam soil and a silt loam soil. Slurry from lactating Holstein dairy cows (Bos taurus) fed two levels of dietary CP (low CP [LCP], 155 g kg; high CP [HCP], 168 g kg) each fed at four levels of dietary tannin extract, a mixture from red quebracho (Schinopsis lorentzii) and chestnut (Castanea sativa) trees (0 tannin [0T]; low tannin [LT], 4.5 g kg; medium tannin [MT], 9.0 g kg; and high tannin [HT], 18.0 g kg) were applied to soil-containing lab-scale chambers, and NH emissions were measured 1, 3, 6, 12, 24, 36, and 48 h after slurry application. Emissions from the HCP slurry were 1.53 to 2.57 times greater ( < 0.05) than from the LCP slurry. At trial's end (48 h), concentrations of inorganic N in soils were greater ( < 0.05) in HCP slurry-amended soils than in LCP slurry-amended soils. Emissions from HT slurry were 28 to 49% lower ( < 0.05) than emissions from 0T slurry, yet these differences did not affect soil inorganic N levels. Emissions from the sandy loam soil were 1.07 to 1.15 times greater ( < 0.05) than from silt loam soil, a result that decreased soil inorganic N in the sandy loam compared with the silt loam soil. Larger-scale and longer-term field trails are needed to ascertain the effectiveness of feeding tannin extracts to dairy cows in abating NH loss from land-applied slurry and the impact of tannin-containing slurry on soil N cycles.  相似文献   
513.
Modeling is a common practice to evaluate factors affecting water quality in environmental systems impaired by point and nonpoint losses of N and P. Nevertheless, in situations with inadequate information, such as ungauged basins, a balance between model complexity and data availability is necessary. In this paper, we applied a simplified analytical model to an artificially drained floodplain in central-western Italy to evaluate the importance of different nutrient sources and in-stream retention processes and to identify critical source areas. We first considered only a set of chemical concentrations in water measured from February through May 2008 and from November 2008 through February 2009. We then broadened available data to include water discharge and hydraulic-head measurements to construct a hydrogeological model using MODFLOW-2000 and to evaluate the reliability of the simplified method. The simplified model provided acceptable estimates of discharge (ranging from 0.03-0.75 m s) and diffuse nutrient inputs from water table discharge and in-stream retention phenomena. Estimates of PO-P and total P retention (ranging from 1.0 to 0.6 μg m s and from 1.18 to 0.95 μg m s for PO-P and total P, respectively) were consistent with the range of variability in literature data. In contrast, the higher temporal variability of nitrate concentrations decreased model accuracy, suggesting the need for more intensive monitoring. The model also separated the dynamics of different reaches of the drainage network and identified zones considered critical source areas and buffer zones where pollutant transport is reduced.  相似文献   
514.
ABSTRACT: The cost of sanitary facilities is a major cost item in recreation areas at water resources projects. Often planners wish to analyze the tradeoffs in providing different levels of service at these areas. Cost functions are presented in this paper to enable the planner to make quick comparisons between different types of buildings, sizes, exterior treatments, and number of fixtures.  相似文献   
515.
ABSTRACT: A large-scale simulation/optimization model provides schedules for operation of water and power for the California State Water Project (SWP). The SWP consists of a series of reservoirs linked by rivers, pumping plants, canals, tunnels, and generating plants and is operated by the California Department of Water Resources. The Department provides water to municipal and agricultural users, and manages its electrical loads and resources. The model, therefore, performs hydraulic and electrical computations leading to optimal operation of the entire system. It consists of hydraulic network programming components to meet the storage objectives at all the reservoirs, a linear programming component to determine the schedules at pumping and generating plants, an electrical network programming component to balance electrical loads and resources, and a number of other simulation components. It operates on yearly, weekly, and daily bases. It is primarily used for real-time operation of the SWP and can provide hourly detail schedules which are implemented by the SWP staff via a computerized system.  相似文献   
516.
    
Global warming is negatively affecting the environment of the planet. This situation has led to the development of international standards, such as the International Organisation for Standardization's DIS 14064‐1 and the Greenhouse Gas Protocol Corporate Standard, both of which measure corporate carbon footprints. These standards provide guidelines that can be applied to different organizational sectors. However, these are not sufficient for controlling the reduction of carbon emissions, because although they propose the use of indicators, they do not explicitly define them. In addition, in the case of emissions from wastewater treatment, they only suggest that the emissions associated with this process be considered. In the present study, an eight‐step, unified methodology based on these two international standards is proposed, focusing on direct emissions. Moreover, the step‐by‐step to data collection, calculations, and the required indicators to control the emissions are defined. The first scope considers direct emissions from sources that are owned or controlled by the company. Methane generation measurement from wastewater treatment has been included in the methodology within Scope 1, as it is the second most polluting gas after carbon dioxide, both of which give rise to global warming. The proposed methodology was tested as a case study in one of the most important companies in the food sector in Colombia.  相似文献   
517.
Pollock, Michael M. and Timothy J. Beechie, 2010. Reply to Discussion –“Stream Temperature Relationships to Forest Harvest in Western Washington” by George Ice, George Brown, John Gravelle, C. Rhett Jackson, Jeff Light, Timothy Link, Douglas Martin, Dale McGreer, and Arne Skaugset. Journal of the American Water Resources Association (JAWRA) 46(4): 843-847. DOI: 10.1111/j.1752-1688.2010.00456.x  相似文献   
518.
519.
    
This study assesses the impact of farmer field schools (FFS) on the productivity of vegetable farming in vegetable‐producing areas of East Java and Bali, Indonesia. The FFS have equipped over 3,000 vegetable farmers with integrated crop management knowledge applicable to chilies and tomatoes. The FFS are expected to enhance farmers’ capacity such that they can increase production. This study employs a difference‐in‐differences (DiD) method to overcome selection bias. A survey of 250 FFS‐graduated farmers and 250 non‐FFS farmers were purposively randomly selected from the overall community of farmers. Focus group discussion was used to support the survey. The results indicate that FFS were successful for enhancing farmers’ capability in vegetable farming. Farmers who participated in FFS have higher productivity than those who did not. Farmers also could adapt and adopt the knowledge gained from FFS as they underwent a process of learning by doing. The impacts of the increase in farmers’ capacity can be more evident if weaknesses during the FFS preparation and implementation can be overcome, to ensure more participation, flexibility to fit different conditions/needs and continuous learning.  相似文献   
520.
    
Across the western United States, environmental water transaction programs (EWTPs) restore environmental flows by acquiring water rights and incentivizing changes in water management. These programs have evolved over several decades, expanding from relatively simple two‐party transactions to multiobjective deals that simultaneously benefit the environment and multiple water‐using sectors. Such programs now represent an important water management tool and provide an impetus for collaboration among stakeholders; yet, most evaluations of their effectiveness focus exclusively on environmental outcomes, without adequate attention to impacts on other water users or local economies. To understand how these programs affect stakeholders, a systematic, multiobjective evaluation framework is needed. To meet this need, we developed a suite of environmental and socioeconomic indicators that can guide the design and track the implementation of water transaction portfolios, and we applied them to existing EWTPs in Oregon and Nevada. Application of the indicators quantifies impacts and helps practitioners design water transaction portfolios that avoid unintended consequences and generate mutually beneficial outcomes among environmental, agricultural, and municipal interests.  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