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   991篇
  免费   33篇
  国内免费   8篇
安全科学   53篇
废物处理   43篇
环保管理   265篇
综合类   87篇
基础理论   270篇
环境理论   3篇
污染及防治   200篇
评价与监测   68篇
社会与环境   35篇
灾害及防治   8篇
  2023年   8篇
  2022年   16篇
  2021年   14篇
  2020年   6篇
  2019年   14篇
  2018年   21篇
  2017年   22篇
  2016年   30篇
  2015年   31篇
  2014年   26篇
  2013年   63篇
  2012年   59篇
  2011年   58篇
  2010年   41篇
  2009年   62篇
  2008年   70篇
  2007年   59篇
  2006年   57篇
  2005年   33篇
  2004年   42篇
  2003年   40篇
  2002年   36篇
  2001年   9篇
  2000年   15篇
  1999年   15篇
  1998年   15篇
  1997年   12篇
  1996年   16篇
  1995年   15篇
  1994年   19篇
  1993年   14篇
  1992年   10篇
  1991年   7篇
  1990年   6篇
  1989年   9篇
  1988年   5篇
  1987年   2篇
  1986年   3篇
  1985年   8篇
  1984年   4篇
  1983年   9篇
  1982年   8篇
  1981年   3篇
  1980年   4篇
  1979年   3篇
  1978年   2篇
  1977年   4篇
  1976年   2篇
  1971年   1篇
  1957年   2篇
排序方式: 共有1032条查询结果,搜索用时 15 毫秒
361.
As conservation increases its emphasis on implementing change at landscape-level scales, multi-agency, cross-boundary, and multi-stakeholder networks become more important. These elements complicate traditional notions of learning. To investigate this further, we examined structures of learning in the Landscape Conservation Cooperatives (LCCs), which include the entire US and its territories, as well as parts of Canada, Mexico, and Caribbean and Pacific island states. We used semi-structured interviews, transcribed and analyzed using NVivo, as well as a charrette-style workshop to understand the difference between the original stated goals of individual LCCs and the values and purposes expressed as the collaboration matured. We suggest double-loop learning as a theoretical framework appropriate to landscape-scale conservation, recognizing that concerns about accountability are among the valid points of view that must be considered in multi-stakeholder collaborations. Methods from the social sciences and public health sectors provide insights on how such learning might be actualized.  相似文献   
362.
Mitigation and Adaptation Strategies for Global Change - Soils hold the largest pool of organic carbon (C) on Earth; yet, soil organic carbon (SOC) reservoirs are not well represented in climate...  相似文献   
363.
A well-developed suction pump in the head represents an important adaptation for nectar-feeding insects, such as Hymenoptera, Lepidoptera and Diptera. This pumping organ creates a pressure gradient along the proboscis, which is responsible for nectar uptake. The extremely elongated proboscis of the genus Prosoeca (Nemestrinidae) evolved as an adaptation to feeding from long, tubular flowers. According to the functional constraint hypothesis, nectar uptake through a disproportionately elongated, straw-like proboscis increases flower handling time and consequently lowers the energy intake rate. Due to the conspicuous length variation of the proboscis of Prosoeca, individuals with longer proboscides are hypothesised to have longer handling times. To test this hypothesis, we used field video analyses of flower-visiting behaviour, detailed examinations of the suction pump morphology and correlations of proboscis length with body length and suction pump dimensions. Using a biomechanical framework described for nectar-feeding Lepidoptera in relation to proboscis length and suction pump musculature, we describe and contrast the system in long-proboscid flies. Flies with longer proboscides spent significantly more time drinking from flowers. In addition, proboscis length and body length showed a positive allometric relationship. Furthermore, adaptations of the suction pump included an allometric relationship between proboscis length and suction pump muscle volume and a combination of two pumping organs. Overall, the study gives detailed insight into the adaptations required for long-proboscid nectar feeding, and comparisons with other nectar-sucking insects allow further considerations of the evolution of the suction pump in insects with sucking mouthparts.  相似文献   
364.
The source data for QRA’s is important to assure meaningful risk assessment results, particularly when the result is to be compared against quantitative risk acceptance criteria. The author’s company is one of the largest global QRA providers and we have concluded that the UK HSE Hydrocarbon Release Database (HCRD) provides the basis for the best leak frequency data as it offers complete leak data collection in a systematic manner, against a known population of equipment and facilities in the UK sector of the North Sea for which there is an accurate parts count estimate. The LEAK program is described. It screens HCRD to remove leak events not associated with full operations inventories and flows (e.g. when isolated for maintenance) and further uses a distribution function that permits frequencies for any arbitrary hole size range to be determined (e.g. 25 mm leaks, 50 mm leaks, full-bore ruptures). An important factor is that leak frequency data is not stationary, offshore operators have improved their control of leak events and the HCRD shows a declining leak rate over time.DNV often uses frequency modification techniques, termed MOR - Modification of Risk. This paper reviews 4 methods developed by the company internationally. These are the Manager Method, the API 581 method, a barrier based method, and a proprietary management system based method. These all permit localization of UK North Sea data to apply to other facilities (onshore or offshore) and with different management systems and mechanical integrity programs.Overall, localized data using MOR is considered more accurate than direct use of UK North Sea data, however validation is an issue. There are no direct comparisons that compare leak statistics over a sufficiently long period with static management systems and integrity programs. Thus MOR techniques remain judgment based approaches, but transparent in methodology and assumptions. The barrier based modification technique is the most directly verifiable of the four MOR methods presented.  相似文献   
365.
Detecting water quality improvements following watershed management changes is complicated by flow-dependent concentrations and nonlinear or threshold responses that are difficult to detect with traditional statistical techniques. In this study, we evaluated the long-term trends (1997-2009) in total P (TP) concentrations in the Illinois River of Oklahoma, and some of its major tributaries, using flow-adjusted TP concentrations and regression tree analysis to identify specific calendar dates in which change points in P trends may have occurred. Phosphorus concentrations at all locations were strongly correlated with stream flow. Flow-adjusted TP concentrations increased at all study locations in the late 1990s, but this trend was related to a change in monitoring practices where storm flow samples were specifically targeted after 1998. Flow-adjusted TP concentrations decreased in the two Illinois River sites after 2003. This change coincided with a significant decrease in effluent TP concentrations originating with one of the largest municipal wastewater treatment facilities in the basin. Conversely, flow-adjusted TP concentrations in one tributary increased, but this stream received treated effluent from a wastewater facility where effluent TP did not decrease significantly over the study period. Results of this study demonstrate how long-term trends in stream TP concentrations are difficult to quantify without consistent long-term monitoring strategies and how flow adjustment is likely mandatory for examining these trends. Furthermore, the study demonstrates how detecting changes in long-term water quality data sets requires statistical methods capable of identifying change point and nonlinear responses.  相似文献   
366.
367.
Caruso, Brian S. and Joshua Haynes, 2011. Biophysical‐Regulatory Classification and Profiling of Streams Across Management Units and Ecoregions. Journal of the American Water Resources Association (JAWRA) 00(0):1‐22. DOI: 10.1111/j.1752‐1688.2010.00522.x Abstract: Aquatic resources management in the United States (U.S.) under Clean Water Act Section 404 has become more complex after recent Supreme Court decisions and U.S. Army Corps of Engineers and Environmental Protection Agency (USEPA) guidance. Many intermittent/ephemeral and headwater streams may not be jurisdictional if they lack a significant nexus with navigable waters. Streams in semiarid USEPA Region 8 were classified based on hydrologic permanence and stream order using National Hydrography Dataset (NHD) Plus and GIS to provide information across broad spatial scales to aid with jurisdictional determinations (JDs). Four classes were developed for profiling across management units and ecoregions. Based on medium‐resolution NHDPlus data, intermittent streams comprise >¾, and first order streams constitute >½ of the total stream length in Region 8. Mountain states and ecoregions have the largest percentage of perennial first order streams, whereas the Dakotas, plains, and desert ecoregions have the greatest percentages of intermittent first order and intermittent higher order streams. In the Upper Colorado River Basin, >50% of reaches are intermittent first order, and 9% are perennial first order. NHDPlus data can significantly underestimate the length of headwater and intermittent streams, but can still be a valuable tool to help develop stream classes and for regional JD planning and analysis. Refinement of the stream classes using high resolution NHD data and other key catchment parameters can improve their utility for JDs.  相似文献   
368.
The physiological response of a fish to its environment is mediated through the endocrine axis controlling growth. Therefore, growth-regulating hormone levels can serve as ecologically relevant indicators of fish growth rate. We quantified variation in plasma insulin-like growth factor-I (IGF-I) to evaluate its potential as an indicator of growth in lingcod, an economically and ecologically important bottomfish species in the northeast Pacific. An information-theoretic model selection approach was used to test the hypothesis that variation in lingcod IGF-I is related to season, body size, and gonadal steroid concentration. Season and a length × season interaction were the most important predictors of plasma IGF-I among the variables we evaluated, suggesting that season and body size should be explicitly accounted for when interpreting endocrine patterns in wild fish populations. This is among the few studies that have measured and interpreted patterns of IGF-I in wild fish and the first to describe seasonal endocrine profiles in lingcod.  相似文献   
369.
The southeastern United States supports one of two large loggerhead turtle (Caretta caretta) nesting aggregations worldwide and is therefore critical to global conservation and recovery efforts for the species. Previous studies have established the presence of four demographically distinct nesting populations (management units) corresponding to beaches from (1) North Carolina through northeastern Florida, (2) peninsular Florida, (3) the Dry Tortugas, and (4) northwest Florida. Temporal and geographic genetic structure of the nesting aggregation was examined utilizing partial mitochondrial control region haplotype frequencies from 834 samples collected over the 2002 through 2008 nesting seasons from 19 beaches as well as previously published haplotype data. Most rookeries did not exhibit interannual genetic variation. However, the interannual variation detected did significantly impact the interpretation of spatial genetic structure in northeastern Florida. Based on pairwise F ST comparisons, exact tests of population differentiation, and analysis of molecular variance, the present study upholds the distinctiveness of the four currently recognized management units and further supports recognition of discrete central eastern, southern (southeastern and southwestern), and central western Florida management units. Further subdivision may be warranted, but more intensive genetic sampling is required. In addition, tools such as telemetry and mark-recapture are needed to complement genetic data and overcome limitations of genetic markers in resolving loggerhead turtle rookery connectivity in the southeastern USA.  相似文献   
370.
We validated an existing physically based 3D MIKE SHE groundwater resource model (DK-model) at 175 Danish gauging stations covering different catchment sizes in order to calculate monthly water runoff in the 50% ungauged part of Denmark. Model performance was in most cases good (61% of gauging stations had a Nash-Sutcliffe (NS) coefficient >0.60) but nevertheless showed a large seasonal and georegion specific bias. Therefore, bias correction factors had to be developed before applying the DK-model simulations of runoff in the ungauged areas. Simulated monthly runoff from ungauged areas and the measured monthly runoff from 178 gauging stations were distributed to 2663 smaller Hydrological Units (ca. 15 km(2)) and linked with a new empirical model for flow-weighted monthly total nitrogen (TN) concentrations (R(2) = 0.43; P < 0.0001) developed based on 20 years of observations (1990-2009) in 83 small catchments for calculation of monthly gross diffuse TN-loads from HU's. Nitrogen retention was calculated in streams, lakes and wetlands utilising both lake specific models and rate coefficients to calculate N retention in surface water bodies. The whole model complex was linked in the DK-QN concept for simulation of monthly TN losses from point sources and diffuse sources, TN retention and resulting loadings to Danish coastal waters. The DK-QN model was validated in 118 gauged catchments and the model simulations had for >25% of the observations of monthly discharge weighted TN concentrations a NS larger than 0.26. Catchment specific monthly TN-loadings were modelled with a higher performance as 50% of the catchments had a NS greater than 0.75. The model concept allows calculation of N retention in streams, lakes and wetlands and the average annual model calculated N retention amounted to 21% of the modelled gross riverine TN loadings. The average annual gross TN loading to surface freshwater in Denmark derived from diffuse sources amounted to 97 000 tonnes N (91% of gross TN loadings) which is 54% of the total estimated N-leaching from the root zone on the Danish land area (212 000 tonnes N) during the period 1990-2009.  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