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   2427篇
  免费   304篇
  国内免费   898篇
安全科学   88篇
废物处理   17篇
环保管理   352篇
综合类   1919篇
基础理论   489篇
污染及防治   48篇
评价与监测   162篇
社会与环境   457篇
灾害及防治   97篇
  2024年   29篇
  2023年   89篇
  2022年   205篇
  2021年   243篇
  2020年   201篇
  2019年   185篇
  2018年   174篇
  2017年   211篇
  2016年   199篇
  2015年   189篇
  2014年   164篇
  2013年   168篇
  2012年   221篇
  2011年   192篇
  2010年   138篇
  2009年   110篇
  2008年   92篇
  2007年   135篇
  2006年   109篇
  2005年   112篇
  2004年   76篇
  2003年   70篇
  2002年   49篇
  2001年   62篇
  2000年   51篇
  1999年   33篇
  1998年   30篇
  1997年   23篇
  1996年   16篇
  1995年   7篇
  1994年   8篇
  1993年   6篇
  1992年   2篇
  1991年   3篇
  1990年   2篇
  1989年   1篇
  1988年   4篇
  1987年   5篇
  1986年   4篇
  1985年   4篇
  1983年   1篇
  1980年   1篇
  1978年   1篇
  1977年   2篇
  1976年   1篇
  1972年   1篇
排序方式: 共有3629条查询结果,搜索用时 234 毫秒
991.
城市绿地系统建设的生态对策——以长春市为例   总被引:15,自引:0,他引:15  
绿色空间是所有城市实现可持续发展的物质载体和市民接触自然的主要场所。几乎所有城市不同程度地存在着绿地被挤占、破坏、污染严重等各种问题。以长春市为例,提出了大城市进行绿地系统建设的总量控制、构建经色空间网络体系等生态对策,并针对大城市内部用地紧张的矛盾,探讨了市区内部的绿化用地潜力。  相似文献   
992.
Abstract: Nitrate‐nitrogen (NO3‐N) concentrations in stream water often respond uniquely to changes in inter‐annual conditions (e.g., biological N uptake and precipitation) in individual catchments. In this paper, we assess (1) how the spatial distribution of NO3‐N concentrations varies across a dense network of nonnested catchments and (2) how relationships between multiple landscape factors [within whole catchments and hydrologically sensitive areas (HSAs) of the catchments] and stream NO3‐N are expressed under a variety of annual conditions. Stream NO3‐N data were collected during two synoptic sampling events across >55 tributaries and two synoptic sampling periods with >11 tributaries during summer low flow periods. Sample tributaries drain mixed land cover watersheds ranging in size from 0.150 to 312 km2 and outlet directly to Cayuga Lake, New York. Changes in NO3‐N concentration ratios between each sampling event suggest a high degree of spatial heterogeneity in catchment response across the Cayuga Lake Watershed, ranging from 0.230 to 61.4. Variations in NO3‐N concentrations within each of the large synoptic sampling events were also high, ranging from 0.040 to 8.7 mg NO3‐N/l (March) and 0.090 to 15.5 mg NO3‐N/l (October). Although Pearson correlation coefficients suggest that this variability is related to multiple landscape factors during all four sampling events, partial correlations suggest percentage of row crops in the catchments as the only similar factor in March and October and catchment area as the only factor during summer low flows. Further, the strength of the relationships is typically lower in the HSAs of catchment. Advancing current understanding of such variations and relationships to landscape factors across multiple catchments – and under a variety of biogeochemical and hydrological conditions – is important, as (1) nitrate continues to be employed as an indicator of regional aquatic ecosystem health and services and (2) a unified framework approach for understanding individual catchment processes is a rapidly evolving focus for catchment‐based science and management.  相似文献   
993.
围绕废旧物资增值税税征管理政策调整完善工作,论述了其在废旧物资“产废”、“收废”、“利废”三大基础环节的现状及存在的问题,提出合理平衡三大环节的经济利益及打造三大环节税征链扣的建议,为切实做好废旧物资税征政策调整完善工作提供参考。  相似文献   
994.
Statistical analyses of health effects of air pollution have increasingly used GIS-based covariates for prediction of ambient air quality in “land use” regression models. More recently these spatial regression models have accounted for spatial correlation structure in combining monitoring data with land use covariates. We present a flexible spatio-temporal modeling framework and pragmatic, multi-step estimation procedure that accommodates essentially arbitrary patterns of missing data with respect to an ideally complete space by time matrix of observations on a network of monitoring sites. The methodology incorporates a model for smooth temporal trends with coefficients varying in space according to Partial Least Squares regressions on a large set of geographic covariates and nonstationary modeling of spatio-temporal residuals from these regressions. This work was developed to provide spatial point predictions of PM2.5 concentrations for the Multi-Ethnic Study of Atherosclerosis and Air Pollution (MESA Air) using irregular monitoring data derived from the AQS regulatory monitoring network and supplemental short-time scale monitoring campaigns conducted to better predict intra-urban variation in air quality. We demonstrate the interpretation and accuracy of this methodology in modeling data from 2000 through 2006 in six U.S. metropolitan areas and establish a basis for likelihood-based estimation.  相似文献   
995.
孔宇  甄峰  李兆中  傅行行 《自然资源学报》2019,34(10):2186-2199
智慧社会的出现与发展影响着人地关系,而智能技术作为智慧社会崛起的主要动力与发展的重要支撑,正在改变着国土空间的保护、开发与治理模式,也对国土空间规划编制方法提出了新要求。当前规划编制中,智能技术在国土空间规划中的应用虽然已经有了一定的探索,在技术层面也有很多实践,但缺少对智能技术应用于国土空间规划的整体性思考。基于“生态文明”基础与“以人为本”的核心理念,在对传统规划编制与当前智能技术应用梳理的基础上,从智能感知与收集、智能分析与处理、智能评估和智能决策四个方面,系统地构建全流程的智能技术辅助国土空间规划编制的框架,以适应当前国土空间规划提出的新要求,辅助编制更合理、科学、智慧的方案。  相似文献   
996.
This study examines the role of neighborhood effects in the spatial distributions of selected bird species in Navarre, Spain. We employed a geographic information system (GIS) to organize the data on bird distributions and relevant environmental variables and to analyze their spatial patterns. Three bird species were selected for analysis: the European honey-buzzard (Pernis apivorus), the Eurasian hobby (Falco subbuteo), and the European pied flycatcher (Ficedula hypoleuca). Selected environmental variables of the study area were digitized to create a comprehensive data base and logistic regression models were used to evaluate the significance of each variable in the spatial distribution. The spatial patterns of bird distributions were used to extract topological relationships and to identify neighborhood effects. Although all the selected species illustrate a pattern of positive spatial autocorrelation in their distributions, the significance of neighborhood effects varies from species to species. Among the selected species, neighborhood effects are most evident in the distribution of the European pied flycatcher and are significant for the Eurasian hobby. The distribution of the European honey-buzzard is not much affected by neighborhood effects. The results suggest that examination of neighborhood effects is a prerequisite for modeling bird distributions.  相似文献   
997.
ABSTRACT: In geohydrology, three-dimensional surfaces are typically represented as a series of contours. Water levels, saturated thickness, precipitation, and geological formation boundaries are a few examples of this practice. These surfaces start as point measurements that are then analyzed to interpolate between the known point measurements. This first step typically creates a raster or a set of grid points. In modeling, subsequent processing uses these to represent the shape of a surface. For display, they are usually converted to contour lines. Unfortunately, in many field applications, the (x, y) location on the earth's surface is much less confidently known than the data in the z dimension. To test the influence of (x, y) locational accuracy on z dimension point predictions and their resulting contours, a Monte Carlo study was performed on water level data from northwestern Kansas. Four levels of (x, y) uncertainty were tested ranging in accuracy from one arc degree-minute (± 2384 feet in the x dimension and ± 3036 feet in the y dimension) to Global Positioning Systems (GPS) accuracy (± 20 feet for relatively low cost systems). These span the range of common levels of locational uncertainty in data available to hydrologists in the United States. This work examines the influence that locational uncertainty can have on both point predictions and contour lines. Results indicate that overall mean error exhibits a small sensitivity to locational uncertainty. However, measures of spread and maximum errors in the z domain are greatly affected. In practical application, this implies that estimates over large regions should be asymptotically consistent. However, local errors in z can be quite large and increase with (x, y) uncertainty.  相似文献   
998.
ABSTRACT: Inter-station analysis was employed to evaluate the adequacy of the precipitation network in topographically complex West Virginia. A 25-year period was determined as the minimum lingth of record needed for relatively stable and fairly accurate estimates of long-term (50-year) precipitation and in frequency analysis. Data from the 83 National Weather Service stations with 25-year records were adjusted for consistency and evaluated separately by zones east (31 stations) and west (52 stations) of the Appalachian divide. Correlation coefficients (r) and average standard errors of estimate were computed for all station pairs within 50 miles distance and 1000 feet elevation difference of each other. The third polynomial equation of inter-station distance eliminated using elevation and land slope as the criteria in network design in this mountainous terrain. A network with (r) = 0.9 estimates annual precipitation with accuracy as great as 5 percent, but requires about 250 additional gages (i.e., about 200 percent of the present density).  相似文献   
999.
发展草坪将成为城市绿化建设中愈来愈重要的组成部分.通过草坪浇水、施肥、修剪、除草及病虫害防治,详细阐述了北方地区草坪的养护管理技术,充分说明了科学养管的重要性.  相似文献   
1000.
城镇是人类活动影响地表的最主要形式,是土地利用/土地覆盖变化的主要方面.山东省是中国东部沿海大省,土地利用处于不断地调整过程中.在全省城镇空间分布格局现状的基础上,结合中心地理论,运用数据包络分析(DEA)方法进行了具体分析,对全国土地利用/土地覆盖的深入研究具有重大意义.  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