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   1987篇
  免费   177篇
  国内免费   164篇
测绘学   40篇
大气科学   312篇
地球物理   340篇
地质学   648篇
海洋学   355篇
天文学   4篇
综合类   67篇
自然地理   562篇
  2024年   6篇
  2023年   29篇
  2022年   89篇
  2021年   81篇
  2020年   83篇
  2019年   93篇
  2018年   56篇
  2017年   126篇
  2016年   91篇
  2015年   98篇
  2014年   122篇
  2013年   132篇
  2012年   61篇
  2011年   96篇
  2010年   72篇
  2009年   137篇
  2008年   125篇
  2007年   101篇
  2006年   98篇
  2005年   75篇
  2004年   69篇
  2003年   46篇
  2002年   64篇
  2001年   39篇
  2000年   52篇
  1999年   47篇
  1998年   41篇
  1997年   31篇
  1996年   34篇
  1995年   36篇
  1994年   15篇
  1993年   20篇
  1992年   15篇
  1991年   13篇
  1990年   7篇
  1989年   9篇
  1988年   4篇
  1987年   4篇
  1986年   1篇
  1985年   3篇
  1984年   2篇
  1983年   1篇
  1982年   2篇
  1980年   1篇
  1976年   1篇
排序方式: 共有2328条查询结果,搜索用时 203 毫秒
351.
Collaborative urban governance has increased the role of community organizations in local decision-making processes. These organizations need financial resources in order to participate in urban governance. In this article, I examine the impact of foundation grants on the relationships and agendas of four community organizations in one neighborhood in St. Paul, Minnesota. Drawing on interviews, observations of organizations, and archival research, I demonstrate that in the 1990s, nonprofit foundations had a significant impact on the formation of new organizations and on their agendas in the neighborhood. Foundations are, therefore, an important player in urban governance, shaping a “neighborhood policy regime.”  相似文献   
352.
353.
Wen Lin 《Urban geography》2013,34(7):901-922
While local governments have been one of the major user groups of GIS, there is still little research on how GIS development in local government might be intertwined with urban governance, particularly in non-Western contexts. Drawing upon insights from GIS implementation, critical GIS, and governmentality studies, this article seeks to bridge this gap by examining the implications of Chinese urban government GIS practices amidst China's changing urban governance. Through an in-depth case study of Shenzhen, this article analyzes how urban GIS has been transformed from a practice involving internal organizational workflow automation, into a more active dimension of the governance of urban spaces—reflected in the expanding practice of “geo-coding” the urban landscape. “Geo-coding” here refers to a broadly defined spatial practice of carving and reconstructing a rational urban space. GIS practices have constituted a particular form of geographic rationality that seeks to govern at a distance while simultaneously regulating the urban environment, intersecting with the broader transformations of China's urban governance. These GIS developments have been largely government-centric rather than citizen-centric, yet they provide possibilities for new forms of spatial knowledge production for citizen participation in urban governance.  相似文献   
354.
Tropical cyclones (hurricanes and typhoons) produce high winds that can generate waves capable of damaging coral reefs. As cyclones frequently pass through northeast Australia's Great Barrier Reef (GBR), it is important to understand how the spatial distribution of reef damage changes over time. However, direct measurements of wave damage, or even wave heights or wind speeds, are rare within the GBR. An important factor in estimating whether cyclone damage was possible is the magnitude and duration of high‐energy wind and waves. Thus, before the spatio‐temporal dynamics of past cyclone damage can be modelled, it is necessary to reconstruct the spread, intensity, and duration of high‐energy conditions during individual cyclones. This was done every hour along the track taken by each of 85 cyclones that passed near the GBR from 1969 to 2003, by implementing a cyclone wind hindcasting model directly within a raster GIS using cyclone data available from the Australian Bureau of Meteorology. Three measures of cyclone energy (maximum wind speed—MAX, duration of gales—GALES, and continuous duration of gales—CGALES) were derived from these data. For three cyclones, where field data documenting actual reef damage from cyclone‐generated waves were available, the predictive ability of each measure was assessed statistically. All three performed better in predicting reef damage at sites surveyed along the high‐energy reef front than those surveyed along the more protected reef back. MAX performed best for cyclone Joy (r 2 = 0.5), while CGALES performed best for cyclones Ivor (r 2 = 0.23) and Justin (r 2 = 0.48). Using thresholds for MAX and GALES obtained via comparison with field data of damage, it was possible to produce a preliminary prediction of the risk of wave damage across the GBR from each of the 85 cyclones. The results suggest that while up to two‐thirds of the GBR was at risk from some damage for 30–50% of the time series (~18 out of 35 years), only scattered areas of the region were at risk more frequently than that.  相似文献   
355.
The goal of the present work is to perform a geochemical assessment of High Dam Lake bottom sediments for determining the fate, dispersion and levels of trace elements causing environmental pollution, and provided an access to their probable sources. The sediment samples were analyzed using ICP-MS for 20 elements; Ag, Ba, Cr, Cu, Ga, Hf, Mn, Pb, Rb, Sc, Se, Sn, Sr, Ta Th Ti, T1, U, V and Zr, and their obtained data were treated using statistical, graphical and mapping techniques. The results showed the data set of all analyzed elements affected by outliers and extreme values that caused deviation away from normality. Kruskal-Wallis test revealed that median of some trace element levels in Lake sediments, was not significantly different and other elements rejected the Null hypothesis. Most analyzed elements had high values of median and mean in sediments of Lake Nubia rather than Lake Nasser and their normalization gave the same results of calculated environment factors. Subsequently, Lake Nubia sediments possessed high combined EF levels ofTh, Sc, Sn, Ag, Zr, Hf, Ta, Sr, U, Ti, V, Cr, and Mn, causing significant contamination, which was great potentially related to industrial, agricultural, urbanization and mining activities. Whereas, combined EF of Se, Cu, Ga, Pb, Ba, Rb, and Tl, which are highly elevated in southern Lake Nasser sediments owing to their source are great possibly derived from Lake Nubia and geogenic activities. With decreasing distance towards the High Dam body, the contaminant elements were diminished due to reduction in the environmental factors and Sudanese pollution sources leading to the northern Lake Nasser considered to be less contaminated. Overall, the present study is an environmental alert for contaminated sediments that carried contaminants and considered the secondary source of pollution impact on ecosystem, and subsequently, their environmental risk on Human health.  相似文献   
356.
Leeches (Clitellata: Hirudinida) are abundant predators or ecto-parasites inhabiting various freshwater habitats; however many biotic and abiotic drivers of their assemblage patterns have been deduced rather than directly tested. To study species richness and composition changes in leech assemblages, 109 sites of running and stagnant water bodies were sampled in three regions of the Czech Republic in 2007–2010, together with several explanatory variables that are known or expected to be important predictors of leech distribution. In total, 17 species of leeches were recorded, varying between 0–7 and 0–9 species in lotic and lenitic sites, respectively. These differences in species richness of lotic and lenitic sites were highly significant, contrary to the abundances, which varied between 0–283 and 0–295 individuals. The main change in species composition was controlled by water temperature and morphological characteristics (e.g. substrate and cover of macrophytes), mostly reflecting the differences between lotic and lenitic habitats. We found the density of benthos (i.e. prey availability) to be the best predictor of species composition in both lotic and lenitic sites, together with the percentage of canopy cover. However, the other significant predictors (i.e. the substrate and water conductivity found to be significant in lotic sites, and the mean annual temperature and PO43? in lenitic sites), differed between these habitats. Other than mean annual temperature and water temperature, which had different effects on species richness in lotic and lenitic sites, there were no other differences between lotic and lenitic sites in terms of how species richness and abundance responded to all other analyzed predictors. Our results stress the importance of prey availability and canopy for leech distribution patterns. Differences in the significant predictors of leech assemblage patterns between lotic and lenitic sites raise fundamental questions about the underlying mechanisms and ecological constraints to leech distribution in these main types of aquatic systems.  相似文献   
357.
The heavy metal inventory and the ecological risk of the tidal flat sediments in Haizhou Bay were investigated. Results show that the average concentrations of heavy metals in the surface sediments exceeded the environment background values of Jiangsu Province coastal soil, suggesting that the surface sediments were mainly polluted by heavy metals (Cd, Cr, Cu, Mn, Pb and Zn). In addition, the profiles of heavy metals fluxes can reflect the socio-economic development of Lianyungang City, and heavy metals inputs were attributed to anthropogenic activities. Cr, Cu, Pb and Zn were mainly present in the non-bioavailable residual form in surface sediments, whereas Cd and Mn were predominantly in the highly mobile acid soluble and reducible fractions. The ecological risk of the polluted sediments stemmed mainly from Cd and Pb. According to the Sediment quality guidelines (SQGs), however, the adverse biological effects caused by the heavy metals occasionally occurred in tidal flat.  相似文献   
358.
This paper presents a field study conducted in northwest Turkey and characterizes the NH3 concentration and emission measured in summer season from three chicken farms. The influence of environmental conditions on NH3 concentration and emission was also investigated in this study. Ammonia concentration, temperature, relative humidity and airflow rate were continuously recorded for four sequential days. The environmental conditions were measured using a multifunction temperature and humidity‐meter with a hot wire probe. Portable multiple gas detectors with electro‐chemical sensors were used to measure NH3 concentration. The NH3 emission rates for houses were calculated by multiplying simultaneously measured NH3 concentrations and air flow rates. The average daily mean (ADM) house concentrations of house 1 (H1), house 2 (H2), and house 3 (H3) were measured as 4.43, 3.71, and 6.20 ppm, respectively. NH3 concentration was inversely proportional to temperature (r = ?0.279), relative humidity (r = ?0.063) and airflow rate (r = ?0.554) for all monitored houses. The ADM house NH3 emission was 135 g/(h house) for H1, 255 g/(h house) for H2, and 117 g/(h house) for H3. The combined average emission rate in this study (0.26 g/(d bird)) was lower than the emission rate measured in chicken farms in the USA. However, our results were comparable to rates calculated in European studies because house design, ventilation system and bird diet applied in Turkish chicken farms are very similar to those employed in European countries. The NH3 emissions were significantly correlated to NH3 concentrations (r = 0.45, p ≤ 0.001) and airflow rates (r = 0.97, p ≤ 0.001). A clear diurnal pattern was obtained for NH3 concentrations rather than NH3 emissions at the end of the study.  相似文献   
359.
One mechanism by which biochar application enhances soil nutrient availability is through direct nutrients release from biochar. However, factors controlling the release processes are poorly understood. In this study, the effects of pH, biochar to water ratio, temperature, ionic strength, and equilibration time on the release of PO, NO, NH, K+, Na+, Ca2+, and Mg2+ from biochar were evaluated in simulated experiments. The release of PO, K+, Ca2+, and Mg2+ was significantly affected by extraction pH, suggesting that their release from biochar was pH dependent or an H+‐consuming process. Correlation analysis indicated that PO and Ca2+, PO and Mg2+, and Ca2+ and Mg2+ were co‐solubilized with increasing soil acidity. To a lesser extent, the recovery of the nutrients was also affected by the ratio of biochar to water: more nutrients were soluble with more water supply. In contrast, the release of Na was not affected by pH while the concentration increased with decreasing biochar to water ratio. Meanwhile, other factors (temperature, ionic strength, and equilibration time) had less effect on nutrient release from biochar. Under the influence of pH, the patterns of NO and PO release from biochar were different: extractable NO concentration was not affected by the pH but more PO was released in strongly acidic conditions. Our data suggested that P was mainly retained in inorganic forms while N was in organic forms in biochar. We conclude that environmental factors have marked influences on nutrients release from biochar.  相似文献   
360.
《Urban geography》2013,34(7):694-725
This article explores the possibilities for a political ecology of gentrification. Gentrification research, while firmly rooted in materialist social science, has not yet broadened its interests to consider ecological aspects of, or the role in gentrification of, discourses, social movements, and state policies of the environment. Understanding the political ecologies of gentrification involves recognizing the ways in which material relations and uneven resource consumption, concepts of nature, and the politics of urban environmental management affect gentrification processes. By synthesizing diverse literatures in urban studies, political ecology, urban environmental governance, consumption studies, and gentrification, this study argues that Vancouver, British Columbia represents a well-developed urban crucible for the new political ecologies of gentrification in North America. New developments in Vancouver increasingly contribute to gentrification using languages of sustainability and green consumption in a process of ecological gentrification.  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