全文获取类型
收费全文 | 1808篇 |
免费 | 209篇 |
国内免费 | 195篇 |
专业分类
测绘学 | 51篇 |
大气科学 | 240篇 |
地球物理 | 419篇 |
地质学 | 732篇 |
海洋学 | 345篇 |
天文学 | 10篇 |
综合类 | 73篇 |
自然地理 | 342篇 |
出版年
2024年 | 7篇 |
2023年 | 27篇 |
2022年 | 46篇 |
2021年 | 47篇 |
2020年 | 63篇 |
2019年 | 66篇 |
2018年 | 28篇 |
2017年 | 96篇 |
2016年 | 73篇 |
2015年 | 90篇 |
2014年 | 113篇 |
2013年 | 108篇 |
2012年 | 63篇 |
2011年 | 102篇 |
2010年 | 75篇 |
2009年 | 143篇 |
2008年 | 127篇 |
2007年 | 111篇 |
2006年 | 109篇 |
2005年 | 99篇 |
2004年 | 70篇 |
2003年 | 53篇 |
2002年 | 69篇 |
2001年 | 41篇 |
2000年 | 57篇 |
1999年 | 54篇 |
1998年 | 45篇 |
1997年 | 36篇 |
1996年 | 36篇 |
1995年 | 40篇 |
1994年 | 18篇 |
1993年 | 24篇 |
1992年 | 21篇 |
1991年 | 13篇 |
1990年 | 10篇 |
1989年 | 10篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1980年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有2212条查询结果,搜索用时 468 毫秒
991.
《Marine Policy》2014
Scuba diving allows for underwater visitation of cultural and natural resources. Underwater routes can be used as a tool for guided and supervised underwater visits. Two scuba diving routes were implemented in the Algarve (South of Portugal), the “B24” and “Poço” diving sites. The perceptions of scuba divers regarding several aspects of the routes and the existing support infrastructures were studied following a survey carried out through face-to-face interviews from 2008 to 2012. Divers profile and their perceptions were analysed using 246 valid questionnaires. Divers were mainly Portuguese, over 30 years old and with more than 12 years of formal education. Some of the support infrastructures did not achieve a “good” or “acceptable” grade. This should be carefully considered by diving operators and managers, because perceptions tend to circulate throughout the diving tourists. All features of interpretative slates were graded as highly satisfactory. Overall, diver satisfaction increased slightly after route implementation, with an average ranking of “good”. These findings support the implementation of underwater routes as a way to promote diving activity, and to increase divers' environmental education and awareness. 相似文献
992.
Specimens of Mora moro were collected in two seasons and three localities of the Balearic Sea (NW Mediterranean Sea) and parasitological, dietary (to prey-species level), enzymatic and histological data were obtained, alongside with environmental information (T, S, O2). The relationships among fish parasite load, condition indices, diet, enzymatic activity of muscular acetylcholinesterase (AChE) and lactate dehydrogenase (LDH), intensity of splenic melano-macrophage centres (MMC) and hepatic granulomas were tested. M. moro showed a rich and abundant parasite fauna, and was a new host record for 17 out of the 18 different endoparasite taxa found. Significant differences were detected among locality-season groups, in turn related to different environmental variables, for Anisakidae gen. sp., Anisakis Type II and Tetraphyllidea fam. gen. sp.; thus, they are proposed as potentially useful as biological tags for geographical discrimination of M. moro in the NW Mediterranean Sea. Detailed relationships were found between parasite taxa and prey ingested (e.g. Anisakidae gen. sp. related with meso-bathypelagic crustaceans; Anisakis Type I with benthopelagic squids). Most parasites were linked to samples with highest levels of near-bottom O2, which is consistent with direct relationships found between near-bottom O2 and zooplankton biomass in the Balearic Basin. Total parasite abundance and the abundance of Tetraphyllidea fam. gen. sp. showed a significant relationship with the activity of AChE and the abundance of Anisakis Type II with LDH. AChE was associated with hepatosomatic index (HSI) and condition factor (K); LDH with gonadosomatic index (GSI), K and fish total length (TL). LDH activity showed differences among sampling groups. Splenic MMC and hepatic granulomas were not associated with fish parasite load. A positive relationship was found between MMC area and fish TL and LDH activity. 相似文献
993.
Rapid growth of metropolitan areas is associated with increased flows of nitrogen (N) and phosphorus (P) in the food production–consumption system. However, quantitative analyses of these flows during urban expansion and information about their controlling factors are scarce.Here, we report on N and P flows in the food system of Beijing, which experienced a remarkable growth in population number between especially 1978–2008, using a combination of statistical data bases, surveys and the NUFER model (nutrient flow in the food system, environment and resource). The N (or P) cost of food is defined as the amount of ‘new’ N (or P) used in food production for the delivery of 1 kg N (or P) in the food entering household. ‘New’ N (P) includes fertilizer N (P), biological N fixation, atmospheric N deposition, and imports of N (P) via feed and food. Recycled N (P) includes N (P) in crop residues, manures and wastes.We found that the rapid increase in temporary migrants greatly increased food imports to Beijing metropolitan areas and thereby led to an apparent decrease of the N and P cost of food. The input of ‘new’ N to the food system of Beijing metropolitan areas increased from 180 to 281 Gg, and for P from 33.5 to 50.4 Gg during 1978–2008, as a result of increases in population and changes in food consumption patterns per capita. The food and feed imports in per cent of total ‘new’ N and P inputs increased from 31 to 63% for N and from 18 to 46% for P during 1978–2008. The N and P cost of the food was relatively low compared to the mean of China, and decreased over time. About 52% of the new N input and 85% of the new P input was not recycled in 2008, it accumulated as wastes (in crop residues, animal excreta, and human excreta and household wastes). The N and P use efficiencies in crop and animal production were low, i.e., only 17% for N and 11% for P in 2008. Total losses of ammonia (NH3) and nitrous oxide (N2O) to air and of N to groundwater and surface waters increased by a factor of about 3, and losses of P to groundwater and surface waters increased by a factor of 37 in the period 1978–2008.Key measures for decreasing N and P accumulation and losses are (1) developing satellite towns, (2) expelling animal production to rural areas, and (3) effective collection of the wastes and animal manure, and the utilization of these in rural areas outside Beijing. These findings may also portend changes in other metropolitan areas in China and elsewhere in the rapidly developing world. 相似文献
994.
International environmental policy has evolved from a focus on single issues to more integrated approaches under the framework of sustainable development. This transition has been accompanied by a growing use of targets among international organisations. Targets have long been used in industry and corporate planning, but some have questioned their relevance in the ambit of environmental and sustainable development policy. This paper addresses the question of whether international targets help advance environmental policy integration in international governance. It explores whether the international target of significantly reducing the rate of biodiversity loss by 2010, adopted by the Conference of the Parties to the Convention on Biological Diversity (CBD) in 2002, enabled co-ordination and coherence in international biodiversity governance. The effects of the Target on the cluster of biodiversity-related conventions and their implementation in countries of Latin America and the Caribbean are examined. The analysis is based on official documents and interviews with secretariat officials, international experts and national focal points conducted between September 2011 and April 2012. A claim is made that the 2010 Biodiversity Target was, in essence, a conservation goal that did not fully honour the CBD's sustainable development mission. The Target triggered increased co-operation in the biodiversity cluster without bringing greater alignment of policies and implementation activities around the CBD's sustainability principles. The study suggests that, if targets are to advance EPI among international institutions, they need to be appropriated by relevant stakeholders and supported by implementation strategies that secure their continuous commitment. 相似文献
995.
A recent surge of research has investigated the potential of pro-environmental behavior interventions to affect other pro-environmental behaviors not initially targeted by the intervention. The evidence evaluating these spillover effects has been mixed, with some studies finding evidence for positive spillover (i.e., one pro-environmental behavior increases the likelihood of performing additional pro-environmental behaviors) and others finding negative spillover (i.e., one pro-environmental behavior decreases the likelihood of additional pro-environmental behaviors). Different academic disciplines have investigated this question, employing different methodologies and arriving at divergent findings. This paper provides a unifying theoretical framework and uses the framework to review the existing research on pro-environmental behavior spillover. Our framework identifies different decision modes as competing mechanisms that drive adoption of initial pro-environmental behaviors, with different consequences for subsequent pro-environmental behaviors, leading to positive, negative, or no spillover. Attribution of the initial pro-environmental behavior to either an external motivator (e.g., a price signal) or internal motivator (e.g., self-identity) also matters. In addition, the characteristics of and similarity between initial and subsequent pro-environmental behaviors can be expected to moderate predicted spillover effects. We explore the implications of our model for policymakers and practitioners, and suggest key areas where future research on the topic would be most beneficial. 相似文献
996.
????“?й?????????????????”2009-2011??GPS?????????????????????????η???????????????????????????????????????????????????????????????顣??????????仯??????????????????????????????????????????????????????????????????????????????????????????????????????????????????Ms8.0?????????????????????????α????????????????????????????????????? 相似文献
997.
This study examined the temporal trends of runoff and sediment load and their differential response to human activities in the Lishui river,a tributary of the Yangtze river in southern China.The long-term observation data at four gauging stations,generally involving two periods from 1954 to 1985 and from 2007 to 2011,were used.We detected no significant temporal trend for both the annual runoff volume(Q) and the annual suspended Sediment Load(SL) over more than 30 years before 1985.The flow duration curves and the Suspended Sediment Concentration(SSC) also hold constant before 1985.Compared with the period before 1985,SL has decreased by about 80% though Q remains unchanged for the period after 2007.Detailed examination shows that the flow duration curves after 2007 have changed with a significant decrease in the high-flow component,which acts as a major cause for the decreasing SL.In addition,SSC has decreased by several times,which also contributes to the decrease in SL after 2007.Both decreases in high-flow discharges and in SSC can be linked with recent human activities,mainly including vegetation establishment and dam constructions.The constant Q and the decreasing SL are also reported for the main stream of the Yangtze River and other major rivers in southern China,although they are orders of magnitude larger than our study area in drainage area size.The present study highlights the importance of high-flow discharges on SL and suggests that the use of SL is more appropriate to reflect environmental change than Q. 相似文献
998.
This paper investigates the spatial heterogeneity of three landscapes along an altitudinal gradient and different human land use. The main aim was the identification of appropriate landscape indicators using different extents. ASTER image was used to create a land cover map consisting of three landscapes which differed in altitude and land use. A number of landscape metrics quantifying patch complexity, configuration, diversity and connectivity were derived from the thematic map at the landscape level. There were significant differences among the three landscapes regarding these four aspects of landscape heterogeneity. The analysis revealed a specific pattern of land use where lowlands are being increasingly utilized by humans (percentage of agricultural land = 65.84%) characterized by physical connectedness (high values of Patch Cohesion Index) and relatively simple geometries (low values of fractal dimension index). The landscape pattern of uplands was found to be highly diverse based upon the Shannon Diversity index. After selecting the scale (600 ha) where metrics values stabilized, it was shown that metrics were more correlated at the small scale of 60 ha. From the original 24 metrics, 14 individual metrics with high Spearman correlation coefficient and Variance Inflation Factor criterion were eliminated, leaving 10 representative metrics for subsequent analysis. Data reduction analysis showed that Patch Density, Area-Weighted Mean Fractal Dimension Index and Patch Cohesion Index are suitable to describe landscape patterns irrespective of the scale. A systematic screening of these metrics could enhance a deeper understanding of the results obtained by them and contribute to a sustainable landscape management of Mediterranean landscapes. 相似文献
999.
1000.
Abstract Landsat Thematic Mapper (TM) data have been used to monitor land cover types and to estimate biophysical parameters. However, studies examining the spatial relationships between land cover change and biophysical parameters are generally lacking. With the integration of remote sensing and Geographic Information Systems (GIS), these relationships can be better explored. The research reported in this paper applies this integrated approach for detecting urban growth and assessing its impact on vegetative greenness in the Zhujiang Delta, China. Multi‐temporal Landsat TM data were utilized to map urban growth and to extract and identify changes in vegetative greenness. GIS analyses were conducted to examine the changing spatial patterns of urban growth and greenness change. Statistical analyses were then used to examine the impact of urban growth on vegetative greenness. The results revealed that there was a notably uneven urban growth pattern in the delta, and urban development had reduced the scaled Normalized Difference Vegetation Index (NDVI) value by 30% in the urbanized area. 相似文献