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   331篇
  免费   16篇
  国内免费   4篇
测绘学   5篇
大气科学   8篇
地球物理   100篇
地质学   77篇
海洋学   64篇
天文学   69篇
综合类   2篇
自然地理   26篇
  2021年   5篇
  2020年   2篇
  2019年   2篇
  2018年   6篇
  2017年   5篇
  2016年   5篇
  2014年   9篇
  2013年   18篇
  2012年   11篇
  2011年   16篇
  2010年   12篇
  2009年   24篇
  2008年   13篇
  2007年   15篇
  2006年   12篇
  2005年   16篇
  2004年   14篇
  2003年   10篇
  2002年   14篇
  2001年   16篇
  2000年   5篇
  1999年   2篇
  1998年   6篇
  1997年   6篇
  1996年   12篇
  1993年   3篇
  1991年   5篇
  1990年   6篇
  1988年   4篇
  1987年   2篇
  1985年   4篇
  1984年   3篇
  1982年   6篇
  1981年   3篇
  1980年   6篇
  1979年   6篇
  1978年   2篇
  1977年   3篇
  1976年   4篇
  1975年   3篇
  1974年   2篇
  1973年   2篇
  1972年   2篇
  1971年   2篇
  1968年   3篇
  1947年   2篇
  1921年   2篇
  1910年   1篇
  1908年   1篇
  1871年   3篇
排序方式: 共有351条查询结果,搜索用时 203 毫秒
121.
Cumming  Gabriel  Campbell  Lisa  Norwood  Carla  Ranger  Sue  Richardson  Peter  Sanghera  Amdeep 《GeoJournal》2021,87(2):209-221

Stakeholders in natural resource management decisions are also multifaceted individuals and members of communities; as such, they bring complex histories, experiences, values, aspirations, and relationships to public participation processes. When these processes fail to take this social context into account, multiple problems can result, including a perceived lack of process trustworthiness; perceived focus on issues that seem immaterial or irrelevant; failure to equitably represent and take account of diverse voices; and failure to engage participants in productive dialogue. In this article we evaluate the Community Voice Method (CVM) as a way of addressing those problems by better situating public participation in place. CVM is a mixed-method approach to public participation in which stakeholders are interviewed and the interview data is presented through a film, which is then screened at public meetings to catalyze dialogue. We draw on 14 years of CVM projects addressing natural resource management issues in the United States, the United Kingdom, and the Caribbean. Through an overview of nine projects and their results, and more in-depth consideration of three, we elucidate how this method fosters trustworthy, relevant, representative, and productive public participation that has resulted in community capacity-building, institutional capacity-building, and stakeholder-guided policymaking.

  相似文献   
122.
Three quarters of the global human population will live in coastal areas in the coming decades and will continue to develop these areas as population density increases. Anthropogenic stressors from this coastal development may lead to fragmented habitats, altered food webs, changes in sediment characteristics, and loss of near-shore vegetated habitats. Seagrass systems are important vegetated estuarine habitats that are vulnerable to anthropogenic stressors, but provide valuable ecosystem functions. Key to maintaining these habitats that filter water, stabilize sediments, and provide refuge to juvenile animals is an understanding of the impacts of local coastal development. To assess development impacts in seagrass communities, we surveyed 20 seagrass beds in lower Chesapeake Bay, VA. We sampled primary producers, consumers, water quality, and sediment characteristics in seagrass beds, and characterized development along the adjacent shoreline using land cover data. Overall, we could not detect effects of local coastal development on these seagrass communities. Seagrass biomass varied only between sites, and was positively correlated with sediment organic matter. Epiphytic algal biomass and epibiont (epifauna and epiphyte) community composition varied between western and eastern regions of the bay. But, neither eelgrass (Zostera marina) leaf nitrogen (a proxy for integrated nitrogen loading), crustacean grazer biomass, epifaunal predator abundance, nor fish and crab abundance differed significantly among sites or regions. Overall, factors operating on different scales appear to drive primary producers, seagrass-associated faunal communities, and sediment properties in these important submerged vegetated habitats in lower Chesapeake Bay.  相似文献   
123.
The behavior of a number of solar wind, radiation belt, auroral and geomagnetic parameters is examined during the recent extended solar minimum and previous solar cycles, covering the period from January 1972 to July 2010. This period includes most of the solar minimum between Cycles 23 and 24, which was more extended than recent solar minima, with historically low values of most of these parameters in 2009. Solar rotational periodicities from 5 to 27 days were found from daily averages over 81 days for the parameters. There were very strong 9-day periodicities in many variables in 2005?–?2008, triggered by recurring corotating high-speed streams (HSS). All rotational amplitudes were relatively large in the descending and early minimum phases of the solar cycle, when HSS are the predominant solar wind structures. There were minima in the amplitudes of all solar rotational periodicities near the end of each solar minimum, as well as at the start of the reversal of the solar magnetic field polarity at solar maximum (~?1980, ~?1990, and ~?2001) when the occurrence frequency of HSS is relatively low. Semiannual equinoctial periodicities, which were relatively strong in the 1995?–?1997 solar minimum, were found to be primarily the result of the changing amplitudes of the 13.5- and 27-day periodicities, where 13.5-day amplitudes were better correlated with heliospheric daily observations and 27-day amplitudes correlated better with Earth-based daily observations. The equinoctial rotational amplitudes of the Earth-based parameters were probably enhanced by a combination of the Russell-McPherron effect and a reduction in the solar wind-magnetosphere coupling efficiency during solstices. The rotational amplitudes were cross-correlated with each other, where the 27-day amplitudes showed some of the weakest cross-correlations. The rotational amplitudes of the >?2 MeV radiation belt electron number fluxes were progressively weaker from 27- to 5-day periods, showing that processes in the magnetosphere act as a low-pass filter between the solar wind and the radiation belt. The A p/K p magnetic currents observed at subauroral latitudes are sensitive to proton auroral precipitation, especially for 9-day and shorter periods, while the A p/K p currents are governed by electron auroral precipitation for 13.5- and 27-day periodicities.  相似文献   
124.
To trace offshore surface low-salinity water (LSW) in the northeastern Gulf of Mexico, a proxy was developed using the surface water beam attenuation coefficient (c p), and salinity matched with synchronous Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite data from three annual summer cruises (July 1998–August 2000) using a two-step empirical approach. First, a relationship between in-situ salinity and c p was obtained. Second, in-situ c p was matched with SeaWiFS radiance ratios of all available blue-to-green wavelengths. Finally, satellite-derived surface salinity was determined directly by combining the two empirical relationships, providing a robust estimate over a range of salinities (26–36). This significantly improves the limited spatial and temporal resolution of surface salinity distribution obtained by shipboard sampling. The resulting correlation is best explained as mixing between low-salinity plume waters and normal salinity Gulf waters. The empirical relationships were used to map satellite-derived salinity using the average of SeaWiFS images during each summer cruise. As expected for summer, spatial patterns of LSW plumes with high c p, particulate matter (PM), particulate organic carbon (POC), and chlorophyll-a (Chl-a) were connected to the mouth of the Mississippi River Delta and extended to the east-southeast. Normal salinity Gulf water with lower c p, PM, POC, and Chl-a was confined to the shelf and upper slope in the eastern part of the study area, outside the plumes. This proxy approach can be applied throughout the region of shipboard sampling for more detailed coverage and analysis.  相似文献   
125.
The genetic structure of populations is often shaped by the reproductive system and larval properties of the species. The waratah anemone (Actinia tenebrosa) reproduces through both asexual clones, which have very short-distance dispersal, and sexual larvae, which are believed to disperse much greater distances. The impact of this mixed strategy on the New Zealand population structure of Actinia tenebrosa was investigated using microsatellite markers. The analysis incorporated 24 sampling locations from around New Zealand and one Australian location, using four microsatellite markers, n = 420. We observed low connectivity and high genetic differentiation between all locations sampled, with a distinct pattern of isolation by distance. The most distinct grouping of locations sampled was the north-east of the North Island from Cape Reinga to East Cape, which was identified by SAMOVA and STRUCTURE analyses as being moderately diverged from the remainder of the country. A tentative correlation was observed between genetic clustering and biogeographic regionalisation, where the distribution of a number of genetic clusters matched previously defined biogeographic regions. Within each location sampled, large numbers of clones were present and a latitudinal cline was observed in the relative contribution of asexually and sexually generated recruits, with an increase in asexual recruits on the South Island's east coast.  相似文献   
126.
Banded kokopu populations were surveyed seasonally over 1 year in five Coromandel streams to determine the influence of food supply on diet, growth, abundance, and movement. Streams with high benthic invertebrate abundance and drift of aquatic origin had high fish abundance. However, terrestrial drift was the dominant food for adult banded kokopu over all seasons and streams, on average comprising 75% of their diet by number and 89% by weight. Annual growth rates of tagged individuals ranged from 3 mm to 16.5 mm, with smaller fish growing faster than large fish. Low growth rates of large fish were consistent with the recapture of a fish that had only grown from 195 mm to 215 mm in 7 years. Our observations confirmed the strong association between banded kokopu, pool habitat, and cover, and also indicated that high fish abundance restricted growth rates and movement and increased interaction.  相似文献   
127.
Fish abundance data from selected electric‐fishing records in the New Zealand Freshwater Fish Database (NZFFD) were used to identify riverine fish communities and to examine their relationship to environmental variables included in the NZFFD. Only 21 fish species, 18 native species, and three introduced salmonids, were present at 1% or more of the sites. We defined 12 fish communities characterised by a dominant species: two salmonid communities, two non‐diadromous native communities, and eight communities characterised by diadromous native species. Altitude and distance inland were the two most significant variables, reflecting differences between communities dominated by diadromous and non‐diadromous species. The next most important variables were related to the geographic location of the site. Other environmental variables that were highly correlated with the fish community assignments were stream width and percentage of native forest or farming land use in the catchment upstream of the site. Of the local habitat variables, percentage of cascade habitat and percentage of sand substrate were the most important discriminators between communities. Despite having only a limited set of broad‐scale environmental variables, we achieved 47% success in the prediction of community membership using multiple discriminant analysis, with another 21% of sites being near misses. A greater knowledge of barriers to migration of diadromous species and fine‐scale variables describing in‐stream habitat would probably increase the predictive ability of the model, although collection of such data is time consuming and therefore impractical on a national scale.  相似文献   
128.
The US Army Corps of Engineers (USACE) and the South Florida Water Management District (SFWMD) are partners in an ambitious plan to restore water flows throughout the Everglades ecosystem. An important component of the restoration plan involves storing excess stormwater deep underground in the Floridan Aquifer System using aquifer, storage and recovery (ASR) wells. In order to determine the optimal ASR system and to assess environmental impacts, USACE spent over 11 years and significant resources to develop a three-dimensional groundwater model of the Floridan Aquifer System covering a large portion of the Florida peninsula. This SEAWAT model is capable of evaluating changes in aquifer pressures and density-dependent flows in the entire study area. The model has been used to evaluate the Everglades ASR system already but could also be used by water managers for other important water resources studies in Florida including water supply estimates and adaptation to climate change. As part of an effort to make the model more readily available for other important studies, this study documents and summarizes the overall development of the SEAWAT model including a discussion regarding the intensive calibration and validation efforts undertaken during model development. The paper then demonstrates the use of the model using Everglades ASR project alternatives. Lastly, the paper outlines potential future uses of the model along with its overall limitations. Supplementary online resources are also included that provide researchers with further detail regarding the model development effort beyond the scope of this summary article as well as model development databases.  相似文献   
129.
龙门山彭县-灌县断裂的活动构造与地表破裂   总被引:14,自引:10,他引:4       下载免费PDF全文
2008年5月12日在龙门山发生了8.0级特大地震,彭县-灌县断裂亦发生了同震地表破裂.在前期对龙门山活动构造研究的基础上,汶川特大地震发生后,在灾区进行了多次的野外调查和国际合作考察,重点对汶川地震的地表破裂和地质灾害开展了详细的详细野外地质填图,利用全站仪和GPS对地表破裂进行了精确的测量,研究了的地表破裂地貌错位、构造组合和运动学,已实地测得地表破裂数据70余组(其中彭县-灌县断裂地表破裂数据20余组).文章以彭县-灌县断裂地表破裂为切入点,在彭县-灌县断裂的关键部位开展了详细的野外地貌测量,主要测量了彭州磁峰、白鹿、绵竹金花和汉旺等地的地表破裂,标定了彭县-灌县断裂破裂带的垂向断距和水平断距,结果表明该地表破裂南西起于彭州磁峰,向北东延伸经白鹿、绵竹金花至绵竹汉旺,全长约40~50km.地表破裂带沿彭县-灌县断裂带的走向断续分布,单个破裂长度在几米到500余米不等,破裂带切割了多种类型的地貌单元,包括山脉基岩、河流阶地、冲洪积扇、公路、桥梁等,同时也使道路发生拱曲、破坏和桥梁垮塌或移位.其以脆性破裂为特征,以逆冲一右旋走滑为特点,断面倾角较陡,北西盘为上升盘,南东盘为下降盘,垂直位错介于0.39~2.70m之间,水平位错介于0.20~0.70m,平均垂直位错为1.6m,平均水平位错为0.6m;地表最大错动量的地点位于彭州白鹿镇,其中最大垂直断错为2.7±0.2m,最大水平断错为0.7±0.2m.垂直位错与水平位错量之间的比值为2:1,表明该地震地表破裂带不仅存在逆冲运动分量和右旋走滑运动分量,而且逆冲运动分量大于右旋走滑运动分量,显示了彭县-灌县断裂破裂带具有以逆冲和缩短作用为主、右旋走滑作用为辅的破裂性质.其与映秀-北川断裂带的地表破裂相比较,该断裂的地表破裂程度远小于映秀-北川断裂带的地表破裂程度,主要表现在地表破裂的长度较短,垂直位错和水平位错也相对较小,而且为以逆冲作用为主.初步研究结果表明,彭县-灌县断裂与映秀-北川断裂地表破裂的平面组合样式显示为两条在平面上近于平行的北东向地表破裂带,其间由一条南北向的次级地表破裂带(小鱼洞断裂)将它们相连结,地下破裂面的剖面组合样式显示为叠瓦状,并在汶川地震震源附近或震源的上方相连的,是同"根"的.  相似文献   
130.
Macrinite is a, generally, rare inertinite maceral, often incorporating remnants and fragments of other macerals, including vitrinite, liptinite, and other inertinite. The associated inertinites include multiple forms of funginite. Funginite is also commonly found in association with vitrinite of slightly elevated reflectance and with degraded varieties of vitrinite. Together with the highly degraded macrinite, the latter two associations are here inferred to be part of a continuum of fungal and microbial degradation of peat. In any case, the origin of some macrinite is potentially distinct from that of inertinite generated by fire.  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