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   771篇
  免费   23篇
  国内免费   4篇
测绘学   13篇
大气科学   63篇
地球物理   158篇
地质学   269篇
海洋学   87篇
天文学   91篇
综合类   2篇
自然地理   115篇
  2022年   4篇
  2021年   10篇
  2020年   12篇
  2019年   8篇
  2018年   22篇
  2017年   22篇
  2016年   33篇
  2015年   17篇
  2014年   24篇
  2013年   58篇
  2012年   30篇
  2011年   51篇
  2010年   40篇
  2009年   35篇
  2008年   42篇
  2007年   35篇
  2006年   21篇
  2005年   20篇
  2004年   32篇
  2003年   32篇
  2002年   23篇
  2001年   13篇
  2000年   10篇
  1999年   15篇
  1998年   11篇
  1997年   12篇
  1996年   11篇
  1995年   11篇
  1994年   10篇
  1993年   8篇
  1992年   12篇
  1991年   10篇
  1990年   8篇
  1989年   6篇
  1988年   6篇
  1987年   10篇
  1986年   3篇
  1985年   4篇
  1984年   11篇
  1983年   4篇
  1981年   8篇
  1980年   6篇
  1979年   3篇
  1978年   4篇
  1977年   4篇
  1976年   5篇
  1975年   4篇
  1973年   3篇
  1972年   3篇
  1968年   2篇
排序方式: 共有798条查询结果,搜索用时 125 毫秒
161.
162.
163.
Access to climate information has the potential to build adaptive capacity, improve agricultural profitability, and help manage risks. To achieve these benefits, knowledge of the local context is needed to inform information development, delivery, and use. We examine coffee farming in the Jamaican Blue Mountains (BM) to understand farmer livelihoods, opportunities for climate knowledge to benefit coffee production, and the factors that impinge on farmers’ ability to use climate information. Our analysis draws on interviews and 12 focus groups involving 143 participants who largely cultivate small plots. BM farmers currently experience stresses related to climate, coffee leaf rust, and production costs that interrelate concurrently and with time lags. Under conditions that reduce income, BM farmers compensate by adjusting their use of inputs, which can increase their susceptibility to future climate and disease stresses. However, farmers can also decrease impacts of future stressors by more efficiently and effectively allocating their limited resources. In this sense, managing climate, like the other stresses, is an ongoing process. While we identify climate products that can help farmers manage climate risk, the local context presents barriers that argue for interactive climate services that go beyond conventional approaches of information production and delivery. We discuss how dialogs between farmers, extension personnel, and climate scientists can create a foundation from which use can emerge.  相似文献   
164.
From July to October 2004, five sites in the Hampton–Seabrook Estuary in New Hampshire were sampled with beam and otter trawls. The goals were to describe winter flounder (1) abundance in the estuary, (2) size class distributions, (3) spatial distribution by different size classes, and (4) distribution patterns. Of the 19 species caught, winter flounder was the most abundant and was dominated by young-of-the-year (YOY) fish. The five sites were fairly homogenous in depth, bottom type, salinity, and temperature. However, YOY abundance ranged from 2.1 to 32.1 fish 1,000 m?2 depending on the site. Benthic community was the best indicator of juvenile winter flounder abundance. Catch data of other organisms fluctuated, but no one species was a strong predictor of winter flounder abundance and distribution. During late summer and early fall, the estuary is used primarily by YOY winter flounder, indicating that this estuary functions as a nursery ground.  相似文献   
165.
166.
Elemental and isotopic ratio analyses of U ore concentrate samples, from the 3 operating U mining facilities in Australia, were carried out to determine if significant variations exist between their products, thereby allowing the U ore concentrate’s origin to be identified. Elemental analyses were conducted using inductively coupled plasma mass spectrometry (ICP-MS) and X-ray fluorescence spectrometry (XRF). Lead isotope ratios were measured using ICP-MS and U isotope analyses were conducted using thermal ionisation mass spectrometry (TIMS). Minute quantities of sample, such as that obtained from a swipe, were also examined for elemental concentrations using secondary ion mass spectrometry (SIMS). The results of multivariate statistical analysis show clear patterns in the trace elemental composition of the processed U ores, indicating that it is possible to use this feature as a unique identifier of an Australian U ore concentrate’s source. Secondary ion mass spectrometry analyses also allow individual particles to be differentiated using this ‘fingerprinting’ technique. Isotope ratios determined using TIMS reveal that there is a significant difference in the n(234U)/n(238U) isotope ratio between the U ore concentrate from each mine.  相似文献   
167.
Headless submarine canyons with steep headwalls and shallowly sloping floors occur on both the second and third landward vergent anticlines on the toe of the Cascadia accretionary complex off central Oregon (45 °N, 125° 30′W). In September 1993, we carried out a series of nine deep tow camera sled runs and nine ALVIN dives to examine the relationship between fluid venting, structure and canyon formation. We studied four canyons on the second and third landward vergent anticlines, as well as the apparently unfailed intercanyon regions along strike. All evidence of fluid expulsion is associated with the canyons; we found no evidence of fluid flow between canyons. Even though all fluid seeps are related to canyons, we did not find seeps in all canyons, and the location of the seeps within the canyons differed. On the landward facing limb of the second landward vergent anticline a robust cold seep community occurs at the canyon’s inflection point. This seep is characterized by chemosynthetic vent clams, tube worms and extensive authigenic carbonate. Fluids for this seep may utilize high-permeability flow paths either parallel to bedding within the second thrust ridge or along the underlying thrust fault before leaking into the overriding section. Two seaward facing canyons on the third anticlinal ridge have vent clam communities near the canyon mouths at approximately the intersection between the anticlinal ridge and the adjacent forearc basin. No seeps were found along strike at the intersection of the slope basin and anticlinal ridge. We infer that the lack of seepage along strike and the presence of seeps in canyons may be related to fluid flow below the forearc basin/slope unconformity (overpressured by the impinging thrust fault to the west?) directed toward canyons at the surface.  相似文献   
168.
The mass of nutrients (nitrogen, phosphorus, potassium, and calcium) imported during 1984 and 1985 to the North. Inlet Estuary, Georgetown County, South Carolina, by precipitation and runoff was compared with that imported by a colonial-nesting wading bird, the white ibis (Eudocimus albus). From late March through late June of both years, breeding ibises imported nutrients to the North Inlet Estuary study site from freshwater bottomland forest swamps, where they fed on crayfishes (Procambaridae). Although 1984 was a relatively wet year, in 1985 the ibis breeding season was preceded by a severe winter-spring drought. In 1984 ibises nested in higher numbers, had higher per-pair breeding success, and imported 11 times more nutrients than in 1985. Nutrient input from atmospheric sources was substantially lower in 1984 than in 1985. The 1984 ibises imported 9% as much nitrogen, 33% as much phosphorus, 0.4% as much potassium, and 0.3% as much calcium to the estuary as did atmospheric sources. In 1985 nutrient input from ibises amounted to only 0.2% of the nitrogen, 2.9% of the phosphorus, and ?1% of the potassium and calcium imported by atmospheric sources. Our results show that nutrient inputs to estuaries from colonial-nesting wading birds can be substantial when compared, with those from atmospheric sources and can vary considerably among years. They also suggest that nutrient regimes in estuaries with large assemblages of wading birds may differ significantly from those lacking such colonies. *** DIRECT SUPPORT *** A01BY058 00006  相似文献   
169.
The regulatory requirements for characterization of the Martinsville Alternative Site (MAS) were fulfilled by applying an iterative approach to the groundwater flow modeling of the site and surrounding area. The approach consisted of field data collection and development of an initial conceptual model. The numerical model was then constructed to be consistent with the data and conceptual model. Next, the calibration results were evaluated statistically, and visually by a groundwater modeling review committee, to determine if the model accurately represented groundwater flow at the site. Initial results failed acceptance criteria because the values of numerical model input parameters had to be varied beyond observed data ranges to calibrate the results, and therefore the model was inconsistent with the initial conceptual model. This led to additional field data collection in areas where the numerical model deviated most from field-determined data. The new data provided sufficient information to revise the conceptual model and calibrate the numerical model successfully. Model calibration was followed by validation. Validation of the numerical model provided additional assurance that the model correctly simulated the observed system. No additional data were found to be necessary during validation of the MAS numerical model. The iterative approach proved to be successful for calibrating and validating this groundwater flow model and should be implemented from the onset of characterization planning in other applications.  相似文献   
170.
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