全文获取类型
收费全文 | 242篇 |
免费 | 10篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 7篇 |
大气科学 | 21篇 |
地球物理 | 62篇 |
地质学 | 64篇 |
海洋学 | 23篇 |
天文学 | 28篇 |
综合类 | 1篇 |
自然地理 | 48篇 |
出版年
2023年 | 1篇 |
2022年 | 4篇 |
2021年 | 5篇 |
2020年 | 8篇 |
2019年 | 7篇 |
2018年 | 9篇 |
2017年 | 8篇 |
2016年 | 9篇 |
2015年 | 7篇 |
2014年 | 9篇 |
2013年 | 15篇 |
2012年 | 9篇 |
2011年 | 9篇 |
2010年 | 15篇 |
2009年 | 11篇 |
2008年 | 5篇 |
2007年 | 7篇 |
2006年 | 7篇 |
2005年 | 3篇 |
2004年 | 11篇 |
2003年 | 4篇 |
2002年 | 8篇 |
2001年 | 7篇 |
2000年 | 3篇 |
1999年 | 4篇 |
1998年 | 12篇 |
1997年 | 3篇 |
1996年 | 3篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1993年 | 6篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 1篇 |
1977年 | 1篇 |
1976年 | 4篇 |
1971年 | 1篇 |
1961年 | 1篇 |
排序方式: 共有254条查询结果,搜索用时 15 毫秒
1.
Wetland hydroperiod classification in the western prairies using multitemporal synthetic aperture radar
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Joshua S. Montgomery Chris Hopkinson Brian Brisco Shane Patterson Stewart B. Rood 《水文研究》2018,32(10):1476-1490
Wetlands represent one of the world's most biodiverse and threatened ecosystem types and were diminished globally by about two‐thirds in the 20th century. There is continuing decline in wetland quantity and function due to infilling and other human activities. In addition, with climate change, warmer temperatures and changes in precipitation and evapotranspiration are reducing wetland surface and groundwater supplies, further altering wetland hydrology and vegetation. There is a need to automate inventory and monitoring of wetlands, and as a study system, we investigated the Shepard Slough wetlands complex, which includes numerous wetlands in urban, suburban, and agricultural zones in the prairie pothole region of southern Alberta, Canada. Here, wetlands are generally confined to depressions in the undulating terrain, challenging wetlands inventory and monitoring. This study applied threshold and frequency analysis routines for high‐resolution, single‐polarization (HH) RADARSAT‐2, synthetic aperture radar mapping. This enabled a growing season surface water extent hyroperiod‐based wetland classification, which can support water and wetland resource monitoring. This 3‐year study demonstrated synthetic aperture radar‐derived multitemporal open‐water masks provided an effective index of wetland permanence class, with overall accuracies of 89% to 95% compared with optical validation data, and RMSE between 0.2 and 0.7 m between model and field validation data. This allowed for characterizing the distribution and dynamics of 4 marsh wetlands hydroperiod classes, temporary, seasonal, semipermanent, and permanent, and mapping of the sequential vegetation bands that included emergent, obligate wetland, facultative wetland, and upland plant communities. Hydroperiod variation and surface water extent were found to be influenced by short‐term rainfall events in both wet and dry years. Seasonal hydroperiods in wetlands were particularly variable if there was a decrease in the temporary or semipermanent hydroperiod classes. In years with extreme rain events, the temporary wetlands especially increased relative to longer lasting wetlands (84% in 2015 with significant rainfall events, compared with 42% otherwise). 相似文献
2.
Jennifer M. Galloway Christine T. Doherty R. Timothy Patterson Helen M. Roe 《第四纪科学杂志》2009,24(4):322-335
A pollen‐based study from Tiny Lake in the Seymour‐Belize Inlet Complex of central coastal British Columbia, Canada, permits an evaluation of the dynamic response of coastal temperate rainforests to postglacial climate change. Open Pinus parklands grew at the site during the early Lateglacial when the climate was cool and dry, but more humid conditions in the later phases of the Lateglacial permitted mesophytic conifers to colonise the region. Early Holocene conditions were warmer than present and a successional mosaic of Tsuga heterophylla and Alnus occurred at Tiny Lake. Climate cooling and moistening at 8740 ± 70 14C a BP initiated the development of closed, late successional T. heterophylla–Cupressaceae forests, which achieved modern character after 6860 ± 50 14C a BP, when a temperate and very wet climate became established. The onset of early Holocene climate cooling and moistening at Tiny Lake may have preceded change at more southern locations, including within the Seymour‐Belize Inlet Complex, on a meso‐ to synoptic scale. This would suggest that an early Holocene intensification of the Aleutian Low pressure system was an important influence on forest dynamics in the Seymour‐Belize Inlet Complex and that the study region was located near the southern extent of immediate influence of this semi‐permanent air mass. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
3.
4.
Experiments were conducted in a wave flume to study the differences between harmonic evolution of monochromatic waves as they propagate over a submerged impermeable or porous step under non-breaking conditions. Results are used as a preliminary analysis to establish some engineering design criteria on harmonic generation on submerged porous structures. The root-mean-squared wave height evolution is also studied and compared to linear models as a first approximation. It is shown that porous structure increases the effective relative depth and decreases the relative wave height, resulting in a lower Ursell number and a lower chance to generate harmonics. The effective water depth over a step as defined in the paper, provides information to evaluate the potential harmonic generation. 相似文献
5.
Stable isotope values of Costa Rican surface waters 总被引:3,自引:0,他引:3
Stable isotope data of surface waters from the humid tropics in general, and Costa Rica in particular, are scarce. To improve our understanding of the spatial distribution of stable isotopes in surface waters, we measured δ18O and δD in river and lake (n=63) and precipitation (n=3) samples from Costa Rica. We also present data from the IAEA/WMO isotopes in precipitation network as context for our study. Surface water isotope values do not strongly correlate with elevation, stream head elevation, stream length, distance from Caribbean Sea, or estimated mean annual precipitation for the country as a whole. However, the data show distinct regional trends. The δ18O and δD values downwind of mountain ranges are inversely related to the altitude of the ranges the air masses traverse. In the lee of the high Talamanca Range, δ18O values are 6–8‰ lower, while in the lee of the lower Tilarán Range δ18O values are 2–3‰ lower than upwind sites along the Caribbean Slope. An altitude effect of −1.4‰ δ18O/km is present on the Pacific slope of southern Costa Rica, equivalent to a temperature effect of −0.3‰/°C. The Nicoya and Osa Peninsulas have higher values than upwind sites, suggesting input of Pacific-sourced moisture, evaporative enrichment, or decreased condensation temperatures. Elevated and increasing d-excess values inland along the Nicaragua Trough suggest a recycled component may be an important contributor to the water budget. These data provide preliminary stable isotope information for Costa Rica, and will benefit paleoclimatic research in the region. More detailed studies would be beneficial to our understanding of the controls on stable isotope composition of tropical waters. 相似文献
6.
To better understand the implications of anthropogenic climate change for three major Mid-Atlantic estuaries (the Chesapeake Bay, the Delaware Bay, and the Hudson River Estuary), we analyzed the regional output of seven global climate models. The simulation given by the average of the models was generally superior to individual models, which differed dramatically in their ability to simulate twentieth-century climate. The model average had little bias in its mean temperature and precipitation and, except in the Lower Chesapeake Watershed, was able to capture the twentieth-century temperature trend. Weaknesses in the model average were too much seasonality in temperature and precipitation, a shift in precipitation’s summer maximum to spring and winter minimum to fall, interannual variability that was too high in temperature and too low in precipitation, and inability to capture the twentieth-century precipitation increase. There is some evidence that model deficiencies are related to land surface parameterizations. All models warmed over the twenty-first century under the six greenhouse gas scenarios considered, with an increase of 4.7 ± 2.0°C (model mean ± 1 standard deviation) for the A2 scenario (a medium-high emission scenario) over the Chesapeake Bay Watershed by 2070–2099. Precipitation projections had much weaker consensus, with a corresponding increase of 3 ± 12% for the A2 scenario, but in winter there was a more consistent increase of 8 ± 7%. The projected climate averaged over the four best-performing models was significantly cooler and wetter than the projected seven-model-average climate. Precipitation projections were within the range of interannual variability but temperature projections were not. The implied research needs are for improvements in precipitation projections and a better understanding of the impacts of warming on streamflow and estuarine ecology and biogeochemistry. 相似文献
7.
8.
Jeremy C. Ely Chris D. Clark Richard C. A. Hindmarsh Anna L. C. Hughes Sarah L. Greenwood Sarah L. Bradley Edward Gasson Lauren Gregoire Niall Gandy Chris R. Stokes David Small 《第四纪科学杂志》2021,36(5):946-960
Palaeo-ice sheets are important analogues for understanding contemporary ice sheets, offering a record of ice sheet behaviour that spans millennia. There are two main approaches to reconstructing palaeo-ice sheets. Empirical reconstructions use the available glacial geological and chronological evidence to estimate ice sheet extent and dynamics but lack direct consideration of ice physics. In contrast, numerically modelled simulations implement ice physics, but often lack direct quantitative comparison with empirical evidence. Despite being long identified as a fruitful scientific endeavour, few ice sheet reconstructions attempt to reconcile the empirical and model-based approaches. To achieve this goal, model-data comparison procedures are required. Here, we compare three numerically modelled simulations of the former British–Irish Ice Sheet with the following lines of evidence: (a) position and shape of former margin positions, recorded by moraines; (b) former ice-flow direction and flow-switching, recorded by flowsets of subglacial bedforms; and (c) the timing of ice-free conditions, recorded by geochronological data. These model–data comparisons provide a useful framework for quantifying the degree of fit between numerical model simulations and empirical constraints. Such tools are vital for reconciling numerical modelling and empirical evidence, the combination of which will lead to more robust palaeo-ice sheet reconstructions with greater explicative and ultimately predictive power. 相似文献
9.
Integrated land use–transportation models predict future transportation demand taking into account how households and firms arrange themselves partly as a function of the transportation system. Recent integrated models require parcels as inputs and produce household and employment predictions at the parcel scale. Block subdivision algorithms automatically generate parcel patterns within blocks. Evaluating block subdivision algorithms is done by way of generating parcels and comparing them to those in a parcel database. Three block subdivision algorithms are evaluated on how closely they reproduce parcels of different block types found in a parcel database from Montreal, Canada. While the authors who developed each of the algorithms have evaluated them, they have used their own metrics and block types to evaluate their own algorithms. This makes it difficult to compare their strengths and weaknesses. The contribution of this paper is in resolving this difficulty with the aim of finding a better algorithm suited to subdividing each block type. The proposed hypothesis is that given the different approaches that block subdivision algorithms take, it’s likely that different algorithms are better adapted to subdividing different block types. To test this, a standardized block type classification is used that consists of mutually exclusive and comprehensive categories. A statistical method is used for finding a better algorithm and the probability it will perform well for a given block type. Results suggest the oriented bounding box algorithm performs better for warped non-uniform sites, as well as gridiron and fragmented uniform sites. It also produces more similar parcel areas and widths. The Generalized Parcel Divider 1 algorithm performs better for gridiron non-uniform sites. The Straight Skeleton algorithm performs better for loop and lollipop networks as well as fragmented non-uniform and warped uniform sites. It also produces more similar parcel shapes and patterns. 相似文献
10.