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261.
262.
Stuart D. Jordan 《Solar physics》1977,51(1):51-59
Calculations performed with several models of the solar chromosphere support Ulmschneider's conclusion that relatively short period acoustic waves heat the low chromosphere in the region just above the temperature minimum. However, these same short period waves (10 period P80 s) are not able to maintain chromospheric temperatures at heights where 5000Å(normal) < 10-6. The calculations also show that an earlier conjecture stating that the H2 population might influence the non-LTE chromospheric H- population is probably not correct, due to lower values of the ratio n
e/n
H inferred from more recent observations. Finally, the calculations support Athay's contention that the Cayrel mechanism alone cannot produce the observed temperature rise, because the magnitude of the radiative cooling in the lines is too great. 相似文献
263.
Thomas E. Jordan David L. Correll Dennis F. Whigham 《Estuarine, Coastal and Shelf Science》1983,17(6):651-667
Tidal exchanges of nitrogen, phosphorus, and organic carbon by a high and a low elevation marsh in the Rhode River estuary were measured throughout the year. Both marshes tended to import particulate matter and export dissolved matter, although they differed in the fluxes of certain nutrients. Compared with tidal exchanges, bulk precipitation was a major source of ammonia and nitrate and a minor source of other nutrients. There was a net retention of nutrients by the portion of the Rhode River that included both marshes and a mudflat. However, the marshes accounted for only 10% of the phosphorus retention and 1% of the nitrogen retention while they released organic carbon amounting to 20% of the retention. This suggests that the mudflat acted as a sink for nutrients. The primary role of the marshes seems to be transformation of particulate to dissolved nutrients rather than nutrient retention or release. 相似文献
264.
Yifeng Cui Reagan Moore Kim Olsen Amit Chourasia Philip Maechling Bernard Minster Steven Day Yuanfang Hu Jing Zhu Thomas Jordan 《Acta Geotechnica》2009,4(2):79-93
Earthquakes are among the most complex terrestrial phenomena, and modeling of earthquake dynamics is one of the most challenging
computational problems in science. Computational capabilities have advanced to a state where we can perform wavefield simulations
for realistic three-dimensional earth models, and gain more insights into the earthquakes that threaten California and many
areas of the world. The Southern California Earthquake Center initiated a major earthquake research program called TeraShake
to perform physics-based numerical simulations of earthquake processes for large geographical regions, at high resolution,
and for high frequencies. For a large scale simulation such as TeraShake, optimization problems tend to emerge that are not
significant in smaller scale simulations. This involves both large parallel computation and also massive data management and
visualization coordination. In this paper, we describe how we performed single-processor optimization of the TeraShake AWM
application, optimization of the I/O handling, and optimization of initialization. We also look at the challenges presented
by run-time data archive management and visualization. The improvements made to the TeraShake AWM code enabled execution on
the 40k IBM Blue Gene processors and have created a community code that can be used by seismologists to perform petascale
earthquake simulations. 相似文献
265.
Testing alarm-based earthquake predictions 总被引:1,自引:0,他引:1
266.
We present herein clear field evidence for the persistence of a coarse surface layer in a gravel‐bed river during flows capable of transporting all grain sizes present on the channel bed. Detailed field measurements of channel topography and bed surface grain size were made in a gravel‐bed reach of the Colorado River prior to a flood in 2003. Runoff produced during the 2003 snowmelt was far above average, resulting in a sustained period of high flow with a peak discharge of 27 m3/s (170% of normal peak flow); all available grain sizes within the study reach were mobilized in this period of time. During the 2003 peak flow, the river avulsed immediately upstream of the study reach, thereby abandoning approximately one half kilometer of the former channel. The abandonment was rapid (probably within a few hours), leaving the bed texture essentially frozen in place at the peak of the flood. All locations sampled prior to the flood were resampled following the stream abandonment. In response to the high flow, the surface median grain size (D50s) coarsened slightly in the outer part of the bend while remaining nearly constant along the inner part of the bend, resulting in an overall increase from 18 to 21 mm for the study reach. Thus, the coarse bed surface texture persisted despite shear stresses throughout the bend that were well above the critical entrainment value. This may be explained because the response of the bed texture to increases in flow strength depends primarily upon the continued availability of the various grain size percentiles in the supply, which in this case was essentially unlimited for all sizes present in the channel. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
267.
Rebecca C. Jordan Angela M. Gospodarek Eric T. Schultz Robert K. Cowen Kamazima Lwiza 《Estuaries and Coasts》2000,23(5):683-689
Estuaries are critical habitats for larvae and juveniles of many marine fishes, possibly because they promote high growth
rates and survival rates. We investigated spatial and temporal changes in growth rate of larval bay anchovy (Anchoa mitchilli), in the middle Hudson River estuary where abundance of larvae is high. In two consecutive summer seasons, we sampled larvae
at 4 sites evenly spaced over 45 km, at weekly intervals for up to a month. We examined otoliths to determine age in days
and then used age-length regressions to estimate growth rate. In 1995, larval anchovy growth rates varied from 0.39 to 0.88
mm d−1 (median=0.48 mm d−1). In 1996, growth rates varied from 0.41 to 0.77 mm d−1 (median=0.55 mm d−1). In both years, we found significant spatial and temporal variation in growth rate. Larvae collected in the upper portion
of Haverstraw Bay tended to grow more slowly than larvae collected in other sites. The dates on which the most rapidly growing
larvae were collected varied from site to site. Neither temperature nor salinity variations explained growth rate differences.
Growth rate variation, probably governed by patches of zooplankton, occurred on temporal scales of a week and spatial scales
of 15 km. 相似文献
268.
Crew variability in topographic surveys for monitoring wadeable streams: a case study from the Columbia River Basin
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Sara Bangen Joe Wheaton Nicolaas Bouwes Chris Jordan Carol Volk Michael B. Ward 《地球表面变化过程与地形》2014,39(15):2070-2086
Digital elevation models (DEMs) derived from ground‐based topographic surveys have become ubiquitous in the field of fluvial geomorphology. Their wide application in spatially explicit analysis includes hydraulic modeling, habitat modeling, and morphological sediment budgeting. However, there is a lack of understanding regarding the repeatability and precision of DEMs derived from ground‐based surveys conducted by different, and inherently subjective, observers. This is of particular concern when we consider the proportion of studies and monitoring programs that are implemented across multiple sites and over time by different observers. We used a case study from the Columbia Habitat Monitoring Program (CHaMP), where seven field crews sampled the same six sites, to quantify the magnitude and effect of observer variability on DEMs interpolated from total station surveys. We quantified the degree to which DEM‐derived metrics and measured geomorphic change were repeatable. Across all six sites, we found an average elevation standard deviation of 0.05 m among surveys, and a mean total range of 0.16 m. A variance partition between site, crew, and unexplained errors for several topographically derived metrics showed that crew variability never accounted for > 1.5% of the total variability. We calculated minor geomorphic changes at one site following a relatively dry flow year between 2012 and 2011. Calculated changes were minimal (unthresholded net changes ±1–3 cm) with six crews detecting an indeterminate sediment budget and one crew detecting a minor net erosional sediment budget. While crew variability does influence the quality of topographic surveys, this study highlights that when consistent surveying methods are employed, the data sets are still sufficient to support derivation of topographic metrics and conduct basic geomorphic change detection. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
269.
An increasingly common method to restore eroding beaches is nourishment, a process by which lost sand is replaced with terrestrial or offshore sediments to widen beaches. The southeastern Florida coastline contains shore-parallel coral reef communities adjacent to eroding beaches. Scleractinian corals and other reef-associated organisms are known to demonstrate sensitivity to elevated sedimentation levels. Sediment traps were used to examine spatio-temporal sedimentation patterns and assess the effects of nourishment (dredge and fill) activities. Several environmental variables correlated with among-site spatial variability of sediment parameters. Intra-annual variability correlated with wind velocity and direction. Nourishment activities showed localized effects, with sites in close proximity to dredging areas exhibiting significantly higher collection rates and lower percent fines than control sites. A regional increase in sedimentation occurred while nourishment activities were ongoing. Due to concurrent impacts of hurricanes, only one during-construction sampling interval revealed substantially higher collection rates relative to corresponding pre-construction sampling intervals. 相似文献
270.