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141.
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Guang Han GuiFang Zhang Li You Liang Zhou Lin Yang XueYong Zhao YuLin Li TongHui Zhang 《寒旱区科学》2015,7(1):0029-0039
By viewing satellite imagery, a striking large-scale dunefield can be clearly perceived, with a size of nearly 63 km long and 11 km wide, and trending NE–SW, on the right flank of the lower Laoha River, Northeast China. By means of remote sensing imagery analysis and field observation as well as a comparison with a small-scale dunefield on the right flank of the lower Xiangshui River, analogous to the case of the lower Laoha River, this paper presents a new mechanism for its origin and development. The results show that:(1) the large-scale dunefield bears a tile-style framework overwhelmingly composed of transverse barchanoid ridges perpendicular to the predominant winds, and inlaid diverse blowouts.(2) The small-scale dunefield, referred to as a primary structural unit of the large one, is typical of an incipient dunefield, following the same rules of evolution as the larger.(3) A succession of barchanoid ridge chains can steadily migrate downwind in much the same manner as surface wave propagation in air or water stimulated by an incised valley, and ultimately tend to bear roughly the same wavelength and amplitude under stable climate and hydrologic regimes.(4) The first ridge chain acquires its sand source substantially from the downwind escarpments exposing the loose Quaternary sandy sediments to the air, while the ensuing ridges derive their sands dominantly from in situ deflation of the underlain Quaternary loose sandy sediments in blowouts, partly from the upwind ridges through northern elongated horns. Theoretically, the sands from riparian escarpments can be transported by wind to the downwind distal end of a dunefield after sufficient long duration.(5) The lower Laohahe region experienced probably three significant climatic changes in the past, corresponding to the three active dune belts, suggesting that once a large-scale dunefield occurs, it is nearly impossible to be completely stabilized, at least in its central portions. At present, seasonal shrinkage and stagnation of the lower Laoha River, widespread farming and afforestation in the valley, and establishing windbreaks downwind of the valley as well as surrounding the dunefield, appear to have significantly modified local flow fields and sand sources, engendering significant degradation of the dunefield. 相似文献
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1 IntroductionTheArcticarea ,asthebackgroundofglobalclimateandenvironment,drawspeo ple’sattentionwithitsimpactonglobalchange (IPCC 1 990 ) .Greatamountofenergyandmassareexchangedbetweentheseaandairwithspecialinterface,air ice seasurface ,inthisarea .Thereforeiti… 相似文献
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1 IntroductionArcticclimateandenvironmentareveryimportantintheglobalchangesystems(WMO/UNEP 1 990 ;WMO 1 991 ) .Observationsandmodelingresultsindicatethattheglobalwarmingwillbeenhancedinthepolarregions ,especiallyintheNorthHemi sphere ,withapredictedwarmingofabout… 相似文献
146.
Young-June Han 《Dynamics of Atmospheres and Oceans》1984,8(2):107-140
A new six-layer world ocean general circulation model based on the primitive system of equations is described in detail and its performance in the case of a homogeneous ocean is described. These test integrations show that the model is capable of reproducing the observed mean barotropic or vertically-integrated transport, as well as the seasonal variability of the major ocean gyres. The surface currents, however, are dominated by the Ekman transport, and such non-linear features as the western boundary currents and the equatorial countercurrents are poorly represented. The abyssal boundary countercurrents are also absent due to the lack of thermohaline forcing. The most conspicuous effect of the bottom topography on a homogeneous ocean is seen in the Southern ocean where the calculated Antarctic circumpolar transport through the Drake passage ( ≈ 10 Sv, with bathymetry included) greatly underestimates the observed transport (≈ 100 Sv). 相似文献
147.
本文通过对榆林地区米脂、绥德、子洲、清涧、吴堡和榆林市黄土窑洞建窑环境进行考察,总结分析了在黄土介质中修建的土窑洞崩塌灾害多发性原因,并提出了可操作性的防治对策。该对策可广泛适用于我国黄土窑洞分布区。 相似文献
148.
Hydrodynamic change recorded by diatoms in sediments of Liuxihe Reservoir, southern China 总被引:1,自引:0,他引:1
Diatoms are used widely for paleolimnological studies in lakes, but their use for studying the environmental history of reservoirs
has not been tested extensively. Reservoirs have hydrodynamic characteristics intermediate between those of rivers and lakes.
This study assessed the utility of diatom assemblages as recorders of long-term changes in hydrodynamics and spatial gradients
in Liuxihe Reservoir, an impoundment in southern China. Four sediment cores were collected in the reservoir, from the riverine,
transition and lacustrine zones. Each core was sectioned at 2-cm intervals to investigate the stratigraphic distribution of
accumulated diatoms. Varve counting was used to develop a chronology for one of the cores. The unique characteristics of Liuxihe
Reservoir, including its large size, great depth, long narrow morphology and strong thermal stratification for 10 months of
the year, limit secondary sedimentation processes and preserve the varves, enabling development of an accurate chronology.
Damming profoundly altered the physical environment of the former river, especially in the lacustrine zone, where the change
is clearly illustrated by diatoms in the sediment. Diatom abundance increased as a consequence of nutrient enrichment after
construction of the dam in 1958, but later decreased as the new impoundment stabilized. After damming, relative abundance
of Cyclotella increased along with a simultaneous decrease of Navicula and Achnanthes in the lacustrine zone, most significantly in 1963, when Cyclotella replaced Navicula as the dominant genus. This switch was indicative of a general shift from a lotic to a lentic habitat. A longitudinal gradient
was apparent in the patterns of sedimentation and diatom accumulation at different sites in the reservoir, with diatom abundance
highest in the transition zone. In the long term, water discharge from the reservoir showed a weak, but significant negative
correlation with diatom abundance in the lacustrine zone (r = −0.320, P = 0.03). In summary, diatom assemblages in the Liuxihe Reservoir sediments recorded past changes in hydrodynamics, suggesting
that paleolimnological study of some impoundments is feasible. 相似文献
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