全文获取类型
收费全文 | 167771篇 |
免费 | 2886篇 |
国内免费 | 2808篇 |
专业分类
测绘学 | 5245篇 |
大气科学 | 12875篇 |
地球物理 | 34176篇 |
地质学 | 60535篇 |
海洋学 | 13819篇 |
天文学 | 33098篇 |
综合类 | 2586篇 |
自然地理 | 11131篇 |
出版年
2021年 | 1284篇 |
2020年 | 1418篇 |
2019年 | 1499篇 |
2018年 | 7042篇 |
2017年 | 6321篇 |
2016年 | 5910篇 |
2015年 | 2652篇 |
2014年 | 3714篇 |
2013年 | 7608篇 |
2012年 | 4861篇 |
2011年 | 7730篇 |
2010年 | 6685篇 |
2009年 | 8429篇 |
2008年 | 7280篇 |
2007年 | 7458篇 |
2006年 | 5507篇 |
2005年 | 4673篇 |
2004年 | 4811篇 |
2003年 | 4498篇 |
2002年 | 4222篇 |
2001年 | 3552篇 |
2000年 | 3421篇 |
1999年 | 2970篇 |
1998年 | 2984篇 |
1997年 | 2882篇 |
1996年 | 2519篇 |
1995年 | 2410篇 |
1994年 | 2204篇 |
1993年 | 2026篇 |
1992年 | 1949篇 |
1991年 | 1819篇 |
1990年 | 2023篇 |
1989年 | 1831篇 |
1988年 | 1697篇 |
1987年 | 1997篇 |
1986年 | 1767篇 |
1985年 | 2178篇 |
1984年 | 2529篇 |
1983年 | 2322篇 |
1982年 | 2217篇 |
1981年 | 2135篇 |
1980年 | 1869篇 |
1979年 | 1804篇 |
1978年 | 1778篇 |
1977年 | 1715篇 |
1976年 | 1549篇 |
1975年 | 1455篇 |
1974年 | 1495篇 |
1973年 | 1561篇 |
1972年 | 953篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
811.
Debasish Chakraborty Gautam Kumar Sen Sugata Hazra 《Journal of Earth System Science》2009,118(5):609-617
Texture in high-resolution satellite images requires substantial amendment in the conventional segmentation algorithms. A measure is proposed to compute the Hölder exponent (HE) to assess the roughness or smoothness around each pixel of the image. The localized singularity information is incorporated in computing the HE. An optimum window size is evaluated so that HE reacts to localized singularity. A two-step iterative procedure for clustering the transformed HE image is adapted to identify the range of HE, densely occupied in the kernel and to partition Hölder exponents into a cluster that matches with the range. Hölder exponent values (noise or not associated with the other cluster) are clubbed to a nearest possible cluster using the local maximum likelihood analysis. 相似文献
812.
813.
A.K.L. Johnson 《The Australian geographer》2004,35(1):39-57
Rainfall regimes with strong spatial and temporal variation are characteristic of many coastal regions of north and eastern Australia. In coastal regions of north eastern Australia, regimes vary considerably over short distances. This occurs because of changes in local topography, including the height and orientation of mountain ranges and the direction of the coastline with respect to the prevailing moist south east air stream. Northern Australia experiences a tropical monsoon climate with rainfall occurring predominantly during the summer months. Areas with a closer proximity to the coast typically experience the heavier rainfalls. While networks of rainfall gauges have been established and continuous records are available for most of these stations from the 1890s, their low distribution density relative to the complexity of rainfall pattern they are required to represent means that there remains a poor understanding of the spatial and temporal distribution of rainfall in the wet tropics. An enhanced knowledge of rainfall distribution in both space and time has the potential to deliver significant economic and environmental benefits to managers of natural resources. This paper reports on the application of a technique for estimating mean annual and mean monthly rainfall across the Herbert River catchment of north east Australia's dry and wet tropics. The technique utilises thin plate smoothing splines to incorporate both location and elevation into estimates of rainfall distribution. We demonstrate that the method can be applied successfully at the meso scale and within the domain of routinely available data. As such, the method has broad relevance for decision making. 相似文献
814.
815.
Richard L. Reynolds Joseph G. Rosenbaum Josh Rapp Michael W. Kerwin J. Platt Bradbury Steven Colman David Adam 《Journal of Paleolimnology》2004,31(2):217-233
Petrological and textural properties of lacustrine sediments from Upper Klamath Lake, Oregon, reflect changing input volumes of glacial flour and thus reveal a detailed glacial history for the southern Cascade Range between about 37 and 15 ka. Magnetic properties vary as a result of mixing different amounts of the highly magnetic, glacially generated detritus with less magnetic, more weathered detritus derived from unglaciated parts of the large catchment. Evidence that the magnetic properties record glacial flour input is based mainly on the strong correlation between bulk sediment particle size and parameters that measure the magnetite content and magnetic mineral freshness. High magnetization corresponds to relatively fine particle size and lower magnetization to coarser particle size. This relation is not found in the Buck Lake core in a nearby, unglaciated catchment. Angular silt-sized volcanic rock fragments containing unaltered magnetite dominate the magnetic fraction in the late Pleistocene sediments but are absent in younger, low magnetization sediments. The finer grained, highly magnetic sediments contain high proportions of planktic diatoms indicative of cold, oligotrophic limnic conditions. Sediment with lower magnetite content contains populations of diatoms indicative of warmer, eutrophic limnic conditions. During the latter part of oxygen isotope stage 3 (about 37–25 ka), the magnetic properties record millennial-scale variations in glacial-flour content. The input of glacial flour was uniformly high during the Last Glacial Maximum, between about 21 and 16 ka. At about 16 ka, magnetite input, both absolute and relative to hematite, decreased abruptly, reflecting a rapid decline in glacially derived detritus. The decrease in magnetite transport into the lake preceded declines in pollen from both grass and sagebrush. A more gradual decrease in heavy mineral content over this interval records sediment starvation with the growth of marshes at the margins of the lake and dilution of detrital material by biogenic silica and other organic matter. 相似文献
816.
R. G. Bill Jr. E. Chen R. A. Sutherland J. F. Bartholic 《Boundary-Layer Meteorology》1979,16(1):23-33
A steady-state, two-dimensional numerical model is used to simulate air temperatures and humidity downwind of a lake at night. Thermal effects of the lake were modelled for the case of moderate and low surface winds under the cold-air advective conditions that occur following the passage of a cold front. Surface temperatures were found to be in good agreement with observations. A comparison of model results with thermal imagery indicated the model successfully predicts the downwind distance for which thermal effects due to the lake are significant. 相似文献
817.
Tamarix nabkha is one of the most widespread nabkhas,distributing in the arid region of China.Based on the observations outdoors and the simulation experiments in laboratories,analysis in this paper refers to the biological geomorphologic features and growth process of Tamarix nabkhas in the middle and lower reaches of the Hotan River,Xin- jiang.And the results indicate that the ecological type of Tamarix in the study area is a kind of Tugaic soil habitat based on the deep soil of the Populus Diversifolia f... 相似文献
818.
Ground-based thermal emission measurements of the zenith sky have been made at Peterborough, Ontario since January 1993. In this paper, the measurement of the 6 band of atmospheric CFC-12, an important greenhouse gas, is presented for a cold, clear day in January 1994. A spectrum of the non-CFC-12 emission features has been simulated using the FASCD3P radiation code and measured radiosonde profiles of temperature, pressure and humidity. This has enabled a satisfactory subtraction of the interfering emission features from the CFC-12 emission spectrum. A comparison of the observed and simulated 6-bands of CFC-12 shows good agreement at all frequencies of emission. From these spectra the total downward greenhouse radiative flux from the CFC-12 6 emission based for a very cold day has been estimated to be 0.27 W m−2±10%. 相似文献
819.
N. I. Alekseevskii K. M. Berkovich R. S. Chalov S. R. Chalov 《Geography and Natural Resources》2012,33(3):192-199
We assess the spatiotemporal changes in channel processes on rivers of Russia, determine the causes for vertical (incision or directional sediment accumulation) and horizontal (displacement of channel forms) deformations and show the distribution of stream channel of different morphodynamical types and with a different reconfiguration rate. The conditions are revealed, under which the channel types change over time. Particular emphasis is placed on the analysis of spatiotemporal changes in channels caused by anthropogenic disturbances and by direct technogenic interferences in the life of rivers (hydroelectric schemes, quarries in the river channels, and waterway dredging). 相似文献
820.