全文获取类型
收费全文 | 32387篇 |
免费 | 435篇 |
国内免费 | 373篇 |
专业分类
测绘学 | 1254篇 |
大气科学 | 2453篇 |
地球物理 | 6274篇 |
地质学 | 11393篇 |
海洋学 | 2500篇 |
天文学 | 7740篇 |
综合类 | 156篇 |
自然地理 | 1425篇 |
出版年
2021年 | 287篇 |
2020年 | 286篇 |
2019年 | 336篇 |
2018年 | 843篇 |
2017年 | 793篇 |
2016年 | 1041篇 |
2015年 | 607篇 |
2014年 | 967篇 |
2013年 | 1714篇 |
2012年 | 1052篇 |
2011年 | 1287篇 |
2010年 | 1087篇 |
2009年 | 1400篇 |
2008年 | 1216篇 |
2007年 | 1167篇 |
2006年 | 1185篇 |
2005年 | 988篇 |
2004年 | 872篇 |
2003年 | 878篇 |
2002年 | 862篇 |
2001年 | 772篇 |
2000年 | 748篇 |
1999年 | 677篇 |
1998年 | 620篇 |
1997年 | 632篇 |
1996年 | 572篇 |
1995年 | 540篇 |
1994年 | 508篇 |
1993年 | 434篇 |
1992年 | 389篇 |
1991年 | 418篇 |
1990年 | 414篇 |
1989年 | 395篇 |
1988年 | 363篇 |
1987年 | 437篇 |
1986年 | 362篇 |
1985年 | 447篇 |
1984年 | 479篇 |
1983年 | 466篇 |
1982年 | 448篇 |
1981年 | 360篇 |
1980年 | 363篇 |
1979年 | 315篇 |
1978年 | 308篇 |
1977年 | 294篇 |
1976年 | 260篇 |
1975年 | 256篇 |
1974年 | 283篇 |
1973年 | 307篇 |
1972年 | 194篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
Christopher J. Brown Brandon K. Poiencot Nick Hudyma Barry Albright Richard A. Esposito 《Environmental Earth Sciences》2014,71(2):793-806
One alternative to reduce global greenhouse gas emissions is to store the emissions in underground geologic sequestration repositories. The efficacy of this approach has been favorably evaluated by numerous authors over the last 15 years. This paper discusses an assessment of the overall feasibility of storing emissions in three different repositories in the Florida panhandle located in the Southeastern United States. The feasibility assessment evaluates both saline aquifers and oil reservoirs located in the panhandle region. The overall feasibility is driven by the available geologic sequestration capacity, the transportation cost to deliver emissions to a respective repository, and other engineering and regulatory issues. The geologic sequestration capacity is generally controlled by the so-called storage efficiency, a variable dependent on the site-specific geology, reservoir conditions, and the injected fluid characteristics. For this paper, storage efficiency for saline repositories was assessed in more detail using numerical modeling. Based on the work completed, the 3 repositories studied have at least 4.55 gigatonnes of capacity to sequester CO2. 相似文献
82.
P. K. Manoharan 《Astrophysics and Space Science》1996,243(1):221-224
Interplanetary scintillation measurements obtained inside 200 R using the Ooty Radio Telescope during August 1986–April 1991 have been analysed to study the interplanetary disturbances (or events) and their occurrence rates at various phases of the solar cycle. The disturbances are identified by the increase in the level of scintillation compared with the expected value. In total, 735 events have been identified. The results show a rate of 0.49 events per day near solar maximum and a low rate of 0.16 events per day during minimum of activity. The results are compared with coronal mass ejection (CME) rates and transients rates obtained from the Doppler scintillation measurements. 相似文献
83.
K. Hummel 《International Journal of Earth Sciences》1928,19(3):223-228
Ohne ZusammenfassungVortrag, gehalten in der Hauptversammlung der Geologischen Vereinigung in Frankfurt a. M., am 7. I. 1928. Ausführlichere Mitteilungen über dieses Thema werden im Rahmen einer größeren Abhandlung über die südalpine Mitteltrias an anderer Stelle veröffentlicht werden. 相似文献
84.
Rice research for food security and sustainable agricultural development in Asia: Achievements and future challenges 总被引:1,自引:0,他引:1
Rice is the major staple food of Asia, and an important source of employment and income in rural areas, particularly in low-income countries. Research has contributed significantly in achieving food security by increasing the yield potential of rice in irrigated systems, reducing the crop maturity period and achieving yield stability by developing resistance against major insects and diseases in the modern high-yielding varieties. Poverty is, however, still extensive in fragile rainfed rice ecosystems where rice yield has remained low, as scientists have yet to develop high-yielding varieties resistant to abiotic stresses and problem soils. Rice production needs to be increased by another 70% over the next 30 years to meet growing food needs. This has to be achieved with less land, less water, and less labor to accommodate the demand for these inputs from the expanding nonagricultural sectors. The challenge to the rice research community is to make further shifts in yield potential of rice for the irrigated systems, to close the yield gaps in the rainfed systems through developing resistance of high yielding varieties to abiotic stresses, and greater understanding of the interactions between genotypes and environment, developing durable resistance against pests and diseases to reduce farmers' dependence on harmful agrochemicals, and to increase efficiency in the use of water, labor and fertilizers. As further intensification of rice cultivation is inevitable, scientists must understand the negative environmental side-effects of increasing rice productivity, to develop appropriate mitigation options. 相似文献
85.
Summary. The contributions from the oceanic and ionospheric dynamos, Lo and LI respectively, to the geomagnetic lunar daily variation, L , at Alibag, a coastal station in the Indian equatorial region, are calculated from the L harmonics derived from a 41–yr long series of observations. The analysis in the calendar months shows a steady and significant ocean dynamo contribution in the vertical component, Z, in all the months except April. Examination, by an analysis of the data year by year, of the association of Lo and LI with varying solar and magnetic activities reveals, surprisingly, a stable correlation between the magnetic activity index A P and the oceanic part in the horizontal and vertical components but not in declination, which probably indicates the influence of induced currents, along the latitudes, on L o . 相似文献
86.
In this paper we compute the rate of solar EUV heating in the upper atmosphere by photo-dissociation and photo-ionization, taking care to include properly the effects of oblique incidence of solar flux, sphericity of the atmosphere and ellipticity of the Earth's orbit. The time and latitudinal variations of the computed heat function are revealed by numerical Fourier analysis of the heat function into harmonics of the yearly cycle. It is shown that EUV absorption contains a ‘latitude independent’ semi-annual component of heating with the ‘proper phase’ to account for the semi-annual density variations. Further, the annual component of the heat function predicts the existence of ‘summer polar’ density increases in the northern and southern hemispheres. 相似文献
87.
In the studies reteted to surface energy balance, satellite data provides important inputs for estimating regional surface albedo and evapotranspiration. The paper describes the use of satellite data in determining the surface emissivity over heterogeneous a’reas by taking Normalized Difference Vegetation Index (NDVI) as modulating parameter at pixel resolution. The estimated emissivity values have been used to find the surface temperature at the pixel scale. Landsat-TM-visible, NIR, TIR bands data and some ground meteorological data have been used in an energy balance model for estimating surface albedo and evapotranspiration. The ET values derived from the model are in good agreement with the values obtained with. ‘CENTURY MODEL’ and ground observations over the area, suggesting the possible use of this approach fot regional scale studies on evapotranspiration. 相似文献
88.
A. H. W. Kearsley R. Forsberg A. Olesen L. Bastos K. Hehl U. Meyer A. Gidskehaug 《Journal of Geodesy》1998,72(10):600-605
Two detailed geoids have been computed in the region of North Jutland. The first computation used marine data in the offshore
areas. For the second computation the marine data set was replaced by the sparser airborne gravity data resulting from the
AGMASCO campaign of September 1996. The results of comparisons of the geoid heights at on-shore geometric control showed that
the geoid heights computed from the airborne gravity data matched in precision those computed using the marine data, supporting
the view that airborne techniques have enormous potential for mapping those unsurveyed areas between the land-based data and
the off-shore marine or altimetrically derived data.
Received: 7 July 1997 / Accepted: 22 April 1998 相似文献
89.
90.
Diwakar Paudel Jay Krishna Thakur Sudhir Kumar Singh Prashant K. Srivastava 《国际地球制图》2015,30(2):218-241
Soil is a vital part of the natural environment and is always responding to changes in environmental factors, along with the influences of anthropogenic factors and land use changes. The long-term change in soil properties will result in change in soil health and fertility, and hence the soil productivity. Hence, the main aim of this paper focuses on the analysis of land use/land cover (LULC) change pattern in spatial and temporal perspective and to present its impact on soil properties in the Merawu catchment over the period of 18?years. Post classification change detection was performed to quantify the decadal changes in historical LULC over the periods of 1991, 2001 and 2009. The pixel to pixel comparison method was used to detect the LULC of the area. The key LULC types were selected for investigation of soil properties. Soil samples were analysed in situ to measure the physicochemical soil properties. The results of this study show remarkable changes in LULC in the period of 18?years. The effect of land cover change on soil properties, soil compaction and soil strength was found to be significant at a level of <0.05. 相似文献