全文获取类型
收费全文 | 29553篇 |
免费 | 494篇 |
国内免费 | 225篇 |
专业分类
测绘学 | 735篇 |
大气科学 | 2257篇 |
地球物理 | 5754篇 |
地质学 | 10866篇 |
海洋学 | 2555篇 |
天文学 | 6287篇 |
综合类 | 65篇 |
自然地理 | 1753篇 |
出版年
2021年 | 273篇 |
2020年 | 299篇 |
2019年 | 302篇 |
2018年 | 727篇 |
2017年 | 726篇 |
2016年 | 840篇 |
2015年 | 467篇 |
2014年 | 802篇 |
2013年 | 1558篇 |
2012年 | 925篇 |
2011年 | 1223篇 |
2010年 | 1053篇 |
2009年 | 1363篇 |
2008年 | 1184篇 |
2007年 | 1173篇 |
2006年 | 1160篇 |
2005年 | 841篇 |
2004年 | 839篇 |
2003年 | 767篇 |
2002年 | 768篇 |
2001年 | 712篇 |
2000年 | 701篇 |
1999年 | 550篇 |
1998年 | 518篇 |
1997年 | 598篇 |
1996年 | 468篇 |
1995年 | 473篇 |
1994年 | 471篇 |
1993年 | 380篇 |
1992年 | 387篇 |
1991年 | 357篇 |
1990年 | 373篇 |
1989年 | 342篇 |
1988年 | 344篇 |
1987年 | 368篇 |
1986年 | 325篇 |
1985年 | 423篇 |
1984年 | 414篇 |
1983年 | 445篇 |
1982年 | 415篇 |
1981年 | 365篇 |
1980年 | 402篇 |
1979年 | 324篇 |
1978年 | 302篇 |
1977年 | 294篇 |
1976年 | 266篇 |
1975年 | 264篇 |
1974年 | 269篇 |
1973年 | 254篇 |
1971年 | 173篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
K. Nageswara Rao G. Murali Krishna D. Ramprasad Naik B. Hema Malini 《Journal of the Indian Society of Remote Sensing》2003,31(2):71-80
Interpretation of IRS LISS II and LISS III imagery has revealed the various landforms as well as land use/land cover features in a part of the Godavari delta coastal belt. A comparative analysis of geomorphological vs. land use/land cover maps suggested that the landforms exert a certain degree of control over human land use activities even in this monotonously plain area. Further, an analysis of the sequential imagery pertaining to 1992 and 2001 aimed at detecting the land use/land cover change has indicated that the aquaculture has phenomenally increased by 9,293.5 ha during the 9-year period. At the same time, the cropland which occupied about 29,104 ha in 1992 has been reduced to 19,153.9 ha by 2001 mainly due to the encroachment of aquaculture. Village level data on temporal variation in land use/land cover extracted through GIS analysis revealed that in 14 out of the total 39 villages in the area, the conversion of cropland into aquaculture ponds was more than 30% with the highest conversion rate of 89.8% in Gondi village. These fourteen villages, which are designated as ‘aquaculture hotspots’ are grouped into 4 priority classes based on the intensity of conversion. 相似文献
892.
High-resolution simulations of global climate, part 2: effects of increased greenhouse cases 总被引:6,自引:0,他引:6
We report results from the highest-resolution simulations of global warming yet performed with an atmospheric general circulation model. We compare the climatic response to increased greenhouse gases of the National Center for Atmospheric Research (NCAR) climate model, CCM3, at T42 and T170 resolutions (horizontal grid spacing of 300 and 75 km respectively). All simulations use prescribed sea surface temperatures (SST). Simulations of the climate of 2100 ad use SSTs based on those from NCAR coupled model, Climate System Model (CSM). We find that the global climate sensitivity and large-scale patterns of climate change are similar at T42 and T170. However, there are important regional scale differences that arise due to better representation of topography and other factors at high resolution. Caution should be exercised in interpreting specific features in our results both because we have performed climate simulations using a single atmospheric general circulation model and because we used with prescribed sea surface temperatures rather than interactive ocean and sea-ice models. 相似文献
893.
The Multi frequency Scanning Microwave Radiometer (MSMR) onboard Oceasat-1 was used to develop a retrieval method fornear-surface specific humidity by means of multivariate regressiontechnique. The MSMR measures the microwaveradiances in 8 channels at the frequencies of 6.6, 10.7, 18 and 21 GHzfor both vertical and horizontal polarizations. Regression coefficients were derived using the ship reports of the Comprehensive Ocean-Atmosphere Data Set (COADS) for the months of July, October and December, in 1999. Daily near-surface specific humidity data from COADS in 2° × 2° latitude/longitude bins and collocated brightness temperature data from MSMR were used to derive the coefficients. The derived coefficients werevalidated with humidity given in COADS.A linear relationship is established to determine the near-surface specifichumidity from MSMR brightness temperature (Tb) with an rms error of 1.2 g kg-1 for individual situations and an rms errorof 0.84 g kg-1 for monthly time scales over global oceans.The retrieval algorithm is validonly for the open sea regions. 相似文献
894.
M. Krämer Ri. Müller H. Bovensmann J. Burrows J. Brinkmann E. P. Röth J.-U. Grooß Ro. Müller Th. Woyke R. Ruhnke G. Günther J. Hendricks E. Lippert K. S. Carslaw Th. Peter A. Zieger Ch. Brühl B. Steil R. Lehmann D. S. McKenna 《Journal of Atmospheric Chemistry》2003,45(1):51-77
Several stratospheric chemistry modules from box, 2-D or 3-D models, have been intercompared. The intercomparison was focused on the ozone loss and associated reactive species under the conditions found in the cold, wintertime Arctic and Antarctic vortices. Comparisons of both gas phase and heterogeneous chemistry modules show excellent agreement between the models under constrained conditions for photolysis and the microphysics of polar stratospheric clouds. While the mean integral ozone loss ranges from 4–80% for different 30–50 days long air parcel trajectories, the mean scatter of model results around these values is only about ±1.5%. In a case study, where the models employed their standard photolysis and microphysical schemes, the variation around the mean percentage ozone loss increases to about ±7%. This increased scatter of model results is mainly due to the different treatment of the PSC microphysics and heterogeneous chemistry in the models, whereby the most unrealistic assumptions about PSC processes consequently lead to the least representative ozone chemistry. Furthermore, for this case study the model results for the ozone mixing ratios at different altitudes were compared with a measured ozone profile to investigate the extent to which models reproduce the stratospheric ozone losses. It was found that mainly in the height range of strong ozone depletion all models underestimate the ozone loss by about a factor of two. This finding corroborates earlier studies and implies a general deficiency in our understanding of the stratospheric ozone loss chemistry rather than a specific problem related to a particular model simulation. 相似文献
895.
B. Kasietczuk 《Journal of Geodesy》1997,71(3):131-136
This paper presents an extension of the geodetic network adjustment model. The proposed extension makes possible the estimation
of the 3-rd and 4-th central moments for the vector of measurement errors in the process of network adjustment by the least-squares
method with application of orthogonal matrices. It allows to estimate the asymmetry and kurtosis of the measurement errors
distribution.
Received 13 April 1993; Accepted 8 July 1996 相似文献
896.
This paper explores the potential of a digital camera to produce multiple images suitable for plotting. The objective was to build a three dimensional database by linking triangulated images from a Kodak DCS420 digital camera with a computer aided measurement system. The system then supplied X, Y, Z data from x, y image co-ordinates captured off two or more images. Relative accuracy in object space was around 1;50 000. 相似文献
897.
介绍了正在建设中的全球湖泊数据库的情况,以及该数据库的水位资料所反映的晚更新世末期以来全球湿润状况的变化。通过大尺度湖水位变化的时空分析,结果显示当今湖泊水位状态较历史时期而言,位于低纬干旱或半干旱地区的湖泊水位较低,而中纬及高纬湿润地区的湖泊水位较高。自末次冰盛期以来,北美大陆中南部地区湖泊水位自高至低,反映出该地区气候条件由湿变干,至早、中全新世达最干旱。而在非洲及南亚季风地区,冷期偏干,暖期偏湿。特别是在早、中全新世的温暖时期,为历史上最湿润时期。北半球中纬度地带的气候干湿变化与北半球冰盖的存在及其消融导致的西风带的南北摆动有关;而北半球季风区在早、中全新世出现的高潮面与北半球夏季辐射的增加有密切联系。 相似文献
898.
A method is described for evaluating the ‘partial derivatives’ of globally averaged top-of-atmosphere (TOA) radiation changes
with respect to basic climate model physical parameters. This method is used to analyse feedbacks in the Australian Bureau
of Meteorology Research Centre general circulation model. The parameters considered are surface temperature, water vapour,
lapse rate and cloud cover. The climate forcing which produces the changes is a globally uniform sea surface temperature (SST)
perturbation. The first and second order differentials of model parameters with respect to the forcing (i.e. SST changes)
are estimated from quadratic least square fitting. Except for total cloud cover, variables are found to be strong functions
of global SST. Strongly non-linear variations of lapse rate and high cloud amount and height appear to relate to the non-linear
response in penetrative convection. Globally averaged TOA radiation differentials with respect to model parameters are also
evaluated. With the exception of total cloud contributions, a high correlation is generally found to exist, on the global
mean level, between TOA radiation and the respective parameter perturbations. The largest non-linear terms contributing to
radiative changes are those due to lapse rate and high cloud. The contributions of linear and non-linear terms to the overall
radiative response from a 4 K SST perturbation are assessed. Significant non-linear responses are found to be associated with
lapse rate, water vapour and cloud changes. Although the exact magnitude of these responses is likely to be a function of
the particular model as well as the imposed SST perturbation pattern, the present experiments flag these as processes which
cannot properly be understood from linear theory in the evaluation of climate change sensitivity.
Received: 16 January 1997/Accepted: 9 May 1997 相似文献
899.
H. E. Scheel H. Areskoug H. Geiss B. Gomiscek K. Granby L. Haszpra L. Klasinc D. Kley T. Laurila A. Lindskog M. Roemer R. Schmitt P. Simmonds S. Solberg G. Toupance 《Journal of Atmospheric Chemistry》1997,28(1-3):11-28
Surface ozone data from 25 Europeanlow-altitude sites and mountain sites located between79°N and 28°N were studied. The analysiscovered the time period March 1989–February 1993.Average summer and winter O3 concentrations inthe boundary layer over the continent gave rise togradients that were strongest in the north-west tosouth-east direction and west-east direction, respectively. WintertimeO3 ranged from 19 to 27 ppbover the continent, compared to about 32 ppb at thewestern border, while for summer the continentalO3 values ranged between 39 and 56 ppb and theoceanic mixing ratios were around 37 ppb. In the lowerfree troposphere average wintertime O3 mixingratios were around 38 ppb, with only an 8 ppbdifference between 28°N and 79°N. For summerthe average O3 levels decreased from about 55 ppbover Central Europe to 32 ppb at 79°N. Inaddition, O3 and Ox(= O3 + NO2)in polluted and clean air were compared. Theamplitudes of the seasonal ozone variations increasedin the north-west to south-east direction, while thetime of the annual maximum was shifted from spring (atthe northerly sites) to late summer (at sites inAustria and Hungary), which reflected the contributionof photochemical ozone production in the lower partsof the troposphere. 相似文献
900.
A widely used method of evaluating effects of climate change on flow regime is to perturb the climate inputs to a rainfall–runoff model and examine the effect on a statistic of the modelled flows. Such studies require four elements: a method of perturbing the climate, a rainfall–runoff model, a study catchment and a flow index. In practice the direction and magnitude of the estimated effects depend on each of the four elements, leading to concern over the usefulness and generality of the results. To investigate these uncertainties two climate scenarios and eight climate sensitivity tests have been applied to three UK catchments using two conceptual rainfall–runoff models in order to quantify effects of climate change on three flow indices representing mean runoff, flood magnitudes and low flows. The sensitivity tests were found to be useful to assess the suitability of the models to simulate flows outside the conditions experienced in their calibration. Both models gave internally consistent results but, on close examination, one model was found inappropriate for this application. Results show that the effect of climate change on flow varies between catchments and that different flow response indices can change in opposite directions, e.g. floods increased in magnitude while low flows reduced. Contrasting results were obtained from the two climate scenarios. 相似文献