首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   38707篇
  免费   737篇
  国内免费   237篇
测绘学   747篇
大气科学   2578篇
地球物理   7620篇
地质学   14057篇
海洋学   3545篇
天文学   8715篇
综合类   86篇
自然地理   2333篇
  2022年   257篇
  2021年   437篇
  2020年   498篇
  2019年   576篇
  2018年   1010篇
  2017年   1014篇
  2016年   1078篇
  2015年   581篇
  2014年   1029篇
  2013年   1849篇
  2012年   1190篇
  2011年   1629篇
  2010年   1442篇
  2009年   1795篇
  2008年   1629篇
  2007年   1688篇
  2006年   1556篇
  2005年   1058篇
  2004年   1075篇
  2003年   1112篇
  2002年   995篇
  2001年   868篇
  2000年   797篇
  1999年   724篇
  1998年   715篇
  1997年   723篇
  1996年   587篇
  1995年   572篇
  1994年   499篇
  1993年   453篇
  1992年   408篇
  1991年   423篇
  1990年   437篇
  1989年   390篇
  1988年   369篇
  1987年   398篇
  1986年   415篇
  1985年   507篇
  1984年   545篇
  1983年   542篇
  1982年   496篇
  1981年   454篇
  1980年   429篇
  1979年   408篇
  1978年   375篇
  1977年   384篇
  1976年   344篇
  1975年   353篇
  1974年   340篇
  1973年   370篇
排序方式: 共有10000条查询结果,搜索用时 739 毫秒
431.
432.
This is a study of the sensitivity of model results (atmospheric content of main gas constituents and radiative characteristics of the atmosphere) to errors in emissions of a number of atmospheric gaseous pollutants. Groups of the model variables most dependent on these errors are selected. Two variants of emissions are considered: one without their evolution and the other with their variation according to the IPCC scenario. The estimates are made on the basis of standard statistical methods for the results obtained with the detailed onedimensional radiative—photochemical model of the Main Geophysical Observatory (MGO). Some approaches to such estimations with models of higher complexity and to the solution of the inverse problem (i.e., the estimation of the necessary accuracy of external model parameters for obtaining the given accuracy of model results) are outlined.  相似文献   
433.
Particle-bound phosphorus along an urbanized coastal plain estuary   总被引:1,自引:0,他引:1  
The distribution of particle-bound phosphorus in the suspended sediment of the Delaware Estuary was examined with a sequential chemical leaching technique. The phosphorus content of particles was highest in the tidal river (140–250 μmol g−1) near major anthropogenic inputs. Despite this enrichment of river particles with phosphorus, suspended particles within the salinity gradient had a phosphorus content more similar to the world's average. Sequential chemical leaches revealed that particulate phosphorus was associated with organic matter, aluminum oxides, iron oxides, and apatite in all areas of the estuary. However, ‘excess’ particle-bound phosphorus in the tidal river was associated mainly with iron oxides (27%), aluminum oxides (23%), and organic matter (50%). Within the salinity gradient, particulate phosphorus associated with iron oxides, aluminum oxides, and apatite all decreased with increasing salinity. Estuarine mixing was simulated to determine whether the observed decreases in particle-bound phosphorus pools in field samples were due to release into solution. During simulated mixing, particulate phosphorus associated with iron and aluminum oxides decreased, but no change was observed in apatite-bound phosphorus. The results of the mixing study combined with the observed particle-bound phosphorus distributions suggest that phosphate concentrations along the Delaware Estuary may be partially ‘buffered’ by aluminum and iron oxide phases.  相似文献   
434.
The Marine Physical Laboratory of the Scripps Institution of Oceanography has developed an acoustic relay transponder for precise relative positioning of near-bottom instruments and geologic sampling devices. Although specifically designed to position equipment lowered on standard wire ropes without a need to maintain direct electrical contact with the surface ship, the relay transponder may be used to track free vehicles, such as deep submersibles, from the surface. The relay transponder is positioned relative to an array of bottom-anchored acoustic transponders. It is interrogated acoustically from the surface ship; it then sequentially interrogates the bottom transponders which, in turn, reply to the ship. From the measurement of the total travel time (ship to relay transponder to bottom transponder to ship) and assuming, or knowing, the sound velocity of the water, we obtain a relayed range measurement. These relayed ranges, used in conjunction with ship to bottom-transponder ranges, allow us to calculate the position of the relay transponder. A recent application of this technique is described in which several gravity core samples from the crest of the Horizon Guyot were positioned with respect to the detailed bathymetry and the geology within the area. The estimated error in positioning the samples is less than 20 m inside a navigational net extending over 100 km2.Contribution of the Scripps Institution of Oceanography, new series.  相似文献   
435.
Data collected from 20 years of observations by the P. P. Shirshov Institute of Oceanology (about 250 comprehensive ecosystem stations) are summarized to evaluate the primary production and biomass of the principal groups of plankton in the Pacific Ocean. The stations were classified into geographical regions according to satellite determined criteria. The areas of these regions were evaluated according to CZCS data from 1978 to 1986. The total value of primary production was evaluated as 26·9 Gt C year−1. Taking the “bottle effect” correction into account, this value may be as high as 45.6 Gt C year−1. Total biomass values of phytoplankton (134 Mt C), bacteria (86 Mt C), protozoa (26 Mt C) and mesoplankton (184 Mt C) were also calculated. Seasonal variabilities of all these values were remarkably insignificant.  相似文献   
436.
The problem of simulation of possible scenarios of the development of processes running in ecological economic systems is studied as a part of the general problem of control over natural economic complexes. The consumption of mineral, food, and ecological resources is regarded as credits given by the society to the producer and then refunded as a part of his gains. If this nature-protective tax does not compensate the damage to the environment caused by production, then the debt of the producer (called accumulated credit) appears and, finally, the process of consumption of the resources is terminated. A universal simulation model is constructed by using a new method of adaptive balance of causes (ABC method). We present basic concepts of this method and some examples of application of the simulation model to the analysis of various scenarios of consumption of marine bioresources under variable economic conditions for a marine environment with variable assimilation capacity.  相似文献   
437.
438.
It is often desirable or necessary to store collected seawater samples prior to analysis for dissolved inorganic nutrients. It is therefore important to establish preservation and storage techniques that will ensure sample integrity and will not alter the precision or accuracy of analysis. We have performed a series of experiments on the storage of nutrient samples collected at the oligotrophic North Pacific benchmark Station ALOHA, using both standard autoanalyses and low-level techniques. Our results reveal that for oligotrophic oceanic waters, the immediate freezing of an unfiltered water sample in a clean polyethylene bottle is a suitable preservation method. This procedure is simple, it avoids potentially contaminating sample manipulations and chemical additions, and it adequately preserves the concentrations of nitrate + nitrite, soluble reactive phosphate, and soluble reactive silicate within a single water sample.  相似文献   
439.
Nepheloid layers and internal waves over continental shelves and slopes   总被引:2,自引:0,他引:2  
Cacchione  D. A.  Drake  D. E. 《Geo-Marine Letters》1986,6(3):147-152
Theoretical and laboratory results indicate that bottom velocities within shoaling internal gravity waves intensify upslope approximately inversely proportional to the water depth. The elevated velocities (and bottom stresses) caused by shoaling and, possibly, breaking internal waves might explain the generation and maintenance of near-bottom nepheloid zones and attached turbid plumes that have been observed over certain continental shelves and slopes. This process is proposed as an explanation of zones of relatively low transmissibility that emanate from the upper continental slope near Newport submarine canyon off southern California.  相似文献   
440.
A method of determination of atmospheric dynamic characteristics from the data of remote sensing from a geostationary satellite is described. The method is based on the use of inhomogeneities in the concentration field of a conservative additive as tracers and on the application of correlation-extreme algorithms. Unlike the common methods used abroad, this method is able to determine not only the vector field of wind velocity but also the coefficient of turbulent diffusion and vorticity. Results of computations of the fields of the horizontal component of wind velocity and the effective coefficient of horizontal mesoscale turbulent diffusion from the Meteosat-8 SEVIRI water-vapor channel data are presented. It is shown that the average values of the effective coefficient of mesoscale horizontal turbulent diffusion in the areas with a predominantly turbulized air-mass motion are 1.5 times greater than in the areas where a laminar motion dominates. Specific features of the calculated values of the upper-troposphere dynamic characteristics in different stages of the North Atlantic TC Helene (September 2006) are analyzed.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号