全文获取类型
收费全文 | 17172篇 |
免费 | 3186篇 |
国内免费 | 3674篇 |
专业分类
测绘学 | 935篇 |
大气科学 | 1873篇 |
地球物理 | 4497篇 |
地质学 | 9427篇 |
海洋学 | 2876篇 |
天文学 | 559篇 |
综合类 | 1143篇 |
自然地理 | 2722篇 |
出版年
2024年 | 138篇 |
2023年 | 319篇 |
2022年 | 522篇 |
2021年 | 770篇 |
2020年 | 742篇 |
2019年 | 818篇 |
2018年 | 678篇 |
2017年 | 731篇 |
2016年 | 706篇 |
2015年 | 821篇 |
2014年 | 1025篇 |
2013年 | 1289篇 |
2012年 | 981篇 |
2011年 | 1132篇 |
2010年 | 1042篇 |
2009年 | 1091篇 |
2008年 | 1099篇 |
2007年 | 1130篇 |
2006年 | 1174篇 |
2005年 | 994篇 |
2004年 | 944篇 |
2003年 | 820篇 |
2002年 | 743篇 |
2001年 | 643篇 |
2000年 | 593篇 |
1999年 | 470篇 |
1998年 | 446篇 |
1997年 | 400篇 |
1996年 | 318篇 |
1995年 | 291篇 |
1994年 | 270篇 |
1993年 | 206篇 |
1992年 | 159篇 |
1991年 | 124篇 |
1990年 | 82篇 |
1989年 | 93篇 |
1988年 | 54篇 |
1987年 | 38篇 |
1986年 | 26篇 |
1985年 | 30篇 |
1984年 | 17篇 |
1983年 | 14篇 |
1982年 | 5篇 |
1981年 | 11篇 |
1980年 | 11篇 |
1979年 | 4篇 |
1978年 | 14篇 |
1973年 | 1篇 |
1971年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
1.
Forests in the Southeastern United States are predicted to experience future changes in seasonal patterns of precipitation inputs as well as more variable precipitation events. These climate change‐induced alterations could increase drought and lower soil water availability. Drought could alter rooting patterns and increase the importance of deep roots that access subsurface water resources. To address plant response to drought in both deep rooting and soil water utilization as well as soil drainage, we utilize a throughfall reduction experiment in a loblolly pine plantation of the Southeastern United States to calibrate and validate a hydrological model. The model was accurately calibrated against field measured soil moisture data under ambient rainfall and validated using 30% throughfall reduction data. Using this model, we then tested these scenarios: (a) evenly reduced precipitation; (b) less precipitation in summer, more in winter; (c) same total amount of precipitation with less frequent but heavier storms; and (d) shallower rooting depth under the above 3 scenarios. When less precipitation was received, drainage decreased proportionally much faster than evapotranspiration implying plants will acquire water first to the detriment of drainage. When precipitation was reduced by more than 30%, plants relied on stored soil water to satisfy evapotranspiration suggesting 30% may be a threshold that if sustained over the long term would deplete plant available soil water. Under the third scenario, evapotranspiration and drainage decreased, whereas surface run‐off increased. Changes in root biomass measured before and 4 years after the throughfall reduction experiment were not detected among treatments. Model simulations, however, indicated gains in evapotranspiration with deeper roots under evenly reduced precipitation and seasonal precipitation redistribution scenarios but not when precipitation frequency was adjusted. Deep soil and deep rooting can provide an important buffer capacity when precipitation alone cannot satisfy the evapotranspirational demand of forests. How this buffering capacity will persist in the face of changing precipitation inputs, however, will depend less on seasonal redistribution than on the magnitude of reductions and changes in rainfall frequency. 相似文献
2.
根据印度洋西北部鸢乌贼(Sym plectoteuthis oualaniensis)作业渔场的现场调查数据.分析了作业水深、作业时段、摄食等级和月相等因子对手钓上钩率的影响。统计表明.晚上、午夜和凌晨3个不同时段和不同作业水深对手钓上钩率均有显著性影响。3个时段中,其摄食等级呈先低后高再低的现象。午夜和凌展,上钩率的变化趋势与摄食等级的变化趋势呈显著的负相关。在月相的影响下.离朔日三四天之前,手钓产量呈上升趋势;离望日五六天之前。手钓产量呈明显的下降趋势。 相似文献
3.
本文以丰富的实际资料,论证了地下水的卤素元素(F、Cl、Br、I)的形成、含量及其分布规律与含水介质成分、上覆岩土性质、地下水退流条件、氧化还原环境、地下水矿化度之间的关系。根据江汉平原东部区和鄱阳湖区地下水中Br、I元素的调查研究结果及它们形成的控制因素与分布规律,结合长江三角洲南部区水文地球化学环境条件分析对比,指出该区是一个形成Br、I矿泉水的有利地区。 相似文献
4.
A. G. Nikoghossian 《Astrophysics》2006,49(2):228-239
The problem of finding statistical averages which characterize the radiation field in a semi-infinite atmosphere has been
discussed by the author in a series of papers. In this paper some of the results obtained there are generalized to media of
finite optical thickness. The average number of scattering events undergone by a photon during its random walk in an atmosphere
and the average time required for this are determined. Three different types of averages are considered, depending on the
photon characteristics over which the averages are taken. Special attention is devoted to the asymptotic behavior of these
quantities for media with large optical depths taking the influence of continuum absorption into account.
__________
Translated from Astrofizika, Vol. 49, No. 2, pp. 263–275 (May 2006). 相似文献
5.
A. G. Nikoghossian 《Astrophysics》2004,47(3):412-421
The approach proposed in the previous parts of this series of papers is used to solve the radiative transfer problem in scattering and absorbing multicomponent atmospheres. Linear recurrence relations are obtained for both the reflectance and transmittance of these kinds of atmospheres, as well as for the emerging intensities when the atmosphere contains energy sources. Spectral line formation in a one-dimensional inhomogeneous atmosphere is examined as an illustration of the possibility of generalizing our approach to the matrix case. It is shown that, in this case as well, the question reduces to solving an initial value problem for linear differential equations. Some numerical calculations are presented. 相似文献
6.
This paper proposes a multi‐level parallelized substructuring–frontal combined algorithm for the analysis of the problem of thermo/hydraulic/mechanical behaviour of unsaturated soil. Temperature, displacement, pore water pressure and pore air pressure are treated as the primary variables in a non‐linear analysis. Details are given firstly of the substructuring–frontal combined approach. The incorporation of the algorithm in a multi‐level parallel strategy is then discussed. The parallel processing can thus be carried out at different substructural levels. The method thus developed impacts, in a positive way, on both computer storage requirement and execution time. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
7.
8.
CHEN Bing-lu ZHANG Yun-ni CHEN Xin-geng WANG Zhi-gang YANG Guang-xing 《中国地理科学(英文版)》2001,11(2):155-162
1 HYDROLOGIC FEATURES Lingdingyang Estuary, located at the middle south of Guangdong Province, is a bell-shaped estuary with a north-south direction. Its area is about 2100km2. The north of Qi′ao Island and Inner-Lingding Island, and the south of Humen are grouped as Neilingdingyang Estuary, having an area of 1041km2. Affected by topography, runoff and tide, its dynamic condition is very complicated. Different water areas have different hydrologic features. The topography under … 相似文献
9.
1 INTRODUCTIONIn the previous paper (on et al. 1999, hereafter Paper I), we investigated the wavelength-dependence of four colltribution functions (CFs) derived from dmerent formal solutions andreferring to different emergellt quantities in the unpolarized case. Because one cannot generallyassign a single formation region to the whole line band in a real stellar atmosphere, e.g., the solaratmosphere, instead, the line formation region can be defined as the layers deviating farthestfrom t… 相似文献
10.
D.W. Crawford D.A. Purdie A.P.M. Lockwood P. Weissman 《Estuarine, Coastal and Shelf Science》1997,45(6):799-812
In the Southampton Water estuary (southern England, U.K.), red-tides caused by the planktonic, phototrophic ciliateMesodinium rubrum(=Myrionecta rubra) occur during most summers and sometimes in autumn. These events were investigated in detail between 1985 and 1987 and were characterized by levels of chlorophylla(chl a) of over 100 μg l−1, cell numbers ofM. rubrumof over 1×103 ml−1, oxygen saturations of around 150%, and depleted numbers of macrozooplankton. Initiation of red-water did not appear to be triggered by irradiance or nutrients, but coincided with an increase in temperature and water column stability. This enhanced stability was promoted by increased surface to bottom gradients of both temperature and salinity, and by reduced mixing during neap tides. Development of red-water was accompanied by removal of most of the dissolved NH+4from the water column, whereas some NO−3persisted, presumably maintained by freshwater input. NO−3and NH+4gradually returned to pre-bloom concentrations as the red-water declined in late summer. Maximal biomass ofM. rubrumappeared to be limited by irradiance, and self-shading probably imposed an upper limit of around 300 mg chl a m−2within the water column. At the observed levels of chl a, irradiance values within the population maximum between 1 and 3 m depth were only just of the order (≈15 μmol photons m−2 s−1) required to balance estimated respiratory demands. Oxygen concentration became undersaturated during the late bloom phase, with minimal values of 20–30% saturation recorded in deeper waters; however, despite this and reduced numbers of macrozooplankton, direct deleterious effects on other organisms were not observed. 相似文献