全文获取类型
收费全文 | 1750篇 |
免费 | 44篇 |
国内免费 | 24篇 |
专业分类
测绘学 | 57篇 |
大气科学 | 104篇 |
地球物理 | 367篇 |
地质学 | 613篇 |
海洋学 | 115篇 |
天文学 | 379篇 |
综合类 | 17篇 |
自然地理 | 166篇 |
出版年
2022年 | 8篇 |
2021年 | 9篇 |
2020年 | 23篇 |
2019年 | 23篇 |
2018年 | 32篇 |
2017年 | 24篇 |
2016年 | 32篇 |
2015年 | 26篇 |
2014年 | 41篇 |
2013年 | 77篇 |
2012年 | 51篇 |
2011年 | 72篇 |
2010年 | 92篇 |
2009年 | 98篇 |
2008年 | 83篇 |
2007年 | 72篇 |
2006年 | 63篇 |
2005年 | 60篇 |
2004年 | 52篇 |
2003年 | 67篇 |
2002年 | 59篇 |
2001年 | 28篇 |
2000年 | 41篇 |
1999年 | 27篇 |
1998年 | 20篇 |
1997年 | 23篇 |
1996年 | 22篇 |
1995年 | 24篇 |
1994年 | 28篇 |
1993年 | 29篇 |
1992年 | 24篇 |
1990年 | 22篇 |
1989年 | 25篇 |
1988年 | 26篇 |
1987年 | 24篇 |
1986年 | 21篇 |
1985年 | 36篇 |
1984年 | 35篇 |
1983年 | 30篇 |
1982年 | 26篇 |
1981年 | 38篇 |
1980年 | 25篇 |
1979年 | 20篇 |
1978年 | 17篇 |
1977年 | 22篇 |
1976年 | 15篇 |
1975年 | 15篇 |
1974年 | 22篇 |
1973年 | 18篇 |
1971年 | 8篇 |
排序方式: 共有1818条查询结果,搜索用时 15 毫秒
101.
The Qinghai-Tibet Plateau encompasses a large quantity of wetlands, some of which have been degraded to varying severity levels. In the literature, a number of degradation indicators have been proposed to evaluate ecological health of wetlands, but their effectiveness in the plateau environment remains unknown. In this study, we assessed the effectiveness of three degradation indicators, soil moisture content at 10 cm deep, vegetative cover, and density of pika burrows. The degradation severity of wetlands in Maduo County on the Qinghai-Tibet Plateau is enumerated at four levels, intact, slight, moderate and severe. Analysis of 106 samples collected in the field demonstrates that the density of pika burrows is the least reliable indicator. By comparison, vegetative cover and underlying soil moisture content are more reliable, even though neither is a perfect indicator as the difference among adjacent levels of severity as revealed by t-test is not always statistically significant. The imperfection of vegetative cover as an indicator is due to its variation among different types of wetlands. The limitation of moisture content is attributed to its non-linear relationship with wetland degradation. Above the threshold of about 50% in moisture content wetlands are unlikely to be degraded. It is recommended that moisture be measured at the point near the surface and vegetative cover be further differentiated by species in order to improve their effectiveness. 相似文献
102.
Abstract The detailed characteristics of a CASPII warm frontal passage are presented in this article. This storm, Intensive Operating Period (IOP) 13 (February 26–27, 1992), was observed in detail with an array of diverse instruments. It has the advantage over earlier freezing precipitation studies of having simultaneous, in situ and remote sensing measurements by aircraft and ground‐based Doppler radar. The associated precipitation was in the form of banded structures parallel to the front. Within these bands were embedded precipitation cores, some parallel to the band, some perpendicular. The warm front itself was characterized by major perturbations in its kinematic and thermodynamic features. The cores oriented parallel to the front were the result of embedded convection generated, at least in part, by the irregularities in the frontal surface. The cores oriented perpendicular to the front were closely associated with the 0°C isotherm on the underside of the frontal inversion. Precipitation phase changes played a significant role in the occurrence of wide near 0°C regions, both vertically and horizontally. These regions had a profound influence on the observed precipitation types and led to complex precipitation‐thermodynamic‐dynamic interactions. Instabilities produced by these interactions are seen in wave‐like features observed by the Doppler radar in these regions, both parallel and perpendicular to the frontal zone. 相似文献
103.
Nathalie van Vliet Ole Mertz Andreas Heinimann Tobias Langanke Unai Pascual Birgit Schmook Cristina Adams Dietrich Schmidt-Vogt Peter Messerli Stephen Leisz Jean-Christophe Castella Lars Jørgensen Torben Birch-Thomsen Cornelia Hett Thilde Bech-Bruun Amy Ickowitz Kim Chi Vu Kono Yasuyuki Jefferson Fox Christine Padoch Alan D. Ziegler 《Global Environmental Change》2012,22(2):418-429
This meta-analysis of land-cover transformations of the past 10–15 years in tropical forest-agriculture frontiers world-wide shows that swidden agriculture decreases in landscapes with access to local, national and international markets that encourage cattle production and cash cropping, including biofuels. Conservation policies and practices also accelerate changes in swidden by restricting forest clearing and encouraging commercial agriculture. However, swidden remains important in many frontier areas where farmers have unequal or insecure access to investment and market opportunities, or where multi-functionality of land uses has been preserved as a strategy to adapt to current ecological, economic and political circumstances. In some areas swidden remains important simply because intensification is not a viable choice, for example when population densities and/or food market demands are low. The transformation of swidden landscapes into more intensive land uses has generally increased household incomes, but has also led to negative effects on the social and human capital of local communities to varying degrees. From an environmental perspective, the transition from swidden to other land uses often contributes to permanent deforestation, loss of biodiversity, increased weed pressure, declines in soil fertility, and accelerated soil erosion. Our prognosis is that, despite the global trend towards land use intensification, in many areas swidden will remain part of rural landscapes as the safety component of diversified systems, particularly in response to risks and uncertainties associated with more intensive land use systems. 相似文献
104.
105.
Alan K. Whitfield 《Estuaries and Coasts》1988,11(3):152-159
The Swartvlei estuary possesses a prolific growth of both intertidal and subtidal eelgrass,Zostera capensis. During 1984 less than 12% of the eelgrass beds were located in the upper half of the estuary, yet deposition ofZostera/macroalgal wrack in this region, when the estuary was linked to the sea (open phase), was similar to that in the lower half. Over a period of 20 semidiurnal tidal cycles there was a net gain of 2.5 tonnes dry mass of plant material into the upper reaches. Export of aquatic macrophytes and filamentous algae from the lower reaches toward the sea over 20 tidal cycles amounted to 1.6 tonnes dry mass. The amount of plant material transported during spring tides was 2 to 3 times greater than that carried during neap tides. Shallowing of the estuary mouth due to sand deposition resulted in a decline in the tidal prism and a decrease in macrodetrital flux. Total export ofZostera and associated algae amounted to 0.87 g ash-free dry mass m?2d?1 and represented a monthly export of 18% ofZostera bed biomass. Deposition of plant wrack during the 1984–1985 closed phase amounted to 63 g dry mass per meter of shore per day at the lower reaches site but only 10 g m?1d?1 was recorded at the upper reaches site. The relatively low latter value was attributed to the absence of tidal action which transports macrodetritus from the lower and middle reaches into the upper part of the system. During the 1984 open phase 70 g m?1d?1 was deposited at the lower reaches site and 68 g m?1d?1 at the upper reaches site. The role of tides in the redistribution of aquatic macrophyte primary production in the Swartvlei estuary was therefore clearly underscored. 相似文献
106.
Summary Continuous releases of fluorescent dye were made in the southern North Sea and the power law dependence of the plume width, represented by
y
, against the diffusion time determined. The spreading was non-Fickian and could be represented in the form
y
2
=B
2
t whereB was a diffusion velocity of magnitude 1.4×10–2 m/s. Such spreading was reproduced in both particle tracking and finite difference plume models by allowing the horizontal diffusivity,K
H, to depend linearly on diffusion time. The weakness of this method is that it is not clear how the diffusion parameters can be extrapolated to weather conditions that are different to those prevailing at the time of the experiments. However, comparable spreading rates were obtained by combining a Fickian diffusion model with an advective field that represented the near-surface current shears due to wind and waves. The resulting shear diffusion effect produced realistic simulation of the observed dispersion rates. An advantage of this approach is that it enables predictions to be made over a range of weather conditions provided that the wind and wave shears can be accurately parameterized.
相似文献
Ausbreitung von Tracerwolken an der Oberfläche in der südlichen Nordsee
Zusammenfassung Fluoreszierender Farbstoff wurde in der südlichen Nordsee kontinuierlich freigesetzt und die Breite der Wolke, repräsentiert durch y , in Abhägigkeit von der Ausbreitungszeit bestimmt. Die Ausbreitung folgte nicht dem Fickschen Ansatz und konnte in der Form y 2 =B 2 t dargestellt werden, wobeiB eine Ausbreitungsgeschwindigkeit von der Größenordnung 1.4×10–2 m/s darstellt. Eine derartige Ausbreitung wurde sowohl mit Modellen des Partikel-Tracking als auch mit Modellen der finiten Differenzen reproduziert, wobei die horizontale AusbreitungsgrößeK H linear von der Diffusionszeit abhängig war. Die Schwäche dieser Methode ist, daß es nicht klar ist, wie die Diffusions-Parameter für andere Wetterbedingungen als zur Zeit des Experiments zu extrapolieren wären. Vergleichbare Ausbreitungsverhältnisse werden durch die Kombination eines Fickschen Diffusionsmodells mit einem advektiven Feld erreicht, welches die oberflächennahen durch Wind und Wellen verursachten Stromscherungen berücksichtigt. Der so resultierende Scher-Diffusions-Effekt erbrachte ein realistische Simulation der beobachteten Ausbreitung. Ein Vorteil dieses Vorgehens ist, daß es Vorhersagen über einen Bereich von Wetterbedingungen ermöglicht, vorausgesetzt, daß die durch Wind und Wellen verurschte Scherung zutreffend parameterisiert werden kann.
La dispersion de la panache de surface dans la partie sud de la Mer du Nord
Résumé A partir de déversements continus de produits fluorescents effectués dans la partie sud de la Mer du Nord, on a pu déterminer l'expression de la largeur du panache y , par son carré en fonction du temps de diffusion. La dispersion n'était pas du type Fickian et était représentée sous la forme y 2 =B 2 t aB était une vitesse de diffusion de grandeur égale 1.4×10–2 m/s. Une telle dispersion peut être reproduite par une modélisation du panache à la fois par suivi de particules et aux différences finies en admettant que le coefficient de diffusion horizontaleK H dépendait linéairement du temps. L'inconvénient de cette méthode est qu'il n'est pas facile de savoir comment les paramètres de diffusion peuvent être extrapolés à des conditions météorologiques différentes de celles prévalant au moment des expériences. Quoi qu'il en soit, des taux de dispersion comparables ont été obtenus en combinant un modèle de diffusion Fickian avec un champ d'advection qui représentait les cisaillements de courant de surface dus au vent et aux vagues. L'effet de diffusion du cisaillement résultant produisit une simulation réaliste des taux de dispersion observés. Un avantage de cette approche est de rendre possible des prédictions réalisées à travers un éventail de conditions météorologiques à condition que les cisaillements liés au vent et aux vagues puissent être paramétrisés avec précision.
相似文献
107.
108.
Alan D. Hecht Director of the National Climate Program Office, an Office created by Congress in 1978 to coordinate a U.S. program of climate research and services. The office is located in the National Oceanic and Atmospheric Administration within the Department of Commerce; Bo R. Döös Member of the Swedish Academy of Sciences, is a visiting UCAR scientist (University Corporation for Atmospheric Research) in the NCPO. Views expressed in this editorial are of the authors and do not reflect any official views of UCAR or NOAA, DOC, or other federal agencies within the NCP. 相似文献
109.
Arsenic in Ground Water of the Western United States 总被引:12,自引:0,他引:12
110.
R. W. Parkinson T. C. Wang J. R. White J. R. David M. E. Hoffman 《Environmental Geology》1993,22(1):26-32
This project was designed to: (1) document the distribution and migration of organochlorine pesticide residues within marsh substrates of 18 St. Lucie County mosquito control impoundments located along the Indian River Lagoon estuary, and (2) evaluate the impact of water management techniques on residue mobility. Our results indicate that detectible concentrations of organochlorine compounds, applied between the late 1940s and early 1950s, are present in 16 of the 18 St. Lucie County mosquito control impoundments. These compounds are primarily restricted to the surficial, organic-rich wetland sediment, which, based upon geotechnical analysis, was exposed to the atmosphere at a time when the impoundments were subjected to pesticide treatment. Contaminated sediments are present below the surficial, organic-rich layer, suggesting that some vertical migration of pesticides has occurred. It is unlikely that leaching associated with the downward percolation of impounded water was responsible for this migration as pesticide residues were never detected within thein situ pore waters. An alternative explanation is that biological processes (e.g., rooting, burrowing) facilitated the downward flux of organochlorine compounds into sediment horizons not subjected to direct treatment. Eighty-eight surface water samples obtained from two impoundments subjected to contrasting water management techniques were analyzed for pesticide content. None of the surficial water samples collected in association with these impoundments contained detectible concentrations of organochlorine compounds. These samples were unfiltered and contained as much as 25 mg/1 of particulate organic matter. This suggests that the currently preferred management technique (RIM), which is designed to maintain water quality, limit mosquito production, and provide for ecological continuity, does not hydraulically mobilize pesticide residues into the Indian River Lagoon estuary. 相似文献