全文获取类型
收费全文 | 2686篇 |
免费 | 73篇 |
国内免费 | 49篇 |
专业分类
测绘学 | 273篇 |
大气科学 | 257篇 |
地球物理 | 492篇 |
地质学 | 1231篇 |
海洋学 | 132篇 |
天文学 | 320篇 |
综合类 | 48篇 |
自然地理 | 55篇 |
出版年
2023年 | 16篇 |
2022年 | 52篇 |
2021年 | 59篇 |
2020年 | 62篇 |
2019年 | 65篇 |
2018年 | 244篇 |
2017年 | 226篇 |
2016年 | 215篇 |
2015年 | 128篇 |
2014年 | 188篇 |
2013年 | 248篇 |
2012年 | 154篇 |
2011年 | 150篇 |
2010年 | 131篇 |
2009年 | 130篇 |
2008年 | 117篇 |
2007年 | 71篇 |
2006年 | 65篇 |
2005年 | 45篇 |
2004年 | 36篇 |
2003年 | 29篇 |
2002年 | 21篇 |
2001年 | 16篇 |
2000年 | 26篇 |
1999年 | 19篇 |
1998年 | 14篇 |
1997年 | 16篇 |
1996年 | 9篇 |
1995年 | 9篇 |
1994年 | 18篇 |
1993年 | 13篇 |
1992年 | 6篇 |
1991年 | 23篇 |
1990年 | 18篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 17篇 |
1986年 | 14篇 |
1985年 | 15篇 |
1984年 | 11篇 |
1982年 | 6篇 |
1980年 | 5篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1975年 | 6篇 |
1974年 | 11篇 |
1973年 | 5篇 |
1972年 | 8篇 |
1971年 | 5篇 |
1969年 | 5篇 |
排序方式: 共有2808条查询结果,搜索用时 15 毫秒
881.
Sudhir Kumar Subrata Halder D. C. Singhal 《Journal of the Geological Society of India》2011,78(6):587-598
Groundwater utilization in Nadia district, West Bengal has been subjected to rapid exploitation in the wake of increasing
urbanization and production of agricultural commodities. It is, therefore, necessary to evaluate the existing trend and availability
of groundwater in time and space and its movement for proper planning in future. In the present study, an attempt has been
made develop a groundwater management model using Visual MODFLOW software. The groundwater flow model for the study area was
formulated by using input hydrogeological data and appropriate boundary conditions. The groundwater flow pattern of the study
area indicates the occurrence of base flow which feed both the Rivers Bhagirathi and Jalangi throughout the year. The computed hydraulic heads were calibrated by comparing with observed groundwater level data for years
2004 to 2006 and were verified with the data of 2007. The outcome of modeling shows that this model can be used for prediction
purpose in the future by updating input boundary conditions and hydrologic stresses during the preceding year. The model optimized
unit draft for deep tube well as 556.5 m3/day and the same for shallow tube well as 41 m3/day keeping the existing tube well structures in running condition and maintaining the present and recent past trends of
groundwater level. The model can be further improved if more spatial and temporal input parameters are available and can be
incorporated into the model for more realistic characterization of groundwater flow. 相似文献
882.
Chemical weathering of the Indo-Gangetic alluvium with special reference to release of fluoride in the groundwater,Unnao district,Uttar Pradesh 总被引:1,自引:0,他引:1
In the central part of Indo-Gangetic alluvium in the Unnao district, Uttar Pradesh there are many pockets where groundwater
shows high fluoride content. Drinking of fluorinated ground water has effected a large population and in many villages more
than 80% of the population is suffering from fluorosis. The source of this fluoride appears to be the alluvial sediments deposited
in the geological past as no hard rock terrain is present in the nearby areas. The area is dominantly made up of mud with
pockets of sand. The sand fraction is made up of quartz, plagioclase, microcline, muscovite and biotite along with some accessory
minerals like garnet, epidote, chlorite, tourmaline, hornblende, kyanite and a few opaque minerals. Moreover, the fluoride
content in the groundwater varies both spatially and with depth indicating a sporadic occurrence. The surface water is devoid
of high content of fluoride but is reported in hand pumps and in the dug wells. This paper deals with the geochemical study
of the sediments up to a depth of 45m as most of the hand pumps are up to this depth to understand the source of fluoride.
14C dates of calcretes have suggested that the 45 m thick succession must have been formed in about 45000 years. 相似文献
883.
N.?NatarajanEmail author G.?Suresh Kumar 《Geotechnical and Geological Engineering》2011,29(4):477-491
In this study, the behavior of thermal fronts along the fracture is studied in the presence of fracture-skin in a coupled
fracture-matrix system. Cold water is injected into the fracture, which advances gradually towards production well, while
extracting heat from the surrounding reservoir matrix. The heat conduction into the fracture-skin and the rock-matrix from
the high permeability fracture is assumed to be one dimensional perpendicular to the axis of fluid flow along the fracture.
Constant temperature cold water is injected through an injection well at the fracture inlet. The fluid flow takes place along
the horizontal fracture which ensures connectivity between the injection and production wells. Since the rock-matrix is assumed
to be tight, the permeability of fracture-skin as well as the rock-matrix is neglected. The present study focuses on the heat
flux transfer at the fracture-skin interface as against the earlier studies on fracture-matrix interface, and the sensitivity
of additional heterogeneity in the form of fracture skin in a conventional fracture-matrix coupled system is studied. The
behavior of thermal fronts for various thermal conductivity values of the fracture-skin and rock-matrix is analyzed. Spatial
moment analysis is performed on the thermal distribution profiles resulting from numerical studies in order to investigate
the impact on mobility and dispersion behavior of the fluid in the presence of fracture-skin. The presence of fracture skin
affects the heat transfer significantly in the coupled fracture-matrix system. The lower order spatial moments indicate that
the effective thermal velocity increases with increase in skin thermal conductivity and a significant thermal dispersion is
observed at the inlet of the fracture owing to the high thermal conductivity of the fracture-skin at the early stages. Furthermore
the higher spatial moments indicate that the asymmetricity increases with decrease in skin thermal conductivity unlike the
case with half fracture aperture and fluid velocity and the kurtosis is maximum with higher skin thermal conductivity which
implies enhanced heat extraction from the fracture-skin into the fracture. Results suggest that the amount of heat extraction
by the circulating fluid within the fracture from the reservoir not only depends on the rock-matrix module of the reservoir
characteristics but also the fracture-skin characteristics of the system and subsequently influence the reservoir efficiency. 相似文献
884.
The role of biofilm proteins in modulating cyprid metamorphosis is not well understood. In the present investigation this possibility was explored by disrupting biofilm proteins using commercially available protease. The influence of natural biofilm and artificial biofilms of Aeromonas salmonicida salmonicida and Bacillus brevis and their culture supernatants and exopolysaccharides obtained under different nutritional conditions was evaluated. Natural biofilm facilitated cyprid metamorphosis in Balanus amphitrite, whereas monospecies biofilms did not. The treatment of biofilms with protease resulted in a decline in metamorphosis, except in B. brevis, where although metamorphosis was negated initially, it increased with the protease concentration and observation days. As the response of cyprids to exopolysaccharides produced by A. salmonicida salmonicida was dependent on protein concentration, protease treatment resulted in a decrease in metamorphosis, whereas in case of B. brevis biofilms, protease hydrolysis products could be important in providing settlement cues. Thus, biofilm bacterial components play an important role in the cyprid metamorphosis of Balanus amphitrite. 相似文献
885.
Halder Debjani Kheroar Shyamal Srivastava Rajiv Kumar Panda Rabindra Kumar 《Theoretical and Applied Climatology》2020,140(3):823-838
Theoretical and Applied Climatology - Temperature and CO2 are the two most important climate parameters that affect crop yield directly. In this study, the impact of these two parameters on the... 相似文献
886.
The kinetics of hydrogen atom abstraction reactions of HFE-227pc by OH and Cl was studied by ab initio method. The structural optimization and frequency calculation of the titled compound and the species formed during the abstraction reactions were performed with density functional theory using hybrid meta density functional MPWB1K with 6–31?+?G(d,p) basis set. The energy of the species was further refined by making a single point energy calculation at G3B3 level of theory. The standard enthalpies of formation of reactant and the radical formed after H-atom abstraction was calculated using isodesmic method. The rate constants of abstraction reactions were calculated using Conventional Transition State Theory (CTST) and were found to be 1.5?×?10?15 and 0.53?×?10?16 cm3molecule?1 s?1 for OH and Cl respectively. The calculated value for the abstraction by OH is close to the experimental value of 2.26?×?10?15 cm3molecule?1 s?1 whereas the same for Cl is found to be about five times lower than that of 2.70?×?10?16 cm3molecule?1 s?1. The theoretical studies yielded the enthalpies of formation and the rate constants that are vital in determining the lifetime of HFE-227pc. 相似文献
887.
P. C. S. Devara Sumit Kumar G. Pandithurai P. D. Safai S. Dipu 《Meteorology and Atmospheric Physics》2013,120(3-4):189-200
To comprehend the characteristics of heterogeneous aerosols, apart from some thematic multi-institutional, multi-platform and multi-parameter campaigns conducted at several places over the globe, presently two major ground-based networks, involving Cimel (AERONET) and Prede (SKYNET) Sun/sky photometers/radiometers have been in progress. In this paper, we report the results of a study that has been undertaken to compare the performance and data products of Cimel and Prede instruments, which were operated concurrently for a period of about 2 years, at the Indian Institute of Tropical Meteorology, Pune, India. The results show a good agreement in the direct Sun observations (aerosol optical depth). The results are also substantiated by making comparison with surface-level black carbon aerosol mass concentration, apart from comparison of other parameters. With regard to the retrieved products such as aerosol size distribution, Prede shows more or less equal concentration of fine mode and relatively more concentration of coarse mode aerosol as compared to those measured by the Cimel. Moreover, the single scattering albedo values measured by the Prede overestimate those observed by the Cimel. These deviations are found to be primarily due to the nature of aerosol loading (turbidity) in the sensing region which is sensitive to the data retrieval techniques including model assumptions employed in both networks. The results of the present study are found useful for multidimensional mapping of aerosol characteristics by integrating the products from both AERONET and SKYNET monitoring stations and thereby help understanding better the impact of aerosols on climate. 相似文献
888.
Emily E. Riddle Amy H. Butler Jason C. Furtado Judah L. Cohen Arun Kumar 《Climate Dynamics》2013,41(3-4):1099-1116
Lagged ensembles from the operational Climate Forecast System version 2 (CFSv2) seasonal hindcast dataset are used to assess skill in forecasting interannual variability of the December–February Arctic Oscillation (AO). We find that a small but statistically significant portion of the interannual variance (>20 %) of the wintertime AO can be predicted at leads up to 2 months using lagged ensemble averages. As far as we are aware, this is the first study to demonstrate that an operational model has discernible skill in predicting AO variability on seasonal timescales. We find that the CFS forecast skill is slightly higher when a weighted ensemble is used that rewards forecast runs with the most accurate representations of October Eurasian snow cover extent (SCE), hinting that a stratospheric pathway linking October Eurasian SCE with the AO may be responsible for the model skill. However, further analysis reveals that the CFS is unable to capture many important aspects of this stratospheric mechanism. Model deficiencies identified include: (1) the CFS significantly underestimates the observed variance in October Eurasian SCE, (2) the CFS fails to translate surface pressure anomalies associated with SCE anomalies into vertically propagating waves, and (3) stratospheric AO patterns in the CFS fail to propagate downward through the tropopause to the surface. Thus, alternate boundary forcings are likely contributing to model skill. Improving model deficiencies identified in this study may lead to even more skillful predictions of wintertime AO variability in future versions of the CFS. 相似文献
889.
In this paper first ever we have developed a class of well behaved charged fluid spheres expressed by a space time with its hypersurfaces $t = \operatorname {const}$ . as spheroid for the case 0<K<1 with surface density 2×1014 gm/cm3. The same utilized to construct a superdense star and seen that star satisfies all well behaved condition for 0<K≤0.038. The maximum mass occupied and the corresponding radius are found to be 4.830982M Θ and 20.7612 km respectively. The redshift at the center and on the surface is given z 0=0.425367 and z a =0.240901. 相似文献
890.
Susanta Kumar Bisoi P. Janardhan D. Chakrabarty S. Ananthakrishnan Ankur Divekar 《Solar physics》2014,289(1):41-61
Possible precursor signatures in the quasi-periodic variations of solar photospheric fields were investigated in the build-up to one of the deepest solar minima experienced in the past 100 years. This unusual and deep solar minimum occurred between Solar Cycles 23 and 24. We used both wavelet and Fourier analysis to study the changes in the quasi-periodic variations of solar photospheric fields. Photospheric fields were derived using ground-based synoptic magnetograms spanning the period 1975.14 to 2009.86 and covering Solar Cycles 21, 22, and 23. A hemispheric asymmetry in the periodicities of the photospheric fields was seen only at latitudes above ±?45° when the data were divided into two parts based on a wavelet analysis: one prior to 1996 and the other after 1996. Furthermore, the hemispheric asymmetry was observed to be confined to the latitude range of 45° to 60°. This can be attributed to the variations in polar surges that primarily depend on both the emergence of surface magnetic flux and varying solar-surface flows. The observed asymmetry along with the fact that both solar fields above ±?45° and micro-turbulence levels in the inner-heliosphere have been decreasing since the early- to mid-nineties (Janardhan et al. in Geophys. Res. Lett. 382, 20108, 2011) suggest that around this time active changes occurred in the solar dynamo that governs the underlying basic processes in the Sun. These changes in turn probably initiated the build-up to the very deep solar minimum at the end of Cycle 23. The decline in fields above ±?45°, for well over a solar cycle, would imply that weak polar fields have been generated in the past two successive solar cycles, viz. Cycles 22 and 23. A continuation of this declining trend beyond 22 years, if it occurs, will have serious implications for our current understanding of the solar dynamo. 相似文献