全文获取类型
收费全文 | 25954篇 |
免费 | 253篇 |
国内免费 | 926篇 |
专业分类
测绘学 | 1448篇 |
大气科学 | 2087篇 |
地球物理 | 4908篇 |
地质学 | 12173篇 |
海洋学 | 1239篇 |
天文学 | 1778篇 |
综合类 | 2171篇 |
自然地理 | 1329篇 |
出版年
2023年 | 6篇 |
2022年 | 19篇 |
2021年 | 38篇 |
2020年 | 37篇 |
2019年 | 40篇 |
2018年 | 4792篇 |
2017年 | 4091篇 |
2016年 | 2656篇 |
2015年 | 314篇 |
2014年 | 168篇 |
2013年 | 128篇 |
2012年 | 1073篇 |
2011年 | 2839篇 |
2010年 | 2090篇 |
2009年 | 2413篇 |
2008年 | 1981篇 |
2007年 | 2426篇 |
2006年 | 127篇 |
2005年 | 251篇 |
2004年 | 470篇 |
2003年 | 457篇 |
2002年 | 282篇 |
2001年 | 72篇 |
2000年 | 74篇 |
1999年 | 25篇 |
1998年 | 43篇 |
1997年 | 14篇 |
1996年 | 10篇 |
1995年 | 13篇 |
1994年 | 13篇 |
1993年 | 8篇 |
1992年 | 9篇 |
1991年 | 9篇 |
1990年 | 5篇 |
1989年 | 10篇 |
1987年 | 5篇 |
1986年 | 9篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1982年 | 7篇 |
1981年 | 26篇 |
1980年 | 22篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1976年 | 7篇 |
1975年 | 5篇 |
1973年 | 3篇 |
1965年 | 3篇 |
1962年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
982.
The influence of topography and land use on water quality of Xiangxi River in Three Gorges Reservoir region 总被引:5,自引:0,他引:5
A self-organizing map (SOM) was used to cluster the water quality data of Xiangxi River in the Three Gorges Reservoir region.
The results showed that 81 sampling sites could be divided into several groups representing different land use types. The
forest dominated region had low concentrations of most nutrient variables except COD, whereas the agricultural region had
high concentrations of NO3N, TN, Alkalinity, and Hardness. The sites downstream of an urban area were high in NH3N, NO2N, PO4P and TP. Redundancy analysis was used to identify the individual effects of topography and land use on river water quality.
The results revealed that the watershed factors accounted for 61.7% variations of water quality in the Xiangxi River. Specifically,
topographical characteristics explained 26.0% variations of water quality, land use explained 10.2%, and topography and land
use together explained 25.5%. More than 50% of the variation in most water quality variables was explained by watershed characteristics.
However, water quality variables which are strongly influenced by urban and industrial point source pollution (NH3N, NO2N, PO4P and TP) were not as well correlated with watershed characteristics. 相似文献
983.
N. Janardhana Raju 《Environmental Geology》2007,52(6):1067-1074
In the management of water resources, quality of water is just as important as its quantity. In order to know the quality
and/or suitability of groundwater for domestic and irrigation in upper Gunjanaeru River basin, 51 water samples in post-monsoon
and 46 in pre-monsoon seasons were collected and analyzed for various parameters. Geological units are alluvium, shale and
quartzite. Based on the analytical results, chemical indices like percent sodium, sodium adsorption ratio, residual sodium
carbonate, permeability index (PI) and chloroalkaline indices were calculated. The pre-monsoon waters have low sodium hazard
as compared to post-monsoon season. Residual sodium carbonate values revealed that one sample is not suitable in both the
seasons for irrigation purposes due the occurrence of alkaline white patches and low permeability of the soil. PI values of
both seasons revealed that the ground waters are generally suitable for irrigation. The positive values of Chloroalkaline
indices in post-monsoon (80%) and in pre-monsoon (59%) water samples indicate absence of base-exchange reaction (chloroalkaline
disequilibrium), and remaining samples of negative values of the ratios indicate base-exchange reaction (chloroalkaline equilibrium).
Chadha rectangular diagram for geochemical classification and hydrochemical processes of groundwater for both seasons indicates
that most of waters are Ca–Mg–HCO3 type. Assessment of water samples from various methods indicated that majority of the water samples in both seasons are suitable
for different purposes except at Yanadipalle (sample no. 8) that requires precautionary measures. The overall quality of groundwater
in post-monsoon season in all chemical constituents is on the higher side due to dissolution of surface pollutants during
the infiltration and percolation of rainwater and at few places due to agricultural and domestic activities. 相似文献
984.
湘江中下游农田土壤和蔬菜的重金属污染 总被引:11,自引:0,他引:11
A total of 219 agricultural soil and 48 vegetable samples were collected from the midstream and downstream of the Xiangjiang River(the Hengyang-Changsha section)in Hunan Province.The accumulation characteristics,spatial distribution and potential risk of heavy metals in the agricultural soils and vegetables were depicted.There are higher accumulations of heavy metals such as As,Cd,Cu,Ni,Pb and Zn in agricultural soils,and the contents of Cd(2.44 mg kg^-1 ),Pb(65.00 mg kg^-1 )and Zn(144.13 mg kg^-1 )are 7.97,3.69 and 1.63 times the corresponding background contents in soils of Hunan Province,respectively. 13.2%of As,68.5%of Cd,2.7%of Cu,2.7%of Ni,8.7%of Pb and 15.1%of Zn in soil samples from the investigated sites exceeded the maximum allowable heavy metal contents in the China Environmental Quality Standard for Soils(GB15618-1995,Grade Ⅱ).The pollution characteristics of multi-metals in soils are mainly due to Cd.The contents of As,Cd,Cu,Pb and Zn in vegetable soils are significantly higher than the contents in paddy soils.95.8%, 68.8%,10.4%and 95.8%of vegetable samples exceeded the Maximum Levels of Contaminants in Foods(GB2762-2005)for As,Cd,Ni and Pb concentrations,respectively.There are significantly positive correlations between the concentrations of Cd,Pb and Zn in vegetables and the concentrations in the corresponding vegetable soils(p〈0.01).It is very necessary to focus on the potential risk of heavy metals for food safety and human health in agricultural soils and vegetables in the midstream and downstream of the Xiangjiang River,Hunan Province of China. 相似文献
985.
In October of 2004, a 3-d observational program to measure flow and sediment resuspension within a coastal intertidal salt
marsh was conducted in the North Inlet/Winyah Bay National Estuarine Research Reserve located near Georgetown, South Carolina.
Current and acoustic backscatter profiles were obtained from a moored acoustic Doppler current profiler (ADCP) deployed in
a shallow tidal channel during the spring phase of the tidal cycle under high discharge conditions. The channel serves as
a conduit between Winyah Bay, a large brackish estuary, and North Inlet, a saline intertidal coastal salt marsh with little
freshwater input. Salinity measurements indicate that the water column is vertically well mixed during flood, but becomes
vertically stratified during early ebb. The stratification results from brackish (15 psu) Winyah Bay water entering North
Inlet via the tidal channel, suggesting an exchange mechanism that permits North Inlet to receive a fraction of the poor water
quality and high discharge flow from upland rivers. Although maximum flood currents exceed maximum ebb currents by 0.2 m s−1, suspended sediment concentrations are highest during the latter ebb phase and persist for a longer fraction of the ebb cycle.
Even though the channel is flood-dominated, the higher concentrations occurring over a longer fraction of the ebb phase indicate
net particulate transport from Winyah Bay to North Inlet during spring tide accompanied by high discharge. Our evidence suggests
that the higher concentrations during ebb result from increased bed friction caused by flow asymmetries and variations in
water depth in which the highest stresses occur near the end of ebb near low water despite stronger maximum currents during
flood. 相似文献
986.
K. K. Sinha Pradeep Pandey C. L. Bhairam P. S. Parihar 《Journal of the Geological Society of India》2011,77(2):183-189
In the western part of Bundelkhand massif, a caldera with intra-caldera sediments, known as Dhala Formation, occurs as an
outlier in and around Mohar village of Shivpuri district, Madhya Pradesh. For the first time, occurrence of peperite is being
reported from the basal part of the Dhala sediment. Two types of peperites have been recognized: blocky and fluidal or globular
with variable morphology. In peperitic zones, features like soft sediment deformations, presence of sediment into the rhyolite
along cracks, vesiculation of the sediments and other evidences suggestive of sediment fluidization are some definite characteristics
of interaction of hot magma with wet sediments forming peperite. The occurrence of peperites reflects the contemporaniety
of deposition of the Dhala sediments and volcanism, which is well in accordance to the volcanic origin of Dhala structure.
Further, the nature of unconformity between the Dhala and overlying Kaimur which is characterized by merely a few centimeter
thick pebbly/conglomeratic bed does not appear to represent a large hiatus as expected between the Semri and Kaimur of Vindhyan
Supergroup. So, the contemporaniety of the Dhala Formation (at least the lower part) as reflected by occurrence of peperites,
coupled with the available age of the rhyolite and the nature of the unconformity between the Dhala and overlying Kaimur provide
convincing evidence to correlate the Dhala Formation with the Lower part of the Kaimur and unlikely with the Semri Group or
Bijawar as proposed earlier. 相似文献
987.
This paper presents a robust H∞ output feedback control approach for structural systems with uncertainties in model parameters by using available acceleration measurements and proposes conditions for the existence of such a robust output feedback controller. The uncertainties of structural stiffness, damping and mass parameters are assumed to be norm-bounded. The proposed control approach is formulated within the framework of linear matrix inequalities, for which existing convex optimization techniques, such as the LMI toolbox in MATLAB, can be used effectively and conveniently. To illustrate the effectiveness of the proposed robust H∞ strategy, a six-story building was subjected both to the 1940 El Centro earthquake record and to a suddenly applied Kanai-Tajimi filtered white noise random excitation. The results show that the proposed robust H∞ controller provides satisfactory results with or without variation of the structural stiffness, damping and mass parameters. 相似文献
988.
To mitigate the impact of natural or man-made hazards on the services of an infrastructure facility, it is important to quantitatively assess its available capacity. For example, in a post-disaster scenario, critical infrastructure is likely to experience (i) excessive demand for the service of an infrastructure and/or (ii) compromised capacity because of damage to the infrastructure and the failure of infrastructure interdependencies. As the demand grows and nears the capacity limit of an infrastructure facility, a shortage of services required for the community’s recovery will occur. The development of mitigation strategies and an assessment of their effectiveness require a systematic approach. In this paper, a functional stress–strain principle for infrastructure facilities is proposed to quantitatively assess their serviceability in post-disaster scenarios. Functional stress in infrastructure management represents a service-related demand on an infrastructure facility, while strain indicates its coping capacity. The dynamic nature of infrastructure services will be considered depending on the relationship between demand and available capacity. The allowable range of functional stress is then defined, considering plastic and elastic patterns of responses of a facility during recovery to explore strain capacity variations. The proposed principle facilitates a systematic understanding of how infrastructure facilities can adapt themselves to growing stress and the maximum level of stress they can handle. The application of the proposed functional stress–strain principle is demonstrated through case studies of two infrastructure facilities in a post-earthquake scenario: a medical facility and a power facility. 相似文献
989.
Svend Pedersen Tom Andersen Jens Konnerup-Madsen William L. Griffin 《International Journal of Earth Sciences》2009,98(5):1151-1171
We report U–Pb dates and Lu–Hf isotope data, obtained by LAM-ICPMS, for zircons from metamorphic rocks of the Setesdalen valley,
situated in the Telemark block south of the classic Telemark region of southern Norway. The samples include infracrustal rocks
from the metamorphic basement, metaigneous rocks and metasediments from the Byglandsfjorden supracrustal cover sequence, and
metaigneous rocks which intruded the whole succession. The main crustal evolution took place from 1,550–1,020 Ma, beginning
with the emplacement of juvenile tonalitic melts; the contribution of older crustal material increased with time. Around 1,320 Ma,
further addition of juvenile material occurred, involving both mafic and felsic melts, metamorphism and deformation. Acid
magmas with high FeO*/MgO were intruded at 1,215 Ma, coinciding with underplating elsewhere in South Norway. The period starting
at 1,215 Ma is represented by supracrustal rocks, principally metarhyolites with minor mafic material and immature sediments
of the Byglandsfjorden Group. The crust generation processes ended with the intrusion of diorites and granodiorites at 1,030 Ma,
late in the Sveconorwegian orogeny. Regional processes of metamorphism and deformation (around 1,290 and 1,000 Ma) can be
related to the assembly of Rodinia.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
990.
The sparse polynomial chaos expansion is employed to perform a probabilistic analysis of the tunnel face stability in the spatially random soils. A shield tunnel under compressed air is considered which implies that the applied pressure is uniformly distributed on the tunnel face. Two sets of failure mechanisms in the context of the limit analysis theory with respect to the frictional and the purely cohesive soils are used to calculate the required face pressure. In the case of the frictional soils, the cohesion and the friction angle are modeled as two anisotropic cross-correlated lognormal random fields; for the purely cohesive soils, the cohesion and the unit weight are modeled as two anisotropic independent lognormal random fields. The influences of the spatial variability and of the cross-correlation between the cohesion and the friction angle on the probability density function of the required face pressure, on the sensitivity index and on the failure probability are discussed. The obtained results show that the spatial variability has an important influence on the probability density function as well as the failure probability, but it has a negligible impact on the Sobol’s index. 相似文献