全文获取类型
收费全文 | 28964篇 |
免费 | 1106篇 |
国内免费 | 1976篇 |
专业分类
测绘学 | 1580篇 |
大气科学 | 3078篇 |
地球物理 | 5833篇 |
地质学 | 13880篇 |
海洋学 | 1433篇 |
天文学 | 1897篇 |
综合类 | 2553篇 |
自然地理 | 1792篇 |
出版年
2024年 | 31篇 |
2023年 | 113篇 |
2022年 | 238篇 |
2021年 | 293篇 |
2020年 | 229篇 |
2019年 | 253篇 |
2018年 | 4957篇 |
2017年 | 4233篇 |
2016年 | 2805篇 |
2015年 | 464篇 |
2014年 | 377篇 |
2013年 | 295篇 |
2012年 | 1258篇 |
2011年 | 2980篇 |
2010年 | 2270篇 |
2009年 | 2566篇 |
2008年 | 2108篇 |
2007年 | 2570篇 |
2006年 | 219篇 |
2005年 | 368篇 |
2004年 | 530篇 |
2003年 | 513篇 |
2002年 | 368篇 |
2001年 | 171篇 |
2000年 | 208篇 |
1999年 | 252篇 |
1998年 | 196篇 |
1997年 | 186篇 |
1996年 | 168篇 |
1995年 | 155篇 |
1994年 | 107篇 |
1993年 | 124篇 |
1992年 | 81篇 |
1991年 | 58篇 |
1990年 | 29篇 |
1989年 | 55篇 |
1988年 | 43篇 |
1987年 | 24篇 |
1986年 | 15篇 |
1985年 | 14篇 |
1984年 | 16篇 |
1983年 | 7篇 |
1982年 | 12篇 |
1981年 | 29篇 |
1980年 | 27篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1976年 | 7篇 |
1974年 | 2篇 |
1958年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
General purpose Computational Fluid Dynamics (CFD) solvers are frequently used in small-scale urban pollution dispersion simulations
without a large extent of ver- tical flow. Vertical flow, however, plays an important role in the formation of local breezes,
such as urban heat island induced breezes that have great significance in the ventilation of large cities. The effects of
atmospheric stratification, anelasticity and Coriolis force must be taken into account in such simulations. We introduce a
general method for adapting pressure based CFD solvers to atmospheric flow simulations in order to take advantage of their
high flexibility in geometrical modelling and meshing. Compressibility and thermal stratification effects are taken into account
by utilizing a novel system of transformations of the field variables and by adding consequential source terms to the model
equations of incompressible flow. Phenomena involving mesoscale to microscale coupled effects can be analyzed without model
nesting, applying only local grid refinement of an arbitrary level. Elements of the method are validated against an analytical
solution, results of a reference calculation, and a laboratory scale urban heat island circulation experiment. The new approach
can be applied with benefits to several areas of application. Inclusion of the moisture transport phenomena and the surface
energy balance are important further steps towards the practical application of the method. 相似文献
52.
Mohd Rizaludin Mahmud Mazlan Hashim Mohd Nadzri Mohd Reba 《Asia-Pacific Journal of Atmospheric Sciences》2017,53(3):375-384
We investigated the potential of the new generation of satellite precipitation product from the Global Precipitation Mission (GPM) to characterize the rainfall in Malaysia. Most satellite precipitation products have limited ability to precisely characterize the high dynamic rainfall variation that occurred at both time and scale in this humid tropical region due to the coarse grid size to meet the physical condition of the smaller land size, sub-continent and islands. Prior to the status quo, an improved satellite precipitation was required to accurately measure the rainfall and its distribution. Subsequently, the newly released of GPM precipitation product at half-hourly and 0.1° resolution served an opportunity to anticipate the aforementioned conflict. Nevertheless, related evidence was not found and therefore, this study made an initiative to fill the gap. A total of 843 rain gauges over east (Borneo) and west Malaysia (Peninsular) were used to evaluate the rainfall the GPM rainfall data. The assessment covered all critical rainy seasons which associated with Asian Monsoon including northeast (Nov. - Feb.), southwest (May - Aug.) and their subsequent inter-monsoon period (Mar. - Apr. & Sep. - Oct.). The ability of GPM to provide quantitative rainfall estimates and qualitative spatial rainfall patterns were analysed. Our results showed that the GPM had good capacity to depict the spatial rainfall patterns in less heterogeneous rainfall patterns (Spearman’s correlation, 0.591 to 0.891) compared to the clustered one (r = 0.368 to 0.721). Rainfall intensity and spatial heterogeneity that is largely driven by seasonal monsoon has significant influence on GPM ability to resolve local rainfall patterns. In quantitative rainfall estimation, large errors can be primarily associated with the rainfall intensity increment. 77% of the error variation can be explained through rainfall intensity particularly the high intensity (> 35 mm d-1). A strong relationship between GPM rainfall and error was found from heavy (~35 mm d-1) to violent rain (160 mm d-1). The output of this study provides reference regarding the performance of GPM data for respective hydrology studies in this region. 相似文献
53.
Seasonal change in body length of important small copepods and relationship with environmental factors in Jiaozhou Bay,China 总被引:1,自引:0,他引:1
Differences among species in prosome length and in species’ response to environmental factors do exist. Therefore, it is useful to examine prosome length for different copepod species in variable environments. Seasonal variations in prosome length of four small copepods and their copepodite stages in the Jiaozhou Bay were compared and the relative influence of temperature, salinity, and chlorophyll concentration were examined. Two peaks were found in the mean prosome length of Paracalanus parvus (in early winter and May). For Acartia bifilosa, the maximum values of all copepodites occurred mainly from February to April, and decreased to the bottom in July. Prosome length of Acartia pacifica peaked when it first appeared in June, then reached to the minimum in July. Parvocalanus crassirostris only appeared from late summer to autumn and the mean prosome length showed no clear changes. Correlations of adult prosome length with environmental factors were evaluated. For the four species, temperature was negatively correlated to prosome length except for P. crassirostris. But the different species varied markedly in their responds to temperature. A. bifilosa showed a more definite trend of size variation with temperature than P. parvus and A. pacifica. Correlations of prosome length with salinity were significantly positive for almost all the small copepods. The relationship between chlorophyll concentration and prosome length was complicated for these copepods, but for P. parvus, chlorophyll concentration was also an important affecting factor. Furthermore, investigation needs to be done on food quality for some copepod. These results are essential to estimate the biomass and the production, and to understand these small copepods’ population dynamics in this human-affected bay. 相似文献
54.
Measures of Parameter Uncertainty in Geostatistical Estimation and Geostatistical Optimal Design 总被引:2,自引:2,他引:2
Wolfgang Nowak 《Mathematical Geosciences》2010,42(2):199-221
Studies of site exploration, data assimilation, or geostatistical inversion measure parameter uncertainty in order to assess
the optimality of a suggested scheme. This study reviews and discusses measures for parameter uncertainty in spatial estimation.
Most measures originate from alphabetic criteria in optimal design and were transferred to geostatistical estimation. Further
rather intuitive measures can be found in the geostatistical literature, and some new measures will be suggested in this study.
It is shown how these measures relate to the optimality alphabet and to relative entropy. Issues of physical and statistical
significance are addressed whenever they arise. Computational feasibility and efficient ways to evaluate the above measures
are discussed in this paper, and an illustrative synthetic case study is provided. A major conclusion is that the mean estimation
variance and the averaged conditional integral scale are a powerful duo for characterizing conditional parameter uncertainty,
with direct correspondence to the well-understood optimality alphabet. This study is based on cokriging generalized to uncertain
mean and trends because it is the most general representative of linear spatial estimation within the Bayesian framework.
Generalization to kriging and quasi-linear schemes is straightforward. Options for application to non-Gaussian and non-linear
problems are discussed. 相似文献
55.
56.
Fukun Chen Xiang-Hui Li Xiu-Li Wang Qiu-Li Li Wolfgang Siebel 《International Journal of Earth Sciences》2007,96(6):1179-1194
The Baoshan block of the Tethyan Yunnan, southwestern China, is considered as northern part of the Sibumasu microcontinent.
Basement of this block that comprises presumably greenschist-facies Neoproterozoic metamorphic rocks is covered by Paleozoic
to Mesozoic low-grade metamorphic sedimentary rocks. This study presents zircon ages and Nd–Hf isotopic composition of granites
generated from crustal reworking to reveal geochemical feature of the underlying basement. Dating results obtained using the
single zircon U–Pb isotopic dilution method show that granites exposed in the study area formed in early Paleozoic (about
470 Ma; Pingdajie granite) and in late Yanshanian (about 78–61 Ma, Late Cretaceous to Early Tertiary; Huataolin granite).
The early Paleozoic granite contains Archean to Mesoproterozoic inherited zircons and the late Yanshanian granite contains
late Proterozoic to early Paleozoic zircon cores. Both granites have similar geochemical and Nd–Hf isotopic charateristics,
indicating similar magma sources. They have whole-rock T
DM(Nd) values of around 2,000 Ma and zircon T
DM(Hf) values clustering around 1,900–1,800 and 1,600–1,400 Ma. The Nd–Hf isotopic data imply Paleoproterozoic to Mesoproterozoic
crustal material as the major components of the underlying basement, being consistent with a derivation from Archean and Paleoproterozoic
terrains of India or NW Australia. Both granites formed in two different tectonic events similarly originated from intra-crustal
reworking. Temporally, the late Yanshanian magmatism is probably related to the closure of the Neotethys ocean. The early
Paleozoic magmatism traced in the Baoshan block indicates a comparable history of the basements during early Paleozoic between
the SE Asia and the western Tethyan belt, such as the basement outcrops in the Alpine belt and probably in the European Variscides
that are considered as continental blocks drifting from Gondwana prior to or simultaneously with those of the SE Asia. 相似文献
57.
ShaoHua Zhao ZhiFei Liu Quan Chen XingXing Wang JiangNan Shi HaiYan Jin JingJing Liu ZhiMin Jian 《中国科学:地球科学(英文版)》2017,60(7):1368-1381
Sediment components and their fluxes of Cores MD12-3428(water depth: 903 m), MD12-3433(water depth: 2125 m),and MD12-3434(water depth: 2995 m), obtained along a transect on the continental slope of the northern South China Sea, have been conducted to reveal the spatiotemporal variations and the controlling factors of the sediment components and of their fluxes.Results show that deep-sea sediments in the northern South China Sea are composed mainly of terrigenous(59–89%) and carbonate(6–38%) particles, with minor components of opal(1.6–9.4%) and organic matter(0.7–1.9%). Fluxes of terrigenous and carbonate particles reach up to 2.4–21.8 and 0.4–6.5 g cm–2 kyr–1, respectively, values that are one to two orders of magnitude higher than the fluxes of opal and organic matter. Temporal variations of the percentages and fluxes of deep-sea sediment components have displayed clear glacial-interglacial cyclicity since the last glaciation. Terrigenous, opal, and organic matter percentages and their fluxes increas clearly during marine isotope stage 2, while carbonate percentages and fluxes show an opposite variation pattern or are characterized by an unremarkable increase. This implies that deep-sea carbonate in the South China Sea is affected by the dilution of terrigenous inputs during the sea-level lowstand. With increasing water depth along the transect, the terrigenous percentage increases but with largely decreased fluxes. Both the percentage and flux of carbonate decrease, while the percentages and fluxes of opal and organic matter display much more complicated variational features. The spatiotemporal variations of deep-sea sediment components and of their fluxes since the last glaciation in the northern South China Sea are strongly controlled by sea-level fluctuations. Simultaneously, terrigenous supply associated with monsoonal rainfall, marine primary productivity,and the dilution effect between terrigenous and biogenic particles, also play interconnected roles in the sediment accumulation processes. 相似文献
58.
Juan Pablo Corella Adel El Amrani Javier Sigró Mario Morellón Eugenio Rico Blas Lorenzo Valero-Garcés 《Journal of Paleolimnology》2011,46(3):469-485
We present a high-resolution, multiproxy reconstruction of the depositional history of Lake Arreo, northern Spain, for the
last 60 years. We conducted sedimentological, geochemical and diatom analyses in short cores and made a detailed comparison
with regional instrumental climate data (1952–2007), limnological monitoring of the lake (1992–2008) and recent land use changes
that affect the lake catchment. Chronology is based on “floating” discontinuous varve counts and 137Cs and 14C dates. Four periods were identified in the Lake Arreo recent history: (1) prior to 1963, varved facies intercalated with
fine turbidite deposits, and diatom assemblages dominated by Cyclotella taxa indicate predominantly meromictic conditions, (2) from 1964 to 1978, permanent anoxia persisted in bottom waters, as shown
by similar facies and diatom assemblages as before, though detrital layers were coarser, (3) from 1979 to 1994, sediment delivery
to the lake increased and laminated, clastic facies were deposited, and (4) from 1995 to 2008, dominance of massive facies
and an increase in Fragilaria tenera and Achnanthes minutissima reflect relatively lower lake levels, less frequent bottom anoxia with more frequent water column mixing, similar to modern
conditions. The period 1952–1979 was a time of meromixis and varved facies deposition, and was characterized by higher rainfall
and less intense agricultural pressure in the watershed. There were two short humid periods (1992–1993 and 1996–1998) when
monitoring data show more anoxic weeks per year and relatively higher lake levels. Increased cultivation of small landholdings
in 1963, and particularly after 1979, caused a large increase in sediment delivery to the lake. The inferred lake evolution
is in agreement with monitoring data that suggest a transition from dominantly meromictic conditions prior to 1993–1994 to
a predominantly monomictic pattern of circulation since then, particularly after 2000. The synergistic effects of intensive
water extraction for irrigation and lower rainfall since 1979, and particularly since 1994, brought the long period of meromictic
conditions in Lake Arreo to an end. Water balance and sediment delivery to the lake are dominant factors that control the
limnological and mixing conditions in Lake Arreo and they must be considered in management and restoration plans. 相似文献
59.
60.
Manzoor Ahmad Badar Safdar Hussain Shanawer Niaz Saif ur Rehman 《Arabian Journal of Geosciences》2016,9(9):545
The optical anomalies, and surface and lamellar textures of a birefringent grossular garnet crystal from the Eden Mills, Belvidere Mountain, Vermont, USA, have been investigated by optical polarizing microscope, electron-probe micro-analyzer, back-scattered electron imaging, infrared spectroscopy, and single-crystal X-ray diffractometer from the standpoint of crystal growth. This grossular shows one-to-one correlation between natural surface features and its internal textures under crossed polarizers. Electron-probe micro-analyzer (EPMA) gave average chemical composition in (110) thin section, of bright lamella {Ca2.97Mn0.06}∑3.03 [Al1.59Fe0.37Ti0.01]∑1.97(Si3.00)∑3.00 (Gros79.5And18.9Sps1.6) and of dark host {Ca2.99Mn 0.06}∑3.05 [Al1.73Fe0.26 Ti0.01]∑2.00(Si2.97OH0.03)∑3.00 (Gros85.4And13Sps1.6). The correspondence of surface features and the internal textures with spiral or pyramidal growth mechanism suggest that the internal textures of the Eden Mills grossular are formed during growth process. The optical vibrational orientations and the growth steps inclination along [001] and \( \left[\overset{-}{1}10\right] \) directions predict monoclinic symmetry. With X-ray diffractometer (XRD) method, pseudocubic parameters are a = 11.839(2) Å, b = 11.855(1) Å, and c = 11.868(2) Å with interaxial angles α = 90.00(1)°, β = 89.99(1)°, and γ = 90.02(2)° that show orthorhombic symmetry of this crystal. Lamellar texture of Al3+-rich host with Fe3+-rich lamella infers cation ordering at octahedral site of the garnet structure. IR data favors the non-cubic orientation of [(OH) 4] at tetrahedral position in this grossular structure. 相似文献