全文获取类型
收费全文 | 30077篇 |
免费 | 349篇 |
国内免费 | 975篇 |
专业分类
测绘学 | 1514篇 |
大气科学 | 2454篇 |
地球物理 | 5824篇 |
地质学 | 13434篇 |
海洋学 | 1585篇 |
天文学 | 2564篇 |
综合类 | 2172篇 |
自然地理 | 1854篇 |
出版年
2021年 | 47篇 |
2020年 | 66篇 |
2019年 | 73篇 |
2018年 | 4841篇 |
2017年 | 4114篇 |
2016年 | 2687篇 |
2015年 | 332篇 |
2014年 | 207篇 |
2013年 | 339篇 |
2012年 | 1158篇 |
2011年 | 2952篇 |
2010年 | 2230篇 |
2009年 | 2514篇 |
2008年 | 2097篇 |
2007年 | 2535篇 |
2006年 | 280篇 |
2005年 | 374篇 |
2004年 | 586篇 |
2003年 | 577篇 |
2002年 | 410篇 |
2001年 | 163篇 |
2000年 | 157篇 |
1999年 | 116篇 |
1998年 | 116篇 |
1997年 | 85篇 |
1996年 | 88篇 |
1995年 | 92篇 |
1994年 | 86篇 |
1993年 | 76篇 |
1992年 | 86篇 |
1991年 | 70篇 |
1990年 | 98篇 |
1989年 | 85篇 |
1988年 | 75篇 |
1987年 | 100篇 |
1986年 | 75篇 |
1985年 | 96篇 |
1984年 | 131篇 |
1983年 | 108篇 |
1982年 | 97篇 |
1981年 | 121篇 |
1980年 | 105篇 |
1979年 | 100篇 |
1978年 | 71篇 |
1977年 | 86篇 |
1976年 | 77篇 |
1975年 | 70篇 |
1974年 | 55篇 |
1973年 | 61篇 |
1972年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
The Samborombon Bay wetland is located on the west margin of the Rio de la Plata estuary, in the Province of Buenos Aires,
Argentina. This paper analyses the geological, geomorphologic, soil and vegetation characteristics of the southernmost sector
of this wetland and their influence on surface water and groundwater. The study area presents three hydrologic units: coastal
dunes, sand sheets and coastal plain. Coastal dunes and sand sheets are recharge zones of high permeability with well-drained,
non-saline soils, and a few surface water flows. Changes in the water table are related to rainfall. Groundwater in coastal
dunes is Ca–Mg–HCO3 to Na–HCO3, and of low salinity (590 mg/l). Groundwater in sand sheets is mainly Na–HCO3 with a salinity of about 1,020 mg/l. The coastal plain exhibits medium to low permeability sediments, with submerged saline
soils poorly drained. Groundwater is Na–Cl with a mean salinity of 16,502 mg/l. A surface hydrological network develops in
the coastal plain. Surface water levels near the shoreline are affected by tidal fluctuations; far from the shoreline water
accumulates because of poor drainage. Both sectors have Na–Cl water, but the former is more saline. Human intervention and
sea level rise may affect the wetland severely. 相似文献
72.
By using small scale model tests, the interference effect on the vertical load-deformation behavior of a number of equally
spaced strip footings, placed on the surface of dry sand, was investigated. At any stage, all the footings were assumed to
(i) carry exactly equal magnitude of load, and (ii) settle to the same extent. No tilt of the footing was permitted. The effect
of clear spacing (s) among footings on the results was explored. A new experimental setup was proposed in which only one footing
needs to be employed rather than a number of footings. The bearing capacity increases continuously with decrease in spacing
among the footings. The interference effect becomes further prominent with increase in soil friction angle. In contrast to
an increase in the bearing capacity, with decrease in spacing of footings, an increase in the footing settlement associated
with the ultimate state of shear failure was observed. The present experimental observations were similar to those predicted
by the available theory, based on the method of characteristics. As compared to the theory, the present experimental data,
however, indicates much greater effect of interference especially for larger spacing among footings. 相似文献
73.
K. Vinod kumar R. C. Lakhera Tapas R. Martha R. S. Chatterjee A. Bhattacharya 《Environmental Geology》2008,55(4):789-799
Mass movements such as landslides in mountainous terrains are natural degradation processes and one of the most important
landscape-building factors. Varunawat Parbat overlooking Uttarkashi town witnessed a series of landslides on 23 September
2003 and the debris slides and rock falls continued for 2 weeks. This landslide complex was triggered due to the incessant
rainfall prior to the event, and its occurrence led to the blockage of the pilgrim route to Gangotri (source of the Ganges
river) and evacuation of thousands of people to safer places. Though there was no loss of lives due to timely evacuation,
heavy losses to the property were reported. High-resolution stereoscopic earth observation data were acquired after the incidence
to study the landslide in detail with emphasis on the cause of the landslide and mode of failure. Areas along the road and
below the Varunawat foothill region are mapped for landslide risk. It was found that the foothill region of the Varunawat
Parbat was highly disturbed by man-made activities and houses are dangerously located below steep slopes. The potential zones
for landslides along with the existing active and old landslides are mapped. These areas are critical and their treatment
with priority is required in order to minimise further landslide occurrences. 相似文献
74.
Eduardo E. Alonso Sarah M. Springman Charles W. W. Ng 《Geotechnical and Geological Engineering》2008,26(6):817-826
Two large-scale “in situ” demonstration experiments and their instrumentation are described. The first test (FEBEX Experiment)
involves the hydration of a compacted bentonite barrier under the combined effect of an inner source of heat and an outer
water flow from the confining saturated granite rock. In the second case, the progressive de-saturation of Opalinus clay induced
by maintained ventilation of an unlined tunnel is analyzed. The paper shows the performance of different sensors (capacitive
cells, psychrometers, TDR’s) and a comparison of fill behaviour with modelling results. The long term performance of some
instruments could also be evaluated specially in the case of FEBEX test. Capacitive sensors provide relative humidity data
during long transient periods characterised by very large variations of suction within the bentonite. 相似文献
75.
76.
77.
Robert Alan Lake 《Meteorology and Atmospheric Physics》1967,16(2-3):242-259
Summary The heat balance equation for the bottom surface of floating sea ice is evaluated on the basis of observations of ice temperature, water temperature, current velocity, and ablation or accretion of ice. Assuming equality of the eddy diffusivities for momentum, heat, and salt (average 24 cm2 sec–1) it is shown that the temperature gradient in the oceanic boundary layer is extremely small (averages between 2.10–5 and 4.10–4°C/meter) and difficult to measure directly. It is suggested that a large part of the heat transfer from the relatively warm Atlantic water to the arctic atmosphere may occur through open leads in the ice cover.
With 4 Figures
Contribution 141, Department of Atmospheric Sciences, University of Washington, Seattle. This is a thesis submitted by the author in partial fulfillment of the requirements for the degree of Master of Science. 相似文献
Zusammenfassung Die Wärmebilanzgleichung für die Unterseite von schwimmendem Meereis wird an Hand von Beobachtungen der Eistemperatur, der Wassertemperatur, der Strömungsgeschwindigkeit und der Eisdickenänderung ausgewertet. Unter der Annahme gleicher Austauschkoeffizienten für Bewegungsgröße, Wärme und Salzgehalt (im Mittel 24 cm2 sec–1) ergibt sich für die ozeanische Grenzschicht ein außerordentlich kleiner Gradient der Wassertemperatur (durchschnittlich zwischen 2.10–5 und 4.10–4°C/Meter), der durch direkte Beobachtungen schwer nachzuweisen ist. Es ist zu vermuten, daß ein beträchtlicher Teil der Wärmeabgabe von der relativ warmen atlantischen Wassermasse an die arktische Atmosphäre durch Öffnungen in der Meereisdecke erfolgt.
Résumé On établit l'équation du bilan thermique valable pour la surface inférieure de la glace marine dérivante. Pour ce faire, on se sert d'observations de la température de la glace, de celle de l'eau, de la vitesse du courant et des variations de l'épaisseur de la glace. En admettant que les coefficients d'échange sont les mêmes pour la quantité de mouvement, la chaleur et le taux de salinité (en moyenne 24 cm2 sec–1), il résulte pour la couche limite un gradient extrêmement faible de la température de l'eau (situé en moyenne entre 2·10–5 et 4·10–4°C/m); un tel gradient est difficile à prouver au moyen d'observations directes. On peut supposer qu'une partie importante de la chaleur transmise par l'eau relativement chaude de l'Atlantique à l'atmosphère arctique passe au travers des lacunes de la couche de glace recouvrant l'océan.
With 4 Figures
Contribution 141, Department of Atmospheric Sciences, University of Washington, Seattle. This is a thesis submitted by the author in partial fulfillment of the requirements for the degree of Master of Science. 相似文献
78.
79.
Iron solubility equilibria were investigated in seawater at 36.22‰ salinity and 25°C using several filtration and dialysis techniques. In simple filtration experiments with 0.05 μm filters and Millipore ultra-filters, ferric chlorides fluorides, sulfates, and FeOH2+ species were found to be insignificant relative to Fe(OH)2+ at p[H+] = ?log [H+] greater than 6.0. Hydrous ferric oxide freshly precipitated from seawater yielded a solubility product of . Solubility studies based on the rates of dialysis of various seawater solutions and on the filtration of acidified seawater solutions indicated the existence of the Fe(OH)30 species. The formation constant for this species can be calculated as . The Fe(OH)4? species is present at concentrations which are negligible compared to Fe(OH)2+ and Fe(OH)30 in the normal pH range of seawater. However, there is at least one other significant ferric complex in seawater above p[H+] = 8.0 (possibly with bicarbonate, carbonate, or borate ions) in addition to the Fe(OH)2+ and Fe(OH)30 species. 相似文献
80.
The prevalence of dermal neoplasia among neotenic tiger salamanders (Ambystoma tigrinum) collected from the Reese Air Force Base sewage lagoon far exceeded that among salamanders from nearby uncontaminated lagoons. Perylene was the predominant pglycyclic aromatic hydrocarbon contaminant in this lagoon. Perylene was shown to be a substrate for monooxygenases in skin and liver homogenates and formed DNA-binding metabolites. Perylene exposure appeared to augment subsequent perylene metabolism by dermal enzymes but it was not a strong inducer of hepatic monooxygenases. Aroclor 1254 induced benzo(a)pyrene and perylene metabolism by both skin and liver enzymes. However, perylene apparently was not activated by salamander enzymes, as suggested by its lack of in vitro mutagenicity and failure to induce tumors. 相似文献