全文获取类型
收费全文 | 3611篇 |
免费 | 148篇 |
国内免费 | 25篇 |
专业分类
测绘学 | 116篇 |
大气科学 | 236篇 |
地球物理 | 1556篇 |
地质学 | 1024篇 |
海洋学 | 169篇 |
天文学 | 495篇 |
综合类 | 11篇 |
自然地理 | 177篇 |
出版年
2021年 | 38篇 |
2020年 | 70篇 |
2019年 | 49篇 |
2018年 | 102篇 |
2017年 | 103篇 |
2016年 | 143篇 |
2015年 | 125篇 |
2014年 | 128篇 |
2013年 | 181篇 |
2012年 | 117篇 |
2011年 | 149篇 |
2010年 | 154篇 |
2009年 | 166篇 |
2008年 | 143篇 |
2007年 | 107篇 |
2006年 | 100篇 |
2005年 | 66篇 |
2004年 | 66篇 |
2003年 | 71篇 |
2002年 | 64篇 |
2001年 | 74篇 |
2000年 | 52篇 |
1999年 | 57篇 |
1998年 | 56篇 |
1997年 | 44篇 |
1996年 | 60篇 |
1995年 | 43篇 |
1994年 | 50篇 |
1993年 | 43篇 |
1992年 | 49篇 |
1991年 | 33篇 |
1990年 | 49篇 |
1989年 | 46篇 |
1988年 | 48篇 |
1987年 | 35篇 |
1986年 | 47篇 |
1985年 | 41篇 |
1984年 | 47篇 |
1983年 | 52篇 |
1982年 | 49篇 |
1981年 | 41篇 |
1980年 | 37篇 |
1979年 | 48篇 |
1978年 | 41篇 |
1977年 | 33篇 |
1976年 | 31篇 |
1975年 | 39篇 |
1973年 | 35篇 |
1972年 | 34篇 |
1971年 | 36篇 |
排序方式: 共有3784条查询结果,搜索用时 93 毫秒
141.
142.
Christiane Neumann Julia Beer Christian Blodau Stefan Peiffer Jan H. Fleckenstein 《水文研究》2013,27(22):3240-3253
Exchange of groundwater and lake water with typically quite different chemical composition is an important driver for biogeochemical processes at the groundwater‐lake interface, which can affect the water quality of lakes. This is of particular relevance in mine lakes where anoxic and slightly acidic groundwater mixes with oxic and acidic lake water (pH < 3). To identify links between groundwater‐lake exchange rates and acid neutralization processes in the sediments, exchange rates were quantified and related to pore‐water pH, sulfate and iron concentrations as well as sulfate reduction rates within the sediment. Seepage rates measured with seepage meters (?2.5 to 5.8 L m‐2 d‐1) were in reasonable agreement with rates inverted from modeled chloride profiles (?1.8 to 8.1 L m‐2 d‐1). Large‐scale exchange patterns were defined by the (hydro)geologic setting but superimposed by smaller scale variations caused by variability in sediment texture. Sites characterized by groundwater upwelling (flow into the lake) and sites where flow alternated between upwelling and downwelling were identified. Observed chloride profiles at the alternating sites reflected the transient flow regime. Seepage direction, as well as seepage rate, were found to influence pH, sulfate and iron profiles and the associated sulfate reduction rates. Under alternating conditions proton‐consuming processes, for example, sulfate reduction, were slowed. In the uppermost layer of the sediment (max. 5 cm), sulfate reduction rates were significantly higher at upwelling (>330 nmol g‐1 d‐1) compared to alternating sites (<220 nmol g‐1 d‐1). Although differences in sulfate reduction rates could not be explained solely by different flux rates, they were clearly related to the prevailing groundwater‐lake exchange patterns and the associated pH conditions. Our findings strongly suggest that groundwater‐lake exchange has significant effects on the biogeochemical processes that are coupled to sulfate reduction such as acidity retention and precipitation of iron sulfides. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
143.
Jan Nyssen Wim Clymans Katrien Descheemaeker Jean Poesen Ine Vandecasteele Matthias Vanmaercke Amanuel Zenebe Marc Van Camp Mitiku Haile Nigussie Haregeweyn Jan Moeyersons Kristine Martens Tesfamichael Gebreyohannes Jozef Deckers Kristine Walraevens 《水文研究》2010,24(13):1880-1895
Impact studies of catchment management in the developing world rarely include detailed hydrological components. Here, changes in the hydrological response of a 200‐ha catchment in north Ethiopia are investigated. The management included various soil and water conservation measures such as the construction of dry masonry stone bunds and check dams, the abandonment of post‐harvest grazing, and the establishment of woody vegetation. Measurements at the catchment outlet indicated a runoff depth of 5 mm or a runoff coefficient (RC) of 1·6% in the rainy season of 2006. Combined with runoff measurements at plot scale, this allowed calculating the runoff curve number (CN) for various land uses and land management techniques. The pre‐implementation runoff depth was then predicted using the CN values and a ponding adjustment factor, representing the abstraction of runoff induced by the 242 check dams in gullies. Using the 2006 rainfall depths, the runoff depth for the 2000 land management situation was predicted to be 26·5 mm (RC = 8%), in line with current RCs of nearby catchments. Monitoring of the ground water level indicated a rise after catchment management. The yearly rise in water table after the onset of the rains (ΔT) relative to the water surplus (WS) over the same period increased between 2002–2003 (ΔT/WS = 3·4) and 2006 (ΔT/WS >11·1). Emerging wells and irrigation are other indicators for improved water supply in the managed catchment. Cropped fields in the gullies indicate that farmers are less frightened for the destructive effects of flash floods. Due to increased soil water content, the crop growing period is prolonged. It can be concluded that this catchment management has resulted in a higher infiltration rate and a reduction of direct runoff volume by 81% which has had a positive influence on the catchment water balance. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
144.
Sébastien Gac Ritske S. Huismans Nina S. C. Simon Yuri Y. Podladchikov Jan Inge Faleide 《地学学报》2013,25(6):459-464
The origin of large subsidence in intracratonic basins is still under debate. We propose a new and self‐consistent model for the formation of those basins, where lithospheric shortening/buckling triggers metamorphism and densification of crustal mafic heterogeneities. We use a forward thermo‐mechanical finite element technique to evaluate this mechanism for the typical example of the East Barents Sea basin (EBB) where a very large and compensated subsidence, accommodating an up to 20‐km‐thick sediment succession, is observed. The lower crust in the dynamic model is modelled with petrologic‐consistent densities for a wet mafic gabbroic composition that depend on pressure and temperature taking into account dehydration at high PT conditions. The model successfully explains the main characteristics of the EBB, notably the large anomalous and fast subsidence during the Late Permian–Early Triassic, its present‐day geometry and the absence of a significant gravity anomaly. 相似文献
145.
Bas van Geel Daniel C. Fisher Adam N. Rountrey Jan van Arkel Joost F. Duivenvoorden Aline M. Nieman Guido B.A. van Reenen Alexei N. Tikhonov Bernard Buigues Barbara Gravendeel 《Quaternary Science Reviews》2011,30(27-28):3935-3946
Intestinal samples from the one-month-old Siberian mammoth calf ‘Lyuba’ were studied using light microscopy and ancient DNA to reconstruct its palaeo-environment and diet. The palynological record indicates a ‘mammoth steppe’. At least some pollen of arboreal taxa was reworked, and thus the presence of trees on the landscape is uncertain. In addition to visual comparison of 11 microfossil spectra, a PCA analysis contributed to diet reconstruction. This yielded two clusters: one of samples from the small intestine and the other of large-intestine samples, indicating compositional differences in food remains along the intestinal tract, possibly reflecting different episodes of ingestion. Based on observed morphological damage we conclude that the cyperaceous plant remains and some remains of dwarf willows were originally eaten by a mature mammoth, most likely Lyuba’s mother. The mammoth calf probably unintentionally swallowed well-preserved mosses and mineral particles while eating fecal material deposited on a soil surface covered with mosses. Coprophagy may have been a common habit for mammoths, and we therefore propose that fecal material should not be used to infer season of death of mammoths. DNA sequences of trnL and rbcL genes amplified from ancient DNA extracted from intestinal samples confirmed and supplemented plant identifications based on microfossils and macro-remains. Results from different extraction methods and barcoding markers complemented each other and show the value of longer protocols in addition to fast and commercially available extraction kits. 相似文献
146.
147.
A method of landscape evaluation based on the anthropo-ecological approach is worked out. The procedure consists of three stages, viz., general, special and optimization. Within the general stage, the structural arrangement of the landscape is evaluated by means of the concept of geo-ecological sites (GES), which are distributed into four zones (A, B, C, D) according to the degrees of their anthropic disturbance. The zoning performed makes it possible to select those geo-ecological sites where eco-critical relationships can be expected to appear; in order to eliminate such situations, transition zones are calculated for the geo-ecological sites concerned. The outcome of this approach is a proposal of a new structural arrangement of the geo-ecological sites, solving eco-critical situations and serving as a basis for the special and optimization stages of the anthropo-ecological method. 相似文献
148.
Emil M. Apostolov Vojtěch Letfus Reviewer J. Pýcha 《Studia Geophysica et Geodaetica》1989,33(4):379-390
Summary The time variations of the amplitudes and phases of the semi-annual variation in geomagnetic activity, characterized by the linear planetary index aa, have been analysed for the period 1868–1985. The results provide qualitative confirmation of Murayama's conclusions [13] about the systematic f phase in dependence on the changes in the level of solar activity and give support to Russel-McPherron's mechanism [16] concerning the effect of the predominant polarity of the interplanetary magnetic field. A distinctly expressed variation of the phase differences in the course of the sunspot cycle and of the 22-year cycle, and specific variations related to the sequence of four consecutive cycles have been established, as well as a well-defined 90-year period, all of them as a reflection of analogous variations in solar activity. The variations of the phase differences observed around the equinoxes can be explained by the combined effect of the mechanisms of the axial and equinoctial hypothesis. It is assumed that a displacement of the maxima of the semi-annual variation to dates after the equinoxes will be observed in the ascending parts and a reverse displacement towards the equinoxes and earlier dates in the desccending parts of the following sunspot cycles 22 nad 23.On leave from the Geophysical Institute of the Bulgarian Academy of Sciences, Akad. G. Bonchev Str. bl. 3, Sofia 1113, Bulgaria. 相似文献
149.
Summary We find no effect of the interplanetary magnetic field(IMF) sector boundary crossing in ozone at altitudes ranging from the middle stratosphere(10 hPa) to the lower mesosphere(0.4 hPa) at middle latitudes(40°–60° N), based on winter data over the period of December 1979–December 1982.
Резюме Мы не нaшлu нuкaко о эффекma nересеченuя секmорноŭ рaнuцы межnлaнеmно о мa нumно о nоля в озоне в облaсmu высоm меж?rt;у сре?rt;неŭ сmрamосфероŭ(10 Пa) u нuжнеŭ мезосфероŭ(0,4 Пa) нa сре?rt;нuх шuроmaх(40°–60° с.ш.), nрuменяя зuмнuе ?rt;aнные зa nерuо?rt; ?rt;екaбрь 1979 – ?rt;екaбрь 1982 .相似文献
150.
Summary Sets of virtual poles corresponding to sets of geomagnetic field values at equidistant points, lying along circles of latitude, were defined on the basis of the 1980 IGRF extending to degree and order 8. Certain places on the Earth's surface yield virtual poles lying very close together. The segments of the virtual pole paths, corresponding to these places, have a large curvature. The mentioned places of the Earth's surface are supposed to be areas with a low rate of the westward drift. They form continuous zones that show a certain relation to global-tectonics features.
uum ¶rt; 8- n¶rt;a ¶rt;a¶rt; aaumu n 1980 u u nmu uma n, mmmu nm uu n ma, a u¶rt;umam ¶rt; naa m ma nmu nua¶rt;am uma n, m uu ¶rt; ¶rt;. amu u, mmmu mu ma, um uu. ¶rt;naam, m nm ma nmu m amu, m m ana¶rt; ¶rt;a ua. u am n na, m naam a m uua mmuu.相似文献