全文获取类型
收费全文 | 609篇 |
免费 | 25篇 |
专业分类
测绘学 | 28篇 |
大气科学 | 67篇 |
地球物理 | 139篇 |
地质学 | 178篇 |
海洋学 | 55篇 |
天文学 | 93篇 |
综合类 | 2篇 |
自然地理 | 72篇 |
出版年
2021年 | 6篇 |
2020年 | 7篇 |
2019年 | 4篇 |
2018年 | 12篇 |
2017年 | 12篇 |
2016年 | 23篇 |
2015年 | 8篇 |
2014年 | 17篇 |
2013年 | 31篇 |
2012年 | 27篇 |
2011年 | 24篇 |
2010年 | 27篇 |
2009年 | 26篇 |
2008年 | 23篇 |
2007年 | 19篇 |
2006年 | 19篇 |
2005年 | 18篇 |
2004年 | 12篇 |
2003年 | 14篇 |
2002年 | 19篇 |
2001年 | 13篇 |
2000年 | 19篇 |
1999年 | 11篇 |
1998年 | 7篇 |
1997年 | 11篇 |
1996年 | 11篇 |
1995年 | 10篇 |
1994年 | 8篇 |
1993年 | 11篇 |
1992年 | 8篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1989年 | 5篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 13篇 |
1984年 | 10篇 |
1983年 | 14篇 |
1982年 | 13篇 |
1981年 | 11篇 |
1980年 | 16篇 |
1979年 | 6篇 |
1978年 | 10篇 |
1977年 | 10篇 |
1976年 | 7篇 |
1975年 | 7篇 |
1973年 | 4篇 |
1969年 | 2篇 |
1961年 | 2篇 |
排序方式: 共有634条查询结果,搜索用时 105 毫秒
431.
Books reviewed:
MARSHA ALIBRANDI, GIS in the Classroom: Using Geographic Information Systems in Social Studies and Environmental Science
JO WOOD, Java Programming for Spatial Sciences 相似文献
MARSHA ALIBRANDI, GIS in the Classroom: Using Geographic Information Systems in Social Studies and Environmental Science
JO WOOD, Java Programming for Spatial Sciences 相似文献
432.
Nitrate contamination of surface and groundwater has become a serious concern in many agricultural areas throughout the world. Nitrate fertilization is widely used to increase crop yields and quality, but nitrates are highly soluble in water with low retention by soils. Appropriate nitrate application practices can minimize losses, but untimely rainfall can foil management intent. A small dc electrical input has previously been reported to influence nitrate movement in wet soil. Two sets of lysimeter experiments were conducted to evaluate the effect of parallel electrodes on pH and nitrate distribution in field soils subjected to an electrical input. In one experiment, drip irrigation with injected nitrate application was used to grow Sudan grass; in the other, nitrate distribution in wet soil was observed after application of varying levels of electrical current. Increased nitrate concentration, reduced sodium and calcium concentration and lower pH values near the anode, the expected results, were not attained consistently in the test soils with an electrical input of 100–700 mA. The evaluation is being continued with simpler, more controlled conditions in soil column laboratory experiments. 相似文献
433.
Dennis N. Eden 《Boreas: An International Journal of Quaternary Research》1980,9(3):165-177
A thin mantle of cover-loam over much of North-East Essex has been recognised as consisting largely of loess. The cover-loam represents the intermixing, to a varying degree, of a layer of loessial silt with a thin layer of underlying sand which is also of likely aeolian origin. The heavy mineral content of the coarse silt fraction of North-East Essex loess is generally similar to that of last glaciation age loesses elsewhere in Eastern England, Belgium and the Netherlands. This suggests the North-East Essex loess is part of a single loess sheet deposited over Eastern England and parts of Western Europe. Nevertheless, detailed examination of the heavy minerals content from all of these areas reveals slight areal variations especially in the proportion of hornblende. These differences show the coarse silt from North-East Essex loess to have closest affinities with that from Norfolk. A distant source for the loess within the present North Sea Basin is proposed on textural evidence. Loess accumulation in North-East Essex probably commenced in the few thousand years leading up to the maximum extent of Devensian ice (about 18,000 years B.P.) and may have continued to about 14,000 years B.P. 相似文献
434.
This article evaluates some of the factors which limit the human benefits of hazard warnings, with specific reference to flood
warnings, and we conclude by suggesting ways of enhancing these benefits. We focus mainly upon the economic benefits generated
by flood damage savings by households that warnings facilitate; health effects of flooding and flood warnings; and the effects
of warnings on loss of life and physical injury. Our results, based partly upon surveys of flooded households, reveal that
economic benefits are currently more limited than we previously thought, but that for several reasons these benefits are likely
to be under-estimated. We argue that the intangible benefits to public health, safety and security must also be taken into
account in decisions about investment in flood warnings. In England and Wales, the public’s response to flood warnings is
currently low and is a key benefit-limiting factor which could begin to undermine a recent major shift in national flood risk
management policy towards a more people-centred, portfolio approach in which changing human behaviour is viewed as important.
Using a trans-disciplinary approach, we discuss the evidence and literature surrounding this poor response, and suggest a
number of ways in which the issue may be addressed in future. 相似文献
435.
Charles A. Stock Dennis J. McGillicuddy Jr Donald M. Anderson Andrew R. Solow Richard P. Signell 《Continental Shelf Research》2007
Blooms of the toxic dinoflagellate Alexandrium fundyense commonly occur in the western Gulf of Maine but the amount of toxin observed in coastal shellfish is highly variable. In this study, a coupled physical–biological model is used to investigate the dynamics underlying the observed A. fundyense abundance and shellfish toxicity in 1993 (a high toxicity year) and 1994 (low toxicity year). The physical model simulates the spring circulation, while the biological model estimates the germination and population dynamics of A. fundyense based on laboratory and field data. The model captures the large-scale aspects of the initiation and development of A. fundyense blooms during both years, but small-scale patchiness and the dynamics of bloom termination remain problematic. In both cases, the germination of resting cysts accounts for the magnitude of A. fundyense populations early in the spring. Simulations with low net A. fundyense growth rates capture the mean observed concentration during the bloom peak, which is of similar magnitude during both years. There is little evidence that large-scale changes in biological dynamics between 1993 and 1994 were a primary driver of the differences in shellfish toxicity. Results instead suggest that the persistent southwesterly flow of the western Maine Coastal Current led to A. fundyense populations of similar alongshore extent by late May of both years. This period coincides with peak cell abundance in the region. Variations in wind forcing (downwelling favorable in 1993, upwelling favorable in 1994) and subsequent cell transport (inshore in 1993, offshore in 1994) in early June then provides a plausible explanation for the dramatic mid-June differences in shellfish toxicity throughout the western Gulf of Maine. 相似文献
436.
An analysis of soil moisture dynamics using multi-year data from a network of micrometeorological observation sites 总被引:4,自引:0,他引:4
Gretchen R. Miller Dennis D. Baldocchi Beverly E. Law Tilden Meyers 《Advances in water resources》2007
Soil moisture data, obtained from four AmeriFlux sites in the US, were examined using an ecohydrological framework. Sites were selected for the analysis to provide a range of plant functional type, climate, soil particle size distribution, and time series of data spanning a minimum of two growing seasons. Soil moisture trends revealed the importance of measuring water content at several depths throughout the rooting zone; soil moisture at the surface (0–10 cm) was approximately 20–30% less than that at 50–60 cm. A modified soil moisture dynamics model was used to generate soil moisture probability density functions at each site. Model calibration results demonstrated that the commonly used soil matric potential values for finding the vegetation stress point and field content may not be appropriate, particularly for vegetation adapted to a water-controlled environment. Projections of future soil moisture patterns suggest that two of the four sites will become severely stressed by climate change induced alterations to the precipitation regime. 相似文献
437.
The prediction of evaporation from Mediterranean woodland ecosystems is complicated by an array of climate, soil and plant factors. To provide a mechanistic and process-oriented understanding, we evaluate theoretical and experimental information on water loss of Mediterranean oaks at three scales, the leaf, tree and woodland. We use this knowledge to address: what limits evaporation from Mediterranean oak woodlands – the supply of moisture in the soil, physiological control by plants or the demand by the atmosphere? 相似文献
438.
We analyzed monthly boat electrofishing data to characterize the littoral fish assemblages of five regions of the Sacramento-San
Joaquin Delta (northern, southern, eastern, western, and central), California, during two sampling periods, 1980–1983 (1980s)
and 2001–2003 (2000s), to provide information pertinent to the restoration of fish populations in this highly altered estuary.
During the 1980s, almost 11,000 fish were captured, including 13 native species and 24 alien species. During the 2000s, just
over 39,000 fish were captured, including 15 native species and 24 alien species. Catch per unit effort (CPUE) of total fish,
alien fish, and centrarchid fish were greater in the 2000s compared with the 1980s, largely because of increased centrarchid
fish CPUE. These differences in CPUE were associated with the spread of submerged aquatic vegetation (SAV), particularly an
alien aquatic macrophyte,Egeria densa. Native fish CPUE declined from the 1980s to the 2000s, but there was no single factor that could explain the decline. Native
fish were most abundant in the northern region during both sampling periods. Nonmetric multidimensional scaling indicated
similar patterns of fish assemblage composition during the two sampling periods, with the northern and western regions characterized
by the presence of native species. The separation of the northern and western regions from the other regions was most distinct
in the 2000s. Our results suggest that native fish restoration efforts will be most successful in the northern portion of
the Delta. Management decisions on the Delta should include consideration of possible effects on SAV in littoral habitats
and the associated fish assemblages and ecological processes. 相似文献
439.
Summary. Fault zones in wet Westerly granite deformed at temperatures of 300° and 400°C require markedly lower shear stresses for sliding than when dry, even when the effective confining pressure is held constant between the wet and dry tests, provided that the strain rate is lower than 10−7 s−1 . The rate of strength reduction is enhanced by increasing the pore water pressure. The deformation rate is a power function of the applied stress where the stress exponent is approximately 7 for pore water pressure of 100 MPa and 21 for pore water pressure of 20 MPa.
The experimental results are extrapolated to conditions believed to occur at depths of 10 km along the San Andreas Fault Zone. It is suggested that for slow tectonic deformation at strain rates of 10−11 and 10−14 s−1 the shear stress for sliding on faults in granite would be approximately 60 and 20 MPa, respectively, at pore water pressures equal to the hydrostatic head. Fluid overpressures of c. 0.8 lithostatic pressure are required to lower the shear stress for sliding into the 10 MPa range at the slower strain rate. 相似文献
The experimental results are extrapolated to conditions believed to occur at depths of 10 km along the San Andreas Fault Zone. It is suggested that for slow tectonic deformation at strain rates of 10
440.
B. Lokanadham P. K. Subramanian Allan L. Kiplinger B. R. Dennis 《Astrophysics and Space Science》1986,119(1):5-8
The paper presents a detailed study of the high energy X-ray observation of the most unusual solar events observed on 4 and 7 June, 1980 with the Hard X-Ray Burst Spectrometer (HXRBS) on Solar Maximum Mission (SMM) satellite. The hard X-ray data of the events are also compared with the radio microwave fluxes.The X-ray time profiles of these flares are characterized by the occurrence of impulsive phase superposed with a number of narrow spikes before the occurrence of the main energetic events. Studies of the temporal and spectral properties of these events indicated a quasi-oscillatory nature of the sources. Various models for explaining the evolution of the events are considered and the sequential firing loop model seems to be consistent with the observations of the events.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984. 相似文献