全文获取类型
收费全文 | 615篇 |
免费 | 57篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 21篇 |
大气科学 | 100篇 |
地球物理 | 152篇 |
地质学 | 202篇 |
海洋学 | 48篇 |
天文学 | 94篇 |
综合类 | 1篇 |
自然地理 | 67篇 |
出版年
2022年 | 5篇 |
2021年 | 22篇 |
2020年 | 19篇 |
2019年 | 16篇 |
2018年 | 20篇 |
2017年 | 25篇 |
2016年 | 55篇 |
2015年 | 33篇 |
2014年 | 30篇 |
2013年 | 48篇 |
2012年 | 52篇 |
2011年 | 38篇 |
2010年 | 43篇 |
2009年 | 51篇 |
2008年 | 33篇 |
2007年 | 30篇 |
2006年 | 34篇 |
2005年 | 26篇 |
2004年 | 21篇 |
2003年 | 14篇 |
2002年 | 6篇 |
2001年 | 13篇 |
2000年 | 13篇 |
1999年 | 7篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 3篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1989年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1980年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有685条查询结果,搜索用时 265 毫秒
391.
Mark Brenner David A. Hodell Barbara W. Leyden Jason H. Curtis William F. Kenney Binhe Gu Jana M. Newman 《Journal of Paleolimnology》2006,35(1):129-148
We studied the role that submersed aquatic vegetation (SAV) plays in the sedimentation of organic matter (OM) and phosphorus
(P) in Lake Panasoffkee, Florida (USA), a shallow, hard-water, macrophyte-dominated water body. Carbon/Nitrogen ratios (C/N)
and stable isotope signatures (δ13C and δ15N) in algae, higher plants, and surface sediments were measured to identify sources of OM to the lake mud. Pollen, plant macrofossils,
and geochemistry in sediment cores indicated that primary productivity and SAV abundance in Lake Panasoffkee increased in
the late 1800s, probably as a response to increased P loading from human settlement and forest clearance. SAV and associated
periphyton served as temporary sinks for soluble P, maintaining relatively clear-water, low-nutrient conditions in the lake.
P accumulation in Lake Panasoffkee sediments increased together with indicators for greater SAV presence. This suggests that
SAV and associated epiphytes promote P burial and retention in sediments. Although it might be assumed that rooted submersed
macrophytes are directly responsible for P uptake from water and transfer to sediments, C/N and stable carbon isotope results
argue for the importance of other macrophyte growth forms, and perhaps epiphytic algae, in permanent OM and P sequestration.
For instance, high rates of photosynthesis by epiphytes in hard-water systems consume CO2 and promote CaCO3 precipitation. Sloughing of accumulated carbonates from macrophyte leaves transfers epiphytes and associated P to the sediment.
Our paleolimnological findings are relevant to restoration efforts in the Florida Everglades and support the claim that constructed
SAV wetlands remove P from waters effectively. 相似文献
392.
The Doppler effect is the apparent shift in frequency of an electromagnetic signal that is received by an observer moving relative to the source of the signal. The Doppler frequency shift relates directly to the relative speed between the receiver and the transmitter, and has thus been widely used in velocity determination. A GPS receiver-satellite pair is in the Earth’s gravity field and GPS signals travel at the speed of light, hence both Einstein’s special and general relativity theories apply. This paper establishes the relationship between a Doppler shift and a user’s ground velocity by taking both the special and general relativistic effects into consideration. A unified Doppler shift model is developed, which accommodates both the classical Doppler effect and the relativistic Doppler effect under special and general relativities. By identifying the relativistic correction terms in the model, a highly accurate GPS Doppler shift observation equation is presented. It is demonstrated that in the GPS “frequency” or “velocity” domain, the relativistic effect from satellite motion changes the receiver-satellite line-of-sight direction, and the measured Doppler shift has correction terms due to the relativistic effects of the receiver potential difference from the geoid, the orbit eccentricity, and the rotation of the Earth. 相似文献
393.
Jason E. Tanner 《Estuaries and Coasts》2007,30(4):601-606
The European green crab (Carcinus maenas) is a highly successful marine invader, having established populations in a number of areas outside its natural range in
the last 100 years. In South Australia,C. maenas can be abundant on intertidal mud flats, which are used by juveniles of the native blue swimmer crab (Portunus pelagicus) and could have the potential to cause substantial negative effects on this species. The influence of adult blue and green
crabs on habitat selection by juvenile blue crabs was tested to determine if they responded to both predators in a similar
fashion. The presence of predators did not influence habitat selection by juvenile blue crabs in either laboratory or field
experiments, but juvenile behavior in the selected habitat did differ between the two adult species. Many more crabs buried
themselves beneath the substrate when adult conspecifics were present than when adult green crabs were present. Burying in
the presence of adult green crabs was no more frequent than when predators were absent. It remains to be determined if this
makes juvenile blue crabs more vulnerable to predation by green crabs than by adults of their own species, or if the difference
in response is because green crabs pose a different or lesser threat. 相似文献
394.
Chris K. Wendt Jason Beringer Nigel J. Tapper Lindsay B. Hutley 《Boundary-Layer Meteorology》2007,124(2):291-304
Fire scars have the ability to radically alter the surface energy budget within a tropical savanna by reducing surface albedo,
increasing available energy for partitioning into sensible and latent heat fluxes and increasing substrate heat flux. These
changes have the potential to alter boundary-layer conditions and ultimately feedback to local and regional climate. We measured
radiative and energy fluxes over burnt and unburnt tropical savanna near Howard Springs, Darwin, Australia. At the burnt site
a low to moderate intensity fire, ranging between 1,000 and 3,500 kW m−1, initially affected the land surface by removing all understorey vegetation, charring and blackening the ground surface,
scorching the overstorey canopy and reducing the albedo. A reduction in latent heat fluxes to almost zero was seen immediately
after the fire when the canopy was scorched. This was then followed by an increase in the sensible heat flux and a large increase
in the ground heat flux over the burnt surface. Tethered balloon measurements showed that, despite the presence of pre-monsoonal
rain events occurring during the measurement period, the lower boundary layer over the burnt site was up to 2°C warmer than that over the unburnt site. This increase in boundary-layer heating when applied to fire scars at the landscape
scale can have the ability to form or alter local mesoscale circulations and ultimately create a feedback to regional heating
and precipitation patterns that may affect larger-scale processes such as the Australian monsoon. 相似文献
395.
Fritz Petersen Jason A. Hubbart Elliott Kellner Evan Kutta 《Environmental Earth Sciences》2018,77(22):754
A high-spatial resolution study design was used to investigate the relationship between land use practices, stream physicochemistry, hydroclimate, and stream Escherichia (E) coli concentrations in a mixed-land-use watershed in the Appalachian region. Stream samples were collected daily from six monitoring sites and analyzed for total E. coli counts using an enzyme metabolism indicator method. Statistical comparison of E. coli concentration time series showed significant (p?<?0.05) differences between study sites. Although highest average E. coli concentrations were observed at two agricultural sites (534 and 582 colony-forming counts (CFU) per 100 mL, respectively), highest total loadings were observed within the receiving stream, with values increasing downstream (2?×?1012 and 4.2?×?1012 study total CFU for bracketed upstream and downstream sites, respectively). No single physical variable displayed a significant correlation (p?<?0.05) with observed E. coli concentration at every site. However, sites displayed different patterns of significant correlations (p?<?0.05) between E. coli concentration and both physicochemical (e.g. pH, dissolved oxygen saturation) and hydroclimate variables (e.g. streamflow and precipitation). Percent agricultural land cover was the only land use category that showed significant (p?<?0.04) correlation with study average E. coli concentrations, thereby emphasizing the importance of land use practices to stream pathogen regimes. Results validate the analytical method and provide high-resolution, detailed, quantitative characterizations of stream E. coli regimes, thereby supplying land and water resource managers with science-based information to advance management decisions and improve public health. 相似文献
396.
397.
398.
399.
Banglin Zhang Vijay Tallapragada Fuzhong Weng Jason Sippel Zaizhong Ma 《大气科学进展》2015,32(12):1575-1582
The four-dimensional variational(4D-Var) data assimilation systems used in most operational and research centers use initial condition increments as control variables and adjust initial increments to find optimal analysis solutions. This approach may sometimes create discontinuities in analysis fields and produce undesirable spin ups and spin downs. This study explores using incremental analysis updates(IAU) in 4D-Var to reduce the analysis discontinuities. IAU-based 4D-Var has almost the same mathematical formula as conventional 4D-Var if the initial condition increments are replaced with time-integrated increments as control variables.The IAU technique was implemented in the NASA/GSFC 4D-Var prototype and compared against a control run without IAU. The results showed that the initial precipitation spikes were removed and that other discontinuities were also reduced,especially for the analysis of surface temperature. 相似文献
400.
Holley Jason R. McComas Katherine A. Lambert Catherine E. Snider Natalie P. Tucker Grace K. 《Natural Hazards》2022,113(1):615-640
Natural Hazards - Drawing from protection motivation theory (PMT), we examined how place attachment and negative emotions, alongside threat and coping appraisals, personal experiences, and... 相似文献