全文获取类型
收费全文 | 1288篇 |
免费 | 42篇 |
国内免费 | 8篇 |
专业分类
测绘学 | 61篇 |
大气科学 | 100篇 |
地球物理 | 269篇 |
地质学 | 420篇 |
海洋学 | 90篇 |
天文学 | 250篇 |
综合类 | 5篇 |
自然地理 | 143篇 |
出版年
2021年 | 9篇 |
2020年 | 9篇 |
2019年 | 12篇 |
2018年 | 29篇 |
2017年 | 29篇 |
2016年 | 31篇 |
2015年 | 19篇 |
2014年 | 24篇 |
2013年 | 74篇 |
2012年 | 47篇 |
2011年 | 50篇 |
2010年 | 48篇 |
2009年 | 59篇 |
2008年 | 59篇 |
2007年 | 56篇 |
2006年 | 41篇 |
2005年 | 42篇 |
2004年 | 34篇 |
2003年 | 38篇 |
2002年 | 37篇 |
2001年 | 23篇 |
2000年 | 41篇 |
1999年 | 22篇 |
1998年 | 22篇 |
1997年 | 20篇 |
1996年 | 21篇 |
1995年 | 21篇 |
1994年 | 13篇 |
1993年 | 22篇 |
1992年 | 20篇 |
1991年 | 14篇 |
1990年 | 23篇 |
1989年 | 16篇 |
1988年 | 18篇 |
1987年 | 18篇 |
1986年 | 14篇 |
1985年 | 14篇 |
1984年 | 22篇 |
1983年 | 26篇 |
1982年 | 19篇 |
1981年 | 15篇 |
1980年 | 16篇 |
1977年 | 10篇 |
1976年 | 10篇 |
1975年 | 12篇 |
1974年 | 10篇 |
1973年 | 9篇 |
1972年 | 19篇 |
1971年 | 12篇 |
1969年 | 7篇 |
排序方式: 共有1338条查询结果,搜索用时 31 毫秒
291.
Julia L. Barringer Timothy P. Wilson Zoltan Szabo Jennifer L. Bonin Jeffrey M. Fischer Nicholas P. Smith 《Environmental Geology》2008,53(6):1183-1199
Diurnal variations in particulate and dissolved As and metal concentrations were observed in mildly alkaline water from a
wetlands site on the Wallkill River in northwestern New Jersey. The site, underlain by glacial sediments over dolomite bedrock,
is 10 km downstream from a mined area of the Franklin Marble, host to Zn ores, also As and Mn minerals. In mid-September 2005,
maxima and minima in dissolved-oxygen-concentration and pH, typically caused by photosynthesis and respiration, occurred at
2000 and 0800 hours. Concentrations of dissolved As (1.52–1.95 μg/L) peaked at dusk (2000 hours), whereas dissolved Mn and
Zn concentrations (76.5–96.9 and 8.55–12.8 μg/L, respectively) were lowest at dusk and peaked at 1000 hours. These opposing
cycles probably reflect sorption and desorption of As (an anion), and Mn and Zn (cations) as pH varied throughout the 24-h
period. Doubly-peaked cycles of B, Cl, SO4, and nutrients also were observed; these may result from upstream discharges of septic-system effluent. Both recoverable
amd particulate Al, Fe, Mn, and Zn concentrations peaked between 0200 and 0600 hours. The particulate metals cycle, with perturbations
at 0400 hours, may be influenced by biological activity. 相似文献
292.
293.
Summary The aim of this paper is to contribute towards a better understanding of the contemporary position of commons as a land resource, and to evaluate whether commons have a distinct role to play in today's 'post-productivist' countryside. Following the government's recent recognition of the multi-functional role of commons, the paper sets out to investigate whether this can be achieved through existing agri-environmental schemes. A case study of commons in the Cambrian Mountains (Wales) enables an assessment of the compatibility of one recent agri-environmental scheme, the ESA scheme, with commons management. It is concluded that new legislation and policy are needed, and that agri-environmental policy may provide an opportunity to develop a framework within which the management of common land can be improved to meet conservation and amenity as well as farming objectives. 相似文献
294.
An experiment is described showing that an optical scintillation instrument gives reliable values of heat and momentum fluxes in the surface layer, subject to the usual restrictions of homogeneity and steady state. This instrument measures the turbulence inner scale and refractive-index structure parameter, which are used to obtain the fluxes from Monin-Obukhov similarity relationships. The instrument gives space-averaged values over a propagation path that can range in length from tens to hundreds of meters. The history of the use of optical propagation to estimate fluxes is reviewed. 相似文献
295.
R. I. McLean J. K. Summers C. R. Olsen S. L. Domotor I. L. Larsen H. Wilson 《Estuaries and Coasts》1991,14(2):148-156
The Susquehanna River is the major contributor to sediment loadings in the Chesapeake Bay. Because many environmental contaminants are associated with suspended particulates, the degree of particle retention within the reservoirs of the lower Susquehanna River is an important consideration in evaluating contaminant loadings to the Chesapeake Bay. Profiles of weapons-test Cs-137, nuclear power plant-related Cs-134 and Cs-137, and naturally-derived Pb-120 were used to estimate rates of sediment accretion in the conowingo Reservioir, an impoundment of the Susquehanna River along the Maryland-Pennsylvania border. Net accretion rates ranged from about 2 cm yr?1 downstream of a nuclear power plant cooling discharge to a high of about 7 cm yr?1 at the mount of an incoming creek. Slight, but consistent, increases in the annual rate of accretion since the creation of the reservoir in 1928 are apparent. The current net average annual sediment load reatined by the reservoir is estimated to be 0.4×106 to 1.5 × 106 metric tons yr?1. The retained sediment load represents about 8–23% of the long-time average sediment input to the reservoir. 相似文献
296.
297.
298.
We implemented the windbreak model of Wang and Takle to investigate whythose authors could have obtained better agreement with the Bradley andMulhearn wind reduction measurements in the far lee of a fence, than wasobtained by Wilson using a very similar model. According to our experiencethe improvement of the Wang-Takle simulations (relative to Wilson's)largely arises from their having used a too-shallow computational domain(8H, versus Wilson's 47H; H being the windbreak height). 相似文献
299.
300.
The literature suggests that owing to profound difficulties with high school geography curricula, teachers play a vital role in stimulating student interest and in providing a platform for continuation in the study of geography at university. Yet, with little empirical evidence offered in support, it is unclear why students select geography at university and if their high school experience informs their decision. Through a survey of students enrolled in an introductory geography course at an Ontario university, this research questions the relationship between students' high school experience and their enrollment in the course and ponders the implications for the health of the discipline. 相似文献