全文获取类型
收费全文 | 644篇 |
免费 | 35篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 15篇 |
大气科学 | 63篇 |
地球物理 | 161篇 |
地质学 | 232篇 |
海洋学 | 111篇 |
天文学 | 54篇 |
综合类 | 4篇 |
自然地理 | 42篇 |
出版年
2023年 | 2篇 |
2022年 | 6篇 |
2021年 | 16篇 |
2020年 | 16篇 |
2019年 | 19篇 |
2018年 | 31篇 |
2017年 | 33篇 |
2016年 | 44篇 |
2015年 | 30篇 |
2014年 | 42篇 |
2013年 | 42篇 |
2012年 | 30篇 |
2011年 | 64篇 |
2010年 | 43篇 |
2009年 | 46篇 |
2008年 | 45篇 |
2007年 | 32篇 |
2006年 | 22篇 |
2005年 | 19篇 |
2004年 | 24篇 |
2003年 | 11篇 |
2002年 | 13篇 |
2001年 | 8篇 |
2000年 | 7篇 |
1999年 | 6篇 |
1998年 | 5篇 |
1997年 | 2篇 |
1996年 | 1篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1988年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1974年 | 2篇 |
排序方式: 共有682条查询结果,搜索用时 15 毫秒
101.
The aim of the present study was to investigate the ecological quality of tropical estuaries on the northeastern coast of Brazil using the AMBI. Macrofauna classification based on ecological groups was reviewed using the Indicator Value (IndVal) coefficient. The results indicate that the ecosystems exhibit some level of disturbance. Most sites are situated between slightly-moderately disturbed boundaries due to the higher proportion of Nematoda (assigned here as Ecological Group I) and of Oligochaeta and Tubificidae (both classified as Ecological Group V). The AMBI proved efficient in evaluating environmental status, although the applicability of this index requires adjustments regarding some species in ecological groups. The present study also highlights the merits of the IndVal method for examining the assignments of species/taxa to an ecological group and demonstrates the validity of this coefficient is an assessment tool. Moreover, the complementary use of different methods is recommended for the assessment of ecosystem quality. 相似文献
102.
Diopatra neapolitana Delle Chiaje, 1841 (Annelida, Onuphidae) is an important economic natural resource in Ria de Aveiro (northwestern coast of Portugal) and throughout Europe. The species is intensively harvested for use as fresh bait. However, there is only limited knowledge about its life cycle derived from a previous study in Mediterranean Sea. Reproduction and development patterns are known to vary biogeographically, making it important to base management decisions on locally appropriate information. This work examines reproduction patterns for populations from the Eastern Atlantic, which have not previously been assessed, with an eye towards drawing Atlantic–Mediterranean comparisons and informing local management strategies. The study was conducted from May 2007 to April 2009 in Ria de Aveiro. The reproductive biology of D. neapolitana was described from the proportional variation of worms with gametes in the coelom and from the progression of the oocyte diameter. Individuals with gametes inside the coelom were found all year round, but the peak reproductive period occurred between May and August, when almost all individuals had gametes in the coelom and females contained more oocytes than at any other time of the year. The overall male:female ratio was close to 1:1 and the oocyte diameter ranged from 40 to 240 μm. In vitro fertilization was performed and the results compared to other studies. Based on the present results, some protection measures are suggested to implement a sustainable exploitation of the species. 相似文献
103.
104.
Classification of washover dynamics in barrier islands 总被引:1,自引:0,他引:1
Ana Matias
scar Ferreira Ana Vila-Concejo Tiago Garcia Joo Alveirinho Dias 《Geomorphology》2008,97(3-4):655-674
This study systematically classifies washover dynamics with reference to coastal changes along the Ria Formosa barrier islands (Southern Portugal). Identification of washovers using a sequence of 11 sets of aerial photographs dated between 1947 and 2001 allowed a classification to be developed based on: (1) overwash evolution (increasing, decreasing, or constant overwash processes); (2) the mechanisms promoting washover formation (exceptional to infrequent oceanographic conditions, washout processes, structural erosion, inlet dynamics, and human interventions); and (3) the mechanisms promoting washover cessation (berm development, structural erosion, dune development, inlet dynamics, and human interventions). A total of 369 different washovers were observed along the Ria Formosa barriers during the study period, with 209 washovers being formed in various types of dune morphology and 303 being obliterated. The number of washovers was relatively stable from 1947 to 1972, and increased dramatically between 1972 and 1976 probably as a result of the development of immature inlet margins and downdrift starvation. From 1976 to 2001, washover occurrences declined and their spatial dimensions decreased, leading to a decrease in overwash activity over this time. Overall, the dominant formation mechanisms of washovers in the Ria Formosa were inlet dynamics (accounting for 57% of washovers formed) and structural erosion (20%), with human intervention mechanisms accounting for 12%. The cessation of washovers was dominated by dune development (33% of the washovers obliterated) followed by inlet dynamics (24%) and structural erosion (19%), while human intervention mechanisms accounted for 13%. The classification should be of use for the coastal management of barrier systems including the definition of overwash-prone areas and the determination of the relative importance of the mechanisms contributing to washover formation and cessation. 相似文献
105.
106.
107.
108.
A very simple remote sensing-based model for water use monitoring is presented. The model acronym DATTUTDUT (Deriving Atmosphere Turbulent Transport Useful To Dummies Using Temperature) is a Dutch word which loosely translates as “it’s unbelievable that it works”. DATTUTDUT is fully automated and only requires a surface temperature map, making it simple to use and providing a rapid estimate of spatially-distributed fluxes. The algorithm is first tested over a range of environmental and land-cover conditions using data from four short-term field experiments and then evaluated over a growing season in an agricultural region. Flux model output is in satisfactory agreement with observations and established remote sensing-based models, except under dry and partial canopy cover conditions. This suggests that DATTUTDUT has utility in identifying relative water use and as an operational tool providing initial estimates of ET anomalies in data-poor regions that would be confirmed using more robust modeling techniques. 相似文献
109.
Ana Vila-Concejo Michael G. Hughes Andrew D. Short Roshanka Ranasinghe 《Ocean Dynamics》2010,60(2):285-298
Estuarine shorelines are often classified as low-energy coasts and are, therefore, expected to undergo little variation. Port
Stephens (SE Australia) is a ria-like drowned river valley microtidal estuary located on a wave-dominated coast. The outer
part of the estuary is tide-dominated and has a large shallow flood-tide delta, which is also affected by waves. The northern
(predominantly low-energy) shoreline of outer Port Stephens is a continuous stretch of sand comprising areas of high mobility
and areas of relative stability terminating in a western extending sand spit. This paper investigates the effects of periodic
high-energy conditions during which waves penetrate into the estuary by analysing two types of storms, low to moderate (more
frequent type) storms and severe to extreme (low frequency) storms. It is established that low to moderate storms cause generalised
erosion over the northern shoreline. On the contrary, severe to extreme storms, while causing erosion on parts of the beach,
can transport new sediment across the flood-tide delta and deposit it to build a mobile shore attached sandwave. Long-term
(decadal) trends identified in the study area are in agreement with short- and medium-term results. Moving into the estuary
are four complementary zones of sediment transport which include: (1) sandwave formation and westward migration; (2) a relatively
stable area between the sandwave and an erosion zone; (3) an erosion zone undergoing shoreline retreat and finally (4) a depositional
terminus causing westward extension of the sand spit. 相似文献
110.
Extension of observed flood series by combining a distributed hydro-meteorological model and a copula-based model 总被引:2,自引:2,他引:0
Ana I. Requena Isabel Flores Luis Mediero Luis Garrote 《Stochastic Environmental Research and Risk Assessment (SERRA)》2016,30(5):1363-1378
Long flood series are required to accurately estimate flood quantiles associated with high return periods, in order to design and assess the risk in hydraulic structures such as dams. However, observed flood series are commonly short. Flood series can be extended through hydro-meteorological modelling, yet the computational effort can be very demanding in case of a distributed model with a short time step is considered to obtain an accurate flood hydrograph characterisation. Statistical models can also be used, where the copula approach is spreading for performing multivariate flood frequency analyses. Nevertheless, the selection of the copula to characterise the dependence structure of short data series involves a large uncertainty. In the present study, a methodology to extend flood series by combining both approaches is introduced. First, the minimum number of flood hydrographs required to be simulated by a spatially distributed hydro-meteorological model is identified in terms of the uncertainty of quantile estimates obtained by both copula and marginal distributions. Second, a large synthetic sample is generated by a bivariate copula-based model, reducing the computation time required by the hydro-meteorological model. The hydro-meteorological modelling chain consists of the RainSim stochastic rainfall generator and the Real-time Interactive Basin Simulator (RIBS) rainfall-runoff model. The proposed procedure is applied to a case study in Spain. As a result, a large synthetic sample of peak-volume pairs is stochastically generated, keeping the statistical properties of the simulated series generated by the hydro-meteorological model. This method reduces the computation time consumed. The extended sample, consisting of the joint simulated and synthetic sample, can be used for improving flood risk assessment studies. 相似文献