全文获取类型
收费全文 | 3012篇 |
免费 | 78篇 |
国内免费 | 62篇 |
专业分类
测绘学 | 94篇 |
大气科学 | 420篇 |
地球物理 | 580篇 |
地质学 | 1078篇 |
海洋学 | 163篇 |
天文学 | 637篇 |
综合类 | 23篇 |
自然地理 | 157篇 |
出版年
2021年 | 16篇 |
2020年 | 15篇 |
2019年 | 17篇 |
2018年 | 49篇 |
2017年 | 41篇 |
2016年 | 62篇 |
2015年 | 36篇 |
2014年 | 69篇 |
2013年 | 111篇 |
2012年 | 95篇 |
2011年 | 122篇 |
2010年 | 102篇 |
2009年 | 141篇 |
2008年 | 158篇 |
2007年 | 140篇 |
2006年 | 134篇 |
2005年 | 139篇 |
2004年 | 107篇 |
2003年 | 89篇 |
2002年 | 100篇 |
2001年 | 86篇 |
2000年 | 78篇 |
1999年 | 97篇 |
1998年 | 80篇 |
1997年 | 70篇 |
1996年 | 66篇 |
1995年 | 66篇 |
1994年 | 50篇 |
1993年 | 50篇 |
1992年 | 45篇 |
1991年 | 56篇 |
1990年 | 39篇 |
1989年 | 42篇 |
1988年 | 19篇 |
1987年 | 30篇 |
1986年 | 31篇 |
1985年 | 30篇 |
1984年 | 50篇 |
1983年 | 30篇 |
1982年 | 37篇 |
1981年 | 41篇 |
1980年 | 37篇 |
1979年 | 32篇 |
1978年 | 32篇 |
1977年 | 19篇 |
1976年 | 24篇 |
1975年 | 15篇 |
1974年 | 29篇 |
1973年 | 17篇 |
1971年 | 22篇 |
排序方式: 共有3152条查询结果,搜索用时 29 毫秒
1.
Christopher M. U. Neale Maria P. Gonzalez-Dugo Angelica Serrano-Perez Isidro Campos Luciano Mateos 《水文研究》2021,35(6):e14162
Evapotranspiration (ET) is an important parameter in hydrologic processes and modelling. In agricultural watersheds with competing uses of fresh water including irrigated agriculture, estimating crop evapotranspiration (ETc) accurately is critical for improving irrigation system and basin water management. The use of remote sensing-based basal crop coefficients is becoming a common method for estimating crop evapotranspiration for multiple crops over large areas. The Normalized Difference Vegetation Index (NDVI) and the Soil Adjusted Vegetation Index (SAVI), based on reflectance in the red and near-infrared bands, are commonly used for this purpose. In this paper, we examine the effects of row crop orientation and soil background darkening due to shading and soil surface wetness on these two vegetation indices through modelling, coupled with a field experiment where canopy reflectance of a cotton crop at different solar zenith angles, was measured with a portable radiometer. The results show that the NDVI is significantly more affected than the SAVI by background shading and soil surface wetness, especially in north–south oriented rows at higher latitudes and could lead to a potential overestimation of crop evapotranspiration and irrigation water demand if used for basal crop coefficient estimation. Relationships between the analysed vegetation indices and canopy biophysical parameters such as crop height, fraction of cover and leaf area index also were developed for both indices. 相似文献
2.
T. Vinutha V. U. M. Rao Bekele Getaneh Molla Mengesha 《Astrophysics and Space Science》2018,363(9):188
In this paper we have studied the anisotropic Kantowski-Sachs, locally rotationally symmetric (LRS) Bianchi type-I and LRS Bianchi type-III geometries filled with dark energy and one dimensional cosmic string in the Saez-Ballester theory of gravitation. To get physically valid solution we take hybrid expansion law of the average scale factor which is a product of power and exponential type of functions that results in time dependent deceleration parameter (\(q\)). The equation of state parameter of dark energy (\(\omega _{\mathit{de}}\)) has been discussed and we have observed that for the three models it crosses the phantom divide line (\(\omega _{\mathit{de}} = -1\)) and shows quintom like behavior. The density of dark energy (\(\rho _{\mathit{de}}\)) is an increasing function of redshift and remains positive throughout the evolution of the universe for the three models. Moreover in Kantowski-Sachs and LRS Bianchi type-I geometries the dark energy density dominates the string tension density (\(\lambda \)) and proper density (\(\rho \)) throughout the evolution of the universe. The physical and geometrical aspects of the statefinder parameters (\(r,s\)), squared speed of sound (\(v_{s}^{2} \)) and \(\omega _{\mathit{de}}\)–\(\omega ^{\prime }_{\mathit{de}}\) plane are also discussed. 相似文献
3.
Mokievsky V. O. Udalov A. A. Chava A. I. Simakova U. V. Kokarev V. N. Neretina T. V. 《Doklady Earth Sciences》2020,493(2):645-648
Doklady Earth Sciences - The species composition and structure of bottom communities with a predominance of bivalve mollusks of the Portlandia genus, which are widespread on the shelf of the... 相似文献
4.
Tasev G. Serafimovski D. Volkov A. V. Boev B. Djordjevic T. Kolitsch U. Serafimovski T. 《Geology of Ore Deposits》2020,62(5):419-431
Geology of Ore Deposits - The Dudica deposit is located in the southern part of the Republic of North Macedonia (RNM), not far from the border with Greece in the Kozuf–Aridean volcanic... 相似文献
5.
U. M. Leiko 《Kinematics and Physics of Celestial Bodies》2016,32(6):299-306
The results of an analysis of the north–south asymmetry in solar activity and solar magnetic fields are reported. The analysis is based on solar mean magnetic field and solar polar magnetic field time series, 1975–2015 (http://wso.stanford.edu), and the Greenwich sunspot data, 1875–2015 (http://solarscience.msfc.nasa.gov/greenwch.shtml). A long-term cycle (small-scale magnetic fields, toroidal component) of ~140 years is identified in the north–south asymmetry in solar activity by analyzing the cumulative sum of the time series for the north–south asymmetry in the area of sunspots. A comparative analysis of the variations in the cumulative sums of the time series composed of the daily values of the sun’s global magnetic field and in the asymmetry of the daily sunspot data over the time interval 1975–2015 shows that the photospheric large-scale magnetic fields may also have a similar long-term cycle. The variations in the asymmetry of large-scale and small-scale solar magnetic fields (sunspot area) are in sync until 2005.5 and in antiphase since then. 相似文献
6.
7.
Interplay between an axial channel belt,slope gullies and overbank deposition in the Puchkirchen Formation in the Molasse Basin,Austria 下载免费PDF全文
Larisa U. Masalimova Donald R. Lowe Timothy Mchargue Richard Derksen 《Sedimentology》2015,62(6):1717-1748
Deep‐water sediments in the Molasse Basin, Austria, were deposited in a narrow foreland basin dominated by a large channel belt located between the steep Alpine fold and thrust belt to the south and the gentler northern slope off the Bohemian Massif. Several gas fields occur outside the channel belt, along the outer bend of a large meander. Accumulation of these overbank sediments reflects a complicated interplay between slope accommodation and debris‐flow and turbidity‐flow interaction within the channel. The tectonically oversteepened northern slope of the basin (ca 2 to 3°) developed a regionally important erosional surface, the Northern Slope Unconformity, which can be traced seismically for >100 km in an east–west direction and >20 km from the channel to the north. The slope preserves numerous gullies sourced from the north that eroded into the channel belt. These gullies were ca 20 km long, <1 km wide and ca 200 m deep. As the channel aggraded, largely inactive and empty gullies served as entry points into the overbank area for turbidity currents within the axial channel. Subsequently, debris‐flow mounds, 7 km wide and >15 km long, plugged and forced the main channel to step abruptly ca 7 km to the south. This resulted in development of an abrupt turn in the channel pathway that propagated to the east and probably played a role in forming a sinuous channel later. As debris‐flow topography was healed, flows spread out onto narrow area between the main channel and northern slope forming a broad fine‐grained apron that serves as the main gas reservoir in this area. This model of the overbank splay formation and the resulting stratigraphic architecture within the confined basin could be applied in modern and ancient systems or for subsurface hydrocarbon reservoirs where three‐dimensional seismic‐reflection data is limited. This study elucidates the geomorphology of the oversteepened slope of the under‐riding plate and its effects on the sedimentation. 相似文献
8.
9.
In Lake Cuicocha watershed, a young caldera lake, soils consist of volcanic deposits with a high SiO2 and Al2O3 content; these andisols are in an early stage of development; and in drainage water from the watershed, aluminium concentrations
reach 15.0 μmol L−1. Total aluminium concentrations in Lake Cuicocha water raises up to 7.2 μmol L−1, with nearly 70% occurring as filterable Al at neutral to weak alkaline conditions. Al polymerization to gelatinous aggregates
of a few hundred micrometres in diameter as well as the occurrence of Al microcrystals like gibbsite as an ageing product
of gelatinous Al polymers was noted in the lake water. The gelatinous Al leads to the formation of larger aggregates resulting
in flocs of bacteria, algae, microorganisms and detritus. 相似文献
10.
Geochemical processes controlling the spatial distribution of selenium in soil and water,west central South Dakota,USA 总被引:8,自引:0,他引:8
Selenium (Se) is essential in the human diet, but has a low threshold for toxic concentration. It is recommended that nutrients
such as Se should be consumed through foods as part of a normal diet. Se concentrations in crops and meat depend on the amount
of labile Se in the soil where crops and forage are grown. Therefore, managing agriculture for optimal Se in grain crops and
forage requires an understanding of the distribution and mobility of Se. Elevated concentrations of Se occur in waters, soils,
and forage 120 km west of Pierre in west central South Dakota, USA. The research site lies in an elevated, dissected plain
where soils developed on gently dipping Pierre Shale. Soils were sampled along catena transects and waters collected from
soil, ponds, and shallow borings in areas of known elevated forage and crop Se. Soil extracts from saturated-paste extraction
and acid (aqua regia and hydrofluoric acid) extraction were analyzed. Selenate was the dominant Se species in both acid and
saturated-paste extracts; selenite and organic Se were below detection (<0.2 ppb) in the same soil extracts. On average, 98% of soil Se was not water-soluble. The distribution of total Se shows much less spatial variation than water-soluble
Se in the areas sampled. Se shows correlation with organic carbon in soils and waters, suggesting its association with organic
carbon. Ca shows some correlation to Se in acid extracts, but not in saturated-paste extracts or in waters. Total Se shows
no significant correlation to Na, Mg, and total S in the soils. Se in saturated-paste extracts and water samples shows good
correlations with Na, Mg, and SO4, suggesting that evaporitic Na–Mg–sulfate minerals may temporarily concentrate water-soluble Se in shallow soils. The dissolution
and precipitation of these Na–Mg–sulfate salts together with pH and oxidation–reduction conditions apparently control water-soluble
Se distribution and mobilization in shallow subsurface environments. 相似文献