全文获取类型
收费全文 | 10911篇 |
免费 | 1458篇 |
国内免费 | 1544篇 |
专业分类
测绘学 | 956篇 |
大气科学 | 1159篇 |
地球物理 | 3804篇 |
地质学 | 4531篇 |
海洋学 | 1045篇 |
天文学 | 1024篇 |
综合类 | 466篇 |
自然地理 | 928篇 |
出版年
2023年 | 138篇 |
2022年 | 363篇 |
2021年 | 519篇 |
2020年 | 428篇 |
2019年 | 477篇 |
2018年 | 598篇 |
2017年 | 554篇 |
2016年 | 696篇 |
2015年 | 637篇 |
2014年 | 685篇 |
2013年 | 834篇 |
2012年 | 724篇 |
2011年 | 684篇 |
2010年 | 626篇 |
2009年 | 640篇 |
2008年 | 598篇 |
2007年 | 500篇 |
2006年 | 456篇 |
2005年 | 296篇 |
2004年 | 274篇 |
2003年 | 212篇 |
2002年 | 205篇 |
2001年 | 202篇 |
2000年 | 150篇 |
1999年 | 138篇 |
1998年 | 144篇 |
1997年 | 115篇 |
1996年 | 108篇 |
1995年 | 83篇 |
1994年 | 89篇 |
1993年 | 77篇 |
1992年 | 80篇 |
1991年 | 79篇 |
1990年 | 73篇 |
1989年 | 89篇 |
1988年 | 57篇 |
1987年 | 66篇 |
1986年 | 63篇 |
1985年 | 47篇 |
1984年 | 58篇 |
1983年 | 55篇 |
1982年 | 54篇 |
1979年 | 58篇 |
1978年 | 54篇 |
1977年 | 53篇 |
1975年 | 48篇 |
1974年 | 46篇 |
1973年 | 56篇 |
1972年 | 44篇 |
1971年 | 52篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
231.
232.
233.
234.
通过样方法及植被调查法,从纵向和横向两个不同方向对工程破坏后的沙地植被进行比较研究。在纵向上,由于植被恢复时间的不同,其恢复的程度也不同。植被恢复状况可明显的分3个阶段:定居前期(1~6a)主要以多年生羽叶三芒草和小半灌木绢蒿为主;中期(7~11a)主要以多年生小半灌木青杆沙蒿为主;后期(12~16a)主要生长着多年生麻黄及草本沙苔,地表出现大面积的生物结皮。在横向上,将恢复后期的植被与原始植被进行比较。通过对植被发育不同阶段过程中物种重要值、物种多样性、丰富度及生活型的变化等方面的调查,发现在不同阶段,各物种的重要值发生了不同的变化,物种多样性指数,均匀度指数,丰富度指数到后期均增大。 相似文献
235.
236.
Joeri Kaal Yolanda Carrión Marco Eleni Asouti Maria Martín Seijo Antonio Martínez Cortizas Manuela Costa Casáis Felipe Criado Boado 《Quaternary Science Reviews》2011,30(1-2):161-175
The Holocene fire regime is thought to have had a key role in deforestation and shrubland expansion in Galicia (NW Spain) but the contribution of past societies to vegetation burning remains poorly understood. This may be, in part, due to the fact that detailed fire records from areas in close proximity to archaeological sites are scarce. To fill this gap, we performed charcoal analysis in five colluvial soils from an archaeological area (Campo Lameiro) and compared the results to earlier studies from this area and palaeo-ecological literature from NW Spain. This analysis allowed for the reconstruction of the vegetation and fire dynamics in the area during the last ca 11 000 yrs. In the Early Holocene, Fabaceae and Betula sp. were dominant in the charcoal record. Quercus sp. started to replace these species around 10 000 cal BP, forming a deciduous forest that prevailed during the Holocene Thermal Maximum until ~5500 cal BP. Following that, several cycles of potentially fire-induced forest regression with subsequent incomplete recovery eventually led to the formation of an open landscape dominated by shrubs (Erica sp. and Fabaceae). Major episodes of forest regression were (1) ~5500–5000 cal BP, which marks the mid-Holocene cooling after the Holocene Thermal Maximum, but also the period during which agropastoral activities in NW Spain became widespread, and (2) ~2000–1500 cal BP, which corresponds roughly to the end of the Roman Warm Period and the transition from the Roman to the Germanic period. The low degree of chronological precision, which is inherent in fire history reconstructions from colluvial soils, made it impossible to distinguish climatic from human-induced fires. Nonetheless, the abundance of synanthropic pollen indicators (e.g. Plantago lanceolata and Urtica dioica) since at least ~6000 cal BP strongly suggests that humans used fire to generate and maintain pasture. 相似文献
237.
The use of the exponent K(q) function to delimit homogeneous regions in regional frequency analysis of extreme annual daily rainfall
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A. P. García‐Marín J. Estévez C. Sangüesa‐Pool R. Pizarro‐Tapia J. L. Ayuso‐Muñoz F. J. Jimenez‐Hornero 《水文研究》2015,29(1):139-151
The regional frequency analysis of extreme annual rainfall data is a useful methodology in hydrology to obtain certain quantile values when no long data series are available. The most crucial step in the analysis is the grouping of sites into homogeneous regions. This work presents a new grouping criterion based on some multifractal properties of rainfall data. For this purpose, a regional frequency analysis of extreme annual rainfall data from the Maule Region (Chile) has been performed. Daily rainfall data series of 53 available stations have been studied, and their empirical moments scaling exponent functions K(q) have been obtained. Two characteristics parameters of the K(q) functions (γmax and K(0)) have been used to group the stations into three homogeneous regions. Only five sites have not been possible to include into any homogenous regions, being the local frequency analysis of extreme daily rainfall the most appropriate method to be used at these locations. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
238.
Low-order stabilized finite element for the full Biot formulation in soil mechanics at finite strain
Lluís Monforte Pedro Navas Josep Maria Carbonell Marcos Arroyo Antonio Gens 《国际地质力学数值与分析法杂志》2019,43(7):1488-1515
This article presents a novel finite element formulation for the Biot equation using low-order elements. Additionally, an extra degree of freedom is introduced to treat the volumetric locking steaming from the effective response of the medium; its balance equation is also stabilized. The accuracy of the proposed formulation is demonstrated by means of numerical analyses. 相似文献
239.
240.
Pegmatite dikes bearing andalusite crosscut foliation S2 in Alpujarride gneisses and schists. Post‐S2 andalusite is transposed by a foliation S3, defined by fibrolite, which affects the dikes. The dikes represent highly differentiated granitic magmas with low REE and Zr contents and a positive Eu anomaly. U‐Pb SHRIMP dating of magmatic zircons provided Pan‐African ages (cores) and late Variscan ages (rims). However, U‐rich rims also provided metamorphic Alpine ages, supporting a polyorogenic tectonometamorphic history for pre‐Mesozoic Alpujarride rocks. 相似文献