全文获取类型
收费全文 | 14735篇 |
免费 | 3757篇 |
国内免费 | 5626篇 |
专业分类
测绘学 | 2292篇 |
大气科学 | 2546篇 |
地球物理 | 3001篇 |
地质学 | 9278篇 |
海洋学 | 3453篇 |
天文学 | 362篇 |
综合类 | 1295篇 |
自然地理 | 1891篇 |
出版年
2024年 | 123篇 |
2023年 | 343篇 |
2022年 | 878篇 |
2021年 | 1111篇 |
2020年 | 930篇 |
2019年 | 1072篇 |
2018年 | 1019篇 |
2017年 | 915篇 |
2016年 | 941篇 |
2015年 | 1086篇 |
2014年 | 1040篇 |
2013年 | 1262篇 |
2012年 | 1338篇 |
2011年 | 1363篇 |
2010年 | 1306篇 |
2009年 | 1199篇 |
2008年 | 1215篇 |
2007年 | 1153篇 |
2006年 | 1018篇 |
2005年 | 856篇 |
2004年 | 668篇 |
2003年 | 467篇 |
2002年 | 516篇 |
2001年 | 509篇 |
2000年 | 413篇 |
1999年 | 274篇 |
1998年 | 135篇 |
1997年 | 148篇 |
1996年 | 118篇 |
1995年 | 94篇 |
1994年 | 84篇 |
1993年 | 62篇 |
1992年 | 61篇 |
1991年 | 40篇 |
1990年 | 36篇 |
1989年 | 28篇 |
1988年 | 35篇 |
1987年 | 37篇 |
1986年 | 23篇 |
1985年 | 16篇 |
1984年 | 16篇 |
1983年 | 14篇 |
1982年 | 12篇 |
1981年 | 10篇 |
1979年 | 14篇 |
1977年 | 7篇 |
1975年 | 8篇 |
1973年 | 9篇 |
1964年 | 7篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Forests in the Southeastern United States are predicted to experience future changes in seasonal patterns of precipitation inputs as well as more variable precipitation events. These climate change‐induced alterations could increase drought and lower soil water availability. Drought could alter rooting patterns and increase the importance of deep roots that access subsurface water resources. To address plant response to drought in both deep rooting and soil water utilization as well as soil drainage, we utilize a throughfall reduction experiment in a loblolly pine plantation of the Southeastern United States to calibrate and validate a hydrological model. The model was accurately calibrated against field measured soil moisture data under ambient rainfall and validated using 30% throughfall reduction data. Using this model, we then tested these scenarios: (a) evenly reduced precipitation; (b) less precipitation in summer, more in winter; (c) same total amount of precipitation with less frequent but heavier storms; and (d) shallower rooting depth under the above 3 scenarios. When less precipitation was received, drainage decreased proportionally much faster than evapotranspiration implying plants will acquire water first to the detriment of drainage. When precipitation was reduced by more than 30%, plants relied on stored soil water to satisfy evapotranspiration suggesting 30% may be a threshold that if sustained over the long term would deplete plant available soil water. Under the third scenario, evapotranspiration and drainage decreased, whereas surface run‐off increased. Changes in root biomass measured before and 4 years after the throughfall reduction experiment were not detected among treatments. Model simulations, however, indicated gains in evapotranspiration with deeper roots under evenly reduced precipitation and seasonal precipitation redistribution scenarios but not when precipitation frequency was adjusted. Deep soil and deep rooting can provide an important buffer capacity when precipitation alone cannot satisfy the evapotranspirational demand of forests. How this buffering capacity will persist in the face of changing precipitation inputs, however, will depend less on seasonal redistribution than on the magnitude of reductions and changes in rainfall frequency. 相似文献
2.
PRELIMINARY STUDY ON THE FLOODING AND DROUGHT CLAMITY DURING PAST 1500 YEARS IN THE HAI′AN REGION,JIANGSU PROVINCE 总被引:1,自引:0,他引:1
Collection and arrangement of the historical records of climatic changes and environment evolution,espectial-lyin the aspect of calamities,are made on the history documents of past 1500 years about Haiˊan region,Jiangsu Province.There existed two obvious flooding-drought frequently-occurring periods:one was from 1550 AD to 1850 AD and another was 100 AD to 1200AD.The period of 1550 AD to1850 AD is interrupted by two relatively arid and cold climatic periods:one was from 1630 AD to 1700 AD and another was 1750 AD to 1820 AD.The main characteristic of the calamity periods is that they occurred by turns,and sometimes,both drought and flooding occurred in the same year.The instability of the climatic changes in the Little Ice Age may be the main reason of the frequently-occurring flooding and drought in Haiˊan region.Research results also show that the frequently-occurring periods of flooding and drought is in close relationship with the solar activity,and therefore,occurrence of the flooding and drought may be in relation with the intensity of the solar activity.This hypothesis may need further study in the future. 相似文献
3.
4.
隧道工程地质评价的内容和方法 总被引:2,自引:0,他引:2
结合工程隧道实际,首先阐述了隧道工程的基本地质环境和工程地质条件,然后针对隧道工程可能出现的不良地质现象和可采取的工程措施,从大气降水、围岩稳定、围岩压力、洞口稳定、隧道比选等角度探讨隧道工程地质评价的主要方法和一般内容,进行隧道工程地质评价,为隧道施工、支护提供了依据. 相似文献
5.
Baolin Tan Haisheng Ji Guangli Huang Tuanhui Zhou Qiwu Song Yu Huang 《Solar physics》2006,239(1-2):137-148
Using one-minute cadence vector magnetograms from Big Bear Solar Observatory (BBSO), we analyze the temporal behavior of derived
longitudinal electric currents associated with two flares on July 26, 2002. One of the events is an M1.0 flare which occurred
in active region NOAA 10044, while the other is an M8.7 flare in the adjacent region 10039. Rapid changes of magnetic fields
in the form of flux emergence are found to be associated with both of these events. However, the temporal behavior of electric
currents are very different. For the M1.0 flare, the longitudinal electric current density drops rapidly near the flaring
neutral line; while for the M8.7 flare, the current density rapidly increases, confirming the picture of the current-carrying
flux emergence. We offer a possible explanation for such a difference: magnetic reconnection at different heights for the
two events, near the photosphere for the M1.0 flare, and higher up for the M8.7 flare. 相似文献
6.
7.
The Tamtsag Basin is located in the extreme eastern portion of the Mongolia. The Basin and its counterpart in China (the Hailar Basin) are united a whole basin on the structural setting. In recent years, the Tamtsag Basin attracts more and more attention with the important exploration discovered in the 19th block by SOCO and in Hailar Basin of China. This paper discusses the exploration potential of Tamtsag Basin from the viewpoint of petroleum geology. 相似文献
8.
9.
10.
The effect of ocean wave breaking as a non-Bragg mechanism on backscattering cross-section and modulation transfer functions (MTF) of radar was investigated based on Bragg resonance theory and parametric method. The result showed that the additional effect of wave breaking on backscattering cross-section is not more than 20% except for the small incident angle of VV polarized electromagnetic (e.m.) wave but is significant for HH polarized e.m. wave. Breaking waves lead to increase in the modulus of tilt modulation MTF and the larger the wind speed, the faster the increase. For large incident angle, the modulus of tilt modulation MTF with wave breaking decreases quickly with incident angle for HH polarization and approach to that without wave breaking for VV polarization. The hydrodynamic MTF increases 30%-60% when considering wave breaking and the increase is larger for HH polarization than for VV polarization. 相似文献