全文获取类型
收费全文 | 2306篇 |
免费 | 409篇 |
国内免费 | 725篇 |
专业分类
测绘学 | 239篇 |
大气科学 | 382篇 |
地球物理 | 476篇 |
地质学 | 1438篇 |
海洋学 | 394篇 |
天文学 | 110篇 |
综合类 | 129篇 |
自然地理 | 272篇 |
出版年
2024年 | 6篇 |
2023年 | 30篇 |
2022年 | 105篇 |
2021年 | 107篇 |
2020年 | 96篇 |
2019年 | 121篇 |
2018年 | 120篇 |
2017年 | 113篇 |
2016年 | 131篇 |
2015年 | 110篇 |
2014年 | 122篇 |
2013年 | 136篇 |
2012年 | 151篇 |
2011年 | 132篇 |
2010年 | 159篇 |
2009年 | 133篇 |
2008年 | 131篇 |
2007年 | 127篇 |
2006年 | 111篇 |
2005年 | 118篇 |
2004年 | 107篇 |
2003年 | 108篇 |
2002年 | 197篇 |
2001年 | 157篇 |
2000年 | 122篇 |
1999年 | 87篇 |
1998年 | 60篇 |
1997年 | 56篇 |
1996年 | 40篇 |
1995年 | 33篇 |
1994年 | 26篇 |
1993年 | 18篇 |
1992年 | 31篇 |
1991年 | 15篇 |
1990年 | 15篇 |
1989年 | 20篇 |
1988年 | 23篇 |
1987年 | 13篇 |
1986年 | 14篇 |
1985年 | 10篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 8篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1957年 | 1篇 |
1954年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有3440条查询结果,搜索用时 15 毫秒
1.
Soil moisture, a critical variable in the hydrologic cycle, is highly influenced by vegetation restoration type. However, the relationship between spatial variation of soil moisture, vegetation restoration type and slope length is controversial. Therefore, soil moisture across soil layers (0-400 cm depth) was measured before and after the rainy season in severe drought (2015) and normal hydrological year (2016) in three vegetation restoration areas (artificial forestland, natural forestland and grassland), on the hillslopes of the Caijiachuan Catchment in the Loess area, China. The results showed that artificial forestland had the lowest soil moisture and most severe water deficit in 100-200 cm soil layers. Water depletion was higher in artificial and natural forestlands than in natural grassland. Moreover, soil moisture in the shallow soil layers (0-100 cm) under the three vegetation restoration types did not significantly vary with slope length, but a significant increase with slope length was observed in deep soil layers (below 100 cm). In 2015, a severe drought hydrological year, higher water depletion was observed at lower slope positions under three vegetation restoration types due to higher transpiration and evapotranspiration and unlikely recharge from upslope runoff. However, in 2016, a normal hydrological year, there was lower water depletion, even infiltration recharge at lower slope positions, indicating receiving a large amount of water from upslope. Vegetation restoration type, precipitation, slope length and soil depth during a rainy season, in descending order of influence, had significant effects on soil moisture. Generally, natural grassland is more beneficial for vegetation restoration than natural and artificial forestlands, and the results can provide useful information for understanding hydrological processes and improving vegetation restoration practices on the Loess Plateau 相似文献
2.
Zhi-gen Yang Wen-yao Zhu C. K. Shum Feng-chun Shu 《Chinese Astronomy and Astrophysics》2002,26(4):303-502
The vertical deformation rates (VDRs) and horizontal deformation rates (HDRs) of Shanghai VLBI station in China and Kashima and Kashima34 VLBI stations in Japan were re-analysed using the baseline length change rates from Shanghai to 13 global VLBI stations, and from Kashima to 27 stations and from Kashima34 to 12 stations, based on the NASA VLBI global solution glb1123 (Ma, 1999). The velocity vectors of the global VLBI stations were referred to the ITRF97 reference frame, and the Eulerian vectors of different models of plate motion were used for comparative solutions. The VDR of Shanghai station is estimated to be −1.91±0.56 mm/yr, and those of Kashima and Kashima34 stations, −3.72±0.74mm/yr and −8.81±0.84mm/yr, respectively. The difference between the last two was verified by further analysis. Similar estimates were also made for the Kokee, Kauai and MK_VLBA VLBI stations in mid-Pacific. 相似文献
3.
本文简要介绍了陆地系列地球资源卫星的轨道特征,并以陆地—5号资源卫星为例,根据南山站的座标计算出该站对陆地—5号资源卫星的可观测范围及以该站为中心的卫星的覆盖区域。 相似文献
4.
本文简述了国际天球参考架的发展历史和现在射电参考架的现状—基准源选择的标准和参考架的稳定性。描述了地面上光学观测在依巴谷参考架的维持和加密的一系列工作。介绍由天体测量卫星GAIA和SIM给出的天球参考架可能逵到的精度。详述了在今后十年中地面天体测量的作用以及正在开展有关天球参考架的研究课题 ,同时也列出了我国正在和即将开展天体测量的几个研究课题 相似文献
5.
6.
Zhou Haiyan Zhou Xun Chai Rui Yu Lan Liu Chunhui Li Liangping 《Environmental Geology》2008,53(7):1483-1489
Thermal groundwater occurs in bedrock aquifers consisting of the dolomite of the Wumishan Group of the Jixianin System and
the Cambrian carbonate in the Xiaotangshan geothermal field near the northern margin of the North China Plain, China. The
hot water in the geothermal field of basin-type discharges partly in the form of the Xiaotangshan hot spring under natural
conditions. The hot water has TDS of less than 600 mg/L and is of Na·Ca-HCO3 type. The geothermal water receives recharge from precipitation in the mountain area with elevation of about 500 m above
sea level to the north of the spring. Thermal groundwater flows slowly south and southeast through a deep circulation with
a residence time of 224 years estimated with the Ra–Rn method. The Xiaotangshan hot spring dried up in the middle of the 1980s
owing to the increasing withdrawal of the hot water in the geothermal field in the past decades. The water level of the geothermal
system still falls continually at an annual average rate of about 2 m, although water temperature changes very little, indicating
that the recharge of such a geothermal system of basin-type is limited. Over-exploitation has a dramatic impact on the geothermal
system, and reduction in exploitation and reinjection are required for the sustainable usage of the hot water. 相似文献
7.
Understanding of isotopic variations in leaf water is important for reconstruction of paleoclimate and assessment of global biochemical processes. We report here a study of isotopic distributions within a single needle of two pine species, Pinus resinosa Ait and Pinus strobes L., with the objective of understanding how isotopic compositions of leaf water are controlled by environmental and physiological variables. A 2D model was developed to simulate along-leaf isotopic variations and bulk leaf water isotopic compositions. In addition to variables common to all leaf water isotopic models, this 2D model also takes into account the specific geometry and dimensions of pine needles and the isotopic transport in xylem and mesophyll. The model can successfully simulate oxygen isotopic variations along a single needle and averaged over a leaf (bulk leaf water). The simulations suggest that isotopic composition of the bulk leaf water does not always depend only upon the average transpiration rate, which in turn raises questions about using leaf water isotopic values to estimate transpiration rates. An unsuccessful attempt to simulate along-needle hydrogen isotopic variations suggests that certain unknown biological process(es) may not have been incorporated into our 2D model, and if so, it calls for a reevaluation of all other models for hydrogen isotopic simulations of leaf water since they too lack these processes.Existing leaf water isotopic models are reviewed in this work. In particular, we evaluate the most frequently used model, the stomatal boundary layer model (also referred to as the Craig-Gordon model). We point out that discrepancy between the boundary layer model and the measured bulk leaf water seems to depend upon relative humidity. Using our 2D model, we show that this humidity dependency is a result of an interplay between environmental and physiological conditions: if the transpiration rate of plant leaves decreases with increasing relative humidity, our 2D model can reproduce the pattern of isotopic discrepancy between boundary layer model predictions and observations, enabling us to understand better the reason behind this discrepancy. 相似文献
8.
激光粒度仪的广泛应用带来了与历史数据的对比问题,因此需要建立激光粒度仪与早期分析结果之间的关系。根据江苏海岸潮滩沉积物样品的激光粒度仪和移液管-筛析法分析,对粒度参数、粒度组分等进行了对比,对两种方法的差异进行了分析。结果表明,对于江苏潮滩沉积物粒度参数中平均粒径的激光粒度仪与移液管-筛析分析结果之间有良好的线性关系;筛分法测得的粗颗粒物质较激光法偏少,而移液管法法测得的细颗粒物质较激光法偏多;将样品分类之后再进行两种方法所获粒度参数的回归分析,相关性得以提高,说明不同粒度组成的沉积物对分析结果的对比有不同的影响。两种方法之间的换算关系不仅与研究区域有关,而且与沉积物本身的粒度组成有关。 相似文献
9.
一株胶质芽胞杆菌对磷矿石风化作用的实验研究 总被引:1,自引:0,他引:1
以一株质芽孢杆菌为例研究实验条件下微生物对磷矿石的风化作用.以直接作用和间接作用的方式研究培养基中胶质芽孢杆菌对磷矿粉的风化作用,即在装有100目磷矿石粉的液体培养基中接种,研究该菌对磷矿石粉的直接风化作用;同时,将装有100目磷矿石粉的透析袋放入液体培养基中再接入该菌,研究其对磷矿石粉的间接风化作用.按不同时间取培养液上清液,过滤,用电感耦合等离子体发射光谱(ICP-OES)测定滤液中Ca2 、Mg2 、Na 、Mn2 、Al3 、Fe3 和K 等浓度,比色法测定水溶性P(Pws)和水溶性Si (Siws) 的含量;滤膜上的固体物称重并消解后,同上方法测定Ca2 、Mg2 、Na 、Mn2 、Al3 、Fe3 和K 等浓度以及Pws和Siws含量.此外,细菌风化作用后的矿物残渣用电子探针作表面微观形态分析和XRD矿物物相分析.结果表明:胶质芽孢杆菌对磷矿石粉风化的直接作用强度大于间接作用;对不同矿物的风化强度不同,对粘土矿物的风化作用较明显.提出细菌对磷矿石的风化作用源自细菌生长导致的机械破坏作用、胞外分泌物的生化降解作用以及多种因素之间的协同作用. 相似文献
10.