全文获取类型
收费全文 | 1038篇 |
免费 | 228篇 |
国内免费 | 144篇 |
专业分类
测绘学 | 134篇 |
大气科学 | 34篇 |
地球物理 | 713篇 |
地质学 | 361篇 |
海洋学 | 46篇 |
天文学 | 4篇 |
综合类 | 50篇 |
自然地理 | 68篇 |
出版年
2024年 | 3篇 |
2023年 | 14篇 |
2022年 | 32篇 |
2021年 | 39篇 |
2020年 | 54篇 |
2019年 | 64篇 |
2018年 | 49篇 |
2017年 | 51篇 |
2016年 | 50篇 |
2015年 | 49篇 |
2014年 | 86篇 |
2013年 | 60篇 |
2012年 | 71篇 |
2011年 | 78篇 |
2010年 | 69篇 |
2009年 | 68篇 |
2008年 | 70篇 |
2007年 | 71篇 |
2006年 | 72篇 |
2005年 | 53篇 |
2004年 | 48篇 |
2003年 | 39篇 |
2002年 | 39篇 |
2001年 | 28篇 |
2000年 | 34篇 |
1999年 | 25篇 |
1998年 | 23篇 |
1997年 | 14篇 |
1996年 | 12篇 |
1995年 | 11篇 |
1994年 | 9篇 |
1993年 | 4篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 1篇 |
1986年 | 4篇 |
1983年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有1410条查询结果,搜索用时 31 毫秒
1.
Colin P. R. McCarter Stephen D. Sebestyen Susan L. Eggert Randall K. Kolka Carl P. J. Mitchell 《水文研究》2020,34(26):5354-5369
Sustainable fuels legislation and volatility in energy prices have put additional pressures on the forestry sector to intensify the harvesting of biomass for “advanced biofuel” production. To better understand how residual biomass removal after harvest affects forest hydrology in relatively low slope terrain, a Before-After-Control-Impact (BACI) study was conducted in the USDA Forest Service's Marcell Experimental Forest, Minnesota, USA. Hydrological measurements were made from 2010–2013 on a forested hillslope that was divided into three treatment blocks, where one block was harvested and residual biomass removed (Biomass Removed), the second was harvested and residual biomass left (Biomass Left), and the last block was left as an Unharvested Control. The pre-harvest period (2 years) was 2010–11 and post-harvest (2 years) was 2012–13. Water table elevation at the upslope and downslope position, subsurface runoff, and soil moisture were measured between May–November. Mixed effect statistical models were used to compare both the before-after and “control” treatment ratios (ratios between harvested hillslopes and the Unharvested Control hillslope). Subsurface runoff significantly increased (p < .05) at both harvested hillslopes but to a greater degree on the Biomass Left hillslope. Greater subsurface runoff volumes at both harvested hillslopes were driven by substantial increases during fall, with additional significant increases during summer on the Biomass Left hillslope. The hydrological connectivity, inferred from event runoff ratios, increased due to harvesting at both hillslopes but only significantly on the Biomass Left hillslope. The winter harvest minimized soil disturbance, resulting in no change to the effective hydraulic conductivity distribution with depth. Thus, the observed hydrological changes were driven by increased effective precipitation and decreased evapotranspiration, increasing the duration that both harvested hillslopes were hydrologically active. The harvesting of residual biomass appears to lessen hydrological connectivity relative to leaving residual biomass on the hillslope, potentially decreasing downstream hydrological impacts of similar forestry operations. 相似文献
2.
对旋挖钻机、转盘钻机各自特点和施工成本进行对比分析,提出组合施工方法设想,并通过工程实例得到成功实践,为以后类似工程提供借鉴。客观地提出了旋挖钻机的使用条件和面临的问题。 相似文献
3.
公用微机硬盘信息保护的一种有效方法 总被引:1,自引:0,他引:1
张玉彪 《中国海洋大学学报(自然科学版)》1994,(Z2)
介绍一种保护公用微机硬盘信息的有效方法.不仅可以预防各种计算机病毒,更重要的是可以防止人为操作对硬盘信息的破坏。适用于学校等开放性微机实验室的信息安全与保护. 相似文献
4.
5.
6.
A hierarchical real-time control architecture for a semi-autonomous underwater vehicle 总被引:1,自引:0,他引:1
This paper describes a real-time control architecture for Dual Use Semi-Autonomous Underwater Vehicle (DUSAUV), which has been developed at Korea Research Institute of Ships and Ocean Engineering (KRISO) for being a test-bed of development of underwater navigation and manipulator technologies. DUSAUV has three built-in computers, seven thrusters for six DOF motion control, one 4-function electric manipulator, one ballasting motor, built-in power source, and various sensors. A supervisor control system with GUI and a multi-purpose joystick is mounted on the surface vessel and communicates with vehicle through a fiber optic link. Furthermore, QNX, one of real-time operating system, is ported on the built-in control and navigation computers for real-time control purpose, while Microsoft OS product is ported in the supervisor computer for GUI programming convenience. A hierarchical control architecture, which consists of application layer, real-time layer and physical layer, has been developed for efficient control system of above complex underwater robotic system. The experimental results with implementation of the layered control architecture for various motion control of DUSAUV in an ocean engineering basin of KRISO is also presented. 相似文献
7.
8.
Estimating groundwater evapotranspiration rates using diurnal water-table fluctuations in a semi-arid riparian zone 总被引:2,自引:1,他引:1
Laura K. Lautz 《Hydrogeology Journal》2008,16(3):483-497
In semi-arid climates, phreatophytes draw on shallow aquifers, and groundwater evapotranspiration (ETG) is a principal component of groundwater budgets. Diurnal water table fluctuations, which often are a product of ETG, were monitored in the riparian zone of Red Canyon Creek, Wyoming, USA. These fluctuations were higher in a riparian wetland (2–36 mm) than a grass-covered meadow (1–6 mm). The onset and cessation of water-table fluctuations correspond to daily temperatures relative to freezing. Spatial differences were due to vegetation type and specific yield, while temporal changes were due to vegetation dormancy. Ratios of ETG to potential evapotranspiration (PET), K c,GW, were similar to ratios of actual evapotranspiration (ET) to PET, K c, in semi-arid rangelands. Before vegetation senescence, K c,GW increased between precipitation events, suggesting phreatophytes pull more water from the saturated zone as soil moisture decreases. In contrast, K c decreases with soil moisture following precipitation events as ET becomes increasingly water-limited. Error in ETG is primarily from estimates of specific yield (S y), which is difficult to quantify in heterogeneous sediments. ETG values may be more reliable because the range of acceptable S y is smaller than K c and S y does not change with vegetation type or soil moisture. 相似文献
9.
非均匀月壤介质的被动微波辐射传输模拟 总被引:1,自引:1,他引:0
基于非均匀月壤物理模型和辐射传输方程,模拟月壤介质中的微波辐射传输特性,探讨频率、月壤厚度等与月表亮温的关系。结果表明:在低频段,月壤微波辐射亮温的动态变化范围较大,可探测的月壤厚度大,3 GHz时的最大可探测月壤厚度达12.4 m;在高频段对应的可探测月壤厚度较小,特别是从50GHz往后的频率段内,最大可探测月壤厚度均小于2 m。不同频率的亮温-厚度变化曲线没有交叉点,且频率越高,所能探测的月壤厚度越小。根据模拟结果,建立了月壤厚度与亮温的查找表。基于查找表,利用单个波段的亮温数据即可得到月壤厚度信息。 相似文献
10.