全文获取类型
收费全文 | 7294篇 |
免费 | 797篇 |
国内免费 | 775篇 |
专业分类
林业 | 708篇 |
农学 | 631篇 |
基础科学 | 1054篇 |
1720篇 | |
综合类 | 2403篇 |
农作物 | 238篇 |
水产渔业 | 491篇 |
畜牧兽医 | 1278篇 |
园艺 | 88篇 |
植物保护 | 255篇 |
出版年
2025年 | 2篇 |
2024年 | 136篇 |
2023年 | 144篇 |
2022年 | 192篇 |
2021年 | 254篇 |
2020年 | 278篇 |
2019年 | 324篇 |
2018年 | 215篇 |
2017年 | 324篇 |
2016年 | 348篇 |
2015年 | 314篇 |
2014年 | 399篇 |
2013年 | 473篇 |
2012年 | 615篇 |
2011年 | 588篇 |
2010年 | 430篇 |
2009年 | 443篇 |
2008年 | 402篇 |
2007年 | 494篇 |
2006年 | 420篇 |
2005年 | 313篇 |
2004年 | 214篇 |
2003年 | 218篇 |
2002年 | 152篇 |
2001年 | 145篇 |
2000年 | 144篇 |
1999年 | 118篇 |
1998年 | 102篇 |
1997年 | 67篇 |
1996年 | 89篇 |
1995年 | 89篇 |
1994年 | 64篇 |
1993年 | 82篇 |
1992年 | 51篇 |
1991年 | 55篇 |
1990年 | 42篇 |
1989年 | 45篇 |
1988年 | 28篇 |
1987年 | 23篇 |
1986年 | 11篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有8866条查询结果,搜索用时 0 毫秒
11.
Lighting and air conditioning energy consumption of stadium is very large. That how to use natural lighting and natural ventilation is the key pivot for stadium energy efficiency design. A variety of passive energy efficient measures were used in the Multi functional stadium of Guangdong College of Pharmacy, such as lighting windows with vertical blinds in east and west facades, Openable windows on south and north facades, planting roof and etc. It was found that the above mentioned passive designs could 〖JP2〗increase natural illumination of competition venues, prevent the glare, raise wind speed in audience area, improve comfort level and decrease internal surface temperature of affiliated building and reduce heat radiation. 相似文献
12.
In order to realize energy saving of urban nightscape luminance, power consumption was studied with the change of urban luminance. Based on energy consumption indicator(LPD) of 8.9 W/m2, it was calculated that about 0.84W more energy consumption would be for 1 cd/m2 increase of luminance. The effect of 1 to 2 cd/m2 increase was hard to distinguish for human’s visual function under urban nightscape, while the energy consumption was great. Under the same lighting condition, the reflecting characteristics of building surface materials could affect directly the luminance of nightscape. With the study of the reflecting characteristics of different building surface materials, an illumination calculation model for building was proposed based on the formulation of CIE. 相似文献
13.
以农业大省河北省为例,对农民生活能源使用现状进行分析,并进行影响因素分析与聚类分析,在此基础上提出政策建议。 相似文献
14.
Two energy balance closure approaches: applications and comparisons over an oasis-desert ecotone 总被引:1,自引:0,他引:1
Studies of energy balance that rely on eddy covariance(EC) are always challenged by energy balance closure, which is mainly caused by the underestimations of latent heat flux(LE) and sensible heat flux(Hs). The Bowen ratio(BR) and energy balance residual(ER) approaches are two widely-used methods to correct the LE. A comprehensive comparison of those two approaches in different land-use types is essential to accurately correcting the LE and thus improving the EC experiments. In this study, two energy balance approaches(i.e., BR and ER) were compared to correct the LE measured at six EC sites(i.e., three vegetated, one mixed and two non-vegetated sites) in an oasis-desert ecotone of the Heihe River Basin, China. The influences of meteorological factors on those two approaches were also quantitatively assessed. Our results demonstrated that the average energy closure ratio((LE+Hs)/(Rn–Gs); where Rn is the surface net radiation and Gs is the surface soil heat flux) was approximately close to 1.0 at wetland, maize and village sites, but far from 1.0 at orchard, Gobi and desert sites, indicating a significant energy imbalance at those three latter sites. After the corrections of BR and ER approaches that took into account of soil heat storage, the corrected LE was considerably larger than the EC-measured LE at five of six EC sites with an exception at Gobi site. The BR and ER approaches yielded approximately similar corrected LE at vegetated and mixed sites, but they generated dissimilar results at non-vegetated sites, especially at non-vegetated sites with low relative humidity, strong wind, and large surface-air temperature difference. Our findings provide insight into the applicability of BR and ER approaches to correcting EC-based LE measurements in different land-use types. We recommend that the BR-corrected and ER-corrected LE could be seriously reconsidered as validation references in dry and windy areas. 相似文献
15.
The Xihu desert wetland is located in an extremely arid area in Dunhuang,Gansu province of Northwest China.The area is home to an unusual geographic and ecological environment that is considered unique,both in China and the world.Microclimate is not only related to topography,but is also affected by the physical properties of underlying ground surfaces.Microclimate and CO2 flux have different characteristics under different underlying surface conditions.However,until now,few studies have investigated the microclimate characteristics and CO2 flux in this area.The eddy covariance technique(ECT) is a widely used and effective method for studying such factors in different ecosystems.Basing on data from continuous fine days obtained in the Dunhuang Xihu desert wetland between September 2012 and September 2013,this paper discussed and compared the characteristics of daily microclimate variations and CO2 fluxes between the two periods.Results from both years showed that there was a level of turbulent mixing and updraft in the area,and that the turbulent momentum flux was controlled by wind shear under good weather conditions.The horizontal wind velocity,friction wind velocity and vertical wind velocity were commendably consistent with each other.Air temperature in the surface layer followed an initial decreasing trend,followed by an increasing then decreasing trend under similar net radiation conditions.With changes in air temperature,the soil temperature in the surface layer follows a more obvious sinusoidal fluctuation than that in the subsoil.Components of ground surface radiation during the two study periods showed typical diurnal variations.The maximum diurnal absorption of CO2 occurred at around 11:00(Beijing time) in the Xihu desert wetland,and the concentrations of CO2 in both periods gradually decreased with time.This area was therefore considered to act as a carbon sink during the two observation periods. 相似文献
16.
17.
18.
从利用核聚变获取新能源、天然气水合物转化为天然气和输气管道与地下储气库建设之间的关系等方面,分析与预测了21世纪能源的转换趋势。认为在新世纪应加快对新型能源的开发与利用,尤其是我国要大力发展天然气工业,加强对天然气水合物特别是水合物转化为天然气的研究工作。 相似文献
19.
王智 《西北农林科技大学学报(自然科学版)》1987,(4):47-54
本文提出了推算畦灌水流推进过程的一个数学模型。田间试验表明,该模型的平均误差小于4%,最大误差小于10%。根据这个模型,文中还提出了计算改水成数和畦长的公式。 相似文献
20.
对陕西关中棉区受控与自控两类棉田生态系统的能量流动作了比较研究。结果表明:(1)两类棉田系统的食物网结构、营养层、生态锥体、生态效率等基本特征相似,在能量分配方面有量的差异;(2)天敌的利用效率在两类系统间有显著差异;(3)受控棉田系统的现存量与摄能量的营养级金字塔具有更快的递降率。 相似文献