全文获取类型
收费全文 | 587篇 |
免费 | 31篇 |
国内免费 | 90篇 |
专业分类
林业 | 27篇 |
农学 | 66篇 |
基础科学 | 164篇 |
170篇 | |
综合类 | 162篇 |
农作物 | 4篇 |
水产渔业 | 53篇 |
畜牧兽医 | 33篇 |
园艺 | 2篇 |
植物保护 | 27篇 |
出版年
2024年 | 4篇 |
2023年 | 18篇 |
2022年 | 14篇 |
2021年 | 14篇 |
2020年 | 16篇 |
2019年 | 23篇 |
2018年 | 24篇 |
2017年 | 22篇 |
2016年 | 30篇 |
2015年 | 19篇 |
2014年 | 28篇 |
2013年 | 28篇 |
2012年 | 56篇 |
2011年 | 48篇 |
2010年 | 47篇 |
2009年 | 38篇 |
2008年 | 21篇 |
2007年 | 44篇 |
2006年 | 21篇 |
2005年 | 30篇 |
2004年 | 25篇 |
2003年 | 27篇 |
2002年 | 23篇 |
2001年 | 9篇 |
2000年 | 11篇 |
1999年 | 7篇 |
1998年 | 9篇 |
1997年 | 11篇 |
1996年 | 7篇 |
1995年 | 4篇 |
1994年 | 6篇 |
1993年 | 3篇 |
1992年 | 7篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 5篇 |
1987年 | 4篇 |
1986年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有708条查询结果,搜索用时 10 毫秒
161.
162.
基于SWAT模型的内蒙古闪电河流域径流模拟研究 总被引:2,自引:0,他引:2
以内蒙古闪电河流域为研究对象,采用分布式水文模型SWAT对闪电河流域水文过程进行模拟,模型采用1962~1975年月径流资料进行参数率定,将1986-2001年作为模型的检验期对模型的适用性进行评估。结果表明:实测与模拟的逐月流量总体较为接近,率定期和验证期的Nash-Sutcliffe效率系数均大于0.60,实测与模拟的月流量之间的相关系数在0.65以上,率定期的多年平均相对误差小于5%。由于人类活动的影响,验证期的模拟误差相对较大,超过12%。总体来看,SWAT模型参数经多次率定之后,能够较好地模拟干旱半干旱地区的径流过程,在同类地区具有一定的适用性。模拟过程中,对于降水量突然增大的月份,SWAT模型对月径流量的模拟结果往往超过实测值一倍,直接导致整个模拟过程的误差偏大。然而,模拟值与降水量在年内分配上却表现出很好的相关性,降水偏大的月份径流量敏感性大。同时,SWAT模型模拟径流结果受参数影响很大,调整融雪参数值,模拟精度在原有基础上相对提高。 相似文献
163.
Arne Stensvand Håvard Eikemo Robert C. Seem David M. Gadoury 《European journal of plant pathology / European Foundation for Plant Pathology》2009,125(1):173-178
Darkness suppresses ascospore release in Venturia inaequalis, but the impact of light levels during the extended twilight and dusk that typify Nordic spring conditions is poorly understood.
Volumetric spore traps were operated at two different locations in Norway over several years. During the season of asocspore
release (approximately 1 April to 30 June), on 25 occasions when rain started during night (after 23:00 h and before 04:00 h)
and leaves remained wet until at least midnight the following day, the cumulative percentage of spores trapped at sunrise
did not exceed 1%, irrespective of temperature. Three hours after sunrise, cumulative ascospore release reached 0.8%, 3.0%,
and 8.1% at temperatures of 0 to 5°C, 5 to 10°C, and >10°C, respectively, and 50% release occurred at 11, 9, and 8 h after
sunrise. Additional field and laboratory studies indicated that the protracted dawn and dusk of Nordic latitudes, either alone
or in combination with low temperatures, does not substantially alter previously reported patterns of ascospore release. 相似文献
164.
165.
Effects of structural and depositional crusts on soil erosion on the Loess Plateau of China 总被引:2,自引:0,他引:2
Conventional tillage practices used on the Loess Plateau lead to different soil surface micro-topography which results in forming two types of soil crusts. The objective of this study was to explore the formation position, properties and erosion characteristics of structural crusts and depositional crusts under the influences of the microtopography in the rainfall experiments. Two simulated rainstorms were applied in the experiments. The first rainfall event was used for soil crust formation, then the following simulated rainfall storms at 40 mm h?1, 60 mm h?1, and 80 mm h?1 rates were applied to the soil boxes set to a 17.6% (10°) slope under three tillage types (contour tillage, artificial digging, and straight slope conditions) to investigate the resulting runoff discharge rate and sediment yield on crusted soil surface. Results show that: (1) structural crusts formed on the mounds, and depositional crusts formed in the depressions after the first rainfall events; structural crusts exhibit a lower thickness, bulk density, higher porosity and shear strength than depositional crusts; (2) structural crusts increased the runoff yield less and decreased the sediment yield more than depositional crusts; and (3) the runoff yield was significantly greater, and the sediment yield was lower on the crusted soil surface than that on the uncrusted soil surface, regardless of the effect of the tillage treatments. 相似文献
166.
王永其 《湖南农业大学学报(自然科学版)》2010,(4):138-141
随着经济的快速发展,环境污染越来越严重。排污权作为一项权利提出以后,其法律性质及法域归属问题争议颇多,有学者认为排污权是以环境容量为客体的用益物权,归属物权法域。从排污权的特殊性和现有立法规定看,排污权是以环境资源为客体的一种环境权,归属于环境法域。 相似文献
167.
环境保护投资与工业污染控制关系研究 总被引:2,自引:0,他引:2
李若凝 《河南农业大学学报》2002,36(2):172-175
在定性分析基础上,通过建立模型研究了环境保护投资与4种主要污染物排放量的数量关系。结果表明,环境保护投资与污染物排放量呈负相关,且环境保护投资与废水排放量之间的相关程度最高(r=-0.9759),与SO2排放量之间的相关系数最低(r=-0.6543),即投资在废水防治方面取得了良好的效益,在SO2防治方面效果不佳。进一步分析表明,“三同时”投资(新建项目预防污染投资)对各污染物排放量的影响显著性大于老污染源治理投资,说明“三同时”投资发挥了较好的效益。 相似文献
168.
L. K. Smedema 《Irrigation and Drainage Systems》1989,3(2):143-151
Area reduction formulae of the type Q=cAi where Q=discharge, A=area and c and i are empirical coefficients, are still extensively used in land drainage design to estimate
discharges. This paper analyses the structure of these formulae and the meaning of the coefficients. Empirical values of the
coefficient i (area reduction factor) are assessed in the light of results of models studies and available rainfall distribution
indices. It is concluded that in the special case of land drainage design, area reduction type formulae have definite merits. 相似文献
169.
Quaternary organosilicon pyrethroid‐like ethers (five compounds) and alkanes (three compounds) were used for neurophysiological tests. Their activities in inducing repetitive firing in the central nervous cord of the American cockroach (Periplaneta americana) were evaluated by an extra‐cellular recording method. The ethers were more active than the corresponding alkanes. The ability of the compounds to cause conduction blockage was also measured using the same nerve preparations, but the effects were too weak to allow definitive activity values to be determined. The compounds prolonged the sodium tail‐current in the crayfish giant axon under voltage clamp conditions. The rate of decay of the tail‐current changed in parallel with the reported insecticidal activity against P americana. These findings indicated that tail‐current activity was the most useful nerve parameter in predicting insecticidal activity. Regression analysis of the numerical data together with those reported for other alkanes revealed that the higher the tail‐current activity, the higher the insecticidal activity when a structural feature and the hydrophobicity were considered separately. The insecticidal activity of the ethers was about seven‐fold higher than that of the alkanes with equivalent tail‐current activity and hydrophobicity. Variations in insecticidal activity were parabolically correlated with the hydrophobicity [(log P)opt = 9.1] when other factors were similar. © 2001 Society of Chemical Industry 相似文献
170.
A field experiment was carried out to investigate the effects of different emitter discharge rates under drip irrigation on soil salinity distribution and cotton yield in an extreme arid region of Tarim River catchment in Northwest China.Four treatments of emitter discharge rates,i.e.1.8,2.2,2.6 and 3.2 L/h,were designed under drip irrigation with plastic mulch in this paper.The salt distribution in the range of 70-cm horizontal distance and 100-cm vertical distance from the emitter was measured and analyzed during the cotton growing season.The soil salinity is expressed in terms of electrical conductivity(dS/m) of the saturated soil extract(EC e),which was measured using Time Domain Reflector(TDR) 20 times a year,including 5 irrigation events and 4 measured times before/after an irrigation event.All the treatments were repeated 3 times.The groundwater depth was observed by SEBA MDS Dipper 3 automatically at three experimental sites.The results showed that the order of reduction in averaged soil salinity was 2.6 L/h > 2.2 L/h > 1.8 L/h > 3.2 L/h after the completion of irrigation for the 3-year cotton growing season.Therefore,the choice of emitter discharge rate is considerably important in arid silt loam.Usually,the ideal emitter discharge rate is 2.4-3.0 L/h for soil desalinization with plastic mulch,which is advisable mainly because of the favorable salt leaching of silt loam and the climatic conditions in the studied arid area.Maximum cotton yield was achieved at the emitter discharge rate of 2.6 L/h under drip irrigation with plastic mulch in silty soil at the study site.Hence,the emitter discharge rate of 2.6 L/h is recommended for drip irrigation with plastic mulch applied in silty soil in arid regions. 相似文献