全文获取类型
收费全文 | 2697篇 |
免费 | 88篇 |
国内免费 | 400篇 |
专业分类
林业 | 175篇 |
农学 | 150篇 |
基础科学 | 886篇 |
769篇 | |
综合类 | 910篇 |
农作物 | 33篇 |
水产渔业 | 73篇 |
畜牧兽医 | 81篇 |
园艺 | 37篇 |
植物保护 | 71篇 |
出版年
2024年 | 38篇 |
2023年 | 48篇 |
2022年 | 109篇 |
2021年 | 116篇 |
2020年 | 107篇 |
2019年 | 136篇 |
2018年 | 75篇 |
2017年 | 135篇 |
2016年 | 143篇 |
2015年 | 104篇 |
2014年 | 148篇 |
2013年 | 160篇 |
2012年 | 209篇 |
2011年 | 175篇 |
2010年 | 135篇 |
2009年 | 129篇 |
2008年 | 124篇 |
2007年 | 152篇 |
2006年 | 129篇 |
2005年 | 114篇 |
2004年 | 97篇 |
2003年 | 98篇 |
2002年 | 72篇 |
2001年 | 56篇 |
2000年 | 54篇 |
1999年 | 46篇 |
1998年 | 53篇 |
1997年 | 43篇 |
1996年 | 47篇 |
1995年 | 32篇 |
1994年 | 34篇 |
1993年 | 12篇 |
1992年 | 17篇 |
1991年 | 13篇 |
1990年 | 6篇 |
1989年 | 10篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1962年 | 1篇 |
排序方式: 共有3185条查询结果,搜索用时 0 毫秒
71.
The Richards equation has been widely used for simulating soil water movement. However, the take-up of agro-hydrological models using the basic theory of soil water flow for optimizing irrigation, fertilizer and pesticide practices is still low. This is partly due to the difficulties in obtaining accurate values for soil hydraulic properties at a field scale. Here, we use an inverse technique to deduce the effective soil hydraulic properties, based on measuring the changes in the distribution of soil water with depth in a fallow field over a long period, subject to natural rainfall and evaporation using a robust micro Genetic Algorithm. A new optimized function was constructed from the soil water contents at different depths, and the soil water at field capacity. The deduced soil water retention curve was approximately parallel but higher than that derived from published pedo-tranfer functions for a given soil pressure head. The water contents calculated from the deduced soil hydraulic properties were in good agreement with the measured values. The reliability of the deduced soil hydraulic properties was tested in reproducing data measured from an independent experiment on the same soil cropped with leek. The calculation of root water uptake took account for both soil water potential and root density distribution. Results show that the predictions of soil water contents at various depths agree fairly well with the measurements, indicating that the inverse analysis is an effective and reliable approach to estimate soil hydraulic properties, and thus permits the simulation of soil water dynamics in both cropped and fallow soils in the field accurately. 相似文献
72.
基于正交试验的斜击式水涡轮优化与分析 总被引:1,自引:0,他引:1
参考斜击式水轮机结构设计方法,使用正交试验法对JP75卷盘喷灌机水涡轮的主要几何参数进行优化试验,并对优化结果进行相关系数及主成分分析,最后确定影响效率的主要因素及水涡轮最优参数组合.使用优化后的参数建立水涡轮最优模型,并采用CFD数值计算方法对流量为500~900 r/min、转速为18~40 m3/h下优化模型的性能进行计算及分析;在此基础上,分析了水涡轮外特性曲线,优化模型在非设计转速下具有较高的稳定性和高效性.在流量区间内,优化模型最高效率点提升了近31%;研究了水涡轮优化模型的内部流场分布, 相比于原有模型,内部流场更为合理流畅,进口管道内压力分布均匀,且能够形成有效射流,同时,水涡轮叶片的优化设计解决了存在的局部高压区和叶间涡问题;与原有模型进行能量转化率比较,验证了转轮处能量有效利用率明显提升了42.71%. 相似文献
73.
74.
Dimitrios FanourakisSusana M.P. Carvalho Domingos P.F. Almeida Olaf van KootenWouter G. van Doorn Ep Heuvelink 《Postharvest Biology and Technology》2012,64(1):64-73
A constant high relative air humidity (RH) during cultivation can strongly reduce the vase life in some cut rose cultivars. We studied three contrasting cultivars in their tolerance to high RH in order to analyse in detail the water relations during postharvest and better understand this genotypic variation. Plants were grown at moderate (60%) and high (95%) RH, and cut flowers were placed in water immediately after cutting. Flowers of cv. Pink Prophyta grown at high RH did not open throughout vase life, while flower opening of cvs. Frisco and Dream was not affected by preharvest RH. Cultivation at high RH resulted in about 80% shorter vase life in Pink Prophyta, whereas in Dream and Frisco the negative effect was considerably smaller (15 and 9% shorter vase life, respectively). The shorter vase life and reduced flower opening of cut roses grown at high RH was due to a higher rate of transpiration both in the light and dark periods. It was found that the leaves of Pink Prophyta grown at high RH could partly close their stomata upon lowering of the water potential or when flower stalks were fed with abscisic acid, but stomata remained far more open than in leaves grown at moderate RH. The RH during cultivation did not affect stem hydraulic conductivity and its recovery after air emboli induction. Preventing vascular occlusion largely alleviated the high-cultivation-RH effect on vase life and flower opening, showing that the effect of high-cultivation-RH becomes only important if water uptake is limited. 相似文献
75.
In Central Europe, various plant species including large-grain legumes and their mixtures are grown as catch crops, particularly between grains harvested early and subsequent summer crops. This article investigates the question of how soil structure in the topsoil is influenced when catch cropping with large-grain legumes (experimental factor A: without catch crop, with catch crop) under different ploughless tillage conditions during catch crop seeding (experimental factor B: deep tillage/25–30 cm, shallow tillage/8–10 cm). Five one-year trials were performed using standard machinery at various sites in Germany. Soil core samples extracted from the topsoil in the spring after catch crop cultivation served to identify air capacity, saturated hydraulic conductivity and precompression stress. The above-ground and below-ground biomass yields of the catch crops were also determined at most of the sites. In addition, the soil compaction risk for the working steps in the experiments was calculated using the REPRO model.The dry matter yield of the catch crops varied considerably between the individual trial sites and years. In particular, high levels of dry matter were able to form in the case of early seeding and a sufficient supply of precipitation. The soil structure was only rarely affected positively by catch crop cultivation, and catch crops did not contribute in the short term to loosening already compacted topsoils. In contrast, mechanical soil stresses caused by driving over the ground and additional working steps used in cultivating catch crops often led to lower air capacity in these treatments. This is consistent with the soil compaction risks calculated using the REPRO model, which were higher in the treatments with catch cropping. Catch crop cultivation also only resulted in improved mechanical stability at one location. The positive effect of deep ploughless tillage on air capacity and saturated hydraulic conductivity, however, became more clearly evident regardless of catch crop cultivation. In order for catch crop cultivation with large-grain legumes to be able to have a favourable impact on soil structure, it is therefore important that cultivating them does not result in any new soil compaction. In the conditions evaluated, deep tillage was more effective at loosening compacted topsoil than growing catch crops. 相似文献
76.
77.
水平并并下工具模拟试验装置液压系统的功能是提供液压动力源和接受控制指令,按要求完成主液压缸、调整机构和升降机构的往复运动;完成模拟试验装置的运动和拉、压力检测.文章对水平井井下工具模拟试验装置液压系统的设计方案进行设计并对比分析. 相似文献
78.
液压技术在机械制造业、采矿业、环境、电力等行业中有着广泛的应用,尤其是在机械制造业中,是基础领域。文章主要分析了机械制造业中液压技术的重要地位,并且详细论述了液压技术在我国的机械制造业中应用的状况,同时提出,为了提高机械制造业的效率,我们要对液压技术进行不断的研究和提高。 相似文献
79.
综述了国内外拖拉机液压提升能力试验的几种加载方案,并就其优缺点进行了探讨,给出了一种适于基层单位进行液压提升器检测维修的经济型方案。 相似文献
80.