全文获取类型
收费全文 | 6611篇 |
免费 | 385篇 |
国内免费 | 1170篇 |
专业分类
林业 | 441篇 |
农学 | 708篇 |
基础科学 | 2660篇 |
1615篇 | |
综合类 | 2014篇 |
农作物 | 125篇 |
水产渔业 | 179篇 |
畜牧兽医 | 145篇 |
园艺 | 49篇 |
植物保护 | 230篇 |
出版年
2024年 | 38篇 |
2023年 | 165篇 |
2022年 | 284篇 |
2021年 | 271篇 |
2020年 | 283篇 |
2019年 | 334篇 |
2018年 | 230篇 |
2017年 | 353篇 |
2016年 | 426篇 |
2015年 | 344篇 |
2014年 | 391篇 |
2013年 | 440篇 |
2012年 | 563篇 |
2011年 | 527篇 |
2010年 | 419篇 |
2009年 | 377篇 |
2008年 | 327篇 |
2007年 | 387篇 |
2006年 | 336篇 |
2005年 | 281篇 |
2004年 | 192篇 |
2003年 | 195篇 |
2002年 | 131篇 |
2001年 | 108篇 |
2000年 | 101篇 |
1999年 | 93篇 |
1998年 | 88篇 |
1997年 | 79篇 |
1996年 | 71篇 |
1995年 | 57篇 |
1994年 | 50篇 |
1993年 | 51篇 |
1992年 | 37篇 |
1991年 | 26篇 |
1990年 | 37篇 |
1989年 | 23篇 |
1988年 | 25篇 |
1987年 | 12篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1956年 | 1篇 |
排序方式: 共有8166条查询结果,搜索用时 46 毫秒
71.
Farmers in the North Central region of the United States often are reluctant to use shelterbelts because of inadequate information clearly showing their benefits. We developed a computer model, called SBELTS for ShelterBELT and Soybeans, that simulates the influence of a shelterbelt on soybean (Glycine max L.) production across an agricultural field in the midwestern United States. Objectives of this study were to 1) describe the structure of SBELTS, 2) present model simulations, and 3) discuss model limitations. SBELTS is composed of three submodels. The first submodel produces characteristics of a shelterbelt that are passed to the second submodel that estimates daily windrun at user-specified distances leeward (away from the prevailing wind) of the shelterbelt. Estimated daily windruns are merged with other microclimatic information to produce weather files for each specified distance. Finally, the third submodel uses a soybean growth/yield model to estimate soybean yield at specified distances, and the yields are averaged. Data collection was not a part of the present project, so SBELTS was evaluated by comparing predicted results with published information. SBELTS was used to predict soybean yield across a field leeward of a 7.6 m tall shelterbelt, and the predicted yield curve compared well with published yield curves. The sensitivity of SBELTS to variation in rainfall was evaluated by predicting yields for 3.8 m tall and 7.6 m tall shelterbelts in wet, normal, and dry years. Results showed no shelterbelt influence in wet years, some influence in normal years, and a sizable influence in dry years. Results showed that the 7.6 m shelterbelt had more influence than the 3.8 m shelterbelt. Although SBELTS has limited use, it is the first step in the development of more advanced models that will be able to simulate production of soybeans and other crops under the influence of shelterbelts on a variety of soil types.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
72.
Trade-offs among wood production, wood quality and ecological characteristics in the management of harvested forest stands are explored through model simulation of various silvicultural regimes. Long-term production of merchantable wood, production of various types of high-quality wood, and the level of certain quantitative ecological indicators are projected for coniferous forests of Pacific Northwestern USA. The set of ecological indicators used is based on the species composition and physical structure of old, unlogged forest stands. Simulations are performed with an ecological model of forest stand dynamics that tracks the fate of live and dead trees. Short rotations (<50 years) produce the least amount of high-quality wood over the multi-century simulation period. They also fail to generate ecological attributes resembling those of old forest stands. Production of high-quality wood is moderate to high under all rotations of 80 years or more; however, most ecological indicators require longer rotations unless alternatives to clearcutting are applied. Alternatives examined include retention of 15% cover of live tree canopy at each harvest in combination with artificial thinning between harvests. Thinning from below can expedite the development of large live and dead trees, and canopy height diversity without greatly diminishing wood quantity or quality. Proportional thinning retains understory stems, thereby expediting the recruitment of shade-tolerant trees. A possible drawback to thinning, particularly proportional thinning, is the diminished production of clean-bole wood at rotations of 150 and 260 years. It is concluded that most wood quantity, wood quality and ecological objectives can be met with long rotations (ca. 260 years). Certain objectives can be met with shorter rotations (80–150 years) when treatments of thinning and canopy tree retention are applied. 相似文献
73.
74.
Application of an airborne laser scanner to forest road design with accurate earthwork volumes 总被引:1,自引:0,他引:1
In this study we developed a forest road design program based on a high-resolution digital elevation model (DEM) from a light detection and ranging (LIDAR) system. After a designer has located the intersection points on a horizontal plane, the model first generates the horizontal alignment and the ground profile. The model precisely generates cross-sections and accurately calculates earthwork volumes using a high-resolution DEM. The model then optimizes the vertical alignment based on construction and maintenance costs using a heuristic technique known as tabu search. As the distance between cross-sections affects the accuracy of earthwork volume calculations, the results were examined by comparing them with the exact earthwork volume calculated by the probabilistic Monte Carlo simulation method. The earthwork volumes calculated by the Pappus-based method were similar to those calculated by the Monte Carlo simulation when the distance between cross-sections was within 10m. The model was applied to a high-resolution DEM from the LIDAR of Capitol Forest in Washington State, USA. The model generated a horizontal alignment, length 827m, composed of five horizontal curves. We examined the number of grade change points. The results indicated that tabu search found the best solution ($61.42/m) with five grade change points. This was composed of two vertical curves that almost followed the ground profile. As the accuracy of a high-resolution DEM from LIDAR increases, the model would become a useful tool for a forest road designer because it eliminates or at least reduces the time-consuming process of road surveys. 相似文献
75.
基于细胞数字特征的板材材种识别技术 总被引:10,自引:0,他引:10
木材识别是合理利用木材最基本的前提。为了解决人工鉴定失误率高的问题,本文提出基于细胞数学描述理论的板材识别方法,采用木材端面细胞的显微数字图像,利用板材材种的数学仿真技术、图像处理技术、数据库管理和系统辨识方法,提取细胞轮廓形态和尺寸等参数建立基准细胞和材种识别数字化参数库。减少传统的依赖图像像素特征识别比较方法的不确定性,创新材种识别的数字化理论体系,推动板材材种识别的实用化。 相似文献
76.
基于虚拟样机技术的双横臂独立前悬架振动仿真分析及参数优化 总被引:2,自引:0,他引:2
根据振动力学以及多刚体系统动力学原理,分析了悬架系统的振动模型,同时对双横臂独立前悬架做了合理的简化,建立了双横臂独立式前悬架力学及虚拟样机模型,并在虚拟样机软件ADAMS/VIEW模块上进行仿真,在此基础上对前悬架的各个参数进行优化设计,使得悬架振动达到一个最优值。 相似文献
77.
介绍了开放式体系结构数控系统的概念、特点及分类,并简要介绍了开放式数控系统在电火花线切割机上的应用。 相似文献
78.
中国木材工业数控化的普及 总被引:17,自引:0,他引:17
分析我国木材加工机械制造业及其数控化的现状,提出发展分工明确的产业集团,研究进入木工机械数控化的突破点,引进国外数控木材加工机械的先进技术,依靠产学研合作攻关,开发有中国特色的数控机械产品,将是我国木材加工机械数控化发展的方向和模式。 相似文献
79.
80.